Sensors based on porous Pd-doped hematite (α-Fe2O3) for LPG detection
Graphical abstract
Introduction
Semiconductors have achieved an increased importance in recent years due to their wide applications as gas sensors [1], [2], [3], [4]. Particular attention has been paid to sensors based on nanostructured semiconductors because of their high surface-to-volume ratio and their ability to form border grain zones. The formation of these zones improves the physical–chemical properties of the bulk material which drastically enhances sensitivity and selectivity to gas detection in a great range of concentrations at low operation temperatures with suitable stability [5], [6].
Hematite is a semiconductor with a band gap of 2.2 eV which has been widely used for its semiconductor and magnetic properties as a gas sensor [7], [8], [9], [10], [11], [12], catalyst [13], [14], [15], pigment [16] and ionic interchanger [17]. Hematite has proved to be an interesting material as a gas sensor but its low sensitivity and high operation temperatures have restricted potential applications [18]. Modifying the hematite-based nanostructured materials with some noble metals has improved response sign, leading to the detection of various gases [19], [20], [21]. Gas sensor applications show that doping hematite with Au, Pt and Pd improves the sensing properties of the starting material due to the cooperative influence of chemical and electronic effects of metal on the hematite structure [22], [23]. This could be verified by the enhancement of gas sensitivity and the reduction of testing temperatures [24], [25]. Hematite has also been extensively used as a sensor for the detection of organic compounds such as alcohol and amines [26], [27], [28].
Liquefied petroleum gas (LPG) is an inflammable mixture of hydrocarbons, particularly propane and butanes, with extensive use as fuel in heating appliances and vehicles. Detection of LPG is important to prevent leaks and unexpected explosions in homes and offices. Development of hematite based LPG sensors offers great advantages due to the low cost of starting material and easy availability. Hematite material in nanorod form has proved to be a successful sensor for the detection of LPG without the use of surfactants [29] and in the form of an undoped thin film prepared by plasma-enhanced chemical vapor deposition for detection of some gases as LPG, coal gas, hydrogen, and ethanol [30].
The sensitive properties of hematite respond to both an increase in surface area and to doping with noble metal. In order to accomplish final purpose requirements in this work Pd has been selected as a doping metal considering high performances showed over SnO2 film sensors in LPG detection [31], [32], [33]. To the best of our knowledge, this paper could be the first usage of Pd-doped hematite as a sensor for LPG detection. With this aim, some experimental variables leading to high efficiencies as metallic loading, operational temperature, recovery time, reversibility, among others, have been considered in this work.
Section snippets
Preparation of nanoparticles
All the reagents were analytical grade. Pd/α-Fe2O3 nanoparticles were prepared by co-precipitation method from 0.1 M Fe(NO3)3 (MERCK) and 0.5 M PdCl2·4H2O (MERCK, 59% Pd) solutions. Mixed solutions of metallic salts (in calculated proportions) were added dropwise to an aqueous solution of 0.3 M Na2CO3 (anhydrous, Riedel-de Haën) with 0.3 wt.% polyglycol used as surfactant agent under vigorous stirring at 80 °C [21]. Afterwards, enough sodium carbonate was added to the prepared metallic solution in
Structural, morphological and chemical analysis
In order to study temperature effect on the stability of material sensor, TG analysis in parallel with DTA has been performed with an undoped sample, obtained after precipitation and drying treatment. As is observed in Fig. 2, the profile of DTA showed that around 100 °C, a drastic weight loss occurs with an endothermic peak which could be attributed to dehydration and decomposition of metal nitrate precursor [20]. In the range 200–400 °C the weight loss corresponds to the exothermic reaction of
Conclusions
Sensors based on Pd doped α-Fe2O3 nanoparticles have been prepared by chemical coprecipitation for LPG detection. Pd particles were very well dispersed over the hematite surface (Pd/Fe surface ratio is very close to bulk one). Pd played substantial role in the sensitivity of sensors since the Pd-doped samples showed much higher response to LPG than the undoped hematite sample. Gas response of sensors increased linearly with LPG concentration and, as a rule, with the increasing of temperatures
Acknowledgements
The authors gratefully acknowledge the financial support for this work provided by CONCYTEC (Project 371-2012-CONCYTEC-OAJ), National University of Engineering-Faculty of Sciences (Project 2012), IGI-Institute of Research of National University of Engineering (Project IGI 2012) and DGI-PUCP, Research Center of Pontifical Catholic University of Peru (Project DGI 2010-0003). The authors appreciate the collaboration of Esther Ocola, Amanda Marie Bauer and Francisco Tarazona Vasquez for language
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