Low temperature nanocrystalline TiO2–Fe2O3 mixed oxide by a particulate sol–gel route: Physical and sensing characteristics

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Abstract

Nanocrystalline TiO2–Fe2O3 thin films and powders were prepared by a straightforward aqueous particulate sol–gel route at the low temperature of 300 °C. Titanium(IV) isopropoxide and iron(III) chloride were used as precursors, and hydroxypropyl cellulose was used as a polymeric fugitive agent in order to increase the specific surface area. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) revealed that the powder crystallised at the low temperature of 300 °C, containing anatase-TiO2 and hematite-Fe2O3 phases. Furthermore, it was found that Fe2O3 retarded the anatase-to-rutile transformation up to 500 °C. The activation energies for crystallite growth of TiO2 and Fe2O3 components in the binary system were calculated 10.62 and 0.67 kJ/mol, respectively. Moreover, one of the smallest crystallite sizes was obtained for TiO2–Fe2O3 binary mixed oxide, being 6 nm at 300 °C. Field emission scanning electron microscope (FE-SEM) analysis revealed that the deposited thin films had nanostructured morphology. Thin films produced under optimized conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO gas (i.e., 25 ppm) at low operating temperature of 150 °C, resulting in increased thermal stability of sensing films as well as a decrease in their power consumption. Furthermore, TiO2–Fe2O3 sensors follow the power law for the detection of CO gas.

Highlights

► Nanostructured TiO2–Fe2O3 gas sensor was prepared by a new aqueous sol–gel route. ► The sensor exhibited a remarkable response towards low concentrations of CO. ► It also showed fast response and recovery times at low temperature of 150 °C.

Introduction

Transition metal oxide films have found wide applications as gas sensors [1], catalysts [2] and in optical electronics [3]. TiO2 and Fe2O3 are common single metal oxide semiconductors used as gas sensors since their electric conductivity changes when exposed to gases such as trimethylamine, ethanol, oxygen (O2), hydrogen (H2), carbon monoxide (CO) and petrol vapors [4], [5], [6], [7], [8], [9], [10], [11]. Recently, many efforts have been aimed to improve the gas sensing performance by improvements in selectivity, sensitivity and durability. In order to improve these properties, microstructure control by preparing porous, high specific surface area films and doping with hetero components (such as Sn, V, Cr, W, Co, Cu, Fe, Nb, Ta, Ga and Mo) are known to be effective, because active sites for particular gas species can be produced [1], [12], [13], [14], [15]. Another method to improve gas sensing performance of metal oxide semiconductors is to employ binary metal oxide semiconductors. This novel alternative has the potential to form tailored film morphologies, which facilitates gas-film interaction by altering atomic ratio of each element. Furthermore, it is possible to increase the current single metal oxide surface-to-volume ratio and to fabricate stable nano-sized grain morphologies for high performance gas sensing thin films [16].

Sensing properties of binary oxides based on TiO2 such as TiO2–MoO3 [16], TiO2–WO3 [17], TiO2–Cr2O3 [18] TiO2–V2O5 [14] and TiO2–CeO2 [19] reported previously. Binary oxides based on Fe2O3 such as Fe2O3–SnO2 [9], [20], Fe2O3–CeO2 [21], Fe2O3–ZrO2 [9] and Fe2O3–TiO2 [10] for sensing applications have also been studied before. The major disadvantage of TiO2-based gas sensors is their high power consumption which is required to operate the sensors at the required elevated temperature (>400 °C) [22], [23]. In contrast, Fe2O3-based gas sensors usually operate at lower temperatures (e.g., 100 °C) [9], [20]. Therefore, the empirical exploration of mixing TiO2 and Fe2O3 may lead to new gas sensing properties or may simply lead to a material composed of characteristics similar to TiO2 and Fe2O3. Dai [10] studied sensing properties of TiO2–Fe2O3 thin films, deposited by plasma enhanced chemical vapour deposition (PECVD) technique, towards trimethylamine. Tan et al. [9] investigated sensing characteristics of Fe2O3–TiO2 material, prepared by high-energy ball milling process, towards ethanol and oxygen gases.

Binary metal oxides can be obtained by different deposition techniques. Among chemical routes, sol–gel techniques offer important advantages due to low cost synthetic route, excellent compositional control, high homogeneity at the molecular level, feasibility of producing thin films on large and complex shapes and, the most significant one, low crystallisation temperature. TiO2–Fe2O3 mixed oxides prepared by various sol–gel processes have been reported in the literature. For example, Gupta and Ghosh [24] synthesized iron(III)–titanium(IV) oxide powder by slow injection of TiCl4 into hot (60 °C) FeCl3 solution. They studied kinetics behaviour of the material for arsenic removal. Mora et al. [25], Celik et al. [26], Neri et al. [27] and Pal et al. [28] prepared TiO2–Fe2O3 mixed oxides by polymeric sol–gel process, although these were intended for photocatalytic application. Nanostructured iron oxide–titania aerogel was prepared by sol–gel polymerization of iron acetylacetonate and titanium butoxide for photocatalytic application by Wang and Ro [29]. Balek et al. [30] studied thermal behaviour of Fe2O3–TiO2 mesoporous gels, synthesized by polymeric sol–gel method, using tetrabutyl orthotitanate and iron(III) nitrate as precursors and polyethylene glycol (PEG600) as additive. Long and Yang [31] synthesized Fe2O3–TiO2 powder by polymeric sol–gel process from iron nitrate or sulphate and titanium butoxide precursors for selective catalytic oxidation of ammonia to nitrogen. Magnetic characteristics of TiO2–Fe2O3 powder, synthesized by polymeric sol–gel technique, were studied by Kundu et al. [32].

So far, no significant work has been reported on preparation of TiO2–Fe2O3 thin films by particulate sol–gel process for gas sensing application. Therefore, in the present work a straightforward particulate sol–gel route for improvement of TiO2 sensing performance by adding Fe2O3, in the form of TiO2–Fe2O3 binary oxide film is reported. This process can be defined as an environmentally friendly processing as it uses an aqueous solution. One of the advantages of the present method is using an alternative to alkoxide (i.e., iron(III) chloride) as an iron source to produce a low cost product. Besides controlling the phase structure, composition homogeneity, crystallite size, monodispersity and microstructure, the cost of the product is also an important concern. Therefore, starting with a low cost precursor such as iron(III) chloride may reduce the total cost of production. Since the pores in particulate sol–gel processes are much larger than that found in polymeric sol–gel route, the capillary stress and therefore the shrinkage decrease during heat treatment. Therefore, it is possible to produce crack-free thin films with high surface area.

Section snippets

Preparation of TiO2–Fe2O3 sol

Titanium(IV) tetraisopropoxide (TTIP) with a normal purity of 97% (Aldrich, UK) and iron(III) chloride (FeCl3) with a normal purity of 98% (Aldrich, UK) were used as titanium and iron precursors, respectively. Analytical grade hydrochloric acid (HCl, 37%, Fisher, UK) was used as a catalyst for the peptisation and deionised water was used as a dispersing media. Hydroxypropyl cellulose (HPC) with an average molecular weight of 100,000 g/mol )Aldrich, UK( was used as a polymeric fugitive agent

Particle sizes and charges

Fig. 1 shows the mean size of the particles for prepared sol. It can be observed that it had a narrow particle size distribution around 20.5 nm. The particle size of the TiO2 sol reported in our previous study was around 18 nm [37]. Therefore, no significant increase in the mean size of the particles was observed for TiO2–Fe2O3 sol, which confirms that stability of the sol is maintained when a solution of iron chloride is added into TiO2 sol.

The zeta potential of the particles is shown in Fig. 2.

Conclusions

Nanostructured TiO2–Fe2O3 thin films and powders were successfully prepared via a new particulate sol–gel route for gas sensing application. The sol was stable over 3 months, since the constant zeta potential was measured during this period, being 45 mV at pH=2. Moreover, a narrow particle size distribution around 20.5 nm was observed for prepared sol. Based on XRD analysis, the average crystallite size of the powder was calculated around 6.0 nm at 300 °C and reached to 6.8 nm after annealing at 700 

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    Postal address: Department of Materials Science & Engineering, Sharif University of Technology, Azadi Ave., Tehran, Iran.

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