Enhanced response of porous ZnO nanobeads towards LPG: Effect of Pd sensitization

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Abstract

The nanoporous ZnO films consisting of nanostructural beads were prepared by pyrolytic decomposition of an aqueous zinc nitrate solution and their liquid petroleum gas (LPG) sensing properties were studied. The response of the ZnO nanobeads towards LPG was enhanced significantly by palladium (Pd) sensitization which was deposited onto the ZnO nanobeads by a sequential dipping and drying method. The LPG sensing properties of the ZnO nanobeads and Pd-sensitized ZnO were investigated at different operating temperatures and different gas concentrations ranging from 0.1 to 0.4 vol%. The unsensitized ZnO nanobeads exhibited the maximum response of 31% at 673 K upon exposure to 0.2 vol% LPG which was improved up to 63% at the optimum temperature of 548 K by Pd sensitization. Additionally, the Pd-sensitized ZnO was able to respond very quickly to the exposure of LPG.

Introduction

Besides the capabilities of ZnO for various applications, such as solar cells [1], photocatalysis [2], ultraviolet laser [3], [4], transparent conductive oxides [5] and spintronics [6], its competence as a gas sensor has undoubtedly been proved for various oxidizing and reducing gases [7], [8], [9], [10], [11]. The most of the research groups have devoted their efforts towards nanostructured zinc oxide, since reactions at grain boundaries and complete depletion of carriers in the grains can strongly modify the material transport properties. The nanocrystalline ZnO thin films are the most promising metal oxide for solid state chemical sensors, due to small dimensions, low cost, low power consumption and high compatibility with microelectronic processing. Although nanocrystalline zinc oxide itself is active and responds to gases, the principle of which relies on the change in conductivity on exposure to a target gas, its gas-sensing performance can be enhanced by doping of impurity and/or using a small amount of noble metal catalyst, such as palladium (Pd) and platinum (Pt), which not only promotes gas sensitivity but also improves the response time. Different approaches appear in literature for addition of a catalyst layer on the sensing material. Generally, the sputtering or evaporation technique is used to deposit a layer of few angstroms on the metal oxide thin films. Bulk doping with a constant dopant concentration can be achieved using suitable sputtering targets of composite nature. Surface doping is possible by subsequent evaporation of the metals [12]. When a catalyst is optimally distributed on the surface of the semiconductor, the effect on the sensitivity can be quite dramatic [13], [14]. This is particularly interesting for Pd [15], [16], [17]. However, it is critical to control the amount of the catalyst material for thin films. Mitra et al. [12], [18], [19] have demonstrated the simple and inexpensive technique of Pd sensitization of ZnO films by a wet chemical process, where the ZnO film was immersed in a solution of palladium chloride salt for a certain period of time after which the film was withdrawn and subjected to the post-deposition annealing.

In our previous work [20], we could optimize that the ZnO film obtained by pyrolytic decomposition of 0.10 M (25 cm3) of a zinc nitrate solution exhibited the better response with interesting morphology of nanobeads. So, to improve the LPG sensing performance of these films, the Pd catalyst was loaded on the ZnO nanobeads and the effect on the response towards LPG has been presented in this paper. The Pd was coated on the ZnO film in an analogous way to that of Mitra et al. [12], [18], [19]. The effect of Pd sensitization on the response and response-recovery time at different operating temperature and gas concentration was investigated. Here, the Pd-sensitized ZnO film by 10 dipping cycles showed the better stability and response, so the results of the same film are discussed here, since 10 dipping cycles could result in only a thin and discontinuous layer, causing disability to measure the thickness of the Pd layer.

Section snippets

Experimental

Zinc oxide nanobeads were grown on glass substrates by pyrolytic decomposition of an aqueous zinc nitrate solution [20]. The solution (25 cm3) was sprayed through a glass nozzle onto the ultrasonically cleaned glass substrates kept at a temperature of 623 K. The spray rate of 3 ml/min was maintained using air as a carrier gas. The temperature was controlled using an electronic temperature controller. Hazardous fumes evolved during the thermal decomposition of the initial ingredient were expelled

Structural analysis

Fig. 1 shows the X-ray diffraction pattern of a spray deposited ZnO film. The d values of the film were in good agreement with those reported in the PDF for ZnO [PDF No. 79–206, a = 3.2499 Å and c = 5.2065 Å], possessing hexagonal wurtzite structure. It is seen from figure that the ZnO film exhibited a strong orientation along c-axis (0 0 2). The intensity of (0 0 2) plane is significantly high as compared to other peaks. The other orientations corresponding to (1 0 1) and (1 0 2), (1 0 3), etc., are present

Conclusions

The effect of Pd sensitization on the LPG sensing properties of ZnO films of nanoporus morphology has been studied. The (0 0 2)-oriented and nanocrystalline bead-like morphology was obtained by the pyrolytic decomposition of an aqueous zinc nitrate solution. The structural analysis indicated that the ZnO films are oriented along (0 0 2). Surface morphological study revealed the formation of porous nanobead-like morphology. The sensing properties of the ZnO nanobeads and Pd-sensitized ZnO were

Acknowledgements

Authors are very much thankful to CSIR, New Delhi, for providing financial supports through the scheme no. 03(1021)/05/EMR-II. One of the authors V.R.S. is very much thankful to the Shivaji University, Kolhapur for the award of Departmental Research Fellowship (DRF).

V.R. Shinde received her BSc (2001) in general physics, MSc (2003) in solid state physics and PhD (2006) in chemical preparation of ZnO thin films and application in gas sensors, from Shivaji University, Kolhapur (India). She is working as a departmental research fellow (DRF) at Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur, India. Her present research interests include mainly the synthesis of nanocrystalline metal oxide thin films and their applications in

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V.R. Shinde received her BSc (2001) in general physics, MSc (2003) in solid state physics and PhD (2006) in chemical preparation of ZnO thin films and application in gas sensors, from Shivaji University, Kolhapur (India). She is working as a departmental research fellow (DRF) at Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur, India. Her present research interests include mainly the synthesis of nanocrystalline metal oxide thin films and their applications in gas sensors.

T.P. Gujar received his BSc (2001) in general physics, MSc (2003) in materials science and PhD (2006) in oxide film preparation and application in supercapacitors, from Shivaji University, Kolhapur (India). He is working as a senior research fellow (SRF) at Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur, India. His research interests are in the field of synthesis of thin films of metal oxide by vacuum, chemical, electrochemical methods and their applications in supercapacitors and gas sensors.

C.D. Lokhande received his PhD in 1984. He was a Humboldtian (Hahn–Meitner Institute, Berlin, Germany). He is a fellow of Institute of Physics. He is currently a reader in the Department of Physics, Shivaji University, Kolhapur. He has been continuously engaged in the research field from last 25 years. His research interest includes the synthesis of thin films of metal chalcogenides, metal oxides and ferrites by chemical, electrochemical methods and their applications in dye sensitized solar cells, gas sensors, energy storage devices, etc.

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