Sonochemical Approach for Rapid Growth of Zinc Oxide Nanowalls

Avinash P. Nayak1,*

, Aaron M. Katzenmeyer2,†, Yasuhiro Gosho

3,¥, M. Saif Islam2, ‡.

*anayak@ucdavis.edu, †akatz@ucdavis.edu, ¥ygosho@berkeley.edu, ‡sislam@ucdavis.edu.

Department of Electrical Engineering, University of California - Davis, Davis, 95616, USA

Abstract:

In this report, we propose a new approach to synthesize ZnO (zinc oxide) nanowalls (NWall)on

aluminum and alumina coated substrates at room ambient conditions. The synthesized ZnO

NWalls are uniform and highly dense in areas where Al or Al2O3 (Alumina) is present. The

height and thickness of these ZnO NWalls average at 0.8µm-1.5µm to 20nm respectively.

Photoluminescence (PL) measurements, transmission electron microscopy (TEM) images, UV

Vis spectroscopy, SEM-EDS results indicate NWalls composed of ZnO. The sonochemical

synthesis was tried on Si, SiO2, Cr, and Ag but did not result in NWalls. We find that the growth

of ZnO NWalls only form on Al or alumina. We attribute the formation of Al assisted ZnO

nanowall growth to the phase transformations that occur under high-pressure, high-temperature,

and chemical growth kinetics. The deposition method reported is applicable to Al coated non-

metallic surfaces such as glass and we show the as-formed NWalls function as UV

photoconductors.

Keywords: Zinc Oxide, Nanowalls, Sonochemistry, Solution Growth, Ultrasound, Self-

Organization, Selective Deposition.

Introduction:

Zinc oxide (ZnO) has a relatively large bandgap (3.37eV) in comparison to traditional

semiconductors such as silicon (1.12eV), germanium (0.66eV), indium phosphide (1.27eV) and

galium phoshide (2.25eV). This large bandgap is of great interest for photonic applications[1]

,

solar cells[2]

, and gas sensors[3]

. The physical characeteristics of ZnO material have also been

widley explored. Quantum dots[4]

, nanobelts[5]

, nanotubes[6]

, and nanowires[7]

are just a few of

the structures that have been synthesised. Using a metal catalyst, high temperature and vapor-

liquid-process (VLS) process ZnO nanowalls were synthesied[8]

. Using such a process however,

requires extensive time for growth. ZnO nanowalls grown by metalorganic chemical vapor

depositon (MOCVD) has also been reported. This requires an ellaborate setup enclosed in a

pressurized chamber to grow NWalls at relatively low tempereature (~500oC).

[9] This

temperature is too high for many possible device applications. Here we report on a simple

sonochemical method of growth that does not require external input of high temperature or

pressure.

For fast process, high density, uniform growth of ZnO nanowalls, we use a simple and

inexpensive method for growing ZnO nanowalls in ten minutes. Herein, we report on a new

sonochemical deposition technique to selectively deposit ZnO nanowalls on Al or alumina

coated substrates. To our knowledge, no one has adopted this sonochemical method to synthesize

ZnO nanowalls. The room ambient process reported in this paper greatly reduces the time down

to ten minutes for the synthesis of ZnO nanowalls. Scanning electron microscopy (SEM),

absorbtion spectroscopy (UV-Vis), photomuminascence (PL), field emission energy dispersive

spectroscopy (EDS), transmission electron microscopy (TEM) have been used to investigate the

ZnO nanowall structure. The foramtion of these ZnO NWalls is mainly due to the phase

transfomrations that take place under high-pressure and high-temperature caused by the

cavitation bubbles.Growth kinetics of ZnO NWalls are explained by observing growth in three

minute intervals.

Experimental:

The growth of ZnO NWalls was performed using only one system (750W ultrasonic processor,

Sonics and Systems). A solution of 0.02M zinc nitrate hexahydrate (Zn (NO3)2.6H2O, 99.998%)

and 0.02M hexamethylenetetramine (HMT, (CH2) .6N4, 99

+ %) which was first stirred with a

magnetic stir bar at 350 r.p.m for 5 minutes to ensure a mixed solution was used as the only

solution for the growth of ZnO nanowalls. The Al coated substrate was immersed onto the

aqueous solution and sonicated at 50% of the maximum amplitude (21Wcm-2

) of the 20 kHz

ultrasonic probe for 10 minutes. The sample was then rinsed with DI water and dried with a N2

gun for characterization. FE-SEM was performed using a LEO 1550,Hitachi S-4500, or FEI

XL30-SFEG. UV-Vis used was performed using a Perkin Elmer Lambda 750 spectrometer. A

325 nm HeCd laser (Melles Griot) was used to collect the photoluminescence spectrum. All

chemical reagents used in this report were used without further purification.

To understand the growth kinetics, an Al coated Si substrate was immersed into a beaker

containing zinc nitrate hexahydrate and HMTA. The sample was incrementally extracted from

the beaker in three minute intervals.

Sunray 400 SM 100mW/cm2 UVA light (320–390nm) flood-lamp to test for photoconductivity.

Results and discussion:

All zinc oxide nanowalls reported were synthesized using the same experimental parameters

stated in the experimental section except in some cases; a seed layer was first deposited. We find

however that this deposition however is not required for the formation of ZnO nanowalls. Figure

1a shows the ZnO NWalls grown from 6nm of an Al coated Si substrate. The thickness of each

wall is ~10-15nm. The average length of these nanowalls is 0.8µm-1.5µm as seen in the inset.

To understand if similar structures could be synthesized using different substrates, we tried

growth on Si, SiC, Cr, and Ag. As Figure 2 shows, no nanowalls were synthesized. The reason

for sonochemical growth of ZnO nanowires on these substrates is reported elsewhere.[10]

Figure 1. ZnO nanowalls grown on Al coated

Si substrate. The inset shows the cross-

sectional view of ZnO NWalls with good

uniformity in height.

The formation of ZnO nanowalls is only present on Al and alumina coated substrates. To test to

see if ZnO NWalls grew on aluminum and alumina, pure aluminum and alumina were subjected

to the same treatment. The results in figure 3 show that ZnO NWalls do grow on both substrates.

Figure 3. ZnO NWalls on (a) AlOx and (b) Al.

Figure (3a) Alumina Figure (3b) Pure Aluminum

SiC

Figure 2. Attempted growth on a wide variety of chemically active substrates. ZnO

NWs resulted on (a) Cr, (b) Si (c) Ag (d) SiC.

We were able to synthesize ZnO nanowalls on Al wire, Al pallet; Al coated glass, and Al foil.

SEM images of the four are shown in figure 4.

This advantage allows for selective deposition of ZnO nanowalls. Figure 5 shows one such

pattern.

Figure 4. ZnO nanowalls on (a) Al wire (b) Al pallet and (c) Al

coated (~6nm) glass. (d) Al foil

Figure (4a) Al wire Figure (4b) Al pallet

Figure (4c) Glass Figure (4d) Al Foil

In the growth process, elemental Al is consumed as is evident from transmission experiments.

The spectroscopy results showing the transparency of ZnO nanowall electrodes is shown in

figure 6. The transparency increases by ~10% after growth.

AlPtSi02

Al

Al

Al

Si02

Si02

Pt

Al

Si02

Figure 5. Selective deposition of ZnO nanowalls on Al electrodes.

Al

Al Al

SiO2

Dark and UV photocurrents of a NWall film are plotted in figure 7. The measurements were

done on a processed sample of 6nm Al on glass with separation distance between the two

tungsten probe tips was ~10µm. The reason for low conductivity even in the presence of

ultraviolet (UV) light is attributed to the formation of ZnAl2O4 during the growth of these

nanowalls. The chemical growth kinetics for the formation of these ZnO NWalls is explained in

the succeeding section. It has been reported that ZnAl2O4 is lower in electrical conductivity than

ZnO and other oxide mixtures.[11]

Figure 6. Transparency results before and after ZnO NWall growth for different thickness’

of Al. The Al undergoes chemical modification or consumption as a result of the growth

process. The inset shows the optical images of the processed and unprocessed samples.

To examine the composition (i.e. ZnO or ZnAl2O4) of these nanowall structures, UV-Vis and

PL were conducted. The results are seen in figure 8. The absorbance peak at 368nm is observed

as an indication of the inherent property of ZnO. The 378nm PL peak is attributed to the free-

exciton annihilation in ZnO. The broad spectrum of ZnO [8]

368 nm

0%

5%

10%

15%

20%

25%

30%

35%

40%

320 340 360 380 400 420

Absorb

ance (

a.u

.)

Wavelength (nm)

Figure 8 a. The peak absorbance of 368nm light and 8b. the PL peak at 378nm are a good

indication of ZnO nanowall formation. The visible peak in (8b) is indicative of the defect

levels commonly observed in ZnO.

Figure 7. I-V curves of ZnO NWalls with and without UV excitation.

Figure 8a Figure 8b

Energy dispersive spectroscopy (EDS) data with corresponding SEM image (illustrated in figure

9) shows the chemical analysis of an Al electrode on Si and the neighboring SiO2 surface. The

Zn (Lα12) and O (Kα1) signals clearly match the ZnO nanowall growth area. We also see an Al

(Kα1) signals from the Al electrode. On the SiO2 surface, we see the lack of Zn and more Si

(Kα1) and O is clearly seen. The carbon (Kα12) signal is attributed to the EDS system.

Growth mechanism:

We find that no additional seed-layer is necessary beyond Al to get ZnO nanowalls. There are

multiple explanations as to how these ZnO nanowalls form. For ZnO nanowalls grown via VLS

using an Au catalyst, Porter et al.[12]

report that during the nucleation and growth stage, the Au-

Zn alloy forms ripples instead of dots which allows for ZnO nanowall structures to form. Growth

using MBE by Fan et al.[3]

attribute the growth of ZnO nanowalls to the oxygen flow rate and

lattice mismatch of ZnO and Si (111). They state that Zn atoms migrate on the Si surface which

results in a nanowall network. X. Wang et al.[13, 14]

use the CVD approach to growing ZnO

nanowalls and attribute the growth to Zn clusters drifting during ZnO vapor deposition. They

report that when high pressure and supersaturated ZnO is introduced, the deposition of ZnO on

{00-10}, {11-20}, and {0001} surfaces is activated. Due to the ionic polar charges, the {0001}

surface requires the least energy for growth of ZnO nanowalls. Kim et al. [15]

also use the same

technique (CVD) to growth but attribute the growth to the lattice match between the ZnO and the

hexagonal basal plane of the GaN layer on c-plane Al2O3 substrate. Leung et al.[16]

synthesized

ZnO nanowalls using the electrodeposition technique. They suggest that deposition temperature,

potential, time, and electrolyte concentration all play a crucial role in morphology, alignment,

and uniformity on ZnO nanowalls. M. Wang et al.[17]

used a chemical bath deposition (CBD)

technique to synthesize ZnO nanowalls. They reason that growth time increases the density of

ZnO nanowires which then begin to connect with each other and reorganize to form ZnO

nanowalls.

Figure 9. SEM-EDS with the corresponding chemical identification graph. The key

is at the bottom inset of the image. A cross-section EDS of the NWall sample is

shown on the right.

The PL spectra for the ZnO nanowall sample (figure 8b) that we synthesized sonochemically,

indicate that the ZnO nanowall growth is mediated by the presence of aluminum oxide.

Aluminum oxide produced at low temperature shows two PL peaks between 400-500nm.[18]

The

induced high pressure and temperature from the cavitation bubble imploding produces AlOOH

during cavitation.. The zinc/aluminum interface allows for zinc aluminate (ZnAl2O4) or “spinel”

to form.[19]

The reaction between Zn(OH)2 and AlOOH allows for the formation of ZnAl2O4, this

zinc aluminate has been synthesized in a hydrothermal solution at high temperatures.[20]

HMTA

and ZnAl2O4 would then support the vertical growth {0001} of ZnO nanowalls. We find that the

ZnO nanowalls grow perpendicular to the alumina/spinel interface.

The chemical reaction that is undertaken is stated in [equations 1-7][10, 21, 22]

:

(CH2)6N4 + 6H2O 4NH3 + 6HCHO [1]

NH3 + H2O NH4+

+ OH-

[2]

Zn(NO3)6H2O Zn2+

+ 2NO3-

[3]

Zn2+

+ 2OH- Zn(OH)2 [4]

Zn(OH)2 ZnO + H2O [5]

Al + 2H2O 1.5H2 + (AlO)OH [6]

From [4] and [6]:

Zn(OH)2 + 2AlO(OH) ZnAl2O4 + 2H2O [7]

To find at what time the formation of ZnO nanowalls occur, we observe growth in three minute

intervals. The experimental procedure is presented in the experimental section. The results are

shown in figure 10. We see that within nine minutes, ZnO nanowalls have formed. Within six

minutes, spinel clusters form which suppliment the growth of ZnO in the vertical direction.

Figure 10. Growth kinetics for the formation of ZnO NWalls at (a) 3 minutes (b) 6 minutes

(c) 9 minutes (d) 12 minutes (e) 15 minutes and (f) 60 minutes.

a. 3 minutes b. 6 minutes

d. 12 minutes e. 15 minutes

c. 9 minutes

f. 60 minutes

As shown in figure 11, Raman peak was characterized for each time interval. Raman peak for

ZnO is 1157 cm-1

, 1105 cm-1

, 1005 cm-1

, 98 cm-1

, 659 cm-1

, 580 cm-1

, 540 cm-1

,438 cm-1

, 412

cm-1

, 384 cm-1

, 331 cm-1

, 202 cm-1

. Of these, the strongest peak is seen at 438cm-1

which

corresponds to the E2

High nonpolar phonon mode, associated with oxygen. Raman for ZnAl2O4 is

660cm-1

, 419cm-1

. A faint peak at 419cm-1

is observed at 15 minutes. Al2O3 has no detectable

Raman peak.[23]

The 1053cm-1

peak which increases over time is associated with dioxygen.[24]

The Raman peak at 520cm-1

is associated to Si.[25]

The 937cm-1

Raman peak is attributed to

Zn2SiO4 [26]

Conclusion:

In summary, we have successfully fabricated ZnO nanowalls via a novel and simple

sonochemical route at room ambient conditions in the absence of an Au catalyst in less than

thirty minutes. The average thickness of the ZnO nanowalls is 20nm and the length varies from

0.8µm-1.5µm. We find that to grow ZnO nanowalls, an Al layer is essential. We attribute this

growth on Al to the formation of ZnAl2O4 at high temperatures which leads to vertical growth of

ZnO nanowalls. The same growth process was tried on different substrates but resulted in ZnO

nanowires. This substrate selective process allows us to selectively grow ZnO nanowalls in

regions where Al is present. We also find that Al is consumed in the growth process of these

ZnO nanowalls and that transmission increases by ~10% after growth.

Figure 11. Raman peak intensity for 3 minute time intervals and at 60 minutes.

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