Characterization of Aluminum Doped Zinc Oxide Thin films and Nanostructures for H
2Gas Sensing Applications
Victor M. Pantojas, Laura Amadeo, Nelson Granda, Carlos Ortiz, Wilfredo Otaño University of Puerto Rico at Cayey
Cayey, Puerto Rico, 00736 [email protected]
Abstract
Metal oxides, such as ZnO, are especially attractive as sensing elements synthesized in the form of thin films and nanostructures. The influence of Al doping on the gas sensing performance of ZnO films and fibers are explored with the hope of improving the sensitivity or the selectivity of the sensor.
The objective is to obtain faster and more sensitive gas sensors by reducing the dimensions of the sensing elements and by doping. Zinc oxide fibers are formed by preparing a PVA solution containing zinc acetate which is deposited as a thin film by sol-gel or as fibers by electrospinning. A chamber was set up to test the sensitivity of the material to hydrogen gas.
Keywords: Zinc oxide, electrospinning, sol gel, hydrogen sensor Introduction
Zinc oxide (ZnO) is a promising semiconductor for gas sensing applications because of its chemical sensitivity to volatile gases, high chemical stability, and non-toxicity.
ZnO is also easily doped, transparent in the visible, has a high excitonic energy and high optical gain which promises interesting electro- optical applications. Many researchers have studied ZnO thin film and nanostructures for the detection of toxic gases, combustion gases, pollutants and organic vapors [1]. The use of ZnO nanostructures for the detection of hydrogen is of particular interests given its importance as a new energy source [2]. The influence of doping on the gas sensing performance of ZnO films have also been explored with the hope of improving the sensitivity or the selectivity of the sensor [3]. In this work we characterize Al-doped ZnO thin films and nanofibers, prepared by sol-gel and electrospinning respectively. Preliminary work on the influence of Al doping on the sensing of hydrogen will be presented.
Experimental
Aluminum doped zinc oxide thin films were prepared on fuse glass substrates by spin coating a solution containing zinc acetate dihydrate (Zn(CH3COO)2 •2H2O), aluminum nitrate (Al(NO3)3•9H2O), and a mixture of 2- methoxyethanol and monoethanolamine.
Solutions with a concentration of Al ranging from
0 at.% to 10 at % with respect to zinc were prepared. The sol solution is dropped onto the substrate and rotated at 3,000 rpm for 30s. Just after deposition the film is placed on top of a pre-heated hot plate at 250oC for 10 min to remove organics. Such rapid heating has been found to yield a homogeneous microstructure and reduces the possibility of formation of metastable intermediate phases. The coating and heating are repeated until obtaining the desired thickness. Finally, the films are annealed in air at a temperature of 700oC for 1 h.
ZnO fibers are prepared by dissolving zinc acetate dehydrate (AcZn*2H2O) and poly(vynil alcohol) (PVA) in de-ionized water. PVA is a semi-crystalline, hydrophilic polymer with good chemical and thermal stability, and water soluble. The required amount of aluminum nitrate (Al(NO3)3•9H2O) is added to obtain 0.5, 1.0 and 1.5 at% Al. Acetic acid HOAc or ethanolamine was added to the aqueous solution. When the solution becomes clear it is cooled down to room temperature.
Electrospinning is performed with a voltage of 17kV, at a distance of 18cm and a rate 0.05 mL/hr.
Samples are characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy EDS, and optical transmittance.
Electrical characterization is accomplished using a fully computerized I-V and C-V system. Metal contacts are deposited by sputtering.
Ibersensor 2012-October 16-19, Puerto Rico IB12-10
Results
The surface morphology of Al-doped ZnO fibers and thin film studied by SEM are shown in Figure 1. The images present a distribution of pores along the fibers and on the surface of the film which is attributed to the evaporation of the solvent and the decomposition of the organic molecules that generate gases during heat treatment. High porosity and large surface area are desirable in gas sensing applications since the surface in where the gas is adsorbed and reactions occur.
Fig. 1 SEM image of Al-doped ZnO fiber and thin film
Al-doped ZnO thin films show excess oxygen as determined by EDS which is typical of ZnO films prepared by chemical solution based methods such as sol-gel deposition. This zinc deficiency can be reduced by annealing the as-prepared films in an inert gas environment.
The surface roughness appears to decrease with Al content until a minimum of 3.2 nm is reached at 3 at. % Al. This can be related to the known solubility limit of Al in ZnO which is around 2.0 at.%. The optical transmittance evaluated in the range of 400 to 800nm varied from 84.5 to 90.1%. The highest transmittance occurred for 2at. % Al. The optical band gap (Eg) as determined from the optical transmission slightly decreased with Al doping. This may be attributed to the fact that Al3+ ion occupies the divalent Zn2+ sites allowing electrons to move to the conduction band.
In the case of ZnO fibers, two different stabilizers were found to work well, monoethanolamine (MEA) and acetic acid (HOAc). Fibers prepared with acetic acid had an average diameter of 126 nm while for those prepared by MEA was 146 nm. Fibers are deposited on oxidized Si substrates and rectangular Au contacts were deposited 1mm apart. Figure 2 shows a typical experiment where a ZnO fiber mat is exposed to 1% H2 gas
in nitrogen. Characterization of the Al-doped fiber mats as sensor elements is underway.
Fig. 2 Current as a function of time for a ZnO fiber mat hydrogen sensor.
Conclusions
Aluminum doped ZnO thin films and nanofibers were prepared by the sol-gel spin coating and electrospinning. Films with Al concentration ranging from 0 to 10 at. % were prepared.
Compositional analysis shows an excess of oxygen as compared to zinc. The Rms roughness of films decreased with Al concentration showing a minimum for a doping concentration of 2.0%. Higher Al concentration produced films with higher Rms roughness. All doped films exhibited high transmittance in the visible range ranging with a maximum for a concentration of 2 %. Only a slight variation in the band gap energy was observed with Al concentration. Preliminary test of the films and fibers as hydrogen sensors were performed.
Further improvement in sensitivity are been explored by controlling the size, orientation and morphology of the zinc oxide crystalline grains in the films and fibers.
Acknowledgement
This project was supported by DOE-EPSCoR Program grant DE-FG02-08ER46526, NSF- PREM grant NSF-DMR-0934195 and the Institute of Functional Nanomaterials funded by NSF- EPSCoR grant OIA-0701525.
References
[1] N.V. Hien, N.D. Chien, Physica B, Vol. 403, 50-56, 2008. M.-W. Ahn, K.-S. Park, J.-H. Heo, D.-W. Kim, K.J. Choi, J.-G Park, Sensors and Actuators B:Chemical, Vol. 138, 168-173, 2009 [2] O. Lupan, G. Chai, L. Chow, Microelectronics Journal, Vol. 38, 1211-1216, 2007