Mn doped SnO 2 nanobelts (Mn:SnO 2 NBs) and pure SnO 2 nanobelts (SnO 2 NBs) were synthesized by thermal evaporation technique at 1355°C with Ar carrier gas (25 sccm, 150 Torr). The SEM, EDS, XRD, TEM, HRTEM, SAED, XPS, UV-Vis techniques were used to characterize the attained samples. The band gap of Mn doped SnO 2 NBs by UV-Vis was measured to be 3.43 eV at room temperature, lower than that of the pure counterpart with ~3.66 eV. Mn:SnO 2 NB and pure SnO 2 NB sensors were developed. It is found that Mn:SnO 2 NB device exhibits a higher sensitivity with 62.12% to 100 ppm of ethanol at 210°C, which is the highest sensitivity among the three tested VOC gases (ethanol, ethanediol, and acetone). The theoretical detection limit for ethanol of the sensor is 1.1 ppm. The higher response is related to the selective catalysis of the doped Mn ions.
Metal-oxide semiconductor gas sensors have been used to detect gases for their efficiency and spread applicability [
Doping enhances the properties of semiconductors by providing a powerful strategy to control their optical, electronic, transport, and spintronic properties [
In this paper, we systemically investigated the sensing and optical properties of a single Mn:SnO2 NB sensor to volatile organic (VOC) liquids and reported interesting results.
Single SnO2 and Mn:SnO2 NBs were obtained by thermal evaporation method. For the synthesis of Mn:SnO2 NBs, the mixture of pure SnO2 powders (>99.99 wt.%) and MnC2O4∙2H2O powders premixed in the weight ratio of 20:1 was put into a ceramic boat. The ceramic boat was placed into the central position of the horizontal alundum tube, which was put into a high temperature furnace. A silicon substrate coated with about 10 nm Au film was placed into the tube, the distance of silicon substrate and ceramic boat was about 15 cm. After cleaning the tube several times with nitrogen gas, the tube was evacuated by a mechanical pump to a pressure of 1 to 5 Pa. The precursors of SnO2 and MnC2O4∙2H2O powders were maintained at 1355˚C for 2 h and deposited on the Si substrate with Ar carrier gas (25 sccm, the pressure inside the tube is 150 Torr). After the furnace was cooled to room temperature naturally, white wool-like products were obtained. In order to compare the sensing properties of Mn:SnO2 NBs and pure SnO2 NBs, we also prepared pure SnO2 NBs by similar method.
The nanobelts were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution electron microscopy (HRTEM), selected area electron diffraction (SAED), X-ray photoelectron spectrometer (XPS), and ultraviolet and visible spectrophotometer (UV-Vis).
SnO2 NBs and Mn:SnO2 NBs were picked out and then dispersed into ethanol by the tweezers. A few of the resulting suspensions were dropped onto a silicon substrate with thickness of a 500 nm SiO2 layer. The suspensions dried naturally and led to the nanobelts stick to the substrate closely. The mask plate was placed on the top of this substrate to prepare the electrodes. Patterned Ti (10 nm) and Au (100 nm) electrodes were successively deposited on the nanobelts in high vacuum by dual-ion beam sputtering (LDJ-2a-F100-100 series) with Ar carrier gas (10 mA/cm2, 2.2 × 10−2 Pa).
The measurement of the gas sensor was processed with the equipment designed by our laboratory [
The morphology of the as-synthesized materials is displayed in
100 nm, the width is from 250 nm to 1 μm, and the length is about 50 μm. It is also seen that the Mn:SnO2 NBs have a good shape with smooth surface, which are suitable for preparing gas sensors.
For further characterizing their microstructure, we carried out TEM, HRTEM and SAED examination and are presented in
XRD pattern of Mn:SnO2 NBs is presented in
In order to decide whether Mn ions were doped into SnO2 NBs or not, energy-dispersive X-ray diffraction spectroscopy (EDS) pattern of a single Mn:SnO2 NB is conducted, as shown in
The XPS observation of as-prepared Mn:SnO2 sample is shown in
The deconvolution of the O (1s) peak shows two Gaussian peaks centered at 530.9 and 531.7eV, respectively (as illustrated in
SSn and SO are sensitive factors (SSn = 4.095, S0 = 0.711); ISn and IO are the peak areas; nSn and nO are atomic concentrations on behalf of Sn and O elements, respectively. The ratio of nSn and nO is 0.56 by fitting. However, the ratio of nSn and nO in pure SnO2 is 0.5. It is further corroborated that the Mn:SnO2 NBs have oxygen vacancies.
The optical absorption coefficient α of a semiconductor close to the band edge can be expressed by the following Wood-Tauc Equation [
where α is the absorption coefficient, k is a constant about material properties, hν is the energy of a photon, Eg is band gap, and n is a parameter that depends on the nature of the transition. In this case, n is equal to 1/2 for a direct bandgap material. The band gap can be estimated from a plot of (αhν)2 versus photon energy.
The sensor’s response is defined as the relative change of resistance in the surrounding gas atmosphere divided by the resistance in synthetic air [
In the formula of S(%), Ra is the sensor resistance in the air and Rg is the resistance in the tested gas.
The curves exhibit a linear shape, so both doped and pure NBs possess a good Ohmic contact, as shown in
Upon exposed to 100 ppm of ethanol, ethanediol, and acetone gases, the responses of the sensors based on Mn:SnO2 NB and its undoped counterpart have been tested as a function of the temperatures from 50˚C to 300˚C, as shown in
62.12%, 47.92% and 33.33% respectively. However, the best working temperature of the SnO2 NB to the three gases are 220˚C and its responses are only 42.86%, 24.24% and 16.67%. Obviously, the sensitivity of Mn:SnO2 NB is much better than that of the latter, especially to ethanol. The best working temperature of Mn:SnO2 NB is lower and the sensitivity to ethanol reaches 62.12%, far higher to the others. As evidenced from
The response of the Mn:SnO2 NB has also been tested and are illustrated in
As a solid material, the gas-sensing mechanism of SnO2 sensor belongs to surface phenomenon [
O2 (Gas) Û O2 (adsorption) (1)
O2 (adsorption) + e− Û
(adsorption) + e− Û (adsorption) (4)
when ethanol liquid is injected into the equipment and then evaporates, its vapor contacts the sensor and reacts with the surface chemisorbed oxygen. The process is as follows:
C2H5OH +
C2H5OH + 6O− (ads) = 2CO2 + 3H2O + 6e− (6)
C2H5OH + 6O2− (ads) = 2CO2 + 3H2O + 12e− (7)
Obviously, many electrons are released and hence the resistance decreases during the reaction. In our work, the sensitivity to ethanol is availably improved by Mn ions. The reasonable sensitive improvement mechanism is proposed as follows. As shown as the results, Mn ions can promote the crystallinity of SnO2 NBs. Besides, Mn doping can produce more oxygen vacancies and reduces the barrier height of the material [
Mn3+ + H2O → MnO+ + H+ (8)
OO× ↔ VO•• + 2e' + 1/2O2 (9)
2MnO+ + OO× → Mn2O3 + VO•• + 2e' (10)
As expressed by Equations (8)-(10), Mn ions may enhance the surface dehydrogenation resulting in the oxidation of ethanol needs lower energy, so that the liberation of electrons will be promoted [
Mn:SnO2 NBs and pure SnO2 NBs were synthesized by thermal evaporation. The band gap of Mn doped SnO2 nanobelts by UV-Vis was measured and is 3.43 eV respectively at room temperature, lower than that of the pure SnO2 NBs (~3.66 eV). SnO2 NB and Mn:SnO2 NB sensors were developed. It is found that the Mn:SnO2 NB device exhibits a higher sensitivity of 62.12% to 100 ppm of ethanol at 210˚C, which is the highest sensitivity among the three tested VOC gases. The higher response is related to the selective catalysis of doped Mn ions.
This work was supported by the National Natural Science Foundation (Grant No. 11164034), the Key Applied Basic Research Program of Science and Technology Commission Foundation of Yunnan Province (Grant No. 2013FA035), and the Innovative talents of Science and Technology Plan Projects of Yunnan Province (Grant No. 2012HA007).
Huang, J.Q., Liu, Y.K., Wu, Y.M. and Li, X.M. (2017) Influence of Mn Doping on the Sensing Properties of SnO2 Nanobelt to Ethanol. American Journal of Analytical Chemistry, 8, 60-71. http://dx.doi.org/10.4236/ajac.2017.81005