ARTICLE pubs.acs.org/JPCC
Grain Growth Kinetics of ZnO:Al Nanocrystalline Powders During Calcination from SolGels B. Ingham,*,† R. Linklater,†,‡ and T. Kemmitt† †
The MacDiarmid Institute for Advanced Materials and Nanotechnology, Industrial Research Limited, P.O. Box 31-310, Lower Hutt 5040, New Zealand ‡ Department of Chemistry, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand
bS Supporting Information ABSTRACT: The grain growth kinetics of Al-doped ZnO nanocrystalline powders have been studied using in situ, real-time synchrotron X-ray diffraction measurements during calcination at various temperatures (400800 °C) from solgels with varying levels of Al dopant (04 mol %). Development of the crystallite size versus time was described using a relaxation model. A grain growth activation energy of 24 ( 3 kJ/mol was obtained for undoped ZnO, while for all of the Al-doped ZnO experiments the activation energy was 43 ( 4 kJ/mol. This difference is attributed to the segregation of Al to the crystallite interfaces, where it interferes with the ZnO grain growth, by inhibiting the motion of atoms across the grain boundaries.
’ INTRODUCTION ZnO has many optical and electronic applications as a wideband-gap semiconductor, with doped ZnO showing great promise as a transparent conducting oxide (TCO). Doped ZnO films can be prepared via magnetron sputtering, pulsed laser deposition, and chemical vapor deposition.1 Solgel deposition of ZnO has also been proposed, being more cost effective over vacuum deposition techniques for large area TCO applications, with processing details such as choice of solvent and heating profiles influencing the film grain size, morphology, and crystallographic texture and subsequently their optical and electrical performance. 2 However, crystal growth and dopant incorporation from solution processing techniques is driven by different mechanisms to physical deposition techniques, and a full understanding of the processes required to optimize dopant incorporation is not fully developed. Group 13 ions are frequently used to dope ZnO as incorporation of trivalent ions into the zincite lattice sites can result in formation of free carriers and introduce electrical conductivity. Al is often used as a dopant, although its solubility in ZnO is thought to be low. Resistivity measurements on ZnO:Al thin films suggest that Al can generally only be substituted up to 1% before Al-rich phases formed at the ZnO grain boundaries begin to reduce the carrier mobilities.310 Up to the solubility limit, the level of incorporation of Al within the ZnO lattice is expected to be strongly dependent on crystal nucleation and growth during the calcination step. To capture these short time scale processes, in situ synchrotron X-ray diffraction measurements were undertaken to obtain real-time kinetic information of ZnO:Al solgels during calcination for a series of Al concentrations and calcination temperatures. The experiments were performed using bulk r 2011 American Chemical Society
gels; at the nanoscale the processes in a bulk gel are expected to be similar to that of a film (setting aside any effects of crystallographic texture as a result of strongly directional heating). The relative proportions of dopant ions incorporated into zincite lattice sites has previously been related to processing temperatures and heating rates;11 however, this work was limited to a final ex situ ‘snapshot’ of the final products. Understanding the kinetics of ZnO crystallite formation in solgels with and without Al present will assist in determining the optimal processing conditions for calcination of ZnO:Al films.
’ EXPERIMENTAL SECTION ZnO:Al gels were synthesized from reaction of aqueous zinc acetate (Scharlau) with 1 mol equiv of citric acid (Scharlau). The liberated acetic acid was removed by rotary evaporation of samples to dryness at 80 °C, redissolving or resuspending the residue in distilled water, and repeating. A 2 mol equiv amount of ethanolamine (Acros) was added to the citrate to solubilize the Zn citrate salt in water. An aluminum citrate solution was prepared from aluminum s-butoxide (Sigma) and anhydrous citric acid (Applichem) (1:1 ratio) in isopropanol. Removal of the solvents under reduced pressure was carried out by rotary evaporation, and the solid residue was redissolved in distilled water to prepare a standard solution. Aliquots were added to the Zn solutions to form mixtures containing Al in concentrations of 0, 1, 2, and 4 mol % with respect to Zn. The Zn concentration of the solutions was standardized at 0.5 mol L1. Received: July 26, 2011 Revised: September 13, 2011 Published: September 19, 2011 21034
dx.doi.org/10.1021/jp207140g | J. Phys. Chem. C 2011, 115, 21034–21040
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Selected 2θ region of a time series of XRD data for a sample containing 0% Al, heated at 600 °C for 2 h, showing the ZnO 100, 002, and 101 diffraction peaks.
To prepare for synchrotron XRD measurements, a thin layer of each solution was applied to the outside of a 0.3 mm diameter quartz capillary and then dried in an oven in air at 75 °C overnight. Calcination experiments were attempted for samples where 0.5 mm diameter quartz capillaries were filled with solution and then dried; however, these were unsuccessful due to the enclosed geometry, which prevents the organic components from diffusing away as the sample was heated. Synchrotron X-ray diffraction experiments were conducted at the Australian Synchrotron on the Powder Diffraction beamline. The X-ray wavelength was 0.77442 Å (∼16 keV). Samples were heated throughout the measurement using a Cyberstar hot air blower. Over the range of temperatures studied, this has an accuracy of (
