Hydrothermal Synthesis of Flake-Flower NiO and Its Gas Sensing Performance to CO

In this work, flake-flower NiO was successfully prepared via a facile hydrothermal method. The microstructure of the synthesized sample was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). We find that the hierarchical flake-flower structure was assembled by numerous nanosheets with different size and shape. The fabricated sensor based on the obtained microstructure exhibited excellent gas sensing performance including high response, outstanding selectivity and stability toward 5 ppm CO at the optimal working temperature of 250°C. A plausible gas sensing mechanism was given out to explain how the nanosheet assembly morphology affects the gas sensing performance of the flake-flower structure.


INTRODUCTION
Dissolved Gas in Oil Analysis (DGA) is one of the most convenient and effective methods to judge the early latent faults of oil immersed high-voltage electrical equipment at present (Gui et al., 2019;Yang et al., 2019a;Zhou et al., 2019;Wang et al., 2020;Wei et al., 2020a). As one of the most important fault characteristic gases of oil immersed transformer, carbon monoxide (CO), has received considerable attention for its application to provide vital help for judging the operation state of transformer (Zhou et al., 2015(Zhou et al., , 2018aYang et al., 2019b). In this respect, to detect and analyze the dissolved gases, many strategies have been proposed, for instance, gas chromatography, photoacoustic spectrometry and gas sensor (Qu et al., 2016;Liu et al., 2017;Wei et al., 2019b). Among these methods, the design of gas sensor has attracted numerous interest, owing to its low cost, facile route and simple structure Zargouni et al., 2019). However, to ensure the normal operation of transformer and power system, the fabrication of high-performance gas sensors is still a still a challenging work (Wei et al., 2019c;Zhou et al., 2018c;Zhang et al., 2019).
Considering that the morphology of the nanomaterials plays an important role in the gas sensing performance of NiO sensors, there have been considerable efforts in the synthesis of different NiO nanostructures for instance nanoparticle (0-D) Kruefu et al., 2016), nanorod (1-D) Feng et al., 2017), nanosheet (2-D) (Yu et al., 2015;Sta et al., 2016) and nanoflower (3-D) Miao et al., 2017). Compared with low dimensional nanostructures, hierarchical nanostructures have made great progress because of their complicated and beneficial structures (Cao et al., 2015;Wang et al., 2015). For example, Wei et al. (2019a) designed and synthesized 2-D NiO porous nanosheet via a facile hydrothermal method, the gas sensor based on which had excellent gas sensing performance toward 30 ppm H 2 at the optimal working temperature of 225 • C benefited from the special porous nanostructure. Wang et al. (2019b) synthesized hierarchical structure assembled with NiO nanosheets and the sensor based on the nanostructure exhibited excellent gas sensing performance due to its high special surface area. Reasonable design of hierarchical structure of NiO is a challenging but meaningful work to enhance the performance of NiO based sensor to detect the fault characteristic gas in transformer oil (Zhang D. Z. et al., 2016;Balamurugan et al., 2017;Cui et al., 2019).
In this work, hierarchical flake-flower NiO has been prepared with a facile hydrothermal method. The obtained sample was characterized by XRD, SEM, TEM, HRTEM, and SAED and fabricated into gas sensing device. Gas sensing test toward CO was carried out to demonstrate the high-performance of the special hierarchical NiO structure. The gas sensing results indicated that the fabricated sensor showed excellent performances including high response, prominent stability and outstanding selectivity toward 5 ppm CO at the optimal working temperature of 250 • C. A plausible gas sensing mechanism was proposed, demonstrating that the excellent performance might be caused by the hierarchical 3-D structure with high special surface area.

Synthesis of Flake-Flower NiO
All reagents in this work used to prepare the hierarchical flakeflower NiO were of analytical grade and used without any further purification. In a typical hydrothermal procedure, 0.475 g NiCl 2 ·6H 2 O was added into 20 ml mixed solution composed of 10 ml of pure water and 10 ml of ethanol. Then, 0.2 g PVP and 10 ml of EG were added into the mixture under stirring. The pH value of the solution was adjusted to about 10 by dropping NH 3 ·H 2 O. After that, the mixture was magnetically stirred for 5 min to form a homogeneous solution and then poured into a 50 mL Teflon-lined stainless autoclave which was kept at 140 • C for 10 h. After the autoclave cooled to room temperature, the green sample was harvested by centrifugation and washing with pure water and ethanol for several times. After drying at 60 • C for 10 h, the expected product was obtained by calcination at 600 • C for 1 h.

Materials Characterization
The structure and phase purity of the product were characterized by X-ray diffraction (XRD) with a Rigaku D/Max-2550 diffractometry (Cu-Kα radiation, λ = 0.15418 nm, 2θ = 30-85 • ). The morphology of the flake-flower NiO was observed by a Nova 400 scanning electronic microscope (SEM). The high-magnification structure of the flake-flower morphology was investigated by a JEM-2100F field-emission transmission electron microscope including transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED).

Gas Sensor Measurements
In order to fabricate high-performance CO sensing sensor based on the flake-flower NiO, the device was designed with a sideheated structure as shown in Figure 1A. Concretely, the prepared sample was mixed with pure water and ethanol in a ratio of 8:1:1 to obtain a homogeneous solution which was used to form a Frontiers in Materials | www.frontiersin.org sensing film. Then, the formed film was coated onto a ceramic tube which has been designed with a pair of Au electrodes and two pairs of Pt wires. Next, a Ni-Cr wire was inserted into the tube to control the working temperature of the sensor Zhu et al., 2018b). Figure 1B gives out the theoretic diagram of the test circuit, from which one can find that V out represents the output voltage to calculate the resistance and V h represents FIGURE 2 | XRD patterns of the prepared sample.    Frontiers in Materials | www.frontiersin.org the heating voltage to change the working temperature . To ensure the long-term stability and repeatability of the fabricated CO sensor, the device was aged at 300 • C for 1 week in air (Lu et al., 2018). The gas sensing performance of the fabricated sensor was measured with a static analysis system using a Chemical Gas Sensor-8 intelligent system (Beijing Elite Tech Co., Ltd.). Besides, the volume of the test chamber is 20 L and the flux of the test gases was set to 20 ml/min. Gas response in this work were defined as S = R g /R a , in which R g and R a represent the resistances in target gases and in air (Wei et al., 2020b).

Morphology and Structure
The structure and phase purity of the synthesized flake-flower sample were characterized by XRD as shown in Figure 2. From FIGURE 7 | Gas responses of flake-flower NiO based gas sensor toward different concentration of CO from 1 to 100 ppm at the optimal working temperature of 250 • C. the XRD pattern, it is obvious that there are five clear diffraction peaks at 37.1 • , 43.3 • , 63.1 • , 75.1 • , and 79.5 • , which can be will indexed with (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) lattice planes of cubic NiO (JCPDS Card no. 47-1049). Besides, no other diffraction peaks were observed in the spectrum, indicating that the prepared product must be pristine NiO with high-purity. Figure 3 displays the SEM micrographs of the flake-flower NiO with different magnification. As shown in Figure 3a, it can be seen that the synthesized sample have a flower-like microstructure a relatively uniform distribution and a diameter of about 5 µm. From the higher magnification image of the flower-like microstructure (Figure 3b), it is intuitive that the flake-flower structure was assembled by a lot of thin nanosheets. The numerous nanosheets with different shape and size make the NiO structure exhibit a flake-flower shape and hierarchy which will possess large surface sites and abundant reaction rooms for gas molecules.
To observe a more detailed structural information of the flakeflower microstructure, the sample was analyzed by transmission electron microscope with the characterization of TEM, HRTEM and SAED (Figure 4). As presented in Figure 4a, the structure was confirmed to be assembled by numerous irregular and random nanosheets which have been mentioned in Figure 3b. Figure 4b, demonstrating the existence of crystalline NiO. From the clear lattice stripes, the lattice spacing can be calculated to be 0.208 nm which corresponded to the (200) plane of the cubic NiO. Moreover, Figure 4c depicts the SAED image of synthesized NiO sample, from which one can observe that there are a group of diffraction rings, namely (1 1 1), (2 0 0), and (2 2 0) lattice planes of cubic NiO. To confirm the potential impact of the specific surface area on gas sensing properties, the typical nitrogen adsorption-desorption isotherm was calculated. The BET analysis (Figure 5) indicated that the specific surface area of the synthesized flake-flower NiO is 176.5 m 2 g −1 and the average pore size is 27.6 nm, suggesting that the flake-flower microstructure provided a large surface area.

Gas Sensing Performance
As we all know, the working temperature of gas sensor has a great influence on the reaction in the process of gas sensing. To obtain the optimal working temperature of the fabricated CO sensor based on the flake-flower NiO, the device was tested at different temperatures from 100 to 400 • C toward 5 ppm CO. It can be seen from Figure 6 that the response of the flake-flower NiO based sensor toward CO of 5 ppm increased first and then decreased with the increase of temperature, suggesting that the response of the sensor is strongly dependent on the working temperature. This is because the low activation energy at low temperature is not enough to support the gas reaction, and the increased temperature will lead to the higher desorption rate. Given this, the gas response will reach a maximum at a specific temperature, which is the optimum working temperature. In this work, the optimum working temperature of the CO sensor is 250 • C, at which the highest response is 19.5. And subsequent gas sensing experiments will be carried out at this optimal operating temperature.  Figure 7 presents the response of the flake-flower NiO based sensor toward different concentration of CO at the optimal working temperature of 250 • C. Obviously, the response of the fabricated sensor increased almost linearly with the increase of the concentration of CO from 1 to 100 ppm. Besides, the linear fitting function and the linear correlation coefficient were calculated as y = 17.57 + 0.43x and 0.995, respectively. It can be found that the responses of flake-flower NiO based sensor increased obviously linearly with the increasing CO from 1 to 100 ppm at the optimal working temperature of 250 • C. Figure 8 demonstrates the response and recovery characteristic of the prepared sensor with different concentrations of CO (1, 5, 10, 30 ppm) at the optimal working temperature of 250 • C. It is obvious the gas response curve of the sensor increases sharply when CO in, while the gas response curve of the sensor quickly returns to the original state when CO out. The CO responses of the sensor were tested to be 16.9, 19.5, 22.8, and 32.7 under 1, 5, 10, and 30 ppm, respectively. The dynamic response-recovery result indicated that the fabricated sensor possessed excellent reversibility which is an important property for the application of gas sensors.
Considering that selectivity and stability are two important indexes to evaluate the performance of gas sensors, further gas sensing experiments were carried out based on the gas concentration of 5 ppm and the optimal working temperature of 250 • C. As shown in Figure 9A, the sensor based on the flake-flower NiO was exposed toward hydrogen sulfide (H 2 S), acetylene (C 2 H 2 ), hydrogen (H 2 ), ammonia (NH 3 ), methane (CH 4 ), and CO. It can be calculated that the response toward CO is at least 6 times higher than that of other gas, indicating that the NiO based sensor has good selectivity for 5 ppm CO at the optimal working temperature of 250 • C, and can be applied for the effective detection of CO. The long-term stability experiment of fabricated CO sensor was carried out for 1 month. The performance of the sensor toward 5 ppm CO at 250 • C was tested every 5 day. Figure 9B confirms that the prepared device possessed outstanding stability with slight change for 1 month, suggesting the fabricated sensor could be a promising choice for the application to the effective detection of CO.

Sensing Mechanism
As we all know, the basic gas sensing mechanism is demonstrated by the resistance change caused by the reaction between the adsorbed oxygen molecules and the measured gas molecules. Before injecting the CO gases, the oxygen molecules in the air were adsorbed on the surface of the NiO material due to its strong electronegativity (Cao et al., 2015;Chen et al., 2018Chen et al., , 2019. The oxygen molecules captured the electrons from the surface of NiO material and then were reduced to oxygen ions (O − ). For a typical p-type oxide, since the main carrier of NiO is hole, the electrons captured by oxygen mainly come from valence band. This process results in the formation of a hole aggregation layer on the surface of the material, which has a lower resistance compared with the core region (Figure 10). The oxygen adsorption mechanism can be expressed as follows: When the reducing gas CO is introduced, it will react with the oxygen ion adsorbed on the NiO surface to produce CO 2 and release electrons at the same time. This process make the released electrons combine with the holes, leading to the increase of the resistance compared with previous state. The process can be described as follows: Besides, for typical p-type NiO based sensing materials, various strategies have been used to synthesize different nanostructures for the detection of various reducing gases. The gas sensing characteristics of the NiO based sensors of the recently published investigations were listed in Table 1, from which we can find that various NiO based sensors possessed excellent gas sensing performances toward different reducing gases.

CONCLUSION
To summarize, hierarchical flake-flower NiO was successfully synthesized via a facile hydrothermal method. The prepared sample was tested by various structural and morphological characterization, demonstrating that the flake-flower microstructure was assembled by numerous nanosheets with different shape and size. Further CO sensing experiments indicated that the sensor based on the flake-flower NiO exhibited excellent gas sensing performance including high response, outstanding selectivity and stability. A plausible mechanism suggested that the excellent performance was caused by the flakeflower morphology with complicated microstructure and large special surface area. Therefore, the results suggested that flakeflower NiO based CO sensor might be considered as a promising candidate for detecting the fault characteristic gases dissolved in transformer oil.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation, to any qualified researcher.