Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia

Abstract

The elucidation of the fundamental processes in aqueous solution during the chemical bath deposition of ZnO nanowires (NWs) using zinc nitrate and hexamethylenetetramine is of great significance: however, their extrinsic doping by foreign elements for monitoring their optical and electrical properties is still challenging. By combining thermodynamic simulations yielding theoretical solubility plots and speciation diagrams with in situ pH measurements and structural, chemical, and optical analyses, we report an in-depth understanding of the pH effects on the formation and aluminum doping mechanisms of ZnO NWs. By the addition of aluminum nitrate with a given relative concentration for the doping and of ammonia over a broad range of concentrations, the pH is shown to strongly influence the shape, diameter, length, and doping magnitude of ZnO NWs. Tuning the dimensions of ZnO NWs by inhibition of their radial growth only proceeds over a specific pH range, where negatively charged Al(OH) complexes are predominantly formed and act as capping agents by electrostatically interacting with the positively charged m-plane sidewalls. These complexes further favor the aluminum incorporation and doping of ZnO NWs, which only operate over the same pH range following thermal annealing above 200 °C. These findings reporting a full chemical synthesis diagram reveal the significance of carefully selecting and following the pH to control the morphology of ZnO NWs as well as to achieve their thermally activated extrinsic doping, as required for many nanoscale engineering devices.

ZnO Nanostructures in Active Antibacterial Food Packaging: Preparation Methods, Antimicrobial Mechanisms, Safety Issues, Future Prospects, and Challenges

“…[56] The development of strategies that involve the synthesis of nanomaterials with unique properties resulting from the combined effect of structural and morphological modifications has attracted much interest. For example, Verrier et al (2017) comprehensively investigated the influence of pH on the morphology and doping properties of ZnO nanowires (NWs) grown using chemical bath deposition using zinc nitrate Zn(NO 3 ) 2 , hexamethylenetetramine (HMTA), and aluminum nitrate Al(NO 3 ) 3 . [57] For a fixed concentration of Al(NO 3 ) 3 by varying the pH, various ZnO nanostructures, such as nanopencils and nanoneedles, were obtained, which provided a comprehensive idea about the influence of post-deposition annealing conditions and the pH effect of the synthesis medium on the shaping of ZnO nanostructures and the process of extrinsic doping.…”

“…For example, Verrier et al (2017) comprehensively investigated the influence of pH on the morphology and doping properties of ZnO nanowires (NWs) grown using chemical bath deposition using zinc nitrate Zn(NO 3 ) 2 , hexamethylenetetramine (HMTA), and aluminum nitrate Al(NO 3 ) 3 . [57] For a fixed concentration of Al(NO 3 ) 3 by varying the pH, various ZnO nanostructures, such as nanopencils and nanoneedles, were obtained, which provided a comprehensive idea about the influence of post-deposition annealing conditions and the pH effect of the synthesis medium on the shaping of ZnO nanostructures and the process of extrinsic doping. For effective integration of dopants in ZnO, apart from adding Al-based precursors to the aqueous solution, it is necessary to carefully select the pH and the post-deposition annealing conditions.…”

Doping Efficiency in Cobalt-Doped ZnO Nanostructured Materials

“…Zinc and cobalt are mostly in the form of Zn ++ ions and some cobalt complexes, respectively. There are no preferential electrostatic interactions between the surfaces of the ZnO nanorods and the different complexes in aqueous solution for low concentration [23]. So the chances of cobalt incorporation are low.…”

Graphene-based nanocomposites for electronics and photocatalysis

“…To dope the ZnO-NRs with Al, a different strategy was employed by using aluminum nitrate nonahydrate 4 as the Al precursor [29][30][31]. An aqueous solution of zinc nitrate with concentration of 2 mM (in the final growth solution) was prepared and stirred overnight before adding to the growth solution to make sure that the Al is uniformly dissolved and dispersed.…”

“…We used a constant Al concentration bath, with a ratio of [Al(NO3)3]/[Zn(NO3)2] less than 8%, for all the doped samples in order to maximize the conductivity without changing the ZnO morphology [30,31]. Fig.…”