# Hydrothermal Synthesis of Pt-, Fe-, and Zn-dopedSnO2 Nanospheres and Carbon Monoxide Sensing Properties

Authors: Weigen Chen; Qu Zhou; Shudi Peng
Journal: Advances in Materials Science and Engineering
 (2013)

---

## 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.


---

## Body

## 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 SnCl4·5H2O, 0.10 mmol H2PtCl6·6H2O, 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 FeCl3·6H2O and Zn(CH3COO)2·2H2O 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 asS=Ra/Rg, where Ra and Rg 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 1.0×10-6 eV/atom. In the whole process of geometry optimization and energy calculation, atoms were freely allowed to relax in all directions.


## 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 SnCl4·5H2O, 0.10 mmol H2PtCl6·6H2O, 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 FeCl3·6H2O and Zn(CH3COO)2·2H2O 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 asS=Ra/Rg, where Ra and Rg 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 1.0×10-6 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. Figure1 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. Figure2 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].EDS spectra of pure, Fe-, Pt-, and Zn-doped SnO2 nanospheres.
(a)
(b)
(c)
(d)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 Figure3. 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.FESEM images of (a) pure, (b) Pt-, (c) Fe-, and (d) Zn-doped SnO2 nanospheres.
(a)
(b)
(c)
(d)


### 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.Figure5 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. Figure6 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 Figure7, 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. Figure8 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.Figure9(a) shows an established 2×1×1 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 4×2×1 supercell of Sn48O96 was employed.Schematic of (a) SnO2 (110) surface model (b) doping and adsorption model.
(a)
(b)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 Sn5C, Sn6C, sub-Sn5C and sub-Sn6C, cite, respectively. The doping formation energy Ef was defined as [34]
(1)Ef=E(Sn47MO96)-E(Sn48O96)+μSn-μM,
where E(Sn47MO96) and E(Sn48O96) represent the total energy of M-doped and pure SnO2 supercells after geometry optimization, and μSn and μM 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 Ef is, the more stability the doping model has.As shown in Table1, for each metal doping model, the doping formation energy of substitution in Sn5C position is lower than that in Sn6C, sub-Sn5C, and sub-Sn6C cites, respectively. Therefore, Pt, Fe, and Zn atoms are all prior to occupy the Sn5C site on the top layer of SnO2 (110) surface. In the following discussion, we mainly consider Pt, Fe, and Zn atoms substituted on Sn5C site.Table 1
The doping formation energy ofSn47MO96.
Doping formationenergy (eV)
S
n
5C
S
n
6C
Sub-
Sn
5C
Sub-
Sn
6C
E
f(Pt → Sn)
3.52
4.15
5.08
5.79
E
f(Fe → Sn)
3.79
4.37
5.13
6.08
E
f(Zn → Sn)
4.23
4.96
5.68
6.43Figure10 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 onSn5C site with C end [7];  thus, in this study CO gas molecule was perpendicularly imported onto the Sn5C atom with C end, and the initial vertical height between C atom of CO and Sn5C atom of SnO2 (110) surface was set as 2 Å, where C atom could bond with Sn5C atom freely.Figure11 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 Sn5C 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.


## 3.1. Structural Characterization

X-ray powder diffraction measurement was firstly performed to check the crystalline structures of the synthesized nanostructures. Figure1 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. Figure2 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].EDS spectra of pure, Fe-, Pt-, and Zn-doped SnO2 nanospheres.
(a)
(b)
(c)
(d)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 Figure3. 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.FESEM images of (a) pure, (b) Pt-, (c) Fe-, and (d) Zn-doped SnO2 nanospheres.
(a)
(b)
(c)
(d)


## 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.Figure5 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. Figure6 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 Figure7, 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. Figure8 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.Figure9(a) shows an established 2×1×1 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 4×2×1 supercell of Sn48O96 was employed.Schematic of (a) SnO2 (110) surface model (b) doping and adsorption model.
(a)
(b)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 Sn5C, Sn6C, sub-Sn5C and sub-Sn6C, cite, respectively. The doping formation energy Ef was defined as [34]
(1)Ef=E(Sn47MO96)-E(Sn48O96)+μSn-μM,
where E(Sn47MO96) and E(Sn48O96) represent the total energy of M-doped and pure SnO2 supercells after geometry optimization, and μSn and μM 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 Ef is, the more stability the doping model has.As shown in Table1, for each metal doping model, the doping formation energy of substitution in Sn5C position is lower than that in Sn6C, sub-Sn5C, and sub-Sn6C cites, respectively. Therefore, Pt, Fe, and Zn atoms are all prior to occupy the Sn5C site on the top layer of SnO2 (110) surface. In the following discussion, we mainly consider Pt, Fe, and Zn atoms substituted on Sn5C site.Table 1
The doping formation energy ofSn47MO96.
Doping formationenergy (eV)
S
n
5C
S
n
6C
Sub-
Sn
5C
Sub-
Sn
6C
E
f(Pt → Sn)
3.52
4.15
5.08
5.79
E
f(Fe → Sn)
3.79
4.37
5.13
6.08
E
f(Zn → Sn)
4.23
4.96
5.68
6.43Figure10 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 onSn5C site with C end [7];  thus, in this study CO gas molecule was perpendicularly imported onto the Sn5C atom with C end, and the initial vertical height between C atom of CO and Sn5C atom of SnO2 (110) surface was set as 2 Å, where C atom could bond with Sn5C atom freely.Figure11 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 Sn5C 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.



---

*Source: 578460-2013-08-27.xml*