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Controlling Diameter, Length and Characterization of ZnO Nanorods by Simple Hydrothermal Method for Solar Cells

January 2015
World Journal of Nano Science and Engineering 05(01):34-40

DOI:10.4236/wjnse.2015.51005

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CC BY 4.0

Authors:

Yousif Hassan

Salahaddin University - Erbil

Associate Professor Dr Naser Mahmoud Ahmed

Dijlah University College

…

The flow chart showing the procedure for preparation ZnO nanorods.

…

The transmittance spectrum of ZnO nanorods at 90˚C for precursor concentration 35 mM from immersion time 180 min.

…

Plot of (αhν) 2 vs photon energy hν of ZnO nanorods.

…

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Available via license: CC BY 4.0

Content may be subject to copyright.

World Journal of Nano Science and Engineering, 2015, 5, 34-40

Published Online March 2015 in SciRes. http://www.scirp.org/journal/wjnse

http://dx.doi.org/10.4236/wjnse.2015.51005

How to cite this paper: Kurda, A.H., Hassan, Y.M. and Ahmed, N.M. (2015) Controlling Diameter, Length and Characteriza-

tion of ZnO Nanorods by Simple Hydrothermal Method for Solar Cells. World Journal of Nano Science and Engineering, 5,

34-40. http://dx.doi.org/10.4236/wjnse.2015.51005

Controlling Diameter, Length and

Characterization of ZnO Nanorods by Simple

Hydrothermal Method for Solar Cells

Ahmed H. Kurda1, Yousif M. Hassan1, Naser M. Ahmed2

1Physics Department, College of Science, University of Salahaddin, Erbil, Kurdistan of Iraq

2Nano-Optoelectronic Research & Technology Laboratory, School of Physics, Universiti Sains Malaysia,

Penang, Malaysia

Email: ahmedkurda.69@gmail.com, yousif.60@Hotmail.com, naser@usm.my

Received 27 February 2015; accepted 20 March 2015; published 24 March 2015

This work is licensed under the Creative Commons Attribution International License (CC BY).

http://creativecommons.org/licenses/by/4.0/

Abstract

Zinc oxide (ZnO) nanorods have been synthesized by solution processing hydrothermal method in

low temperature using the spin coating technique. Zinc acetate dehydrate, Zinc nitrate hexahy-

drate and hexamethylenetetramine were used as a starting material. The ZnO seed layer was first

deposited by spin coated of ethanol zinc acetate dehydrate solution on a glass substrate. ZnO

nanorods were grown on the ZnO seed layer from zinc nitrate hexahydrate and hexamethylene-

tetramine solution, and their diameters, lengths were controlled by precursor concentration and

development time. From UV-Visible spectrometry the optical band gap energy of ZnO nanorods

was calculated to be 3.3 eV. The results of X-Ray Diffraction (XRD) showed the highly oriented na-

ture of ZnO nanorods the hardest (002) peak reflects that c-axis elongated nanorods are oriented

normal to the glass substrate. The Field Emission Scanning Electron Microscope (FESEM) was em-

ployed to measure both of average diameter of ZnO nanorods, Energy Dispersive X-Ray (EDX) is

used to identify the elemental present and to determine the element composition in the samples.

Keywords

Hydrothermal Method, Nanorods, Spin Coating, ZnO

1. Introduction

Zinc oxide (ZnO) is inexpensive n-type of semiconductor compound, which has shown promise for commercial

applications in photovoltaic cells, [1] nanosensors, [2] [3] photocatalysise, [4] nanolasers [5] and light emitting

diodes [6]. ZnO has a large band gap 3.37 eV, large excitonic binding energy 60 meV and high carrier mobility

A. H. Kurda et al.

at room temperature. ZnO is composed of a hexagonal wurtzite crystal structure with unit cell a = 3.253 Å and c

= 5.215 Å. Many reports have described syntheses ZnO nanorods through demonstrating various chemical

processes that are simple and may also have industrial applications.

Nanostructured ZnO is fabricated using various thin film techniques as spray pyrolysis [7], sputtering [8],

metal organic chemical vapor deposition [9] and hydrothermal method [10]. Hydrothermal method is widely

adopted for the fabrication of transparent and conducting oxide due to its simplicity, safety, no needs costly va-

cuum system and hence cheap method for large area coating. The hydrothermal method also offers other advan-

tages such as high surface area morphology at low crystallization temperature, the easy control of chemical

components and fabrication of thin film at low cost for elucidating the structure and optical properties of ZnO

nanorods.

In this work ZnO nanorods have been produced by thermal method using the solution, zinc acetate dehydrate

in ethanol as a seed layer. The growth of ZnO nanorods, diameter and length are controlled by changing the so-

lution concentration and immersion time in equimolar of zinc nitrate hexahydrate and hexamethylenetetramine

(HMTA) in deionized water at a 90 ̊ C and their morphologies, preferential orientation and optical properties

were examined in particular.

2. Experimental Work

The hydrothermal method synthesis and thin film process arrangement are presented schematically in Figure 1

the glass substrate was cleaned with ethyl alcohol, urine, and acetone several times. The cleaned glass samples

were further treated with UVO for 15 minutes to make rid of organic materials, the ZnO seed layer was first

prepared as follows: A 5 mM ethanol solution of zinc acetate dehydrate (Zn(CH3COO)2∙2H2O, Aldrich, 98%)

Figure 1. The flow chart showing the procedure for preparation ZnO

nanorods.

A. H. Kurda et al.

was spin coated on the cleaned glass at a spinning speed of 2000 rpm for 20 s with a 10 s wait time, then an-

nealing at 150˚C for 15 min. The procedure was repeated three times, and finally the ZnO seed layer was an-

nealed at 350˚C for 15 minutes. The solution of growing ZnO nanorods was prepared by dissolving equimolar

zinc nitrate hexahydrate (Zn(NO3)∙6H2O, Aldrich 98%) and hexamethylenetetramine (HMTA) (C6H12N4, Al-

drich, 99%) in deionized (DI) water. The solution concentration was varied from 15 to 35 mM for controlling

the ZnO nanorods. The ZnO seed layer deposited on glass was immersed in the solution, where the glass was

face down, and the baker was kept at 90˚C for 60, 90, 120, 150, 180 min. Alteration in the immersion time at a

given concentration can control the length of the ZnO nanorods. The ZnO nanorods film was rinsed with Deio-

nized water an ethyl alcohol several times. Ultimately, the film was annealed at 450˚C for 30 minutes.

The average diameter and length of the ZnO nanorods were measured by using the field emission scanning

electron microscope FESEM (Model: FEI Nova NanoSEM 450). The transmission spectra of the films were

measured by a double beam UV/visible (UV-4100) spectrophotometer with a wave length rang 200 nm - 800 nm

and the optical band gap was measured from the transmission spectra.

X-Ray Diffraction (XRD) was utilized for the physical construction of the ZnO thin films. XRD patterns were

obtained with a (Model: PANalytical X’pert PRO MRD PW 3040) single scan diffractometer with CuKα (λ =

1.54050 Å) radiation and scanning range of 2θ set between 20˚ and 80˚. The diameter and length of ZnO nano-

rods were measured using field emission scanning electron microscope FESEM (Model: FEI Nova NanoSEM 450).

Energy Dispersive X-Ray Spectrometer (EDX) used for quantitative detection of elements in the prepared samples.

3. Results and Discussions

3.1. Optical Properties of ZnO Nanorods

Figure 2 shows the optical transmittance spectrum of nanocrystalline ZnO nanorods at 90˚C for precursor con-

centration 35 mM from immersion time 180 minutes annealed at 450˚C for 30 minutes using UV-Visible region

from 200 nm - 800 nm. The transmittance is over 80% in the visible region from 400 nm to 800 nm for all the

samples. Sharp absorption edge is located at 380 nm which is due to the fact that the ZnO is a direct ban gap

semiconductor. The corresponding optical band gap of ZnO thin film is estimated by extrapolation of the linear

relationship between (hν)2 and hν according to Equation [11].

( )

1/2

h A h Eg

αν ν

= −

(1)

where α is the absorption coefficient, hν is the photon energy, Eg is the optical band gap and A is a constant.

Figure 3 depicts the plot of (αhν)2 versus photon energy hν. The value of the direct optical band gap Eg is cal-

culated from the intercept of (αhν)2 vs hν curve had also been plotted. The presence of a single slop in the plot

suggests that the ZnO nanorod has direct and allowed transition. The band gap value of ZnO nanorod is found to

be 3.3 eV which is slightly smaller to bulk ZnO (3.37 eV).

Figure 2. The transmittance spectrum of ZnO nanorods at 90˚C for pre-

cursor concentration 35 mM from immersion time 180 min.

A. H. Kurda et al.

Figure 3. Plot of (αhν)2 vs photon energy hν of ZnO nanorods.

This difference is due to the fact the values of band gap Eg depend on many factors, e.g. the granular structure,

the nature and concentration of precursors, the structural defects and the crystal structure of the films. Moreover,

departures from stoichiometry form lattice defects and impurity stats. Dengue Bao et al. [12] reported that the

band gap difference between the thin film and crystal is due to the grain boundaries and the imperfection of the

polycrystalline thin films. D. L. Zhange et al. [13] reported that this band gap difference between the film and

bulk ZnO is due to the grain boundary, the stress and the interaction potentials between defects and host mate-

rials in the films.

3.2. Structural Analysis of ZnO Nanorods

Figure 4 depicts the X-Ray Diffraction (XRD) pattern of the crystal structure and orientation of the nanocrystal-

line ZnO nanorods deposited on glass substrate using spin coating at 2000 rpm, pre-heated at 150˚C and an-

nealed in air at 450˚C. From the XRD pattern, one can clearly observe a diffraction peak at 2θ = 34.426˚. Strong

preferential growth is observed along c-axis, i.e. (002), suggesting that the prepared ZnO nanorods have the

wurtizit structure.

The unit cell “a” and “c” of the crystalline ZnO nanorods with (002) orientation is calculated using the rela-

tion (2) and (3):

1/ 3 sina

λθ

(2)

sinc

θλ

(3)

The values obtained for the unit cell a = 3.007 Å and c = 5.21 Å are in a good agreement with those reported

in the JCPDS standard data (card no. 80 - 0074). The calculated parameters are given in Table 1.

From the XRD spectrum, grain size (D) of the film is calculated using debay scherrer formula [14].

D k cos

λβ θ

(4)

where k is a constant to be taken 0.49 [14] and, λ, β, and θ are the XRD wave length (λ = 1.5406 Å), full width at

half maximum (FWHM) and Bragg angle respectively. By inserting the different values from Table 2 in the

Scherrer formula grain size of (002) oriented thin film is 44.12 nm which is same as reported in literature [15].

The dislocation density (δ), which represents the amount of defects in the crystal, is estimated from the fol-

lowing equation:

(5)

Strain () of the thin film is determined from the following formula:

A. H. Kurda et al.

Figure 4. X-Ray Diffraction of the ZnO nanorods grown at 90˚C for

180 min from the 35 mM precursor concentration.

Table 1. Lattice parameters of the ZnO nanorods.

a (Å) c (Å)

Standard Calculated Standard Calculated

3.253 3.007 5.215 5.21

Table 2. Structure parameters of the ZnO nanorods.

Plan d (Å) FWHM (β)˚ 2θ˚ D (nm) δ × 10−4 (nm)−2 ε × 10−3

002 2.6055 0.1968 34.426 44.12 5.13 8.049

cos 4

εβ θ

(6)

The calculated structural parameters of the thin film are presented in Table 2.

3.3. Morphological Analysis of ZnO Nanorods

In Figure 5, the (FESEM) shows the average diameter (d) of the ZnO nanorods increases from (57, 64, 83, 120

and 230 nm) as the precursor concentration increase from 15, 20, 25, 30, and 35 mM, respectively, where the

immersion time is fixed for 180 min at 90˚C. Length of the grown ZnO nanorods is about 1 µm regardless of

concentration, which indicates that changes in the precursor concentration at the fixed immersion time can affect

only the diameter of the hexagonal ZnO nanorods. The rate of increase diameter of the ZnO nanorods is esti-

mated to be approximately 34.4 nm/mM.

Length of ZnO nanorods can also be varied when the immersion time changes in the fixed concentration.

Figure 6 shows that the average length of the ZnO nanorods increases from (241, 459, 522, 820 nm and 1.2 µm)

as the immersion time t increases from 60, 90, 120, 150 and 180 min, respectively, at the precursor concentra-

tion of 35 mM. Length of ZnO nanorods indicates that growth rate is 6.3 nm/min.

Figure 7 shows the (EDX) spectrum and atomic composition of the ZnO/glass (002) layers for precursor con-

centration 25 mM at immersion time 180 min. A description of the atomic composition of the elements in the

layers is shown in percentages, as presented in the inset table in Figure 7, the concentration of these elements is

indicated by the peaks, and clearly shows that the elements corresponding to the peaks comprised the layer. No

contaminated element detected in the layers.

4. Conclusion

In this work, we have grown ZnO nanorods on glass substrates by solution processing hydrothermal method in

low temperature using the spin coating technique. The structural, morphological and optical properties were in-

A. H. Kurda et al.

(a) (b) (c)

(d) (e)

Figure 5. Surface FESEM images and diameter of hexagonal ZnO nanorods grown

at 90˚C various concentrations (a) 15; (b) 20; (c) 25; (d) 30; and (e) 35 mM pre-

cursor concentration for 180 min.

(a) (b) (c)

(d) (e)

Figure 6. Surface FESEM images and length of hexagonal ZNO nanorods grown

at 90˚C for immersion time (a) 60; (b) 90; (c) 120; (d) 150; and (e) 180 min from

precursor concentration 35 mM.

Figure 7. The (EDX) of the ZnO nanorodes at 90˚C for precursor concentration 35 mM from immersion time 180 min.

Element

wight%

atomic %

17.16

45.84

82.84

54.16

Total

100

A. H. Kurda et al.

vestigated. The hydrothermal method is a relatively simple technique: there are many factors which affected the

quality of the film. We have optimized different parameters to obtain a good crystalline structure of ZnO nano-

rods with intense and sharp peak. The optical transmittance is over 80% in the wave length range from 400 nm -

800 nm and the band energy band gap is found to be 3.300 eV. According to XRD results, the as deposited films

exhibited a hexagonal wurtized structure with (002) preferential orientation after annealing at 400˚C in air am-

biance for 30 min. The XRD pattern consists of a single (002) peak which occurred due to ZnO crystals and

grows along the c-axis. The grain size estimated to be 44.46 nm. The average diameter and average length of the

ZnO nanorods obtained from the FESEM. The average diameter of ZnO nanorods, which are increasing from

(57, 64, 83, 120 and 230 nm) as the precursor concentration increases at 90˚C for immersion time 180 min, and

the average length of ZnO nanorods increases from (241, 459, 522, 820 nm and 1.2 µm) when the immersion

time was increased at 90˚C for precursor concentration 35 mM. The (EDX) analyses of the samples clearly show

that the sample prepared by above route has pure ZnO nanorod phases.

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Ceramic Science and Engineering Basics to Recent Advancements

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Ceramic Science and Engineering: Basics to Recent Advancements covers the fundamentals, classification and applications surrounding ceramic engineering. In addition, the book contains an extensive review of the current published literature on established ceramic materials. Other sections present an extensive review of up-to-date research on new innovative ceramic materials and reviews recently published articles, case studies and the latest research outputs. The book will be an essential reference resource for materials scientists, physicists, chemists and engineers, postgraduate students, early career researchers, and industrial researchers working in R&D in the development of ceramic materials. Ceramic engineering deals with the science and technology of creating objects from inorganic and non-metallic materials. It combines the principles of chemistry, physics and engineering. Fiber-optic devices, microprocessors and solar panels are just a few examples of ceramic engineering being applied in everyday life. Advanced ceramics such as alumina, aluminum nitride, zirconia, ZnO, silicon carbide, silicon nitride and titania-based materials, each of which have their own specific characteristics and offer an economic and high-performance alternative to more conventional materials such as glass, metals and plastics are also discussed.

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Nanotechnology is the development and manipulation of matter on a near-atomic scale to design and produce new structures, materials, and devices in a useful way. Low-dimensional structures possess novel and unique optical, electronic, or mechanical properties and are subject to intense research. Nanostructures (NSs) with significantly enhanced performance promise a revolution in the fields of nanoscience and nanotechnology since they exhibit tunable physicochemical properties. NSs can be produced by applying the bottom-up or top-down methodologies and their development can be precisely controlled and manipulated. Zinc oxide have been investigated widely because of its profound applications in almost every field of science and significant impact on almost all areas of society. ZnO is a versatile material that exhibits the richest morphologies and semiconducting and piezoelectric dual properties and consequently finds use in mechanical actuators and piezoelectric sensors besides other applications. The ease of preparing diverse ZnO NSs with multifunctional properties has opened a gateway to their applications in novel device implementations. ZnO exhibits probably the richest family of NSs ranging from 0D to 3D among all the other semiconducting oxides. This chapter provides an overview of the sources and classification of nanomaterials. ZnO NSs and their classification, synthesis methods, and applications are also presented. Finally, some simple and inexpensive wet chemical methods for the growth of NSs are described.

Nanostructured ceramics

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Inorganic materials made from metals and nonmetals combined by ionic and/or covalent bonds are known as ceramic materials and can be crystalline, amorphous, or mixture of both. When the size goes below 100 nm, it becomes nanostructured ceramic material. Both top-down and bottom-up approaches have been discussed for the synthesis of different types of nanostructured ceramic materials. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and many other techniques have been used to characterize nanostructured ceramic materials. Optical, electronic, electrical, magnetic, and structural properties of nanostructured ceramic material have been discussed. Applications in different sectors such as thermal barrier coatings, sensors, health, capacitors, automotive, batteries, solid electrolytes for fuel cells, catalysts, cosmetics, corrosion-resistant coatings, bioengineering, optoelectronics, computers, and electronics, etc., have been elaborated in detail.

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Nanostructured zinc oxide (ZnO) nanorods (NRs) with hexagonal wurtzite structures were synthesized using an easy and low-cost bottom-up hydrothermal growth technique. ZnO thin films were prepared with the use of four different solvents, namely, methanol, ethanol, isopropanol, and 2-methoxyethanol, and then used as seed layer templates for the subsequent growth of the ZnO NRs. The influences of the different solvents on the structural and optical properties were investigated through scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, and photoluminescence. The obtained X-ray diffraction patterns showed that the synthesized ZnO NRs were single crystals and exhibited a preferred orientation along the (002) plane. In addition, the calculated results from the specific models of the refractive index are consistent with the experimental data. The ZnO NRs that grew from the 2-methoxyethanol seeded layer exhibited the smallest grain size (39.18 nm), largest diffracted intensities on the (002) plane, and highest bandgap (3.21 eV).

Zinc Oxide Thin Films by Spray Pyrolysis Method

Article

Full-text available

Jan 1999

ZnO thin films were deposited by spraying of zinc acetate solution onto heated glass substrates at 670K. Highly textured in the (002) direction undoped ZnO films, exhibiting exciton emission bands in photoluminescence spectra at 8K, were grown. The initial stages of the thin film growth and effect of doping with In, Ce and Eu were studied. A spraying time of 30s leads to continuous crystalline films with well-shaped grains with narrow granulometric distribution and mean grain size close to 35nm. Depositing times longer than 2min lead to the solid phase sintering process. Wide granulometric distribution of the grains in the range of 50-400nm was found for undoped films. It is shown that the size and orientation of crystallites in the film, the optical and electrical properties of the films are determined by the dopant concentration. As a result dense ZnO films with high optical transmittance were produced.

Low-Temperature Fabrication of Light-Emitting Zinc Oxide Micropatterns Using Self-Assembled Monolayers

Article

Mar 2002
ADV MATER

ZnO micropatterning under very mild conditions is presented. Photolysis (using a mask) of Si–C bonds in a self-assembled phenylsilane layer yields a patterned phenyl/hydroxy surface. ZnO is selectively deposited at 55 °C on the phenyl domains by electroless deposition using a Pd catalyst adhered to the surface. The viability of the ZnO pattern as a phosphor is illustrated by the visible light cathodoluminescence image shown in the Figure and on the cover.

Hydrothermal fabrication of well-ordered ZnO nanowire arrays on Zn foil: Room temperature ultraviolet nanolasers

Article

Dec 2011

Well-ordered nanowires of the hexagonal wurtzite ZnO having an average diameter of 80 nm, a typical length of 12 μm, and a mean packing density of 7.5 nanowires μm−2 have been directly grown on Zn foil in a preferred [0001] direction by a hydrothermal process and employed for room temperature ultraviolet nanolasers. The lasing action of arrayed ZnO nanowires has been observed from 370 to 400 nm with threshold irradiance of 25 kW cm−2. Photoluminescence decays biexponentially: the fast component is attributed to free-exciton decay, and the slow one is to bound-exciton decay. The amplitude of the fast component increases whereas its lifetime decreases with the increment of threshold irradiance, suggesting that ZnO nanowire arrays undergo a change in the lasing mechanism from exciton–exciton scattering to electron–hole plasma recombination.

Influence of dopant concentration on the optical properties of ZnO: In films by sol–gel method

Article

Jul 2009
THIN SOLID FILMS

Undoped and In doped ZnO films have been deposited by sol–gel spin coating method. The effect of indium incorporation on structural and optical properties of ZnO films has been investigated. X-ray diffraction patterns of the films showed the hexagonal wurtzite type polycrystalline structure and that indium incorporation leads to substantial changes in the structural characteristics of ZnO films. The SEM and AFM measurements showed that the surface morphology of the films was affected from the indium incorporation. Optical reflectance and transmittance were recorded with a double beam spectrophotometer with an integrating sphere. The optical band gap of these films was determined. The absorption edge shifted to the lower energy depending on the dopant materials. The optical constants of these films were determined using transmittance and reflectance spectra.

Electron transport properties in ZnO nanowire array/nanoparticle composite dye-sensitized solar cells

Article

Aug 2007

A significant improvement of the efficiency of the ZnO nanowire (NW) dye-sensitized solar cell (DSSC) has been achieved by the chemical bath deposition of the dense nanoparticles (NPs) within the interstices of the vertical ZnO-NW anode. Impedance analyses of the electron transports in DSSCs reveal that the effective diffusion coefficient of an electron in the ZnO-NW array/NP composite anode falls between those in the ZnO-NW and TiO2-NP anodes. The superior performance of the ZnO-NW array/NP composite DSSC to the ZnO-NW cell is mainly ascribed to the enrichment of the light harvesting without significantly sacrificing the electron transport efficiency.

Low‐Temperature Growth of Well‐Aligned ZnO Nanorods by Chemical Vapor Deposition

Article

Feb 2002
ADV MATER

1D single-crystalline zinc oxide nanostructures are promising for nanoscale optoelectronic applications. Here a simple chemical vapor deposition method for the growth of well-aligned ZnO nanorods (see Figure) at low temperature is demonstrated. The nanorods, which grow on fused silica and are preferentially oriented in the c-axis direction, are strongly UV light emitting, peaking at around 386 nm at room temperature. [GRAPHICS] Low-Temperature Growth of Well-Aligned ZnO Nanorods by Chemical Vapor Deposition.

Low-Temperature Fabrication of Light-Emitting Zinc Oxide Micropatterns Using Self-Assembled Monolayer

Article

Mar 2002
ADV MATER

A ZnO micropattern was fabricated by employing a photopatterned phenyltrichlorosilane (PTCS) self-assembled monolayer (SAM) as a template. ZnO was electrolessly deposited selectively on the Pd catalyst attached to the phenyl surfaces. Furthermore, a patterned, monochromatic cathodoluminescence (CL) image of visible light was successfully demonstrated for 1 μm wide lines.

Microstructure, Morphology, and Ultraviolet Emission of Zinc Oxide Nanopolycrystalline Films by the Modified Successive Ionic Layer Adsorption and Reaction Method

Article

Oct 2010
J AM CERAM SOC

Zinc oxide (ZnO) nanopolycrystalline films were successfully prepared by the modified successive ionic layer adsorption and reaction technique, which was based on the alternate immersion of substrate in the alkaline zinc precursor and deionized water. ZnO films were formed through an accumulation of ZnO crystal clusters. The size of the clusters ranged from 200 to 500 nm based on scanning electron micrographic images. Prepared ZnO films exhibited a wurtzite structure, with good microstructure, surface morphology, and optical properties. Ethanolamine was used as a complex reagent, which improved the adsorption of zinc complex with the substrate. Effects of processing parameters on the properties of ZnO nanopolycrystalline films were studied in detail. Intensive and sharp ultraviolet emission peaks at about 400 nm could be observed in the photoluminescence spectra.

ZnO nanoparticulate thin film: Preparation, characterization and gas-sensing property

Article

Sep 2004
SENSOR ACTUAT B-CHEM

ZnO thin films were prepared by sol–gel dip coating method using zinc acetate dihydrate and 2-ethanolamine as the starting materials. The powder and its thin film were characterized by X-ray diffractometer (XRD), atomic force microscopy (AFM) and IR spectra methods. The results indicated that the ZnO particles crystallized in the wurtzite phase, which were well-distributed in the films with the mean particle size of about 10 nm. The IR spectra of the gel and powders annealed at different temperatures were investigated. It showed that the vibration bands of NH and mono-acetate diminished and the stretching mode of ZnO at 471 cm−1 appeared after the dried gel annealed at 500 °C for 1 h. The diameter, D of ZnO particles in the monolayer was also calculated from the optical spectroscopy curve, which is close to that obtained from AFM observation. The gas-sensing properties of the multi-layers for alcohols with different chain lengths were measured at room temperature. The thin films exhibited high sensitivity and rapid response–recovery characteristics to these gases. The film can detect methanol, ethanol and propyl alcohol vapor as low concentration as 1, 10, and 0.5 ppm, respectively.