Synthesis of ultra-long hierarchical ZnO whiskers in a hydrothermal system for dye-sensitised solar cells

Abstract

One-dimensional (1-D) ZnO structures are of great interest for many applications but the direct hydrothermal synthesis of ultra-long ZnO whiskers (>100 μm) remains a great challenge. Herein, we demonstrate the first synthesis of three kinds of ultra-long hierarchical ZnO whiskers, which are defined as ZnO-2 (>100 μm in length), ZnO-3 (>200 μm in length with relatively smooth surface) and ZnO-4 (>200 μm in length with relatively rough surface), via a one-pot hydrothermal process. The maximum length of hierarchical ZnO-4 whiskers can reach up to about 270 μm. The formation of oval-shaped quasi-hollow structural precursors plays a key role for the correct attachment of Zn²⁺-terminated and O²⁻-terminated active surfaces, producing well-ordered Zn²⁺⋯O²⋯Zn²⁺ bonds, and finally promoting the formation of ultra-long ZnO whiskers with hierarchical structures. When the synthesized ultra-long hierarchical ZnO-4 whiskers are mixed with commercial TiO₂ for dye-sensitised solar cells (DSCs), the current density increases significantly from 13.68 mA cm⁻² (commercial TiO₂) to 16.81 mA cm⁻² (TiO₂–ZnO hybrid materials). The hybrid materials show a conversion efficiency of 7.95% which is higher as compared to that of commercial TiO₂ (5.87%). This interesting performance of a hybrid material sheds light on the possibility of preparing ultra-long hierarchical ZnO whiskers (>100 μm) with tunable lengths through hydrothermal approaches and their application in DSCs.