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Systematic study of synthesis of hierarchical ZnO nanostructures grown by hydrothermal process

Systematic study of synthesis of hierarchical ZnO nanostructures grown by hydrothermal process. Surface polarity & ZnO morphology Thermodynamically stable crystallographic faces of ZnO include a polar-terminated (001) face and nonpolar low-symmetry (100) faces.

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Systematic study of synthesis of hierarchical ZnO nanostructures grown by hydrothermal process

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  1. Systematic study of synthesis of hierarchical ZnOnanostructuresgrown by hydrothermal process

  2. Surface polarity & ZnO morphology Thermodynamically stable crystallographic faces of ZnO include a polar-terminated (001) face and nonpolar low-symmetry (100) faces. The nonpolar surfaces are more stable than the polar face → leading to faster growth along the polar surface Variety of one-dimensional (1D) ZnOnanostructures including nanorods/nanowires, nanobelts, nanorings can be readily synthesized

  3. 2-Dimensional ZnOnanomaterials 2D ZnOnanowalls, nanosheets, and nanoplates are also attracted by their interesting properties Primary routes for 2D ZnO nanostructure synthesis include VLS and MOCVD at relatively high temperatures (400~800 °C) {0001}, {0110}, and {1120} surfaces: As dominated by the ionic polar charges, the {0001} surfaces require the lowest energy for ZnOdeposition Moderately supersaturated ZnO vapor is adequate for the growth along the [0001] direction Once higher supersaturated ZnO vapor is introduced, the deposition of ZnO on the other two surfaces can be activated _ _

  4. Low-temperature hydrothermal process for 2D ZnOnanomaterials Solution-based hydrothermal methods Low processing temperature (100 °C) Low cost Large scale Zn(NO3)2 + NaOH (Strongly basic solution)

  5. Low-temperature hydrothermal process for 2D ZnOnanomaterials ZnSO4 + (NH4)2SO4 + NaOH (pH 11.7) _ (0001) O-terminated surface is negatively charged OH− ions are adsorbed on positively charged (0001) surface Zn(OH)42− complex species serving as growth units : growth along [0001] direction is restricted

  6. Low-temperature hydrothermal process for 2D ZnOnanomaterials Zn(NO3)2 + DMAB DMAB is first hydrolyzed, and free electrons are released at the same time NO3- ions can acquire electrons to be reduced to NO2- ions, increase of OH- concentration If concentration of Zn(NO3)2is high, the corresponding concentration of OH- produced by is also high → crystal growth along the <0001> direction is suppressed

  7. Low-temperature hydrothermal process for 2D ZnOnanomaterials Due to thermodynamic restriction of ZnOgrowth, controlling the morphology of ZnOnanostructures remains an important challenge New schemes to influence surface polarity → ALD coating

  8. Surface polarity shielding by ALD ZnO coating: Surface polarity maintained → Continued NR growth AL2O3 + ZnO coating: Polarity partially maintained → NR+NS growth Thicker AL2O3+ ZnO coating: Surface polarity disappears → Only NS growth

  9. Surface polarity shielding by ALD The samples with an Al2O3 layer show a much smaller (002) reflection intensity, along with new (100), (101), and (102) orientations The random intersection of growing crystals produces a high density of grain boundaries, which can provide Zn precipitation sites → Nucleation site of 2D nanosheets

  10. Surface polarity shielding by ALD Nanorod spacing effect: 1) Large separation of surface features (SiN-coated carbon nanofibers) → ZnOnanorods grow uniformly over the entire substrate area and are directed normal to the growth surface 2) Very small separation (ZnOnanorods) → New ZnOnanorods are formed on top of the merged surface

  11. Surface polarity shielding by ALD Summary: 1) Wide gap → ZnONRs emanate normal to the growth surface 2) Narrow gap → 2D ZnOnanosheetsare formed 3) Negligible gap → New ZnOnanorods are formed on top of the merged surface 4) No polarity shielding → Gap width have minimal effect → Continues growth of the vertical nanorods.

  12. Site-Specific Nucleation and Growth Kinetics in Hierarchical Nanosyntheses Organoamine (DAP) Primarily grown nanorod Secondary growth with DAP Preferred adsorption of the DAP on the columnar surfaces of the primary rod Raise the nearby pH and guide the formation of the first a few layers of the precipitation Initiating the site-specific heterogeneous nucleation proces 3+ + H2O → + OH-

  13. Site-Specific Nucleation and Growth Kinetics in Hierarchical Nanosyntheses DAP promote nucleation and also increase growth by: More DAP molecules would produce more OH-ions in the solution, which promotes the formation of more ZnOgrowth units or a faster growth kinetics 3+ + H2O → + OH- Increasing DAP concentration

  14. Site-Specific Nucleation and Growth Kinetics in Hierarchical Nanosyntheses As the DAP concentration increased, the number of the small particles gradually decreased and finally disappeared due probably to the Oswald ripening. Further increase of the DAP concentration would start to raise the pH of the solution, which would make the small ZnO crystals dissolve in the rather basic solution

  15. Site-Specific Nucleation and Growth Kinetics in Hierarchical Nanosyntheses Increased DAP concentration: → Fewer secondary branches → No branches at the very high DAP concentration → Basal facets of both branches and primary ZnO rods gradually became sharper on top Ttaperedmorphology would not be formed by high Miller Index faces but by the multidecked and coaxial nanoplates of ZnO

  16. Site-Specific Nucleation and Growth Kinetics in Hierarchical Nanosyntheses After 6 h, growth almost stopped(2.4 um branch length) longer reaction time does not elicit further growth Linear increment until saturation 5 min nucleation time (obtained from extrapolating to the length to zero)

  17. Site-Specific Nucleation and Growth Kinetics in Hierarchical Nanosyntheses 2) Citrate (Sodium citrate) Citrate anions would preferably bind on the (001) facet of ZnO Prohibit the nucleation and crystal growth along the <001> orientation

  18. Site-Specific Nucleation and Growth Kinetics in Hierarchical Nanosyntheses 3) DAP → Citrate needle-like nanobranches were formed in the secondary growth due to the DAP effect After the tertiary growth, nanoplates were formed due to the citrate

  19. Site-Specific Nucleation and Growth Kinetics in Hierarchical Nanosyntheses 4) Citrate → DAP After the tertiary growth, branches can grow from both edges of platelets and gaps between the plates Needle-like tertiary branches about 2.4um in length were aligned Intercrossing species are formed (Nucleation at nanoplatelet edge)

  20. Future work: systematic study of laterally grown crystalline film • Nanorod density effect • Revealed nanorod length effect • Structure-directing agents effect DAP citrate

  21. Experiment: UV irradiation and PR curing (Irradiation amounts are arbitrary) 1 2 3 4 6 5 7 8 9

  22. Experiment: Etching characteristic 1min 2min 3min 4min

  23. Experiment: Laterial epitaxial growth → Gap is became narrower PR removed →

  24. Next week: ① Selective etching with patterned mold ② Graphene electrode Easy etching by oxygen plasma (No undercut) Highly aligned vertical nanowire array Low-temperature, solution process PDMS mold (Swelled by etchant) AZO

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