Enhanced ethanol response of La2O3-loaded SnO2 nanorods prepared by an improved solid state reaction

La2O3-loaded SnO2 nanorods with rutile structure were successfully synthesized by an improved solid state reaction with the surfactants in the presence of NaCl-KCl. The indirect-heating sensor was prepared using the products as sensing-material to study the response of the La2O3-loaded SnO2 nanorods. The results showed that the performances of the SnO2 nanorods to ethanol was greatly improved after loading La2O3. The significant improvement could be attributed to the excellent catalytic properties of loaded La2O3 and the high surface area.


Introduction
As a well-known substance used for detection of different VOC pollutants owing to chemical and thermal stability, good gas response and low cost [1], SnO 2 has been extensively studied for detection of a wide variety of inflammable and toxic VOCs gases such as ethanol, acetone, isopropanol and so on [2]. However, it is also suffering a serious problem associated with maximum sensitivity, high operating temperature, lack of long-term stability and poor selectivity and so on. In order to overcome these shortcomings, a lot of exploratory work have been devoted to the new material with enhanced gas sensing performances. Recently, one-dimensional (1-D) nanostructured SnO 2 such as nanowires [3], nanobelts [4], nanorods [5] and so on, is attracting a great deal of attention to study the applications in sensor technology due to their high surface-volume ratio and enhanced surface reactivity. Another significant effort, doping Lanthanide has been used to overcome these limitations and improve the performances [6].
In this work, La 2 O 3 -loaded SnO 2 nanorods synthesized by an improved solid state reaction with surfactants in the presence of NaCl-KCl. Indirect-heating sensors based on nanorods were fabricated and investigated for the response to ethanol. The gas sensing experiments of La 2 O 3 -loaded SnO 2 nanorods were carried out compared against the no-loaded SnO 2 nanorods to research the effect of loading La 2 O 3 on the sensing properties.

Experimental
All utilized analytical grade chemical reagents were purchased from the Sinopharm Chemical Reagent Co. No-loaded and La 2 O 3 -loaded SnO 2 nanorods were synthesized via an improved solid state reaction. Typically, a mixture of the SnCl 4 ·5H 2 O added in an amount of LaCl 3 ·6H 2 O (Did not add for no-loaded SnO 2 nanorods) was mixed with KBH 4 in the presence of KCl-NaCl and nonyl phenyl ether (9)/(5) at room temperature. Then, the mixture was fully ground to cause the solid phase reaction. The product kept in a porcelain crucible was annealed at 680 ºC for 2h. After that, the resulting product was washed with distilled water several times to remove NaCl and KCl, and was dried at 80 ºC for 12 h for further characterization. The X-ray diffraction (XRD) using a Rigaku D/MAX-3B powder diffractometer was performed to determine the crystalline structure and phase of SnO 2 nanorods. The morphology and detailed structure was characterized by an FEI Quanta 200 scanning electron microscope (SEM) and a transmission electron microscope (TEM, JEOL 2010, 200 KV). The indirect-heating sensor was elected to study the response of the SnO 2 nanorods [2]. The test of the gas-sensing properties carried out on a JF02F measurement system in a relative humidity range of 40-70%. The response was defined as the ratio of R a /R g , where R a and R g is the resistance in atmospheric air and in target gas, respectively.

Results and discussion
The typical XRD patterns of the as-prepared SnO 2 nanorods were shown in Figure 1(a). As seen from Figure 1(a), all the diffraction peaks of the La 2 O 3 -loaded SnO 2 nanorods can be indexed to the rutile structured SnO 2 (JCPDS file No. 41-1445), which is almost similar to that of the no-loaded SnO 2 nanorods. However, it is obvious that two weak additional diffraction peaks located at 28.42 o and 48.56 o , which could be assigned to the crystalline phases of La 2 O 3 can be detected. Moreover, the relative intensities of the SnO 2 nanorods sample decreased after loading La 2 O 3 , suggesting the growth of SnO 2 nanorod was prevented by La 2 O 3 . Figure 1(b) and (d) showed the morphologies of the SnO 2 nanorods characterized by SEM. Being compared with the no-loaded SnO 2 nanoroads (shown in Figure 1(d)), finer rod-like structured regular morphologies of the La 2 O 3 -loaded SnO 2 nanorods with the length in the range of 500 nm and several micrometers can be observed. It indicates that the La 2 O 3 could prevent the nanorods further growth up [7]. The thinner nanorods could provide a large surface area, contributing to increase the probability of surface trapping to spur the surface reaction. The EDS spectrum of the La 2 O 3 -loaded SnO 2 nanorods shown in Figure 1    It showed that the responses increase with the temperature at first, after reaching a maximum value of 38 and 116 for no-loaded and La 2 O 3 -loaded SnO 2 nanorods at optimum operating temperature of 246 o C, and then decreased rapidly. This performance is associated with the kinetics and mechanism of gas adsorption and desorption of semiconducting metal oxides [8]. From the Figure  3(a), it can be seen that the SnO 2 nanorods sensors have a lower optimum temperature, and the response of La 2 O 3 -loaded SnO 2 nanorods is much larger than that of the no-loaded SnO 2 nanorods. The repeatable characteristic of the response and recovery was shown in Figure 3(c) achieved by the La 2 O 3 -loaded SnO 2 nanorods sensor being orderly exposed to different concentrations at the optimized temperature of 246 o C. It is evident that the response amplitude of the sensors gradationally increases with the concentration changed from 5 to 1000 ppm. The sensor presents a considerable response to low ethanol concentration of 5 ppm. It is worth noting that the characteristics of the SnO 2 (a) 0.342 nm (b) nanorods were remarkably improved after loading La 2 O 3 compared with the no-loaded SnO 2 nanoroads. The response (t Res ) and recovery (t Rec ) time is an important parameter to assess a gas sensor. and recovery (t Rec ) time is short enough for practical application. Furthermore, we performed a series of experiments of SnO 2 nanorods exposed to 300 ppm VOCs gas including acetone, ethanol, toluene, isopropanol, formaldehyde, petrol, carbinol and ammonia in order to explore the selectivity, and the result was shown in Figure 4(c). It showed that sensors exhibited a good selectivity to ethanol. Particularly, the response of the no-loaded SnO 2 nanorods is no greater than 36, while the response of the Ln 2 O 3 -loaded SnO 2 nanorods sensor achieves 84, which is more than 2 times larger than that of the no-loaded SnO 2 nanorods. The greatly improved response could be attributed to the excellent catalytic properties of loaded La 2 O 3 and the restrained growth of the SnO 2 nanorods resulting in a high surface area, being beneficial to increase the amount of active sites on the SnO 2 nanorods surface [6].

Conclusions
In summary, La 2 O 3 -loaded SnO 2 nanorods with rutile structure were successfully synthesized by an improved solid state reaction in the presence of NaCl-KCl and surfactants. The response of the La 2 O 3loaded SnO 2 nanorods to ethanol showed high response, good selectivity, low detection limit, and short response and recovery time. The response improvement of the La 2 O 3 -loaded SnO 2 nanorods to ethanol is remarkable. Doping Lanthanide is effective and significant route to improve the performance of the SnO 2 nanorods for applications in detecting ethanol.