= Solution
A c-axis <wurtzite> ZnO wire has crystallographically polar basal ends, conventionally Zn-terminated $(0001)$ and O-terminated $(000\bar1)$, and approximately nonpolar prismatic side facets such as $\{10\bar10\}$. \b[Crystallographic polarity is not identical to net surface charge in a growth solution]: protonation, adsorbates and ionic speciation can change the latter.
For <face-selective electrostatic growth inhibition>, an oppositely charged additive adsorbs preferentially to a particular face and obstructs incorporation there. Blocking the axial end lowers axial growth relative to radial growth and yields a smaller aspect ratio; blocking sidewalls suppresses radial growth and favours long thin wires.
A primary hydrothermal experiment at pH 11 found negatively charged basal top surfaces and positively charged sidewalls. Positively charged Cd-containing complexes inhibited the top and favoured platelets; negatively charged Al-containing complexes inhibited sidewalls and favoured large aspect ratios. These signs are condition-specific, not fixed by the formal Zn/O termination alone.<Face-selective electrostatic control of hydrothermal zinc oxide nanowire synthesis by Jaebum Joo and others>[2011, Fig. 1 and pH-dependent complex-ion and surface-charge discussion]{ref}
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2022/paper-2-zno-facets.png]
{title=Facet-selective adsorption changes axial and radial ZnO growth}
{description=Original facet-termination diagram and schematic of the cited pH-11 effective surface-charge example. The signs in the adsorption panels denote solution-condition-dependent surface charge, not a universal assignment of charge to crystallographic facets.}
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