Impact of Nb doping on gas-sensing performance of TiO2 thick-film sensors

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Abstract

Using a simple hydrothermal method, the pristine and Nb doped TiO2 is prepared, and their microstructures and gas-sensing responses to the harmful volatile organic compounds are investigated with a special focus on the impact of Nb additive. We find that the gas response of TiO2 is enhanced significantly by doping Nb, which is understood in theory upon proposed adsorption models. Combining experimental measurements with first-principles calculations, the working mechanism underlying such improvement in gas-sensing functions by the Nb additive is discussed.

Introduction

The indoor air quality has become a serious environmental issue in our daily life. Indoor pollutants, which mainly consist of volatile organic compounds (VOCs) such as the formaldehyde, ethanol, xylene and acetone, are known to be one of the causes to deteriorate the health of human being or even result in terrible diseases [1], [2], [3]. To solve this problem, much effort has been devoted to the development of a reliable gas sensor that can pursue an on-site detection of the VOC gases in a smart fashion [4], [5], [6], [7], [8]. Among all the gas-sensing materials, the semiconducting metal oxides have long been recognized as being promising in monitoring the harmful VOC gases on an industrial level owing to their high response as well as low manufacture cost [9], [10], [11], [12], [13], [14], [15]. However, the widespread use of such oxides as gas-sensing materials is currently restrained by the absence of selectivity and durability, the fundamental requirements for a practical application of gas sensors [16], [17].

Of all the traditional semiconducting metal oxides, TiO2 has been demonstrated to hold a substantial promise for sensor devices largely because it is chemically stable even at high temperatures [18], [19], [20]. Further, exotic elements can often be incorporated into TiO2 as additives, which indeed enable a dramatic improvement in its gas-sensing performances, including La, W and Nb [21], [22], [23]. Of these additives, the Nb is found to be very effective in enhancing gas responses of TiO2 to NO2 [24], shortening response time to O [25], and upgrading photosensitivity to NO [26]. One plausible explanation for such enhancement in gas-sensing properties of TiO2 is that the introduction of Nb can mediate elegant shift in electronic structures of TiO2, for instance, displacement of location of Fermi level (EF) with respect to the conduction band of TiO2 [27], [28], and presence of novel occupied states arising from the Nb 4d valence electrons that form nonbonding bands in the bulks, whereas localized dangling bonds on the surfaces [29]. Structurally, Anukunprasert et al. has provided convincing evidence that the addition of Nb can retard the phase transformation of TiO2 from the anatase to rutile as well as inhibit its inner grains from growing. Such structural and electronic modifications due to the Nb have a direct consequence on the gas-sensing ability of TiO2 by enhancing its response to e.g., CO [30], [31] and ethanol [31], [32], whichever method is applied to fabricate such systems. In addition, previous reports have also claimed that the surface morphology of TiO2 can be tailored efficiently by doping Nb as well, such as the pore, particle size, and specific surface area [33], [34], [35]. However, the working mechanism of TiO2 sensors and how the Nb impacts gas-sensing behaviors of TiO2 remain a mystery, in particular, the atomic geometry and chemical environment on the oxide surface, which strongly affect the gas-sensing performances, are hardly accessible.

Here, we have prepared the pristine and Nb doped TiO2 nano-composites via the simple yet efficient hydrothermal method and investigated their gas-sensing responses towards VOCs including methanol, ethanol, acetone and formaldehyde. To gain more insight into gas-sensing mechanism, we conduct a series of first-principles calculation based upon the surface adsorption models inferred from experiments. The Nb doped TiO2 is found to exhibit enhanced gas response to VOCs as compared to pristine TiO2, and our simulations open up a new avenue to understanding gas-sensing origin of the TiO2-based sensing materials.

Section snippets

Preparation of Nb-doped TiO2 nano-composites

All chemical reagents were of analytical grade and applied without any further purification. The pristine and Nb doped TiO2 nano-composites were fabricated using the hydrothermal method, the process of which was illustrated schematically in Fig. 1. The metal salt were first hydrolyzed in dilute ammonium hydroxide solution and then 0.02 M of TiCl4 was dissolved in 50 mL of distilled water, followed by the addition of 0.16 g of NbCl5 in a drop-by-drop fashion under intense magnetic stirring for 30 

Structural characterization

Fig. 4 presents the XRD spectra of the pristine and Nb doped TiO2, where the textural orientations of the detected matters are given as well for easy reference. The diffraction peaks in each sample are consistent well with those of a standard TiO2 with anatase phase (JCPDS 21-1272), indicating that the prepared products are chemically pure. However, no diffraction peak regarding the Nb is detected in the doped samples, which is attributed to the small amount of added Nb that is beyond the

Conclusions

We have applied a simple yet efficient hydrothermal method to prepare pristine and Nb doped TiO2 nano-composites, and investigated their microstructures and gas-sensing behavior to the VOC gases. The Nb doping is found to play an essential role in improving the gas response of anatase TiO2 toward VOCs, the mechanism of which is clarified upon theoretical surface models. In comparison to the clean (1 0 1) surface, the doped one shows enhanced O adsorption and charge transfer, resulting in larger

Acknowledgements

This work was supported in part by the Fundamental Research Funds for the Central University (CDJXS10131154) and the distinguished PhD foundation from the Ministry of Education of China (0903005109044-13).

Wen Zeng received his PhD degree in material science from Chongqing University in China in 2010. He was a visiting scholar at Tohoku University in Japan from 2009 to 2010. He is currently a lecture at the College of Materials Science and Engineering, Chongqing University. His current research includes the synthesis of low-dimensional functional materials, the fabrication of semiconductor sensor, and the first principles calculations.

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    Wen Zeng received his PhD degree in material science from Chongqing University in China in 2010. He was a visiting scholar at Tohoku University in Japan from 2009 to 2010. He is currently a lecture at the College of Materials Science and Engineering, Chongqing University. His current research includes the synthesis of low-dimensional functional materials, the fabrication of semiconductor sensor, and the first principles calculations.

    Tianmo Liu is a professor of College of Materials Science and Engineering at Chongqing University in China since 2001. He received Dr. Eng. from Department of Solid Mechanics, Chongqing Univ. in 1999. His current research interest involves functional gas-sensing materials, magnesium alloys, and theoretical calculations. He is now also holding a group leader position at National Engineering Research Center for Magnesium Alloys at Chongqing University.

    Zhongchang Wang is now an assistant professor at the world premier international research center, advanced institute for materials research, Tohoku University in Japan. He received his PhD in 2007 from The University of Tokyo in Japan. He is currently mainly focusing on gas-sensing materials, interfaces, grain boundaries, dislocations in functional oxides, and quantum transport via combining the advanced transmission electron microscopy with the first-principles calculations.

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