Enhanced response of porous ZnO nanobeads towards LPG: Effect of Pd sensitization
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ZnO film was immersed in a solution of palladium chloride salt for a certain period of time after which the film was withdrawn and subjected to the post-deposition annealing.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
References (25)
- et al.
Chemical-bath ZnO buffer layer for CuInS2 thin-film solar cells
Sol. Energy Mater. Sol. Cells
(1998) - et al.
Investigations on the surface modification of ZnO nanoparticle photocatalyst by depositing Pd
J. Solid State Chem.
(2004) - et al.
Optically pumped ultraviolet lasing from ZnO
Solid State Commun.
(1996) - et al.
Low temperature selective NO2 sensors by nanostructured fibres of ZnO
Sens. Actuators B
(2004) - et al.
Sensing characteristics of tin-doped ZnO thin films as NO2 gas sensor
Sens. Actuators B
(2005) - et al.
Oxygen gas-sensing characteristics for ZnO(Li) sputtered thin films
Sens. Actuators B
(1992) - et al.
ZnO sol–gel derived porous film for CO gas sensing
Sens. Actuators B
(2003) - et al.
A wet-chemical process to form palladium oxide sensitiser layer on thin film zinc oxide based LPG sensor
Sens. Actuators B
(2004) New trends and future prospects of thick- and thin-film gas sensors
Sens. Actuators B
(1991)New approaches for improving semiconductor gas sensors
Sens. Actuators B
(1991)
Activating technology of SnO2 layers by metal particles from ultrathin metal films
Sens. Actuators B
Effects of additives on semiconductor gas sensors
Sens. Actuators
Cited by (157)
ReSe<inf>2</inf>/metal interface for hydrogen gas sensing
2021, Journal of Colloid and Interface ScienceConductometric n-butanol gas sensor based on Tourmaline@ZnO hierarchical micro-nanostructures
2021, Sensors and Actuators, B: ChemicalRecent advancements in liquefied petroleum gas sensors: A topical review
2021, Sensors InternationalThe effect of stabilized ZnO nanostructures green luminescence towards LPG sensing capabilities
2020, Materials Chemistry and PhysicsEffect of low dose-rate industrial Co-60 gamma irradiation on ZnO thin films: Structural and optical study
2020, Microelectronics Reliability
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.