Preparation and Application of TiO2 Nanotube Array Gas Sensor for SF6-Insulated Equipment Detection: a Review
© The Author(s). 2016
Received: 13 April 2016
Accepted: 2 June 2016
Published: 18 June 2016
Since Zwilling and co-workers first introduced the electrochemical anodization method to prepare TiO2 nanotubes in 1999, it has attracted a lot of researches due to its outstanding gas response and selectivity, making it widely used in gas detection field. This review presents an introduction to the sensor applications of TiO2 nanotube arrays (TNTAs) in sulfur hexafluoride (SF6)-insulated equipment, which is used to evaluate and diagnose the insulation status of SF6-insulated equipment by detecting their typical decomposition products of SF6: sulfur dioxide (SO2), thionyl fluoride (SOF2), and sulfuryl fluoride (SO2F2). The synthesis and sensing properties of TiO2 nanotubes are discussed first. Then, it is followed by discussing the theoretical sensing to the typical SF6 decomposition products, SO2, SOF2, and SO2F2, which analyzes the sensing mechanism at the molecular level. Finally, the gas response of pure and modified TiO2 nanotubes sensor to SO2, SOF2, and SO2F2 is provided according to the change of resistance in experimental observation.
Titanium dioxide (TiO2) nanotube has been widely researched due to its distinguished properties, including high surface-to-volume ratios, high surface activity, strong catalytic activity, and high ultraviolet light adsorption and heat conductivity [1–3]. It has been used in fields such as industrial manufacturing, aerospace, ocean exploring, environmental protection, resource development, and medical diagnose [4–7]. To meet the increasing high requirement for gas detection, TiO2 nanotube gas sensors are investigated for qualitative or quantitative gas detection. However, the detection response, selectivity, and accuracy of pure TiO2 are limited for different gases detection. To improve its detection performance, the most used methods are morphology control and surface modification [2, 8, 9], aiming to increase the effective reaction surface and active site. For common gas detection such as O2, H2, SO2 and H2S, the highest detection limit has even reaches parts per million level [10–14].
Sulfur hexafluoride (SF6) insulating gas possesses outstanding arc quenching and insulation performance, which is the most used filled gas in gas-insulated equipment, such as gas-insulated switchgear (GIS), gas-insulated lines (GIL), and gas circuit breaker (GCB) [15–17]. However, SF6 gas will inevitably decomposes to various typical decomposition components: SO2, SOF2, SO2F2, etc. under partial discharge and disruptive discharge (surface flashover and creeping discharge) when insulation defects occurs in production and long term operation process [18–20]. The insulation defects in SF6-insulated equipment show great influence on the stability of entire insulated system. On the one hand, the dielectric strength of filled insulated gas obviously reduces under discharge because of the decomposition of SF6. On the other hand, the decomposition components (low-fluorine sulfides) corrode the surface of SF6-insulated equipment with the action of trace water and oxygen in equipment . Besides, most of the insulation defect-induced discharge is hard to be found by the inspection workers as the discharge is always unsustainable. Therefore, online detection method, which assesses the insulation status automatically in real time, becomes an effective to solve the detection difficulty [22–24]. However, the current detections methods: ultra high frequency (UHF) method, transient earth voltage (TEV) method, ultrasonic method, and fluorescence detection method are easily affected by the environmental interference signal [25–28]. Thanks to the distinguished anti-interference and high detection precision properties of gas sensors detection method, online gas detection becomes a new breakthrough for insulation status assessment of SF6-insulted equipment.
In this paper, we will review the achievements in the filed using TiO2 nanotubes for three typical SF6 decomposition components: SO2, SOF2, and SO2F2 detection. Firstly, pure morphology of TiO2 nanotubes is prepared by adopting different preparation methods. In addition, the surface modification of TiO2 nanotubes is analyzed by experimental study to enhance the gas detection response. Secondly, the gas sensing properties are discussed to analyze the gas detection mechanism by theoretical studies. Finally, the gas sensing property to three typical SF6 decomposition products is discussed by theoretical and experimental studies. Meanwhile, the influence factors such as gas concentration and sensing time are presented in detail.
Synthesis of TiO2 Nanotubes
Due to the features of simple and low-price preparation, hydrothermal treatment method is an effective method to synthesize TiO2 nanotubes in large scale industrial production. The morphology of the produced TiO2 nanotubes are usually composed by small diameter, thin wall, and large surface area nanotubes, and the nanotubes are usually unordered and intertwined as shown in Fig. 1b . For the synthesis of TiO2 nanotubes, firstly, mixing the TiO2 nanoparticles with strong alkali solution under high temperature and high pressure, the single-layer nanosheets of TiO2 appear in the treatment process curl from one dimension to two and three dimensions, which is similar with the formation mechanism of carbon nanotubes. After the chemical reactions, the TiO2 nanotubes is received by ion-exchange and anneal. Finally, the powder-like TiO2 nanotubes is obtained by centrifuged by centrifugal machine after neutralizing the strong alkali solution with weak acid solution. According to the report of Weng et al. in 2006 , TiO2 nanotubes, with an external diameter of around 8 nm and a wall thickness of about 1 nm, was synthesized by hydrothermal treatment method.
The anodization method is one of the effective ways to synthesize ordered alignment and aspect radio TiO2 nanotubes as shown in Fig. 1c . The anodic Ti foil dissolve (metal corrosion or electropolishing) to Ti metal cation under the action of electrolyte and electric field. On the one hand, the produced Ti metal cation reacts with the O2− (produced by water electrolysis) and forms a TiO2 oxidation film on the surface of Ti foil, resulting in the increase of resistance. Therefore, the formation rate of TiO2 oxidation film decreases. On the other side, the produced TiO2 oxidation film is dissolved by the electrolyte. Under the combined action of formation and dissolution, nanotube arrays synthesize on the surface of Ti foil. The nanotube morphology depends on many influence factors: applied voltage, electrolyte composition, and pH value. In 2000, Grimes et al. presented that an aligned and organized TiO2 with an average tube diameter from 25 to 65 nm nanotubes fabricated by anodization method . In addition, Grimes reviewed the fabrication, properties of highly ordered TiO2 nanotube arrays made by anodic oxidation of titanium in fluoride-based electrolytes . They found that the length of TiO2 tends to be longer in the weak acid electrolytes, and the wall of TiO2 nanotubes becomes smoother in organic electrolytes.
Due to the outstanding properties, high surface area, ordered alignment, and morphology adjustability of TiO2 nanotubes, it shows great potential in gas detection. However, the pure TiO2 nanotubes are hard to meet the high detection requirement because of its limitation in gas response, detection range, response time, and operational temperature. In this regard, great efforts have been made to extend the gas detection properties. Currently, a few modification methods: metal decoration, doping, semiconductor composites are introduced to improve the gas sensing properties [38, 39]. Comparing the different modification methods, metal decoration is one of the most effectively method to significantly enhance the gas response of TiO2.
Metal decoration can be achieved in two different ways: metal nanoparticles decoration and metal ions decoration. For metal nanoparticles decoration, the metal nanoparticles are loaded or deposited on the surface of pure TiO2 nanotubes to change the electron distribution in TiO2 nanotubes system. For metal ions decoration, the ions are intruded into TiO2 lattice, resulting in a trace of metal ions take the place of Ti atoms in the TiO2 lattice by physical and chemical approaches. The decorated metal atoms improve the gas sensing properties by changing the electron distribution and energy band. Xiaoxing et al. synthesized Pt atom modified TiO2 nanotubes in H2PtCl6·6H2O (1 g/L) and H3BO3 (20 g/L) electrolytes by pulsed electrodeposition , which show good response to SF6 decomposition products: SO2, SOF2, and SO2F2. Shahin el al. reported that the Au- and Ag-decorated TiO2 nanotubes sensor exhibited a large resistance variation in the presence of very small quantities of H2 gas at 25 °C .
In order to evaluate and diagnose the insulation status of SF6-insulated equipment, it is necessary to precisely detect each kind of decomposition products of SF6: SO2, SOF2, and SO2F2, respectively. However, these three kinds of gas products usually appear at the same time when discharge occurs in SF6-insulated equipment, leading to the cross interference between different products. Therefore, a lot of researches have been made to enhance the gas selectivity while improving the gas response.
(1 0 1) and (0 0 1) Perfect Surface of TiO2
Calculated adsorption energy, charge transfer, and binding distance of the perfect surfaces
(1 0 1) perfect surface
(0 0 1) perfect surface
TiO2 (1 0 1)
TiO2 (0 0 1)
E a (eV)
E g (eV)
Oxygen-Defect (1 0 1) Surface of TiO2
Calculated adsorption energy, charge transfer, and binding distance of (1 0 1) defect surface
(1 0 1) defect surface
TiO2 (1 0 1)
E a (eV)
E g (eV)
Oxygen-Defect (0 0 1) Surface of TiO2
Calculated adsorption energy, charge transfer, and binding distance of the (0 0 1) defect surface
(0 0 1) defect surface
TiO2 (0 0 1)
E a (eV)
E g (eV)
Pt-Decorated (1 0 1) Surface of TiO2
In this section, the gas response (change of resistance) of TiO2 nanotube arrays (TNTAs) to SO2, SOF2 and SO2F2 are discussed at 200 °C, and the negative value of resistance variation (R%) means the reduction of resistance. According to the correspondence between gas response and concentration, it is found that the change between them presents a fitting curve relationship. As a result, we can directly estimate the concentration of gas according to the corresponding gas response.
Comparing the gas sensing speed and magnitude of resistance change at the same gas concentration and temperature, the gas response of TNTAs to SF6 decomposition products is in orders, SO2 > SOF2 > SO2F2, indicating the potential of selective detection between SO2, SOF2, and SO2F2.
TiO2 nanotube arrays (TNTAs) has been widely used as gas sensor for its distinguished properties in large specific surface area, large pore structure, easy synthesis process, and environmentally friendly nature. In order to evaluate and diagnose the insulation status of SF6-insulated equipment, TNTAs gas sensor becomes an effective new method to realize the function by detecting the decomposition components of SF6: SO2, SOF2, and SO2F2. In terms of TNTAs synthesis, three methods, assisted-template method, hydrothermal treatment method, and anodization method, are discussed to analyze the preparation process and the features of prepared TNTAs in detail. Then, recent studies carried out by theoretical simulation have been viewed. The adsorption of SO2, SOF2, and SO2F2 on different surface of TiO2 is reviewed in this section, including (1 0 1) and (0 0 1) perfect surface of TiO2, oxygen-defect (1 0 1) and (0 0 1) surface of TiO2, and Pt-decorated (1 0 1) surface of TiO2. Finally, the experimental researches used to analyze the gas response of TNTAs sensor to SO2 and SOF2 and SO2F2 are discussed. Comparing the gas response to SO2, SOF2, and SO2F2 by different gas sensors (pure TNTAs sensor and Pt, Au-decorated TNTAs sensor), it is found that the metal decoration improves the gas response property to SO2 and SOF2 and SO2F2 and also reduces the working temperature for gas detection. Further, more studies should be investigated to enhance the detection precision and stability of TNTAs, aiming to industrialize the fabrication and application of TNTAs sensor in SF6-insulated equipment.
This work was supported by the National Natural Science Foundation of China under Project no. 51277188.
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