Ag nanoparticles modified TiO2 spherical heterostructures with enhanced gas-sensing performance

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Abstract

Monodispersed TiO2 spherical colloids with diameters of about 250 nm were prepared by a sol–gel method. Heterostructural Ag–TiO2 spheres were manipulated by surface engineering, in which the Ag nanoparticles with an average size of 10 nm were uniformly distributed on the surface of the TiO2 nanospheres by in situ reduction and growth. The gas-sensing properties of the TiO2 nanospheres and heterostructural Ag–TiO2 nanospheres to ethanol and acetone were measured at 350 °C. The results indicated that Ag nanoparticles greatly enhanced the response, stability and response characteristic of TiO2 nanospheres to the tested gases. Response times of Ag–TiO2 sensor to 30 ppm acetone and 50 ppm ethanol were <5 s.

Introduction

It is necessary to monitor colorless organic vapour concentrations in the environment for health and the workplace for safety due to their toxicity and explosive possibility [1]. Nanometer-scaled TiO2, an important multifunctional n-type semiconductor, has stimulated considerable research efforts in seeking morphology-dependent physical properties and applications in gas sensors, solar cells, photocatalysis, etc. [2], [3], [4], [5], [6]. Especially, as an organic vapour sensing material, TiO2 is nontoxic, highly chemically stable and low costing in comparison with other common metal oxides such as SnO2 and ZnO. In fact, a variety of TiO2 nanostructures with different sizes and morphologies, including nanoparticles, nanotubes, nanobelts, and nanowires, have been examined for ethanol and acetone sensing and exhibited good sensing characteristics [7], [8], [9], [10]. However, several drawbacks, such as low response, poor selectivity and stability, and long response and recovery times, limited previously studied TiO2 materials in the practical application. Recently, Hayakawa et al. [11] reported the Pt dispersed-TiO2 nanoparticles with an improved selectivity to trimethylamine and NH3. Hu et al. [12] reported that heterostructural Ag–TiO2 nanobelts exhibited high sensitivity and rapid response for ethanol vapour detection. These two results remind us that the deposition of noble metal nanoparticles onto the surface of oxide nanostructures can improve the response and selectivity, and reduce response time [13], [14], [15], [16]. For exploiting gas sensors, it is desirable to develop high performance, gas-sensing materials; for example, morphologically different, TiO2 heterostructures modified by noble metal nanoparticles.

The nanometer-scaled TiO2 spherical colloids have currently been one of the most active areas of material research due to their high refractive indices, excellent catalytic activities and chemical stabilities, along with potential applications widely spreading from photonic crystals to photocatalysis [17], [18]. In particular, noble metal nanoparticles are easily deposited onto the surface of TiO2 nanospheres through the strong synergistic interactions, which further enhance the catalytic activities and photoelectronic properties of TiO2 nanospheres due to modulation of the energy band structures and surface-to-volume ratios by surface engineering [19], [20], [21]. Because of its unique physical and chemical characteristics, the Ag–TiO2 spherical heterostructure is a remarkable candidate for organic vapour detection. However, the application of the Ag–TiO2 spherical heterostructures in sensors remains unexplored. Here, we successfully prepared the monodispersed TiO2 spherical colloids by a sol–gel method and Ag–TiO2 spherical heterostructures by surface engineering of in situ reduction and growth. The gas-sensing properties of these two materials to vapour phase ethanol and acetone were measured. The results indicated that modified Ag nanoparticles could greatly enhance the response, stability and response characteristic of TiO2 nanospheres to the tested gases.

Section snippets

Synthesis of TiO2 nanospheres

The TiO2 nanospheres were synthesized following the reported procedure [17] with modifications. Tetraethyl titanate (3.5 mL) was added to ethylene glycol (50 mL) and magnetically stirred for 10 h at room temperature. Then, the mixture was poured into a solution containing 170 mL acetone and 2.7 mL water with vigorously stirring for 1 h. The white precipitate was harvested by centrifugation, followed by washing with distilled water and ethanol five times to remove free ethylene glycol from the

Structure and morphology

The as-prepared precursor before calcinations is in an amorphous phase. The XRD pattern of the Ag–TiO2 powders annealed at 500 °C for 1 h is shown in Fig. 1. The d-values calculated are in agreement with those given in the standard data (JCPDS, 21-1272), suggesting that the as-prepared precursor has crystallized in a pure anatase phase. No obvious peaks for metallic silver nanoparticles can be observed because the low diffraction intensity of ounce silver is immersed in that of TiO2. Furthermore,

Conclusion

The thick film sensors based on pure TiO2 and heterostructural Ag–TiO2 nanospheres have been successfully developed. The gas (ethanol and acetone) sensing properties were characterized and the results indicate that the Ag nanoparticles modification significantly enhances the response and stability of TiO2 nanospheres with an ultrafast response of less than 5 s to 50 ppm ethanol and 30 ppm acetone, respectively.

Acknowledgements

This work is supported by Program for New Century Excellent Talents in University (NCET-06-0349), Science and Technology Project of Heilongjiang Province (WB10A204), Innovation Team of Heilongjiang University (Hdtd2010-04), Youth Foundation of Harbin (2005AFXJ059) and Heilongjiang Educational Department (11551337, 11551341).

Xiaoli Cheng was born in 1978. She obtained her MSc degree from School of Chemistry and Materials Science, Heilongjiang University, in 2004. Now she is studying for Ph.D. in the same college with the main interests of inorganic functional materials.

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    Xiaoli Cheng was born in 1978. She obtained her MSc degree from School of Chemistry and Materials Science, Heilongjiang University, in 2004. Now she is studying for Ph.D. in the same college with the main interests of inorganic functional materials.

    Yingming Xu received his PhD in analytical chemistry from Changchun Institute of Applied Chemistry of Chinese Academy of Sciences, China, in 2010. His research interests are mainly directed towards the hierarchical oxide materials and their application as gas sensors.

    Shan Gao finished his postdoctoral work in Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, China, in 2000. He is now the professor of School of Chemistry and Materials Science, Heilongjiang University. His research interests are mainly directed towards the aspects of functional coordination chemistry and nano-materials.

    Hui Zhao received his PhD from Department of Chemistry, Jilin University, China, in 1999. He is now the professor of School of Chemistry and Materials Science, Heilongjiang University. His working interests are the study of oxide electrolyte and electrode materials for SOFC and organic–inorganic catalytic materials.

    Lihua Huo acquired her PhD in physical chemistry from Fuzhou University, China, in 1997. She is now the professor of School of Chemistry and Materials Science, Heilongjiang University. Her research interests are mainly directed towards the development of nano-materials and organic–inorganic hybrid materials and their application as gas sensors.

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