Sonochemically grown 1D ZnO nanostructures and their applications

*[email protected]; phone 90 232 355-0000; fax 90 232 355-0018; gediz.edu.tr

Sonochemically Grown 1D ZnO Nanostructures and Their

Applications Yavuz Bayam*a, Debora Rodriguesb, Ramazan Atalay a, Ceylan Zaferc, Salih Okurd,Rukayya K.

Balaa, Tugba O. Okyayb, Burak Gültekinc, Ihsan Cahaa, Enis E. Tural a, Sinem Duyar a, Cebrail

Özbekd

aDept. of EEE, Gediz University, Seyrek Kampus, Menemen, IZMIR, TURKEY 35665; b Dept. of

Civil and Environmental Engineering N136 Engineering Building 1, Houston, TX, USA 77204-

4003; c Solar Energy Institute, Ege University, Bornova, IZMIR, TURKEY 35100; d Department of

Material Science and Engineering,Izmir Katip Celebi University, 35620, Izmir, Turkey

ABSTRACT

Sonochemical growth technique is based upon the chemical effect of ultrasound on chemical reactions. This process is

carried out at an ambient atmosphere without the need for a complex experimental set up and additional heating. This

method is of significant importance because of it's vital application in various fields. ZnO nanorods were grown on glass

substrates without any additional heat or surfactance by sonochemical growth technique. The grown nanostructures were

characterized by Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy

(EDS). Sonochemically grown ZnO nanorod networks were characterized for their antibacterial properties toward

B.subtilis. These structures were also characterized for their CO sensing properties and photovoltaic performances for

dye sensitized solar cell (DSSC) application. All material characterization and device performances suggest that

sonochemsitry can be utilized as an alternative growth method for 1D ZnO nanostructures.

Keywords: Sonochemistry, ZnO Nanorod, Gas Sensors, Solar Cells, Dye Sensitized Solar Cells, Antibacterial

1. INTRODUCTION

Zinc oxide (ZnO) is a non-toxic, biocompatible inorganic compound that exhibit multiple properties both of

semiconductor, piezoelectric and pyroelectric[1]. It has wide band gap (3.37eV), large exciton binding energy (60

meV)[2], good transparency and high luminescence at room temperature. Due to their unique physical, chemical and

biological properties, nanostructures of ZnO have received tremendous effort from the field of engineering, material

science, and medicine for their potential application in electronic, optoelectronic, and biomedical devices such as photo

detectors[3] and drug delivery vehicles for efficient photodynamic therapy for cancer cells[4]. The high-sensing

capability, high electron mobility, and enhanced analytical performance of ZnO nanostructures are widely explored for

sensor application such as in biosensors for intracellular measurements[5], gas sensors[6], pH and temperature

sensors[7].

Sonochemistry is the study that focuses on the application of ultrasound to chemical reactions. The chemical effects of

ultrasound do not come directly from the interaction of sound waves and the molecular species, rather sonochemistry

arises from a phenomenon known as acoustic cavitation; formation, growth and implosive collapse of bubbles in a liquid

environment [8]. Upon irradiation of liquid with high intensity of sound, the alternate expansion and cooling rate above

1010 K/s.This phenomenon causes high energy chemical reactions which are not accessible or difficult to achieve to take

place within a short period of time, often with emission of light, a phenomenon called sonoluminescence[9].

Sonochemistry has been found to be used in the synthesis of various nanostructures material such as high surface area

transition metals, carbides, alloys, colloids, and oxides as well as biomaterials; notably the protein microspheres[10].

Ultrasonic irradiation of aqueous liquids generates free radicals, and the formation of free radicals by sonolysis of water

has been particularly well-studied for many years. Compression of the acoustic wave create bubbles, these bubbles

oscillate and accumulate ultrasonic energy and grow until they reach an unstable size where they eventually collapse and

release the concentrated energy stored. The implosive collapse of bubbles generate a localized, short-lived hot spot with

very high temperature of ̴ 5000 K and pressure of ̴ 1000 atm, with high heating and cooling rate above 1010 k/s. This

Invited Paper

Low-Dimensional Materials and Devices, edited by Nobuhiko P. Kobayashi, A. Alec Talin, M. Saif Islam, Albert V. Davydov, Proc. of SPIE Vol. 9553, 95530E · © 2015 SPIE

CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2190286

Proc. of SPIE Vol. 9553 95530E-1

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/03/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

phenomenon causes high energy chemical reactions which are not accessible or difficult to achieve to take place within a

short time, often with emission of light, a phenomenon called sonoluminescence[9].

2. EXPERIMENTAL

The glass substrates were first cleaned using a solution of isopropyl alcohol, acetone and distilled water, respectively, in

an ultrasonic bath for 20 minutes, then dried using nitrogen gas. A continuous ZnO seed-layer was first deposited over

the substrates followed by the growth of ZnO nanorods. This method has been reported to be feasible as it does not

require complex setup, it does not produced any hazardous by-product, therefore not harmful to the environment[8, 9].

All chemicals used in this thesis were used directly without any further purification, and all preparations were done at

room temperature.

Seeding Process

Deposition of seed layer is very crucial in the sonochemical growth of ZnO nanorods on any substrate. Several reports

have shown that for ZnO nanorods to be obtained from a reaction between Zinc nitrate and HMT, a catalytic thin layer of

metallic Zn is required[11]. However, this process seems to prolong the process and makes it more difficult because Zn

deposition is not available in most clean rooms. To mitigate these problems, a simple, one-step seeding process was

developed using Zinc acetate dihydrate, which has been shown to form a suitable seed layer for the growth of ZnO

nanorods on both polar and non-polar substrates[12].

To seed the surface of the glass substrate, a 0.005M solution of Zinc acetate dihydrate in isopropyl alcohol was prepared

by dissolving 0.55g Zinc acetate dihydrate(C4H10O6Zn) in a beaker containing 0.5L Isopropyl alcohol (IPA) at room

temperature. The solution was stirred with a magnetic stirrer at 750rpm for 15 min. A clear solution was obtained and the

glass substrate was then immersed into the solution and sonicated for 30 min at 50% of the maximum amplitude of the

24KHz ultrasonic probe working at 400 W.

Growth Process

An aqueous solution of 0.04M Zinc nitrate tetrahydrate (Zn(NO3)2.4H2O), and 0.04M Hexamethylenetetramine

(C6H12N4) was prepared. First, 2.8038g of HMT was dissolved in a beaker containing 0.5L DI water and stirred with a

magnetic stirrer at 750 rpm for 5 min. In a separate beaker containing another 0.5L DI water, 5.2288g of Zn(NO3).4H2O

was dissolved and the solution was also stirred at 750 rpm for 5 min. Equal volume of the two solutions was mixed by

stirring. The substrate was then immersed into the solution and sonicated at 50% of the maximum amplitude of the 24

kHz ultrasonic probe working at 400W for 30 min.

For the solar cell application same experiment was conducted, but in this case the solution was refreshed after 60 min for

6 times in order to increase the length of the nanorods. Due to the fact that grows of ZnO nanowires tend to slowed

down as time increases because of the depletion of Zn2+ ions, introducing a fresh solution has been reported to improve

the aspect ratio[13].

The solution was clear before the growth. After been irradiated with high intensity of sound, the solution was found to be

cloudy with white precipitate. This indicated that the ultrasonic energy alone is strong enough to allow for the dissolution

of the molecular species without being subjected to further treatment. The temperature was found to increase from the

average room temperature (22-25 °C) to 65-80 °C depending on the time of sonication.



T

Glas_ or FTO substrate

Seeding process; Zinc acetatedihvdrate in ?PA - ultrasound

Seeded substrate

Growth process; Zinc nitratete rahvdrate + HMT in DI water +ultrasound

AilZnO iian rid_ ,2n

seeded substrate

))) denotesultrasound

Figure 1. Schematic representation of sonochemical synthesis of ZnO nanorods.

For gas sensor application ZnO nanorods were grown on previously deposited interdigitated Au electrodes on glass

sample.

3. RESULTS

SEM Results

Figure 2 shows the SEM images of the ZnO seed layer. The images show the surface covered with ZnO seed layer, with

few areas devoid of the growth as observed in fig2(a). It can be seen that nanoparticles of similar shape and size were

formed fig2(b). This would serve as the nucleation site for the growth of ZnO nanorods on the substrate.

Figure 3 shows ZnO nanorods of uniform shape and size. The average diameter was found to be ̴ 78nm as estimated

using clemex im-age analysis software.



a

.4

Figure 2. SEM images of ZnO seed layer deposited on glass substrate (a) low resolution and (b) high resolution.

Figure 3. SEM image of sonochemically grown ZnO nanorods a) on glass substrate b) on interdigitated electrode

EDS Results

Figure 4 shows the EDS spectrum of ZnO nanorods grown on a glass substrate. The chemical analysis shows the

presence of Zn and O which indicated that the nanorods are composed purely of Zn and O. How-ever, Si was also

identified but this could be nowhere but from the glass substrate.



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Zn

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0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00ley

1000.0

950.0 -

900 .0 -

850.0-

800.0 -750.0 -

in

d1(LO)

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E(high)

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600 Groove Grating

Spot 1

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600.0 I I

500 100D

Raman Shift (cm-1)

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1500

Figure 4. Energy dispersive X-ray spectroscopy (EDS) of sonochemically grown ZnO nanorods on glass substrate

Raman Spectroscopy Results

Figure 5 shows the raman spectrum of sonochemically grown ZnO nanorods. E2(High) was observed in this sample

which is a vibrational mode of ZnO wurtzite crystal associated with motion of oxygen. A1(LO) which usually arises

due to oxygen deficiency and/or increase in the lateral grain size of the structures[13], was also observed in this sample.

This corresponds to the growth condition, in which the sample was sonicated for the longer period of time. Increase in

sonication time results in the increase number of oxygen vacancies and also leads to crystal lattice deformation [14]. And

the peak at 1045cm-1 is assigned as the second order Raman phonon.

Figure 5. Raman spectroscopy of sonochemically grown ZnO nanorods on glass substrate

Growth of ZnO nanorods was found to be dense and homogeneous. This indicates that the long sonication period allows

for the complete decomposition of Zn(NO3)2 and HMT this provide sufficient Zn2+ and OH ̶ therefore nucleation can be

easily achieved and the growth rate increases[89]. This results suggests that sonication can have a great influence on the

orientation of the ZnO nanorods. The decrease in concentration of Zn2+ in the solution also accounted for the appearance

of ZnO nanorods with sharp tip. In a similarly findings, decrease in the amount of Zn vapor source results in the

formation of needle-like structures[15].



3300

3250

3200

3150

3100

3050

3000o 200 400 600

Time (sec)

800 1000

CO Gas Sensing Results

Conductivity analysis based on gas sensing properties of sonochemically grown ZnO nanorod network is presented

according to the resistance change under the carbon monoxide (CO). When a target gas molecule is adsorbed on the

surface of the ZnO nanorods, it produces an electrical response (the changing in resistance). The change in the resistance

can be detected. The gas response measurement setup was consist of two mass flow controllers, pressure programmer,

and Keithley 2420 Source-meter.

Carbon monoxide (CO) is a reducing gas. So, when it is in contact with the ZnO sensor surface, it reacts with ionically

adsorbed oxygen and release carbon dioxide (CO2) to the air. During the CO exposure, the oxygen concentration of

surface is reduced, and initially trapped electrons by oxygen ions are released back into the ZnO nanorods. As a result of

this conductivity of the ZnO channel decreases.

Figure 6. The Resistance (R) - Time (T) measurement of ZnO nanorod gas sensor under CO.

The adsorption of CO was measured at room temperature in a closed chamber. The CO gas was carried out by flow rate

of 500 sccm. Purified N2 gas was used to separate CO from the sensor. The voltage applied to the sensor was 10 V. N2

and CO was pumped into the measurement chamber by 500sccm flow rate with the 200 seconds time intervals. As it can

be seen from the graph that resistance decreases under CO exposure as expected.

Antibacterial Response Results

Live/Dead Assay

For the antibacterial study Bacilus subtilis 102 a gram-positive bacteria was choosen. Sonochemically grown ZnO

nanorods-coated glass substrates showed greater toxicity compared to glass substrate which was used as a control

sample.

For B. subtilis, over 48% of the cells were dead within the first 2 hours of incubation on the sample. The percentage of

dead cells on ZnO sample was higher than the glass substrate with only about 37% dead cells. As the incubation period increases from 2 hours to 5 hours ZnO nanorod sample showed greater toxicity having 80% of the cells dead. This

indicated that the toxicity increases with increasing time of incubation for ZnO nanorod sample.



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y,rzk \,. & Ztdr/z.&w m,«6, _.

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is

Figure 7. Fluorescence image of B. subtilis on ZnO nanorods substrate after 5 hours of incubation (a) and (b) glass

control (c) and (d) sonochemically grown ZnO nanorods. Green represents total bacterial cells and red represent dead

bacterial cells.

In figures 8, SEM image of B. subtilis after 48 hours of incubation is shown. It can be clearly seen from the image that

substrate with sonochemically grown ZnO nanorods shows high toxicity towards B.subtilis.

Figure 8. SEM image of B.subtilis after 48 h incubation (a) glass control (b) ZnO nanorods

Agar Flipping Test

Agar flipping test was further conducted to monitor the continuous growth of the bacterial cells in the presence of ZnO

nanorods for a long period of time. Growth of cells was measured at both 24 and 48 h. It is worthy of note that ZnOnanorods-coated subtrates greatly reduced the growth of bacterial cell compared to glass substrate. The continuous

growth of bacterial cells follows the same trend as the toxicity test.



Kt a.- CpM-+ol -1!Bs - CD'+krot-2h

&s-2.6-si.1 tos- 27-s1PF

Figure 9. Digital image of agar flipping test of B. subtilis on a) glass substrate b) ZnO nanorods substrate

Figure 9.a shows the growth of B.subtilis on agar plate with the control sample. Growth of cells was found to increase

as the incubation period increases. The growth was observed to be higher in the control plates compared to the ZnO

nanorods samples. This indicated that the ZnO nanorods have to a certain extent suppressed the growth of cells.

Figure 9.b shows the growth of B.subtilis on agar plate in the presence of ZnO coated substrates. It can be seen that the

growth of cells was less than that of the control which shows that the ZnO nanorods coated substrate have some

cytotoxic effect on the bacterial cells by preventing cellular growth compared to the glass substrate.



10

5-

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-5 -

T measurement_1measurement_2

A- dark

-10 -

-150,0 0,2 0,4

Voltage (V)

0,6 08

For DSSC device fabrication, sonochemically grown ZnO nanorods on a FTO substrate was utilized as working

electrode. ZnO nanorod coasted FTO substrate was dipped into dye (Z907) solution and kept there for 24 hours so that

nanorods is coated with dye molecules. Then the substrate was taken from the dye solution and into dilute ethanol

solution. Finally substrate was dried at room temperature. The obtained layer (SnO2: F + ZnO + Dye) formed the

working electrode.

Photovoltaic characterizations of the fabricated device was performed by current density-voltage (J-V) measurement

under standard conditions at simulated 100mWcm-2 irradiation power with AM1.5 spectral distribution. The J-V curve is

presented in Figure 10.

Figure 10. Current density-voltage characteristics of the fabricated ZnO Nanorod dye sensitized solar cell under a

simulated AM1.5 llumination condition.

It has been realized that the length of the ZnO nanorod plays important role. For the earlier trials it was not possible to

obtain a good characteristics from the fabricated solar cell devices because of the length of nanorods was not sufficient

enough. If the length of the nanorods are not as sufficient enough it dye molecules cannot be absorbed by nanorod

network. In order to overcome this problem growths were repeated with the fresh solution for 5 times. Then the grown

nanorod network was utilized as working electrode of a DSSC solar cell device.

The fabricated solar cell device under AM1.5 simulated sunlight of of 100 mW cm−2 exhibited an open-circuit voltage

(Voc) of 0.620 V, a Jsc of 9.06 mA cm−2, and a fill factor (FF) of 0.30, yielding a PCE of 1.70%.

Aknowledgement

This work was supported by the Scientific and Technological Research Council of Turkey (TÜBİTAK),

Grant Number: 114E415

DSSC Results



4. CONCLUSION

ZnO nanorods of different size were successfully synthesized by sonochemical method. It was found that the sonication

period and the amplitude play an essential role in the growth of ZnO nanorods on substrate. The long sonication period at

the maximum amplitude produced longer ZnO nanorods. Sonochemical growth process reduces the growth time

drastically.

The antibacterial properties of the sonochemically synthesized ZnO nanorods toward B.subtilis were investigated. It can

be inferred from the results that sonochemically grown ZnO nanorods have antibacterial effect on B.subtilis. It was also

found that the antibacterial properties of the ZnO nanorods increase with increasing time of incubation.

Sonochemically grown nanorods were also utilized as CO gas sensor. The resulting device showed high sensitivity and a

very good response to CO gas.

Dye sensitized solar cell where sonochemically grown ZnO nanorods were used as working electrode was characterized

for photovoltaic applications. Device showed promising photovoltaic characteristics with (Voc) of 0.620 V, a Jsc of 9.06

mA cm−2, and a fill factor (FF) of 0.30, yielding a PCE of 1.70%.

It can be concluded that sonochemical growth method for 1D nanostructure growth is a very efficient, fast, cost effective

and applicable to mass production. Sonochemistry can be applied to gas sensors, antibacterial coatings and photovoltaic

application in the industry level since it does not require costly and very sophisticated growth systems for 1D

nanostructure growth.

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Sonochemically grown 1D ZnO nanostructures and their applications

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