Enhancement of pH Tolerance in Conductive Al-Doped ZnO Nanofilms via Sequential Annealing

Abstract

Chemically stable and electrically conductive metal oxide nanofilms are promising as robust electrodes for chemical/ biosensors and for photoelectrochemical applications, which require harsh conditions (e.g., acidic or basic environments). Among the various conductive metal oxides, impurity-doped ZnO nanofilms deposited on substrates are chemically nonresistive to acidic and basic environments because of the inevitable etching effects. Herein, we demonstrate a strategy to enhance the pH tolerance of Al-doped ZnO (AZO) nanofilms using a sequential annealing technique for film preparation. This technique involves first annealing in air followed by annealing in Zn vapor atmosphere. Although the as-grown AZO nanofilms rapidly dissolved in acidic and basic solutions, the sequentially annealed AZO nanofilms exhibited excellent pH tolerance toward the chemical etching rate and electrical resistance in buffer solutions, with pH ranging from 3 to 11. This enhancement effect of pH tolerance was considerably weakened when sequential annealing was performed in reverse (Zn vapor/air). The origin of the enhanced pH tolerance of the sequentially annealed AZO nanofilms is discussed in terms of the compensation of the anion/cation vacancies and the surface polarity of the ZnO(0001) surface.

Robust and Electrically Conductive ZnO Thin Films and Nanostructures: Their Applications in Thermally and Chemically Harsh Environments

“…Resistance increase was calculated as ( R – R 0 )/ R 0 , where R and R 0 are the resistance at 25 min immersion in each pH buffer solution and the initial resistance before the stability tests, respectively. Reproduced from ref . Copyright 2021 American Chemical Society.…”

“…Yan et al reported that sequentially annealed AZO thin films (first annealed in air and then in a Zn vapor atmosphere) demonstrated superior pH tolerance in a wide pH range from 3 to 11 (Figure 10). 29 The etching rate of the sequentially annealed AZO thin films at pH 3 was 0.2 nm/min, whereas that of as-grown AZO thin films was 3.2 nm/min. It is noteworthy that the chemical stability of sequentially annealed AZO thin films at pH 3 is comparable to that of the singlecrystalline Zn-polar ZnO (0001) surface, which is known to be much more stable than O-polar ZnO (0001) in an acidic environment (Figure 9).…”

“…Thus, AZO is a promising alternative to indium tin oxide (ITO), which is mass-produced transparent oxide electric material. In addition to these attractive properties, the abundance of resources of pure and doped ZnO is advantageous for rare element-free electronics. , However, ZnO thin films and nanostructures have been widely known as thermally and chemically unstable because of the instability of mobile carriers and their amphoteric nature, which limit their utilization in harsh electronics. − In recent years, a lot of effort has been devoted to achieving stable ZnO thin films and nanostructures toward applications in harsh environments, including chemical sensors, biosensors, and photoelectrochemical devices. ,− Some promising strategies for stable and conductive ZnO have been proposed, accelerating developments of ZnO-based harsh electronics.…”