Alcohol sensor based on a non-equilibrium nanostructured xZrO2–(1−x)α-Fe2O3 solid solution system

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Abstract

Non-equilibrium nanostructured solid solution xZrO2–(1−x)α-Fe2O3 powders have been prepared using the high-energy ball milling technique, and their particle size, structural properties and alcohol gas properties have been systematically characterized using X-ray diffraction (XRD) and gas sensing measurements. Experimental results show that particle size of the powder is drastically milled down to less than 10 nm after 20 h of high-energy ball milling. Our modified structural model, □1/3+ZrO2+yO2→Zr1/34++2(1−y)Os2−+4yOs; with □1/3 denoting the 1/3 available octahedral sites of the corundum structure; for these non-equilibrium nanostructured solid solution xZrO2–(1−x)α-Fe2O3 materials can explain both the lattice expansion of these high energy milled sample as well as the charge neutrality in terms of additional oxygen dangling bonds at the particle surfaces. The screen-printed thick film gas sensors made from such mechanically alloyed materials demonstrate a very high gas sensitivity value of 1097 at 1000 ppm of alcohol gas in air. It is believed that the high gas sensitivity value is related to the enormous oxygen-dangling bonds at the particle surfaces.

Introduction

Many thick film metal oxide semiconductor gas sensors based on their resistivity changes, such as SnO2 and Fe2O3, have been commercially designed to detect various type of gases (e.g., CH4, CO and NO2) [1], [2]. Nano-sized materials have been widely used to produce new semiconductor gas sensors. Owing to the great surface activity provided by their enormous surface areas, they are expected to exhibit higher gas sensitivity. Being a promising gas sensing material for alcohol gas, nano-sized α-Fe2O3 powders have been prepared by different methods, including chemical co-precipitation [3], sol–gel process [4], metallo-organic deposition (MOD) [5], plasma-enhanced chemical vapor deposition (PECVD) [6] and high energy ball milling [7]. These methods are basically processing techniques to build a homogeneous structure on an extremely fine scale of a few nanometers from the molecular level. We have successfully used the high-energy ball milling technique to obtain nano-sized xSnO2–(1−x)α-Fe2O3 based powders as the sensing materials to detect ethanol gas [8], [9]. In this technique, the decrease of the particle size into fine powders of a few nanometers arises from the high-energy impacts during the collisions. In this paper, we report on a new non-equilibrium nanostructured xZrO2–(1−x)α-Fe2O3 solid solution system obtained using the same high energy ball milling method and fabricated into thick film alcohol gas sensors. Such initially immiscible, mechanically alloyed ZrO2–(α-Fe2O3) materials are far from their equilibrium state, and it is believed that the content of Zr4+ ions plays an important role in the gas sensitivity. We describe the structural features of the nanostructured powders in relationship with the high-energy ball milling process, and the sensing properties of the xZrO2–(1−x)α-Fe2O3 based sensors to alcohol gas. We have obtained a good gas sensitivity value of 1097 at 1000 ppm of alcohol gas in air at operating temperature of 240°C for the samples with x=10 mol% of ZrO2 annealed at 400°C. Our non-equilibrium structural model described in Ref. [8] modified for this xZrO2–(1−x)α-Fe2O3 solid solution system can successfully explain the high alcohol gas sensitivity.

Section snippets

Experimental

Samples of the xZrO2–(1−x)α-Fe2O3 system were prepared by mixing powders of hematite (α-Fe2O3) (99.9% purity; particle size <5μm) and zirconia (m-ZrO2) (99.9% purity; 325 mesh). The mixing was carried out using high-energy ball milling in a Fritsch Pulverisette 5 planetary ball milling system as described in our previous paper [10]. Samples were taken at different milling times. Five different mole percents of ZrO2, x=0, 5, 10, 15 and 20 mol%, were used. The structure of these samples was

Results and discussions

The X-ray diffraction (XRD) patterns for the mechanically alloyed xZrO2–(1−x)α-Fe2O3 samples have been analyzed to study their basic structure, as shown in Fig. 1, for x=0.05, 0.10, 0.15, and 0.20, together with that of pure α-Fe2O3. In this figure, the positions of all strong peaks in the XRD patterns for the milled samples correspond to those of the pure hematite structure. This indicates that the basic structure of the high-energy ball milled samples is the same as that of α-Fe2O3 powders,

Conclusion

We have illustrated the preparation of a non-equilibrium nanostructured solid solution xZrO2–(1−x)α-Fe2O3 system for alcohol gas sensing application using the high-energy ball alloying method. In particular, the sensor has shown good alcohol gas sensitivity values of as high as 1097 at 1000 ppm in air at a low operating temperature of 240°C. These excellent experimental results can be explained by the fact that such high-energy mechanically alloyed materials have nanostructured particles and

O.K. Tan received his BEng (1st Class Hons) from the NUSingapore, MSc from the Univ. of Edinburgh, UK, and PhD from Nanyang Tech. Univ. He is currently an Associate Professor at NTU. His areas of research interest include silicon IC designs, thick and thin films, especially semiconductor and ferroelectric films for gas sensor applications and integration on silicon. Dr. Tan is a member of American Ceramic Society and IEEE.

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O.K. Tan received his BEng (1st Class Hons) from the NUSingapore, MSc from the Univ. of Edinburgh, UK, and PhD from Nanyang Tech. Univ. He is currently an Associate Professor at NTU. His areas of research interest include silicon IC designs, thick and thin films, especially semiconductor and ferroelectric films for gas sensor applications and integration on silicon. Dr. Tan is a member of American Ceramic Society and IEEE.

W. Cao received his MEng and BEng from Xi Dian University, China. He is currently doing his PhD on semiconductor oxide based gas sensors with the Sensors and Actuators Group, Microelectronics Centre, NTU. His research interests are oxide semiconductor thick film gas sensors, microelectronics materials, and hybrid microcircuit process.

W. Zhu received his BSc and MSc from Shanghai Jiatong University, China, and PhD from Purdue University, USA. He was a Post-Doctoral Research Associate at Purdue University, Research Fellow at NTU, and currently an Associate Professor at NTU. Dr. Zhu is a member of American Physical Society, American Ceramic Society, IEEE, Materials Research Society, and the New York Academy of Sciences. He has published widely in electronic materials, thin films, ferroelectrics, diamond films, etc. His research interests include ferroelectric materials and memory applications, gas sensors, intelligent/smart materials integrated with silicon devices, diamond films and applications, etc.

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