Phosphorus doped TiO2 as oxygen sensor with low operating temperature and sensing mechanism
Graphical abstract
Highlights
► This work firstly reports an oxygen sensor based on phosphorus doped TiO2. ► The sensitivity and selectivity of TiO2 are enhanced by phosphorus doping. ► The operating temperature is low, and the response time is short. ► The mechanism of oxygen sensing improvement with phosphorus doping is researched. ► Based on this work, nonmetal doping could be as a new filed for gas sensor.
Introduction
Oxygen sensor is utilized to detect and control oxygen concentration for protecting environment and combustion in automobile engines, steel and chemical industries [1]. TiO2 is a good material for oxygen sensor, because of its low cost, stable phase even at high temperature, and high gas sensitivity [2], [3]. But the gas sensor based on pure TiO2 is with high resistance, high operating temperature, and poor selectivity. Further improvement is achieved by addition of noble or transition metals, since these metals can provide more electrons for gas sensor surface, resulting in enhancing the sensitivity [3], [4], [5], [6], [7], [8], [9]. This sensing phenomenon involves the surface processes that adsorbed oxygen molecules, which are chemisorbed on the surface of sensor, capture electrons from the semiconducting sensors at higher oxygen concentration. Consequently, a space-charge region is built on the surface of the sensors, forming an electron depleted layer near the surface and increasing the surface resistance [10], [11]. Contrarily, the adatoms, i.e., chemisorbed oxygen molecules, escape from the surface and inject electrons back into the semiconducting sensors at lower oxygen concentration under an optimized operating temperature, resulting in the decrease of the surface resistance.
The improvement of gas sensing properties through chemical composition control of metal oxide is the main approved method to design advanced sensors. Many efforts have been employed to improve the sensitivity of titanium dioxide and lower the operating temperature. However, up to now, many important problems remained unresolved. Sharma et al. claimed that the rutile phase of Cr3+ doped TiO2 was a potential candidate phase for oxygen sensing, while the rutile was obtained at high calcining temperature (≥700 °C) via phase transferring from anatase [3]. Sensing materials calcined at high temperature may not be helpful as it causes the increase of crystal growth and subsequent loss of surface area. The response of Cr-doped TiO2 to O2 is lower than 7 in the range from 100 ppm to 10% O2 at an operating temperature of 370 °C [12]. Additionally, its response time is about 1–3 min and recovery time is less than 1 min. Platinum has been used to considerably improve not only sensitivity and selectivity but also the response speed of TiO2-based gas sensors. The response time of Pt/TiO2 sensor is about 20 ms exposed to O2, but the working temperature is higher than 400 °C [11]. In order to improve sensitivity for oxygen sensor, Castaneda et al. added three different catalysts into TiO2, while the highest response to O2 is only 2.5 and the optimal operating temperature is 250 °C [13], which was also considered as a low operating temperature by Al-Hardan et al. [14].
Based on the gas-sensing mechanism, the nonmetal ion doping in TiO2 would be used to improve the gas sensing properties as metal doping. Han et al. reported that the resistivity of F-doped SnO2 was decreased compared with pure SnO2; in addition, the sensitivity and selectivity as to H2 were improved by fluorine doping. And also the electrical energy consumption for the heater by fluorine doping was only about 42.6 mW [15]. Zhao et al. prepared F-doped TiO2 as dimethyl methylphosphonate (DMMP) sensor on quartz crystal microbalance (QCM). F-doped TiO2 could further improve the DMMP sensing characteristics, and exhibit a shorter response time (8 s) than pure TiO2 (27 s). They considered that the adsorption of DMMP, which takes place at Lewis acid sites, is enhanced by fluorine doping because of the strong electron withdrawing ability of fluorine [16]. Further, the phosphorus doping for TiO2 can increase surface electronic density and enhance ability of adsorbed oxygen [17]. Therefore, oxygen chemisorptions and oxygen sensitivity could be enhanced with nonmetal doped TiO2.
However, to our best knowledge, it is the first time to synthesize phosphorus doped TiO2 as an oxygen sensor. Yu et al. synthesized phosphated mesoporous titanium dioxide with anatase phase through a surfactant templated approach, where phosphorus from phosphoric acid was directly incorporated into the inorganic framework of TiO2 [18]. Shi et al. described that P-doped TiO2 nanoparticles as visible-light photocatalyst were prepared by sol–gel process [19]. They found that the phosphorus doping stabilizes the anatase TiO2 framework and increases the surface area significantly. In addition, the higher surface acidity of TiO2 is attributed to Ti4+ ions presenting in a tetrahedral coordination through incorporating phosphorus, which could act as Lewis acid sites for adsorbing oxygen molecules [18].
In our work, the phosphorus doping with an appropriate concentration can inhibit the grain growth and provide more active sites for the adsorption of the target gas, and the operating temperature is expected to be among 110–150 °C. Further, the mechanism of oxygen sensing improved with phosphorus doping is also investigated based on microstructures and properties of oxygen response.
Section snippets
Preparation of P-doped TiO2 and fabrication of gas sensors
Pure and P-doped TiO2 nanoparticles were prepared by a sol–gel process. Titanium trichloride (mixture of TiCl3 (15 wt%) and HCl (85 wt%), 99.5% purity, Shanghai Chemical Reagent Corp., China) and phosphoric acid (H3PO4, AR, Sinopharm Chemical Reagent Co. Ltd., China) were mixed with hydrochloric acid. The amount of the doped phosphorus was 2, 5 and 10 mol%, respectively. After stirring at 75 °C for 0.5 h, H2O2 and DBS (CH3(CH2)11OSO3Na) were added into the solution, and the sol was stirred until it
Structural analysis
Fig. 1 shows the XRD patterns of pure and P-doped TiO2 samples calcined at 400 °C for 3 h. The results elucidate that the crystallite phase prefers to transform from rutile to anatase with increasing the concentration of phosphorus dopant. Pure TiO2 is a mixture of rutile and anatase, whereas, the main phase is anatase. The 2 mol% P-doped TiO2 is more amorphous than the others; however, the 5 mol% phosphorus doped nanostructures are mostly anatase and 10 mol% samples are pure anatase. It indicates
Conclusions
The TiO2-based sensing nano-materials doped with phosphorus were synthesized by sol–gel process using TiCl3 as a source of TiO2. The XRD patterns and BET data indicate that the phosphorus dopant can inhibit the grain growth and phase transformation, as the same time, increases the surface area of the materials. Specially, a smaller crystal size (9.2 nm) and bigger surface area (153.9 m2/g) are obtained by 5 mol% P-doped TiO2 compared with other samples. The 5 mol% P-doped TiO2 presents higher
Acknowledgements
The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (91022025, 51072036, and 21203031), Fujian Natural Science Foundation (2012J01204), and Education Term of Fuzhou University (0460-023028).
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