Structural investigation of aluminium doped ZnO nanoparticles by solid-state NMR spectroscopy

Abstract

The electrical conductivity of aluminium doped zinc oxide (AZO, ZnO:Al) materials depends on doping induced defects and grain structure. This study aims at relating macroscopic electrical conductivity of AZO nanoparticles with their atomic structure, which is non-trivial because the derived materials are heavily disordered and heterogeneous in nature. For this purpose we synthesized AZO nanoparticles with different doping levels and narrow size distribution by a microwave assisted polyol method followed by drying and a reductive treatment with forming gas. From these particles electrically conductive, optically transparent films were obtained by spin-coating. Characterization involved energy-dispersive X-ray analysis, wet chemical analysis, X-ray diffraction, electron microscopy and dynamic light scattering, which provided a basis for a detailed structural solid-state NMR study. A multinuclear ( 27 Al, 13 C, 1 H) spectroscopic investigation required a number of 1D MAS NMR and 2D MAS NMR techniques (T 1 -measurements, 27 Al-MQMAS, 27 Al-1 H 2D-PRESTO-III heteronuclear correlation spectroscopy), which were corroborated by quantum chemical calculations with an embedded cluster method (EEIM) at the DFT level. From the combined data we conclude that only a small part of the provided Al is incorporated into the ZnO structure by substitution of Zn. The related 27 Al NMR signal undergoes a Knight shift when the material is subjected to a reductive treatment with forming gas. At higher (formal) doping levels Al forms insulating (Al, H and C containing) side-phases, which cover the surface of the ZnO:Al particles and increase the sheet resistivity of spin-coated material. Moreover, calculated 27 Al quadrupole coupling constants serve as a spectroscopic fingerprint by which previously suggested point-defects can be identified and in their great majority be ruled out.

Polyol synthesized aluminum doped zinc oxide nanoparticles - influence of the hydration ratio on crystal growth, dopant incorporation and electrical properties

“…However we have to note, that the integration of solid-state 27 Al-NMR spectra signals does not provide reliable quantitative data. For all probes we identified amounts smaller 0.2 at.% for Al4 sub that is in accordance with the findings of Avadhut et al [25] and other authors who identified just small amounts of in ZnO solubilized aluminum by 27 Al-NMR [24,28,32]. With increasing hydration ratio the amount of crystalline located aluminum decreases from 0.15 at.% to 0.05 at.% Al and the amount for amorphous located aluminum increases from 1.85 at.% up to 1.95 at.% Al.…”

“…The results of the 27 Al-NMR analysis show, that the addition of water to the precursor solution generally influences the incorporation of the dopant in ZnO. An increase of the hydration ratio results in an increase of amorphous coordinated aluminum and the appearance of the Knight-Shift for h>0.9 could be identified and is evident for an oxidic material with metallic character, and hence hints on an improved conductivity [23,25].…”

“…If we correlate the results of electrical analysis with those of the 27 Al-NMR analysis, it is to note that the amount of amorphous located aluminium increases with increasing hydration ratio. If we assume that the amorphously coordinated aluminum behaves as insulating "shell" as described from Avadhut et al [25], it is possible that this insulating phase additionally hinders the electron transfer and counteracts an improvement of the electron-transfer by minimizing the amount of grain boundaries. But since we can't prove this hypothesis with the help of our analysis, we can just guess about it.…”

“…Beneath these positions, Al 3+ ions can be pentahedral coordinated [23,27], whereas this coordination generally can not be dedicated as a convenient position in the lattice of the ZnO but is known from 27 Al-NMR investigations of different aluminium oxide structures and were often found at the surface of the solids bound to different organic molecules [29,30,31]. Avadhut et al [25] performed multinuclear ( 1 H, 13 C, 27 Al) spectroscopic analysis with the aim to localize the different Al species in AZO. They found Al6, Al5 and Al4 in a disordered, amorphous environment and localized these species near the surface of AZO particles bound to organic molecules as solvent, salt or adsorbed water and just Al4 sub with a small amount of about 0.1 at.% in an ordered, crystalline environment.…”

“…Independent from the hydration ratio the initially inserted aluminum concentration of 2 at.% could be found within the limits of the standard deviation (Table 1). Al-nuclear magnetic resonance spectroscopy ( 27 Al-NMR) the distribution of the aluminum atoms in the host lattice of ZnO can be depicted and characterized [23,24,25,26,27,28]. Related to the crystallographic composition of the wurtzite-structured zinc oxide, different lattice sites can be occupied by the dopant.…”