Vertically aligned zinc oxide nanorod arrays have been synthesized on glass substrate using microwave assisted hydrothermal method. The role of pH on the morphology of zinc oxide nanorods has been studied by varying concentration of ammonia in precursor solution. Structural, morphological changes on synthesis of zinc oxide nanorod arrays due to different pH have been investigated using FESEM and XRD. The study revealed that initial pH of the precursor controls the competitiveness between homogenous nucleation rate and heterogeneous nucleation rate. At lower pH, rate of homogenous nucleation are higher, leading to wastage of precursor solution. At higher pH conditions heterogeneous nucleation rate is also suppressed that reduces the growth process. Hence, an optimum pH of the precursor solution has to maintain to increase zinc oxide nanorod growth rate.

1. 68th Annual session of Indian Institute of
Chemical Engineers,CHEMCON 2015
ROLE OF pH IN RAPID MICROWAVE ASSISTED
HYDROTHERMAL
SYNTHESIS OF ZINC OXIDE NANOROD
ARRAYS
presented by
Surajit Ghosh
Indian Institute of Technology Kharagpur
NT-019

2. Prof. Jayanta Chakraborty, Surajit Ghosh,
Dept. of Chemical Engineering
Indian Institute of Technology Kharagpur
Our team
Dye-sensitized Solar Cells
(DSSCs)
Recent publication: S. Ghosh, J. Chakraborty, Obviating the post annealing step for
production of Al-doped zinc oxide films using sol–gel method: Use of rapid cooling
and gas blanketing, Thin Solid Films, Doi:10.1016/j.tsf.2015.09.062, In press

3. Semiconducting
Layer
Zinc oxide Nanorods array based DSSC

4. Zinc
oxide
DSSC
Gas
sensors
UV-
detector
Field-
effect
transistor
P-N
junction
diode
LEDs
Photo-
catalyst
Anti-
microbial
TCO
Many
more
Application of Zinc oxide Nano-structures

5. Nano-structures of Zinc oxide
Reference:
(a) Zhong Lin Wang, Materials Today, 2004, 7 (6) ,
26–33
(b) Seung Hwan Ko et al, Nanoletters, 2011, 11 (2),
666-671
(c) Rizwan Wahab et al, Applied Surface Science,
2009, 255 (9), 4891-4896
a
b
c

6. Hydrothermal synthesis of Zinc oxide Nanorod arrays
Time Length
(µm)
Dia
(nm)
Growth rate
(length/time)
nm/min
References Hydrothermal
approach
24 h 1 100 0.7 Yu et al.
(2005)
Conventional
6 h 3 300 8.3 Greene et al.
(2003)
Conventional
50 h 17 200 5.6 Law et al
(2005)
Conventional
6 min 0.3 50 75 Unalan et al
(2008)
Microwave
12
min
6 100 30 Mahpeykar et
al (2012)
Microwave
5 h 18 300 60 Liu et al
(2014)
Microwave

7. How microwave assisted heating works ?
Web reference: www.hep.fsu.edu/~berg/teach/phy3091/Talk2Microwave.ppt
as on 27th Dec, 2015
Spin of water molecule

8. Problems associated with
Microwave heating
Homogeneous nucleation VS. Heterogeneous nucleation
• Homogeneous nucleation • Heterogeneous nucleation
• Formation of nucleate in the
solution
• Formation of nucleate on the
seed crystal
• Growth takes place in
solution
• Growth of seed crystal only
• Raw materials depleted
rapidly
• Controlled depletion of raw
materials
• Growth rate reduces • Growth rate remained high
• Undesirable • Desirable

9. Experimental methods

10. ResultsandDiscussions
Appearance of growth solution immediately
after microwave treatment
Appearance of glass
Glass slides after the
treatment
Optical Transmittance of the films

11. FormationofZincoxide
nanorods(effectofammonia)
FESEM image (Top-view) of formed zinc oxide nano-structures on seeded
substrate varying with different Ammonia conc. (white bar=100 nm)

12. FormationofZincoxide
nanorods(effectofammonia)
FESEM image (Cross-sectional view) of formed zinc oxide nano-structures on
seeded substrate varying with different Ammonia conc. (white bar=100 nm)

13. Effect on nanorod structure with ammonia

14. Role of ammonia
1. Web ref: en.wikipedia.org/Zinc_oxide as on 26-12-2015
1
(CH2)6N4+6H2O → 6HCHO+4NH3
Decomposition of
Hexamine
𝑁𝐻3 + 𝐻2 𝑂 → 𝑁𝐻4
+
+ 𝑂𝐻−
𝑍𝑛2+
+ 4𝑂𝐻−
→ 𝑍𝑛(𝑂𝐻)4
2−
𝑍𝑛(𝑂𝐻)4
2−
→ 𝑍𝑛𝑂 + 𝐻2 𝑂 + 2𝑂𝐻−
At excess ammonia 𝑍𝑛2+
+ 4𝑁𝐻3 → 𝑍𝑛(𝑁𝐻3)4
2+
𝑍𝑛(𝑁𝐻3)4
2+
+ 2𝑂𝐻− → 𝑍𝑛𝑂 + 4𝑁𝐻3 + 𝐻2 𝑂

15.  pH control through ammonia plays vital role in zinc oxide nanorod arrays
formation.
 At lower ammonia concentration homogenous nucleation is favorable.
 At higher ammonia concentration, hetergeneous nucleation is favorable
upto 0.8 M concentration.
 More concentration of ammonia reduces heterogeneous nucleation and
therefore arrayed nanorod growth reduces.
 0.8 M concentration of ammonia is optimum to achieve high aspect
ratio ZnO nanorod arrays.
Conclusions

16. Major references
• Lionel Vaysesries, Advanced Materials (2003), vol-15, p-464
• S.M. Mahpeykar, J Koohsorkhi and H Ghafoori-fard, Nanotechnology (2012), vol-
23, p-165602
• Lori E. Greene, Matt Law, Joshua Goldberger, Franklin Kim, Justin C. Johnson,
Yanfeng Zhang, Richard J. Saykally, and Peidong Yang, Angewandte Chemie (Int.
Ed.) (2003), Vol-42, p-3031
• Husnu Emrah Unalan, Pritesh Hiralal, Nalin Rupesinghe,Sharvari Dalal, William I
Milne and Gehan A J Amaratunga, Nanotechnology (2008), Vol-19, p-255608
• Haidong Yu, Zhongping Zhang, Mingyong Han, Xiaotao Hao, and Furong Zhu,
Journal of American Chemical Society (2005), vol-127, p-2378

17. Thank you
Pleasure to have your queries
and
collaborations
Contact us: jayanta@che.iitkgp.ernet.in, surajit@che.iitkgp.ernet.in