Influence of Mn Doping on the Sensing Properties of SnO 2 Nanobelt to Ethanol

Mn doped SnO2 nanobelts (Mn:SnO2 NBs) and pure SnO2 nanobelts (SnO2 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 SnO2 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:SnO2 NB and pure SnO2 NB sensors were developed. It is found that Mn:SnO2 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.


Introduction
Metal-oxide semiconductor gas sensors have been used to detect gases for their efficiency and spread applicability [1] [2].In particular, one-dimensional semiconductor nanostructures, such as nanobelt, nanowire and nanotube, due to their unique physical and chemical properties caused by their nanometer size effect, have aroused great interest due to their unique characteristics [3] [4] [5] [6] [7].Therefore, metal-oxide semiconductor nanostructured materials, such as SnO 2 , In 2 O 3 , WO 3 and ZnO, have been widely manufactured as sensors for de-tecting air composition and organic or poisonous gases due to low cost, easy accessibility, high performance, and reliable stability [8] [9] [10] [11].Among them, SnO 2 with a band gap (3.6 eV), has high sensitivity and fast response [12].Hence, SnO 2 nanomaterials have been investigated to detect many gases, such as ethanol, H 2 S, and so on [13] [14] [15].It is well known that the dopant of rare metals may improve the sensitive performance of nanomaterials [16].For instance, Li et al. have reported that the sensitivity of Er-doped SnO 2 nanobelt device is 9 to the formaldehyde gas [17].Ma et al. have found that the response of Sb-doped SnO 2 nanoribbon device reaches 56 (10) to 100 ppm (ppb) of H 2 S at 150˚C and 19 (1.6) to 100 ppm (ppb) of H 2 S at 25˚C [18].Doping enhances the properties of semiconductors by providing a powerful strategy to control their optical, electronic, transport, and spintronic properties [19].The optoelectronic properties such as photoluminescence and optical band gap of SnO 2 can also be improved by metals doping.Many studies were reported on the SnO 2 doped by Sb, In, and Mn etc. [20] [21] [22].Much emphasis is put on manganese (Mn) due to its large equilibrium solubility and nearly the same ionic radii as with Sn 4+ ion for substitution.
In this paper, we systemically investigated the sensing and optical properties of a single Mn:SnO 2 NB sensor to volatile organic (VOC) liquids and reported interesting results.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:SnO 2 NBs and pure SnO 2 NBs, we also prepared pure SnO 2 NBs by similar method.

The Manufacture and Characterization of a Single Nanobelt Gas Sensor
SnO 2 NBs and Mn:SnO 2 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 SiO 2 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/cm 2 , 2.2 × 10 −2 Pa).
The measurement of the gas sensor was processed with the equipment designed by our laboratory [23].The process was conducted in a hermetic stainless steel box (20 L).Then, the device was put on a heating station, on which its temperature can be accurately controlled.The sensing properties of the device were measured by Keithley 4200 semiconductor test system.The applied bias voltage was 1 V and the testing interval was 200 seconds.The target liquid can be injected into a heater to evaporate the VOC liquid violently and the fan was used to produce a homogeneous atmosphere in the chamber.The optical microscopic image of a single nanobelt device is displayed in Figure 1.

SEM and HRTEM
The morphology of the as-synthesized materials is displayed in Figure 2. Mn:SnO 2 NBs and SnO 2 NBs have similar morphology.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:SnO 2 NBs have a good shape with smooth surface, which are suitable for preparing gas sensors.Figure 2(c) is SEM image of SnO 2 NBs by low magnification, it can be obviously seen that the SnO 2 NBs are band-like structure, the length of them is about 30 -100 μm. Figure 2(d) is a high magnification SEM micrograph of SnO 2 NBs, the SnO 2 NBs have smooth surface, the width is less than 1 μm, and thickness is about 50 -100 nm.
For further characterizing their microstructure, we carried out TEM, HRTEM and SAED examination and are presented in Figure 2(e) and Figure 2(f).TEM image illustrates that the width of a selected NB is about 300 nm. Figure 2(d) presents HRTEM image of a single Mn:SnO 2 NB and its corresponding SAED pattern.It is seen that the interplanar spacing is 0.3362 nm, matching with the (1 0 0) plane for tetragonal structure SnO 2 , no defects have been detected.The regular arrangement of the diffraction spots is shown in the upper-right inset of Figure 2(d), revealing that the growth direction of Mn:SnO 2 NBs is along [1 0 0].

XRD and EDS
XRD pattern of Mn:SnO 2 NBs is presented in Figure 3(a).The diffraction peaks can be indexed as the tetragonal structure SnO 2 with lattice parameters a = b = 0.4738 nm, c = 0.3189 nm (JCPDS file No. 71-0467).
In order to decide whether Mn ions were doped into SnO 2 NBs or not, energy-dispersive X-ray diffraction spectroscopy (EDS) pattern of a single Mn:SnO 2 NB is conducted, as shown in Figure 3(b).It is seen that the doped Mn ions content of SnO 2 NBs is only 1.64 at.%.The Sn and O atomic percentage is 1:1.8, which is smaller than tin dioxide stoichiometric ratio (1:2), indicating that there are oxygen vacancies in the sample.

XPS Analysis
The XPS observation of as-prepared Mn:SnO 2 sample is shown in Figure 4(a).From the global XPS profiles, the lines related to Sn, O, Mn, and C elements are observed.Carbon is ubiquitously presented on all surfaces for XPS spectra.It is well known that the peak at 285.0 eV of carbon C 1 s is used as a reference for charge correction.The Sn(3d) band presents double peaks located at binding energies of 487.0 of Sn(3d 5/2 ) and 495.4 eV of Sn(3d 3/2 ), as shown in Figure 4(b).The separation distance between the two peaks is 8.4 eV, which confirms the formation of Sn 4+ oxidation state in SnO 2 nanobelts [24].The Mn (2p) state can be separated into two peaks.One is centered at 640.0 eV, the other at 644.9 eV, as displayed in Figure 4(c).
The deconvolution of the O (1s) peak shows two Gaussian peaks centered at 530.9 and 531.7eV, respectively (as illustrated in Figure 4(d)).The peak located at the lower binding energy corresponds to the O (1 s) core peak of O 2− bound to Sn 4+ and the other one at higher binding energy originates from O defects [25].

UV-Vis Spectra
The optical absorption coefficient α of a semiconductor close to the band edge can be expressed by the following Wood-Tauc Equation [26]: where α is the absorption coefficient, k is a constant about material properties, hν is the energy of a photon, E g 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.
Figure 5 shows the absorption spectra of Mn:SnO 2 NBs and SnO 2 NBs.The band gaps (Eg) of Mn:SnO 2 NBs and SnO 2 NBs are 3.43 and 3.66 eV, respectively.The shift may be due to the quantum size effect.The plots of (αhν) 2 as a function of the energy (hν) of the incident radiation for Mn:SnO 2 NBs and SnO 2 NBs are presented in the inset of Figure 5.All plots show non-linear nature.Compared with that of SnO 2 NBs, the redshift is observed in the absorption edge of Mn:SnO 2 NBs.The change on the bandgap suggests that the size of the nanobelt has an influence on the optical properties of the materials, which can be tuned by doping.

Sensing Properties
The sensor's response is defined as the relative change of resistance in the surrounding gas atmosphere divided by the resistance in synthetic air [27], which is defined as In the formula of S(%), R a is the sensor resistance in the air and R g 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 Figure 6.The slope of pure SnO 2 NB is less than that of Mn:SnO 2 NB.The resistances of Mn:SnO 2 NB and its pure counterpart are about 1.65 × 10 8 Ω and 1.61 × 10 9 Ω respectively at 210˚C, indicating that the resistance of SnO 2 is greatly reduced after doping.
Upon exposed to 100 ppm of ethanol, ethanediol, and acetone gases, the responses of the sensors based on Mn:SnO 2 NB and its undoped counterpart have been tested as a function of the temperatures from 50˚C to 300˚C, as shown in Figure 7(a), Figure 7(b).The best operating temperature of Mn:SnO 2 NB to the three gases is 210˚C and the responses to ethanol, ethanediol, and acetone are   62.12%, 47.92% and 33.33% respectively.However, the best working temperature of the SnO 2 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:SnO 2 NB is much better than that of the latter, especially to ethanol.The best working temperature of Mn:SnO 2 NB is lower and the sensitivity to ethanol reaches 62.12%, far higher to the others.As evidenced from Figure 7(c), the sensitive responses of Mn: SnO 2 NB for ethanol, ethanediol and acetone are 1.51, 1.45, and 1.25 times as large as those of pure SnO 2 NB.The results manifest that doping of Mn reduces the optimum working temperature, enhances the sensitivity effectively, and greatly improves the selectivity of SnO 2 NB.
The response of the Mn:SnO 2 NB has also been tested and are illustrated in Figure 7(d), Figure 7(e) under ethanol from 10 to 1000 ppm at 210˚C.It is noted that the response increased drastically in the range of 10 -100 ppm, then moderately in 100 -600 ppm, and slowly in 600 -1000 ppm.The response tends to become saturated when ethanol concentration goes up, which can be attributed to the surface coverage of the adsorbed molecules [24].The response of Mn:SnO 2 sensors are provided upon repeatedly ethanol, ethanediol, and acetone gases exposure/removal cycles, as displayed in Figure 7(f).Five cycles are successively recorded, corresponding to 100, 200, 300, 400, and 500 ppm of ethanol gase.For all testing cycles, the resistance returns completely to its original value once the gases are pumped out.It can be seen that Mn:SnO 2 NB has a high sensitivity to ethanol.The corresponding response/recovery time is about 18 s/20 s.
Figure 7(g) shows the fitting curve of response versus ethanol concentration in the range of 100 -500 ppm.The response is approximately linearly, and the slope of it is 0.0334 ppm −1 with a fitting quality of R = 0.9788.The sensor noise is calculated by the variation of the relative response at the baseline with help of root-mean-square deviation (RMSD) [22] [23] [28].Then, 200 data points (N) of Figure 7(f) are collected, and the standard deviation (S) is obtained as 0.17369.According to 2 noise

RMS S N =
, RMS noise is 0.01228 for the ethanol sensor.
Based on the signal-to-noise ratio using ( ) , the theoretical detection limit for ethanol of the sensor is 1.1 ppm.

Gas Sensing Mechanism
As a solid material, the gas-sensing mechanism of SnO 2 sensor belongs to surface phenomenon [29].It is well known that the gas sensing involves three key reaction processes: adsorption, oxidation, and desorption [30].The oxygens are adsorbed in several species such as 2 O − , O − and O 2− through Equations ( 1)-( 4) [31].
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: 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 SnO 2 NBs.Besides, Mn doping can produce more oxygen vacancies and reduces the barrier height of the material [32].
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 [33] [34].Hence, the electric conductivity of SnO 2 NB increases.

Conclusion
Mn:SnO 2 NBs and pure SnO 2 NBs were synthesized by thermal evaporation.The band gap of Mn doped SnO 2 nanobelts by UV-Vis was measured and is 3.43 eV respectively at room temperature, lower than that of the pure SnO 2 NBs (~3.66 eV).SnO 2 NB and Mn:SnO 2 NB sensors were developed.It is found that the Mn:SnO 2 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.

Figure 2 (Figure 1 .
Figure 1.The microscope photograph of a prepared SnO2 NB device.

S
Sn and S O are sensitive factors (S Sn = 4.095, S 0 = 0.711); I Sn and I O are the peak areas; n Sn and n O are atomic concentrations on behalf of Sn and O elements, respectively.The ratio of n Sn and n O is 0.56 by fitting.However, the ratio of n Sn and n O in pure SnO 2 is 0.5.It is further corroborated that the Mn:SnO 2 NBs have oxygen vacancies.

Figure 6 .
Figure 6.I-V curves of pure SnO2 NB and Mn:SnO2 NB devices.

Figure 7 .
Figure 7. (a) and (b) The gas sensitivity of Mn:SnO2 NB and pure SnO2 NB to 100 ppm of various gases at different temperature; (c) Histogram sensitive responses of Mn:SnO2 NB and pure SnO2 NB at 210˚C; (d) The sensitivity of Mn:SnO2 NB to ethanol from 5 to 1000 ppm at 210˚C; (e) The curve of response versus ethanol concentration in the range of 5 -100 ppm; (f) Responses to 100 -500 ppm of ethanol at 210˚C; (g) The fitting curve of response versus ethanol concentration in the range of 100 -500 ppm.

2.1. Synthesis and Characterization of SnO 2 NBs and Mn:SnO 2 NBs Single
SnO 2 and Mn:SnO 2 NBs were obtained by thermal evaporation method.For the synthesis of Mn:SnO 2 NBs, the mixture of pure SnO 2 powders (>99.99 wt.%) and MnC 2 O 4 •2H 2 O 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