Initially, the growth was thus still in plate form.
However, when the pH value drops at a later stage the O-terminated surface becomes less protected and at the same
time the dipole field originating at the center becomes
stronger to guide the growth to a nanorod form and bend
along the field direction (Scheme 2) to a final vertical direction.
When strong simple electrolytes are added, the dipole
field is greatly screened and the structure reverts to a twin-
Scheme 1. Schematic of the formation mechanism of twin-brush ZnO mesocrystals.
Chem. Eur. J. 2012, 18, 16104 – 16113 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.chemeurj.org 16111
FULL PAPER Attachment Of Nanocrystals to ZnO Mesocrystals
plate form. Waltz et al.[50] recently utilized natural polysaccharides
as additives to fabricate ZnO mesocrystals and also
concluded that the high negative charge of the glucuronic
acid-terminated end would predominately interact with
(001) faces of ZnO to form double-disk structures. Another
important point worth investigating is that, by varying the
protecting polymers, one may be able to further control the
shape of the ZnO nanocrystal,[51] and further modulation of
electrooptical properties of the semiconductor may be
gained.
If a positively charged polymer were used in the synthesis
of ZnO, a symmetry argument would lead one to expect
twin crystals to be formed with the Zn-terminal faces as
growth plane. Rana and co-workers have indeed used positively
charged polyamine for morphology control of the
nanostructure of ZnO in a biomimetic synthesis[52] and obtained
twin-crystal ZnO when Zn(OH)2 was used as precursor.
Under more alkaline synthetic conditions, the dominant
intermediate nanoparticles would have been Zn(OH)2,
which is slightly negatively charged due to surface adsorption
of hydroxide. However, the ZnO they obtained does
not seem to be single-crystal-like, but polycrystalline.