UV-Activated Formaldehyde Sensing Properties of Hollow TiO2@SnO2 Heterojunctions at Room Temperature

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Introduction Formaldehyde (HCHO) is an important chemical raw material that has a wide range of applications in interior decoration materials such as adhesives, building materials and coatings. Long-term exposure to formaldehyde can cause headaches, memory loss, especially harmful for pregnant women and infants [1]. Therefore, timely and accurate detection of formaldehyde in indoor environments is very important.In this aspect, titanium dioxide (TiO2) is a typical n-type of semiconductor. Due to its good photocatalytic performance, low cost, non-toxicity and high thermal stability, it has been explored for chemical gas sensors [2]. As reported in literature [3], the construction of heterojunction structure could significantly enhance the gas-sensing performance, and the use of UV irradiation can greatly reduce the working temperature of the sensor, or even down to room temperature [4]. Therefore, the gas sensing properties of SnO2 and TiO2@SnO2 heterojunctions have investigated in this work. Experiment First, the carbon nanospheres templates were prepared by hydrothermal method. Hollow SnO2 nanospheres were obtained by using carbon nanospheres as templates via hydrothermal method at 170 °C for 36 h. Then, hollow SnO2 nanospheres were dispersed into absolute ethanol, tetrabutyl titanate (TBT) and ammonia were added into above solution. Stirred for 24 h at room temperature. Finally, the as-obtained precipitates were calcined at 500 °C for 2 h. The as-prepared hollow SnO2 nanospheres and TiO2@SnO2 heterojunctions powder were dispersed in an alcohol solution. The paste was deposited onto the alumina substrates with Au interdigital electrodes. Then, the sensors were dried at 70 °C for 12 h. Results and conclusions 1. Structure and MorphologyFig.1a shows the XRD patterns of as-synthesized hollow SnO2 nanospheres, hollow TiO2 nanospheres and hollow TiO2@SnO2 heterojuction nanospheres, All the diffraction peaks of TiO2 and SnO2 can correspond to JCPDS card No. 21-1272 and JCPDS card No. 41-1445, respectively. There are no other diffraction peaks of impurity in the XRD pattern. As Fig. 1b shows that the as-prepared SnO2 nanospheres have a distinct hollow structure and the particle size is about 200-300 nm. Fig. 1c presents that the surface of the TiO2@SnO2 nanocomposites is significantly rougher than pure SnO2, and a layer of granular TiO2 is wrapped around the hollow SnO2 nanospheres. From Fig. 1d, the thickness of hollow SnO2 nanospheres inner shell is about 50 nm. Fig. 1d exhibits the lattice fringes of hollow TiO2@SnO2 heterojunctions. The spacing of lattice fringes of TiO2 and SnO2 in the heterojunctions are measured to be 0.357 nm and 0.238 nm, respectively. They can be assigned to (101) lattice plane of anatase TiO2 structure and (200) lattice plane of rutile SnO2 consistent to the XRD results. Fig. 1f show that the element of Ti is uniformly distributed around the Sn element, and the O element is distributed in the whole region.2. Gas-Sensing PropertiesFig. 2a shows the response curves of the hollow TiO2@SnO2 nanospheres and pure TiO2 nanospheres based sensors to different concentrations of formaldehyde under UV at room temperature. The response of the sensors increases as the formaldehyde concentration increases from 0.1 ppm to 10 ppm. Even at lower formaldehyde concentrations, such as 100 ppb, the response of the TiO2@SnO2 nanocomposite based sensor still higher than that of the pure TiO2 based sensor. Fig. 2b shows the response curves of hollow TiO2@SnO2 nanospheres and pure TiO2 based sensors to 10 ppm formaldehyde under UV illumination at room temperature. The response value of TiO2@SnO2 nanocomposites based sensor is about 20, which is much higher than that of TiO2. The response time and recovery time of TiO2@SnO2 nanocomposites are 20 s and 56 s, respectively. The above results show that after incorporated with SnO2, the gas-sensing property of the TiO2 sensor is significantly improved.