Ethanol sensors based on nano-sized α-Fe2O3 with SnO2, ZrO2, TiO2 solid solutions

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Abstract

Nano-sized α-Fe2O3 based solid solutions with different compositions of SnO2, ZrO2 and TiO2, were prepared using high-energy ball milling technique. Their structural properties and ethanol gas sensing properties were characterized using XRD and gas sensing measurements. The experimental results show that the mechanical alloying processes for these powders are the same. The screen-printed thick film gas sensors made from these mechanically alloyed materials demonstrated very high relative resistance. A non-equilibrium structural model was proposed to explain sensing mechanism. The comparison of gas sensing properties was performed for different α-Fe2O3 based solid solutions with optimized compositions of SnO2, ZrO2 and TiO2. Among these three sensors, xTiO2–(1−x)α-Fe2O3 type of gas sensor has much lower relative resistance value for ethanol. This can be elucidated by the different valence states exhibited by titanium ions.

Introduction

Nano-sized materials have been widely used to produce new semiconductor gas sensors, owing to the much greater surface-to-bulk ratio of materials than that of coarse materials. A large fraction of the atoms are present at the surface in nano-sized materials and the surface properties become paramount. Another important effect is associated with the depth of the surface space charge region that is affected by gas adsorption in relation to the particle size [3]. When the particle size gets into nano-range, the depletion layer takes over the bulk, and it becomes difficult to distinguish surface from bulk conduction. These characteristics of nano-sized particle make the materials particularly appealing in applications of gas sensors. Indeed, the grain-size reduction is one of the main factors enhancing gas sensing properties of semiconducting oxides. Thus the use of nano-sized materials in gas sensors is rapidly arousing interest in the scientific community [1], [2], [3], [4], [5], [6], [7], [8], [9], [10].

Nano-sized powders have been prepared, for gas sensor applications, using various methods, including (a) chemical co-precipitation [11], (b) sol–gel process [12], (c) metalorganic deposition (MOD) [13], (d) plasma enhanced chemical vapor deposition (PECVD) [14], (e) atmospheric-pressure chemical vapor deposition (APCVD) [15], (f) physical vapor deposition (PVD) [16], (g) low-pressure flame deposition (LPFD) [17], and (h) laser ablation [18]. Besides the above methods, the high-energy ball milling or mechanical alloying has proved to be a very effective route to prepare nano-sized solid solution for gas sensor application recently [7], [8], [9], [10], [19]. In our research, nano-sized solid solutions, xSnO2–(1−x)α-Fe2O3 and xZrO2–(1−x)α-Fe2O3 powders with grain size of down to 8 nm were prepared using this technique for ethanol gas sensing application [8], [9], [20]. In this paper, we extend our work to xTiO2–(1−x)α-Fe2O3 to study their common and individual characteristics of these three nano-sized solid solutions. Their gas sensing properties could be explained by the non-equilibrium structural model described in [20].

Section snippets

Experimental

For the preparation of xTiO2–(1−x)α-Fe2O3, powders of hematite, α-Fe2O3, (99.9% purity; particle size <5 μm) and titanium(IV) oxide, (99.9% purity, anatase), were mixed with the nominal compositions, 5, 10, 15, 20 mol% TiO2. The mixing was carried out using the planetary ball mill (Fritsch Pulverisette 5). An open container with tungsten carbide (93 wt.% WC and 6 wt.% Co) vials and balls was chosen. This material for the vials and balls have relatively high density of 14.75 g/cm3 to increase the

XRD characterization

The X-ray diffraction (XRD) is an effective method to study the basic structural change during the mechanical alloying. Fig. 1 shows the XRD patterns for the samples of xTiO2–(1−x)α-Fe2O3 milled for 120 h with x=0.05, 0.10, 0.15, and 0.20 corresponding to curves (b), (c), (d), and (e), respectively. As a reference, pattern (a) for the pure α-Fe2O3 powder without ball milling is also attached. Fig. 2 shows the XRD patterns for the sample of 0.10TiO2–0.90α-Fe2O3 milled for different hours, 0, 2,

Conclusions

Nano-sized powders of α-Fe2O3 based with SnO2, ZrO2, and TiO2, respectively, were prepared using mechanical alloying process for gas sensing application. Thick film ethanol gas sensors were fabricated and characterized. The results have shown that the high-energy ball milling is an effective route to prepare nano-sized powders at room temperature for gas sensing applications. The sensing mechanism of α-Fe2O3 based ethanol gas sensors with SnO2, ZrO2, and TiO2, respectively, has been elucidated

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