Advances in Materials Science and Engineering

Volume 2013 (2013), Article ID 578460, 8 pages

http://dx.doi.org/10.1155/2013/578460

Research Article

Hydrothermal Synthesis of Pt-, Fe-, and Zn-doped Nanospheres and Carbon Monoxide Sensing Properties

Weigen Chen,¹ Qu Zhou,¹ and Shudi Peng²

¹State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400030, China

²Chongqing Electric Power Research Institute, Chongqing 401123, China

Received 12 March 2013; Revised 23 July 2013; Accepted 24 July 2013

Academic Editor: Shivakiran Bhaktha

Copyright © 2013 Weigen Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Pure and M-doped (M = Pt, Fe, and Zn) SnO₂ nanospheres were successfully synthesized via a simple and facile hydrothermal method and characterized by X-ray powder diffraction, field-emission scanning electron microscopy, and energy dispersive spectroscopy. Chemical gas sensors were fabricated based on the as-synthesized nanostructures, and carbon monoxide sensing properties were systematically measured. Compared to pure, Fe-, and Zn-doped SnO₂ nanospheres, the Pt-doped SnO₂ nanospheres sensor exhibits higher sensitivity, lower operating temperature, more rapid response and recovery, better stability, and excellent selectivity. In addition, a theoretical study based on the first principles calculation was conducted. All results demonstrate the potential of Pt dopant for improving the gas sensing properties of SnO₂-based sensors to carbon monoxide.

1. Introduction

Carbon monoxide (CO), mainly generated from the burning of fossil fuels and malfunctioning equipments, is a kind of colorless, tasteless, widespread, and toxic gas [1–5]. Simultaneously, CO is an important fault characteristic gas dissolved in power transformer oil and has been widely used in evaluating the insulation performance of running transformer [6–8]. Therefore, it is essential to develop high performance CO gas sensors for environmental monitoring and industrial applications [9–13].

Due to the advantages of simple fabrication process, low maintenance cost, and long useful life, metal oxide semiconductors, such as SnO₂ [14], ZnO [15], In₂O₃ [16], TiO₂ [17], WO₃ [18], and NiO [19], have been widely used for fabricating chemical gas sensors. Among these oxide sensors, SnO₂, as an interesting chemically and thermally stable n-type semiconductor with wide band gap energy and large exciton binding energy, is regarded as one of the most promising sensing materials against CO gas, and the development of SnO₂ sensors for CO detection has been the focus of numerous research works in the world [20–24]. However, there still exist some limitations needed to be further improved, such as low sensitivity, poor selectivity, and bad stability.

At present, numerous efforts have been made to improve the sensing properties of metal oxide semiconductor sensors, among which, the strategy of doping modification with various metallic elements, for example, noble metal [25, 26], rare-earth metal [27], transition metal [28], and metal oxide [29], has been proven effective. Due to unique catalytic and electronic activities, these dopants are highly commended for catalyzing various gas sensing reactions. The current reports on the sensing behaviors of various metals-doped SnO₂ are mainly based on trial-and-error experiments, and its gas sensing mechanism remains controversial. Till now, the first principles theoretical calculation has been successfully used to investigate structural [30], electronic [31], magnetism [32], optical [33], and gas sensing properties [34, 35] of semiconductor SnO₂.

To the best of our knowledge, reports on the synthesis of Pt, Fe and Zn-doped SnO₂ nanospheres and their sensing properties against CO are rare, and there is no literature about an atomic level understanding of its sensing mechanism. For these reasons, an atomic level understanding of CO gas sensing behaviors of Pt-, Fe-, and Zn-doped SnO₂ is performed.

Herein, we present a simple, facile, and green hydrothermal synthesis of pure and M-doped (M = Pt-, Fe-, and Zn) SnO₂ nanospheres. To demonstrate the potential applications, chemical gas sensors were fabricated with the as-synthesized SnO₂ nanospheres, and CO gas sensing properties were investigated in detail. The Pt-doped SnO₂ nanospheres sensor exhibits significantly enhanced CO sensing performances in terms of high sensitivity, low operating temperature, better selectivity, rapid response and recovery time, and good reproducibility in comparison with those of pure, Fe-, and Zn-doped SnO₂ nanospheres. In addition, with the first principles, a theoretical calculation was performed to further understand its sensing mechanism.

2. Experimental

2.1. Synthesis and Characterization of SnO₂ Nanoparticles

Pt-, Fe-, and Zn-doped SnO₂ nanospheres were prepared by a simple and facile hydrothermal synthesis route [14]. All the raw chemicals are analytical-grade reagents purchased from Beijing Chemicals Co. Ltd. and used as received without any further purification.

The detailed synthesis process of Pt-doped SnO₂ nanospheres is as follows. Typically, 20 mL of absolute ethanol, 20 mL of distilled water, 5.0 mmol SnCl₄5H₂O, 0.10 mmol H₂PtCl₆6H₂O, and 30 mmol ammonia hydroxide were mixed together in a 100 mL capacity beaker and magnetically stirred at room temperature for 30 min. Then, the fully mixed precursor was transferred into a 100 mL Teflon-lined stainless steel autoclave, which was completely sealed and subsequently heated at 200°C for 24 h in an electric furnace to produce the precipitate. After cooling to room temperature, the prepared precipitate was harvested by centrifugation and washed with distilled water and absolute ethanol three times, respectively. Finally, the product was dried at 80°C in air for 24 h.

Fe- and Zn-doped SnO₂ nanospheres were obtained in the same process except that 0.10 mmol FeCl₃6H₂O and Zn(CH₃COO)₂2H₂O were added as the source of Fe³⁺ or Zn²⁺ ions, respectively. In this study, pure SnO₂ nanospheres were also synthesized via hydrothermal method for the purpose of comparison.

The crystalline structures of the as-prepared pure, Pt-, Fe-, and Zn-doped SnO₂ nanospheres were performed by X-ray powder diffraction (XRD) at room temperature using a Rigaku D/max-1200X diffractometer (Tokyo, Japan) with Cu Kα1 radiation (40 kV, 200 mA, and λ = 1.5406 Å). A Nova 400 Nano Field-emission scanning electron microscopy (FESEM, FEI, Hillsboro, OR, USA) equipped with an Oxford Inca 250 EDS energy-dispersive spectroscopy detector was used to characterize the surface morphologies and chemical compositions of the prepared samples.

2.2. Fabrication and Measurement of SnO₂-Based Sensors

Chemical gas sensors based on the as-synthesized SnO₂ nanospheres were fabricated as follows. First, the as-synthesized samples were mixed with diethanolamine and absolute ethanol at a weight ratio of 8 : 1 : 1 to form a paste. It was subsequently screen printed onto an alumina ceramic tube, where a pair of Au electrodes has been previously placed at each side. And then, the alumina ceramic tube was dried at 100°C for 2 h in air and sintered at 400°C for 2 h. Finally, an Ni-Cr heating wire was inserted into the tube to form a side-heated gas sensor. The structure diagram of the sensor is similar to that presented in our previous works [7, 14]. The as-prepared sensor was further aged at 200°C for a week in an aging test chamber to improve its stability and repeatability.

The gas sensing properties were measured by a CGS-8 intelligent gas sensing analysis system purchased from Beijing Elite Tech Co., Ltd., China. The operating temperature of the sensor could be adjusted precisely through varying the heating current of the Ni-Cr heater. When a smooth and stable baseline resistance was reached, a certain concentration of CO gas was injected into the test chamber. All tests were performed at constant environment temperature and relative humidity. Gas response in this study was defined as , where and are the sensor resistance in air and in target gas, respectively [26]. The time taken by the sensor to achieve 90% of the total resistance change was designated as the response time in the case of gas adsorption or the recovery time in the case of gas desorption [4].

2.3. Computational Method

Based on the density functional theory (DFT), the first principles calculations were performed using the CASTEP code (Cambridge Sequential Total Energy Package) in this study [36–40]. Generalized gradient approximation (GGA) in the scheme of revised- Perdew-Burke-Ernzerhof (RPBE) was employed to describe the exchange and correlation functional between electrons. Interaction between nuclei and electrons was approximated with planewave ultrasoft pseudopotential [7, 34]. The valence electron configurations for C, O, and Sn atoms were chosen as 2s² 2p², 2s² 2p⁴, and 5s² 5p², respectively, for structural and electronic calculations. The cut-off energy of the planewave was set at 380 eV throughout this work, which ensures a maximum root-mean-square convergent tolerance of  eV/atom. In the whole process of geometry optimization and energy calculation, atoms were freely allowed to relax in all directions.

3. Results and Discussion

3.1. Structural Characterization

X-ray powder diffraction measurement was firstly performed to check the crystalline structures of the synthesized nanostructures. Figure 1 shows the typical XRD patterns of the prepared pure, Pt-, Fe-, and Zn-doped SnO₂ samples. It can be clearly seen in Figure 1 that the synthesized samples are polycrystalline in nature and the XRD patterns of the prepared pure and M-doped SnO₂ powders are well corresponding with JCPDS card number 41-1445. All diffraction peaks can be perfectly indexed to the rutile SnO₂ phases, and due to the low amount of dopants (2 at%) in the samples, there is no indication of the presence of other metal oxide diffraction peaks, implying a high purity of our products.

Figure 1: XRD patterns of pure, Pt-, Fe-, and Zn-doped SnO₂ nanospheres.

An energy dispersive spectroscopy analysis was utilized to confirm the chemical compositions of the synthesized samples. Figure 2 shows the EDS spectra of pure and M-doped SnO₂ nanospheres. As shown in Figure 2(a), only Sn, O, C, and Cu peaks are observed for pure SnO₂, and no other peaks have been found. Nevertheless, characteristic peaks from Fe, Pt, and Zn elements are measured in Figures 2(b), 2(c), and 2(d), respectively. Simultaneously, the radius of Pt⁴⁺ (0.625 Å), Fe³⁺ (0.645 Å), and Zn²⁺ (0.72 Å) is obviously less than or nearly equal to that of Sn⁴⁺ (0.71 Å); so, it is believed Pt⁴⁺, Fe³⁺, and Zn²⁺ could be doped into the SnO₂ matrix. Thus, based on the EDS results, we believe that Pt⁴⁺, Fe³⁺, and Zn²⁺ ions have been successfully incorporated into the SnO₂ nanostructures [26].

(a)

(b)

(c)

(d)

Figure 2: EDS spectra of pure, Fe-, Pt-, and Zn-doped SnO₂ nanospheres.

Field-emission scanning electron microscopy was further used to research the surface structural characteristics of the prepared samples. Typical FESEM micrographs are recorded and shown in Figure 3. As seen in Figures 3(a)–3(d), beautiful nanospheres are observed for pure and M-doped SnO₂ nanostructures. These nanospheres are rather dispersed and highly uniform in shape and size. The diameters of the samples are ranging from 35 to 40 nm. On comparison of the morphology of pure SnO₂ with that of the M-doped samples, similar microstructure and surface morphology are observed, which indicates that the dopant of Pt²⁺, Fe³⁺, and Zn²⁺ ions affects the microstructure and morphology of SnO₂ nanostructures slightly.

(a)

(b)

(c)

(d)

Figure 3: FESEM images of (a) pure, (b) Pt-, (c) Fe-, and (d) Zn-doped SnO₂ nanospheres.

3.2. Gas Sensing Properties

To investigate how the ions doped in SnO₂ nanospheres influence the gas sensing properties of the sensors, we first measured the gas responses of the pure and Pt-, Fe-, and Zn-doped sensors to 100 ppm of CO at a series of operating temperatures ranging from 100°C to 450°C. As shown in Figure 4 for each sensor, the gas response curve increases and arrives to its maximum at a certain temperature, namely, the optimum operating temperature, and then decreases rapidly with further increasing operating temperature. The optimum operating temperatures of the Pt-, Fe-, and Zn-doped SnO₂ sensors are measured to be about 300, 350, and 350°C, respectively. It is also observed that at a same working temperature, the Pt-doped SnO₂ sensor exhibits obviously higher gas response than that of pure, Fe-, and Zn-doped SnO₂ sensors. The highest gas response of the Pt-doped SnO₂ sensor is 33.12 at 300°C, while only 11.15, 13.42, and 16.89 for pure, Zn-, and Fe-doped sensors at 350°C.

Figure 4: Gas responses of the sensors to 100 ppm of CO at various operating temperatures.

Figure 5 represents the gas responses of the pure and Pt-, Fe-, and Zn-doped SnO₂ sensors as a function of CO gas concentration with sensor working at its own optimum operating temperature as measured above. As shown in Figure 5, each sensor exhibits a nearly linear gas response curve against CO with gas concentration in the range of 5–300 ppm. The Pt-doped SnO₂ sensor exhibits enhanced CO sensing response compared with pure, Fe-, and Zn-doped sensors, which implies Pt doping can effectively improve the gas response of SnO₂ sensor to CO gas. Thus, the SnO₂ nanosphere doped with Pt²⁺ is a promising CO sensing material for fabricating high response CO sensors.

Figure 5: Gas responses of the sensors to different CO concentrations.

It is well known that response and recovery characteristics are important for evaluating the performances of semiconductor oxide sensors. Figure 6 shows the response and recovery properties of the Pt-, Fe-, and Zn-doped SnO₂ nanospheres sensors to 100 ppm of CO gas at each optimum operating temperature. It can be seen in Figure 6 that compared with Fe- and Zn-doped SnO₂ nanospheres, the sensor based on 2 at% Pt-decorated SnO₂ nanospheres shows shortest response and recovery time (8 s and 12 s, resp.). Such a rapid response and recovery property could be attributed to the excellent electronic sensitization and catalytic activities of Pt ions.

Figure 6: Response and recovery curves of the sensors to 100 ppm of CO.

The gas responses of the sensors to 200 ppm of CO gas were investigated in two months to investigate its stability and repeatability. The measurements were reported every 10 days. As shown in Figure 7, slight variation is observed for Pt-doped sensor during the long experimental cycle, while much higher fluctuations are performed for pure, Fe-, and Zn-doped sensors. All results demonstrate the Pt-doped sensor exhibits more prominent long-term stability and repeatability against CO than pure, Fe-, and Zn-doped SnO₂ nanospheres sensors.

Figure 7: Stability and repeatability of the sensors to 100 ppm of CO.

Selectivity is another important characteristic for metal oxide semiconductor sensors, especially with potential interference gases. Figure 8 depicts the histogram of the gas responses of Pt-, Fe-, and Zn-doped SnO₂ sensors to 100 ppm of CO and potential interference gases, including CH₃OH, NH₃, H₂S, and NO. As shown in Figure 8, the sensor based on 2 at% Pt-doped SnO₂ nanospheres exhibits considerably lower response values to other potential interface gases at 300°C. All gas sensing properties measured above demonstrate the 2 at% Pt-doped SnO₂ nanospheres sensor particularly interesting for CO detection in practice. Based on the gas measurements above, we can conclude that Pt doping effectively enhances the gas sensing performances of SnO₂-based sensor to CO.

Figure 8: Selectivity of the 2 at% Pt-, Fe-, and Zn-doped SnO₂ sensors to 100 ppm of various gases at 300°C.

3.3. Theoretical Calculations

As a typical n-type semiconductor material, the gas sensing properties of SnO₂ are dominantly controlled by the surface. It is well known that the (110) surface is the most thermodynamically stable plane in tetragonal rutile SnO₂ [30, 31], and it has been widely used to investigate SnO₂ surface properties, such as structural, electronic, optical, magnetic, and gas sensing properties. Therefore, we performed a first principles calculation to go insight into how the doped metallic ions influence the gas sensing performances of SnO₂.

Figure 9(a) shows an established slab model of SnO₂ (110) surface, which is constituted of Sn₁₂O₂₄ with nine layers. To preserve symmetry, the top and bottom layers of the slab were taken to be identical. The central five layers were constrained at their bulk-like sites, and the surface and subsurface layers on either side of the slab were allowed to relax freely in all directions. A surface vacuum slab of 10 Å was added to avoid unnecessary interactions between the slabs, and then a supercell of Sn₄₈O₉₆ was employed.

(a)

(b)

Figure 9: Schematic of (a) SnO₂ (110) surface model (b) doping and adsorption model.

One Sn atom was first substituted by Pt, Fe, or Zn atom in the surface or subsurface layers to obtain the doping model (Sn₄₇MO₉₆) with a doping ratio of 2.08%. Four probable doping configurations were illustrated as shown in Figure 9(b): substitution in , , sub- and sub-, cite, respectively. The doping formation energy was defined as [34]

where and represent the total energy of M-doped and pure SnO₂ supercells after geometry optimization, and and are the chemical potentials of each Sn and M atom in the bulk Sn and M crystal, respectively. According to the definition, the lower the doping formation energy is, the more stability the doping model has.

As shown in Table 1, for each metal doping model, the doping formation energy of substitution in position is lower than that in , sub-, and sub- cites, respectively. Therefore, Pt, Fe, and Zn atoms are all prior to occupy the site on the top layer of SnO₂ (110) surface. In the following discussion, we mainly consider Pt, Fe, and Zn atoms substituted on site.

Table 1: The doping formation energy of .

Doping formation

energy (eV)

(Pt → Sn)
3.52
4.15
5.08
5.79

(Fe → Sn)
3.79
4.37
5.13
6.08

(Zn → Sn)
4.23
4.96
5.68
6.43

Figure 10 shows the calculated total density of states (DOS) of pure, Pt-, Fe-, and Zn-doped SnO₂ (110) surfaces. Compared with pure SnO₂, the DOS of M-doped SnO₂ (110) surface shifted downward, its band gap narrowed, and some new doping levels appeared near the Femi level, which could benefit charge transfer between SnO₂ valence band and condition band and improve the gas sensing properties of the prepared SnO₂ based sensors.

Figure 10: DOS of pure, Pt-, Fe-, and Zn-doped SnO₂ (110) surfaces.

As reported in former works, CO gas molecule prefers to be adsorbed on site with C end [7]; thus, in this study CO gas molecule was perpendicularly imported onto the atom with C end, and the initial vertical height between C atom of CO and atom of SnO₂ (110) surface was set as 2 Å, where C atom could bond with atom freely.

Figure 11 shows the partial density of states (PDOS) of CO gas molecule after adsorption. One can clearly see in Figure 11 that the PDOS of CO has a leftward moving trend and the moving range decreases in the order of Pt-, Fe-, Zn-doped and pure SnO₂ surfaces. As shown in (a), when CO was adsorbed on pure surface, almost no other changes were observed except a little of band shift. For Pt-doped one (b), the PDOS of CO assigned by 5σ orbits nearly vanishes totally, and the 4σ orbits also diminish mostly, which implies a strong chemical reaction and banding between C atom and atom. When CO was adsorbed on Fe-doped (c) and Zn-doped SnO₂ models, similar changes are observed but much slightly.

Figure 11: PDOS of CO molecule after adsorption.

Further quantitative understanding can be obtained by analyzing the charge transfer between CO molecule and adsorption surface [34]. Based on the Mulliken population analysis, the corresponding change transfers are 0.06, 0.22, 0.10, and 0.08 e when CO molecule adsorbed on pure, Pt-, Fe-, and Zn-doped SnO₂ (110) surface. Electrons lost from CO are received by the SnO₂ surface, decreasing the height of barrier in the depletion region of SnO₂ and increasing the concentration of charge carriers. Thus, increasing output voltage and gas response were observed in the gas sensing analysis system.

4. Conclusions

In conclusion, a simple hydrothermal synthesis of pure, 2 at% Pt-, Fe-, and Zn-doped SnO₂ nanospheres was demonstrated in the present paper. The as-synthesized nanostructures were characterized by XRD, EDS, and FESEM. Chemical gas sensors were fabricated, and CO gas sensing properties were discussed in detail. Comparative gas sensing tests show that the 2 at% Pt-doped SnO₂ nanospheres sensor exhibits excellent CO sensing performance in terms of low operating temperature, high response, rapid response and recovery time, and good reproducibility. Moreover, it shows good selectivity to potential interference gases. Based on the first principles calculation, its sensing mechanism was discussed. All results demonstrate that the Pt-doped SnO₂ nanospheres may be a potential candidate for fabricating high-performance CO gas sensors.

Acknowledgments

This work was supported in part by the National Natural Science Foundation of China (nos. 51277185 and 51202302), National Special Fund for Major Research Instrumentation Development (no. 2012YQ160007), the Funds for Innovative Research Groups of China (no. 51021005), and China Postdoctoral Science Foundation (no. 2012M511904).

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