MoO3 structures transition from nanoflowers to nanorods and their sensing performances

Morphology transformation and crystal growth strategies of metal oxide semiconductors are still extensively studied in material science recently, because the morphology and crystallinity significantly affect the physicochemical characteristics of metal oxide nanomaterials. However, understanding the morphology changes of α-MoO3 induced by annealing is still a challenge. Herein, the nanostructure transition of α-MoO3 induced by the annealing temperature is carefully investigated via the XRD and SEM methods. It can be found that crystallization is highly dependent on the annealing temperature. Interestingly, the MoO3 nanoflowers can change into nanosheets at 500 °C. Afterward, the nanosheets turned into microrods with the increase in annealing temperature due to the continuous growth of MoO3 crystal. On the other hand, the sensing performances of various MoO3 nanostructures are studied toward ethanol gas. Compared to the MoO3 nanoflowers and microrods, the MoO3 nanosheets-based sensor exhibits superior sensing performance to ethanol, and the maximum response value is 8.06.


Introduction
In the past few decades, the development of nanotechnologies has created various exotic metal oxide semiconductor (MOS) nanostructures, which open up a new perspective for their exploitation, accordingly, producing many novel and fascinating optoelectronic devices [1][2][3][4][5]. Consequently, metal oxide semiconductors have attracted significant attention in recent years due to their extensive applications to secondary cells, sensors, memories, photodetectors, field-effect Li transistors, and photothermal tumors, etc. [6][7][8][9][10][11][12]. For example, the traditional semiconductors of MoO 3 [13], In 2 O 3 [14], TiO 2 [15], and ZnO [16], owing to their peculiar characteristics such as exotic electronic structure, excellent chemical stability, non-toxicity, rich optoelectronic properties, low cost, and high surface-to-volume ratio at low temperature, have been regarded as the promising materials for the quantum dots sensitized solar cells, lighting emitting components, high frequency devices, and catalysts [17][18][19][20]. On the other hand, the above physicochemical characteristics highly depend on the shape and size [21,22].
Besides, morphology and crystallinity of MOSs are very important to optoelectronic devices [23]. Specifically, nanostructure design strategy can offer interesting and extensive ideas to validly synthesize favorable functional nanomaterials due to their distinct characteristics at the nano-scale. For example, ultrafine NiO nanoparticles were obtained for supercapacitor electrode material [24]; the three-dimensional (3D) metal boron organic polymer converted into one-dimensional (1D) boron organic polymer nanorod array at room temperature [25]. In addition, a facile advance of MgO nanostructures at different annealing temperatures (from 300 to 900°C) was developed [26]. Moreover, crystal quality is one of the other important factors for optoelectronic devices. Recently, Jang and his coworkers observed that the homoepitaxial growth nanowires have constant outer diameters compared to bulk materials [27]. Additionally, excellent photodetector based on p-type Cu 1-x Ni x O films was prepared. The crystal quality, morphology, and grain size of Cu 1-x Ni x O films can be manipulated by the content of Ni dopant [28]. Actually, the crystal growth process and crystal quality are known to be significantly influenced by the crystallization environment.
As a representative n-type semiconductor oxide material with an energy gap of about 3.2 eV, molybdenum oxide (MoO 3 ) has been extensively used in various devices such as sensors, lithium-ion batteries, and photodetectors due to its good thermal and chemical stability [29][30][31]. MoO 3 exhibits various nanostructures such as nanoribbons, nanosheets, nanowires, nanoflowers, and nanorods under different experimental conditions [30][31][32]. However, through controlling the thermal treatment conditions, in situ construction of various MoO 3 nanostructures has not been thoroughly conducted.
In the present contribution, we report an in situ construction of various MoO 3 with high crystallinity method. The various crystalline MoO 3 nanostructures can be obtained from MoS 2 precursor. Specifically, the nanosheets and microrods of MoO 3 can be fabricated in situ from flower-like MoS 2 precursor calcined in the range of 400-900°C in air directly. In addition, the results show that the MoO 3 nanosheetsbased sensor exhibits superior sensing performance toward ethanol gas.

Fabrication of MoO 3 material
The flower-shaped MoS 2 precursor was synthesized by hydrothermal process similar to our previous study [29]. In a typical synthesis, firstly, 1.8 g ammonium molybdate, 1.8 g thiourea, 1 g glucose, 200 mg ammonium fluoride were added into 50 mL distilled water, and then, triethylamine (200 lL) was dropped slowly with a pipette and stirred vigorously for 30 min. Afterward, the resulting mixtures were sealed in a 60 mL Teflon-lined stainless-steel autoclave and heated at 200°C for 24 h, after which the mixtures were cooled to room temperature over 500 min. Thereafter, in the processes of ultrasonicating and centrifuging, the obtained MoS 2 suspension was washed several times with absolute ethanol and deionized water to eliminate redundant ions, respectively. The dark flowerlike powders were obtained after being dried overnight in a vacuum at 80°C; secondly, the MoO3 was obtained via calcining MoS 2 precursor at 400°C, 500°C, 600°C, 700°C, 800°C , and 900°C for 2 h, respectively. All the heating rate is 1°C/min. For convenience, the names of the above samples were defined as MoO 3 -400, MoO 3 -500, MoO 3 -600, MoO 3 -700, MoO 3 -800, and MoO 3 -900 according to the calcination temperature, respectively.
On the other hand, the obtained products (MoO 3 -400, MoO 3 -500, MoO 3 -600, MoO 3 -700, MoO 3 -800, and MoO 3 -900) were grinded thoroughly in an agate mortar to form slurry, respectively. Then, the slurry was uniformly coated on the alumina ceramic tube in turn and annealed in air at 120°C for 2 h. Here, the gas response magnitude of the sensor is defined as S = R a /R g , where R a and R g are the resistance in air and in the detected gas, respectively. Additionally, the response time and the recovery time were expected to be the minimum time required for the gas sensor output to reach 90% of its saturation after the gas was applied or shut off from the chamber.

Characterizations
The morphology of the products was investigated by a field emission scanning electron microscope (FE-SEM, Zeiss Gemini 500). The microstructure of the samples was investigated using X-ray diffraction (XRD, Rigaku Smartlab). X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250xi) was measured to further confirm the surface element composition and chemical state. And the gas sensing properties were measured by the gas sensing system of MA1.0 (Narui Electronics Co. Ltd., China). Figure 1 shows the typical FE-SEM images of the asprepared MoO 3 samples. Specifically, flower-shaped samples with hierarchical nanostructure were successfully synthesized, and the products can be clearly observed by high-resolution FE-SEM, as shown in Fig. 1a-c. Here, Fig. 1a-c belongs to MoO 3 -400, MoO 3 -500, and MoO 3 -600, respectively. As a whole, the above three samples have similar profile of microsphere, and the average diameter of the microspheres was less than 1 lm (981 ± 10 nm, Fig. 1b). On the other hand, the microspheres were constructed by numerous plate-like nanosheets, and the average edge thickness of the nanosheets increased from 12 ± 1 nm (Fig. 1a) to 35 ± 1 nm ( Fig. 1c) with the increase in annealing temperature from 400 to 600°C. In addition, these microspheres interconnected with each other, facilitating carriers such as electrons transport and the ethanol gas molecules diffusion on the surface of the MoO 3 samples. Again, it can be clearly observed that there are many voids on the surface of the MoO 3 samples, which were enclosed by numerous primary ultrathin plate-like nanosheets with clear texture, see Fig. 1c. After annealing at 600°C, the hierarchical flower-like morphology vanished. On the contrary, the microspheres grew up nanosheets (see Fig. 1d-f), and the average thickness of nanosheets increased from 77 ± 5 (Fig. 1d) to 1692 ± 10 nm (Fig. 1f). Generally, the thickness of MoO 3 nanosheet shows a positive relationship to annealing temperature that is the thickness of MoO 3 nanosheets became thicker and thicker with the increase in annealing temperature (Fig. 1e). We suggest that the main reason attributes to the continuous growth of the MoO 3 crystal grain with the increase in annealing temperature. Further, the MoO 3 sheets grew larger and larger after 700°C. Finally, the MoO 3 microrods obtained due to the more thermal energy provided for MoO 3 crystal growth. In terms of crystal growth theory, almost every point on the unpolished surface of a crystal can be filled with atoms to become the point where the crystal grows. According to the FESEM images, the nanosheets and microrods (Fig. 1d-f) have a smoother surface than that of microspheres, because the rough points on the surface of MoO 3 microspheres can be filled by O and Mo atoms. In this processes, the kinetic energy for O and Mo atoms migration and crystal growth was provided via calcination [32]. Finally, the ultrathin plate-like MoO 3 nanosheets grew into long microrods with smooth surface (Fig. 1f).

Result and discussion
To the best of our knowledge, crystallinity, electronic structure and phase stability are strongly influenced by the calcination temperature and composition of the metal oxide semiconductor materials. Figure 2a shows the XRD patterns of MoO 3 samples annealed at different temperature. Evidently, the diffraction peaks of all the samples were consistent with the orthorhombic a-MoO 3 (JCPDS No. 35-0609). The insensitive peaks at 2h = 12.8, 23.3, 25.7, 27.3, and 38.9 corresponding to (020), (110), (040), (021), and (060) planes, respectively. This indicated that the samples grow with preferential orientation of (110). No other diffraction peaks were observed, it is confirmed that a-MoO 3 samples have relatively high crystal purity. On the other hand, the intensity of the diffraction peaks increased gradually with the increase in annealing temperature. It indicated that the grain size of the samples decreased with the increase in annealing temperature according to the Scherrer's formulation (D = kk/Bcosh) [29].
On the basis of the above analysis, the chemical ingredient and the valence state of the MoO 3 -600 were analyzed via XPS investigation. The corresponding results are shown in Fig. 2b and c. The Mo spectrum in Fig. 2b displays two distinct peaks located at 231.8 and 235.1 eV, correspond to Mo 3d 5/2 and Mo 3d 3/2 , respectively [33,34]. Further, the energy separation of two peaks is 3.3 eV, which indicates the successful fabrication of MoO 3 [31]. The peak of O 1s can be deconvoluted into two independent oxygen species at 529.5 and 530.1 eV (Fig. 2c). The peak at 529.5 eV in the O 1s curve can be attributed to the oxygen ions in the crystal lattice that is lattice oxygen O lattice (O 2-) and surface adsorbed oxygen O ads . (e.g., O -) [35]. While the other peak at 530.1 eV was assigned to the oxygen ions  are active to ethanol, so they play a key role in enhancing the sensing performance [36,37].
In order to investigate the optimum working temperature of MoO 3 samples toward ethanol, the responses of six sensors (MoO 3 -400, MoO 3 -500, MoO 3 -600, MoO 3 -700, MoO 3 -800, and MoO 3 -900) were firstly studied as a function of operating temperature (Fig. 3). Evidently, all the sensor's responses increased firstly before 200°C and then decreased drastically with the increase in operation temperature further. Among them, the MoO 3 -600 sensor exhibited a maximum response of 8.06 toward 100 ppm ethanol at 200°C, which is three times higher than those of MoO 3 -400 and MoO 3 -900. According to results, the response values of MoO 3 -400, MoO 3 -500, MoO 3 -700, MoO 3 -800, and MoO 3 -900 are 3.18, 5.34, 5.81, and 4.50 at 200°C, respectively. Obviously, MoO 3 -600 based sensor exhibited the better sensing performance than those of other devices such as MoO 3 -400, MoO 3 -500, MoO 3 -700, MoO 3 -800, and MoO 3 -900. It demonstrated that the nanostructure and annealing temperature of MoO 3 have a significant effect on the device's response. A suitable working temperature is indispensable in that ample thermal energy is a necessary prerequisite to overcome the chemical barrier of gas and the activation barrier of surface reaction. Besides, when further increase the working temperature, the gas desorption rate is higher than that of adsorption rate, which is unfavorable to the response. Figure 4a gives the dynamic response and recovery characteristics of the MoO 3 -600 sensor to different alcohol concentrations from 5 to 500 ppm at 200°C. Obviously, the response of the MoO 3 -600 sensor climbed continually with the increase in ethanol concentration, and then, the response of MoO 3 -600 sensor approached the saturation value when the CH 3 CH 2 OH concentration is over 500 ppm, see Fig. 4b. Similar results have been reported by other literature [37,38]. It can be explained that more ethanol molecules can be physically or chemically adsorbed on the MoO 3 -600 nanosheets, and speeding up the surface reaction rate with the chemisorbed oxygen species such as O -, O 2and O 2 - [30,36]. Additionally, according to the sensing mechanism of MoO 3 , the increase in crystallization has an great influence on its sensing properties [31,38]. On the other hand, the limit of detection (LoD) of ethanol was evaluated by the method of linear extrapolation, specifically, the response sensitivity is a function of ethanol concentration (the inset of Fig. 4b). The detailed calculating formula of the LoD is: LoD = 3 9 (Standard Deviation/Slope), from which the ultra-low ethanol detection concentration is 125 ppb for the MoO 3 -600 sensor. Figure 4c shows a typical repeatability performance of the MoO 3 -600 based sensor toward 100 ppm ethanol at 200°C, exhibiting a superb stability and repeatability. Moreover, the response time (s res ) and recovery time (s recov ) were examined, which were 7 s and 26 s, respectively. The results indicate that the MoO 3 -600 sensor exhibits a very quick response and recovery properties to ethanol (Fig. 4d).
From the aspect of practical applications, selectivity is another very important parameter to gas sensors. Herein, the selectivity of MoO 3 -600 sensor to other VOCs, for example, benzene, isopropanol, chloroform, acetic acid, methanol and acetone, was evaluated at 200°C, see Fig. 5a. Clearly, the maximum gas response value of MoO 3 toward 100 ppm alcohol was 8.06, which was evidently larger than those of other gases. Specifically, the responses to benzene, isopropanol, chloroform, acetic acid, methanol, and acetone were 0.23, 1.47, 1.47, 1.57, 1.74, and 2.31, respectively. Therefore, the sensitivity of the MoO 3 -600 toward ethanol was much higher than that of other VOCs, indicating that the MoO 3 -600 has an excellent selectivity to ethanol. The sensing mechanism of the MOS to ethanol can be illustrated by surface conduction modulation model. According to the literature [16,22], the adsorption and desorption of target gas molecules from the surface of the MOS could regulate the electrical resistance of gas sensor. When the MOS is exposed to fresh air, the oxygen molecules (O 2 ) will capture conductive band electrons (e -1 ) to form chemisorbed oxygen ions species (Oand O 2 -). This process will form an electron depletion layer (EDL) on its surface region of sensing material, which causes the device resistance to increase. When the MOS sensor is exposed to ethanol gas, the ethanol molecules can react with the oxygen species such as Oand O 2 -, resulting in the release of trapped electrons back to the conduction band (E c ), thus, this process can significantly reduce the sensor resistance. This reaction processes can be expressed as Eqs. (1) and (2) [39,40]: In this study, we suggest that O 2 molecules adsorbed on the MoO 3 -600 surface will be ionized into oxygen species (O -, O 2and O 2 -) through capturing electrons (e -1 ) from its E c when the MoO 3 -600 sensor is exposed to air initially. This oxygen adsorption process induces the formation of an EDL and an evident increase in the device resistance. Nevertheless, the oxidation-reduction interaction between the ethanol molecules and the adsorbed oxygen ions will occur on the surface of MoO 3 when the MoO 3 -600 is exposed to ethanol gases. This triggers the decrease in the surface depletion layer width and the resistance of MoO 3 sensor because the captured electrons release back to the E c of MoO 3 . In addition, we found that the sensing performance was influenced by the MoO 3 structures transition. Evidently, the MoO 3 -600 sensor has the best sensing performance because of the rough surface and novel nanostructure.

Conclusion
In conclusion, various MoO 3 nanomaterials were successfully prepared by hydrothermal and calcined processes. The nanostructure morphology and crystal quality of MoO 3 have temperature-dependent relationships. According to the results, the MoO 3 morphology can be manipulated by annealing temperature from nanoflowers, nanosheets to nanorods. In addition, the ethanol sensing performances were carefully investigated by MoO 3 samples. It is found that the MoO 3 -600 has an excellent sensing performance duo to the combination of degree of crystallinity and nanostructure.