# Preparation of Glass Plate-Supported Nanostructure ZnO  Thin Film Deposited by Sol-Gel Spin-Coating Technique and  Its Photocatalytic Degradation to Monoazo Textile Dye

Authors: Mohammad Hossein Habibi; Mohammad Khaledi Sardashti
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Glass plate-supported nanostructure ZnO thin films were deposited by sol-gel spin coating. Films were preheated at275∘C for 10 minutes and annealed at 350, 450, and 550∘C for 80 minutes. The ZnO thin films were transparent ca 80–90% in visible range and revealed that absorption edges at about 370 nm. The c-axis orientation improves and the grain size increases which was indicated by an increase in intensity of the (002) peak at 34.4∘ in XRD corresponding to the hexagonal ZnO crystal. The photocatalytic degradation of X6G an anionic monoazo dye, in aqueous solutions, was investigated and the effects of some operational parameters such as the number of layer and reusability of ZnO nanostructure thin film were examined. The results showed that the five-layer coated glass surfaces have a very high photocatalytic performance.


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## Body

## 1. Introduction

Textile industry wastewater is heavily charged with unconsumed dyes, surfactants, and sometimes traces of metals. These effluents cause a lot of damage to the environment. Many methods, including biosorption [1], conventional activated sludge treatment process [2], electrochemical technologies [3, 4], and reverse osmosis, [5] have been applied
on wastewater treatment. However, new environmental laws may consider the spent
adsorbents or sludge as hazardous wastes that require further treatment.
Consequently, intensive researches on novel technologies with more efficiency
and less energy consumption have been stimulated. Photocatalysis using a
semiconductor as a photocatalyst is an alternative to conventional methods
[6–11]. As a well-known photocatalyst, ZnO has been paid much attention in the
degradation and complete mineralization of environmental pollutants. Since ZnO has approximately the same band gap energy (3.2 eV) as TiO2, its photocatalytic capacity is anticipated to be similar to that of TiO2.
However, the greatest advantage of ZnO is that it
absorbs large fraction of the solar spectrum and more light quanta than TiO2 [12].
Some researches have highlighted the performance of ZnO on degrading some
organic compounds [13]. On the other hand, the use of conventional
powder catalyst results in disadvantages in stirring during the reaction and in
the separation of powder after the reaction. Preparation of film catalysts will
make it possible to overcome these disadvantages and to extend the industrial
applications [14]. The sol-gel
process, as a simple and easy dip-coating means, is one of the versatile
methods to prepare thin film-supported nanosized particles without complicated
instruments such as CVD. Thin film photocatalysts, with their high
photocatalytic ability, high stability, and convenient reuse, have received
more and more attention. However, there has been little success in finding thin
film photocatalyst that can operate effectively with visible light as yet. In
this context, we have paid much attention in preparing thin films of ZnO on
glass plates by a sol-gel
process and studying its photocatalytic ability for the degradation of X6G an
anionic monoazo textile dye. In this paper, a commercial glass-plate was
successfully used as substrate to prepare highest c-axis oriented ZnO
nanostructure thin film by sol-gel dip-coating process. The ZnO/glass films
have been employed in photocatalytic degradation of textile dye. The result
shows that the ZnO/glass film is a photocatalyst with higher activity. It is
very interesting and significant to research promising photocatalysts.


## 2. Experimental

### 2.1. Preparation of Sol

All the chemicals were analytic grade reagents without further purification, and purchased from Merck Company. Light YellowX6G dye
(C. I. Reactive Yellow 2, Rmm872.5) was obtained from Youhao (China) and has chemical structure shown in Figure1. Nanocrystalline  ZnO films were prepared on microscope glass
slide  (75 mm × 25 mm × 1 mm) substrate (washed with ethanol and dilute
acid) by a sol-gel method. Sol solution was prepared by adding 3.10 g Zinc acetate dihydrate (Zn(CH3COO)2·2H2O : ZnAc·2H2O), 0.86 g monoethanolamine (MEA), and adequate deionized
water to 15 mL isopropanol alcohol, then heated to 60°C with continuous stirring for 60 minutes. The coating substrate (microscope glass slid) was preheated at 275°C for 10 minutes in air after each
coating. The sol-gel coating was made usually a day after the sol solution was
prepared and the molar ratio of MEA to Zinc acetate was maintained at 1 : 1.Figure 1
Chemical structure of commercial diazo dye of light yellow (X6G).


### 2.2. Film Deposition

The films of ZnO were prepared by spin coating method onto substrate with 3000 rpm for 45
seconds (the spinner reached 3000 rpm after 5 s which was maintained for 40 s).
Films were annealed at furnace at 550°C for 80 minutes. Precursor
solution did not produce any precipitation after 30 days. This spinning-drying
procedure was repeated from 2 to 10 times. The microscope glass slide (75 mm × 25 mm ×1 mm: cleaned in dilute HCl solution and ethanol) was used as substrate.
Film deposition was carried out in air at room temperature by a rate 3000 rpm for 30 s. After each coating, the films were preheated at 275°C for 10 minutes, and postheated at 550°C for 80 minutes. The deposition
was repeated for 2, 4, 5, 6, 8, and 10 times to obtain a film with different thickness.


### 2.3. Characterization of Thin Films

The structure and crystalline size were determined by XRD   diffraction (Bruker D8 advanced X-ray diffractometer: CuKα radiation, Scan rate 0.03 2θ s−1). X-ray diffraction shows
zincite structure with c-axis orientation (002). Optical transmittance was measured by spectrophotometer (Varian Cary 500 Scan).


### 2.4. Photocatalytic Degradation

The photocatalytic degradation experiments were carried out in a simple (40 cm × 15 cm × 15 cm) oxidation reactor (see Figure2), placed in a 25°C water
bath. Slide with 5 layers that placed in 25 mL 10 ppm X6G solution was irradiated with two 8 W lamps (Philips; λ = 365 nm) placed 5 cm above the
solutions. Concentration is measured by spectrophotometer (Varian Cary 500 Scan). In all experiments, 25 mL of 10 ppm X6G solution was used with stirring during the irradiation.Figure 2
Schematic diagram of photoreactor for degradation.


## 2.1. Preparation of Sol

All the chemicals were analytic grade reagents without further purification, and purchased from Merck Company. Light YellowX6G dye
(C. I. Reactive Yellow 2, Rmm872.5) was obtained from Youhao (China) and has chemical structure shown in Figure1. Nanocrystalline  ZnO films were prepared on microscope glass
slide  (75 mm × 25 mm × 1 mm) substrate (washed with ethanol and dilute
acid) by a sol-gel method. Sol solution was prepared by adding 3.10 g Zinc acetate dihydrate (Zn(CH3COO)2·2H2O : ZnAc·2H2O), 0.86 g monoethanolamine (MEA), and adequate deionized
water to 15 mL isopropanol alcohol, then heated to 60°C with continuous stirring for 60 minutes. The coating substrate (microscope glass slid) was preheated at 275°C for 10 minutes in air after each
coating. The sol-gel coating was made usually a day after the sol solution was
prepared and the molar ratio of MEA to Zinc acetate was maintained at 1 : 1.Figure 1
Chemical structure of commercial diazo dye of light yellow (X6G).


## 2.2. Film Deposition

The films of ZnO were prepared by spin coating method onto substrate with 3000 rpm for 45
seconds (the spinner reached 3000 rpm after 5 s which was maintained for 40 s).
Films were annealed at furnace at 550°C for 80 minutes. Precursor
solution did not produce any precipitation after 30 days. This spinning-drying
procedure was repeated from 2 to 10 times. The microscope glass slide (75 mm × 25 mm ×1 mm: cleaned in dilute HCl solution and ethanol) was used as substrate.
Film deposition was carried out in air at room temperature by a rate 3000 rpm for 30 s. After each coating, the films were preheated at 275°C for 10 minutes, and postheated at 550°C for 80 minutes. The deposition
was repeated for 2, 4, 5, 6, 8, and 10 times to obtain a film with different thickness.


## 2.3. Characterization of Thin Films

The structure and crystalline size were determined by XRD   diffraction (Bruker D8 advanced X-ray diffractometer: CuKα radiation, Scan rate 0.03 2θ s−1). X-ray diffraction shows
zincite structure with c-axis orientation (002). Optical transmittance was measured by spectrophotometer (Varian Cary 500 Scan).


## 2.4. Photocatalytic Degradation

The photocatalytic degradation experiments were carried out in a simple (40 cm × 15 cm × 15 cm) oxidation reactor (see Figure2), placed in a 25°C water
bath. Slide with 5 layers that placed in 25 mL 10 ppm X6G solution was irradiated with two 8 W lamps (Philips; λ = 365 nm) placed 5 cm above the
solutions. Concentration is measured by spectrophotometer (Varian Cary 500 Scan). In all experiments, 25 mL of 10 ppm X6G solution was used with stirring during the irradiation.Figure 2
Schematic diagram of photoreactor for degradation.


## 3. Results and Discussion

### 3.1. Stability of Sol

Sol was prepared by altering the ratio of Zn(Ac)2·2H2O : MEA. In this work, the best sol was obtained with a Zn(Ac)2·2H2O : MEA molar  ratio 1 : 1 in isopropanol. Increasing the water content and decreasing the time of reflux may change the sol to gel immediately. MEA was added to keep the sol solutions stable and clear for a long period (more than 30 days). There
are many factors affecting the crystallization behavior of the films such as
substrate, the time and temperature of reflux, molar ratio of starting material,
and composition of stabilizers such as MEA. In this work, sol was prepared at
60°C for 60 minutes until a clear homogeneous solution appeared. The XRD pattern of the ZnO thin films on microscope glass slide by five spin-coating (5-layer) preheated at 275°C for 10 minutes and postheated for 80 minutes at different temperature, 350°C, 450°C,
and 550°C, was shown in Figure 3. There are three distinct
differences between the XRD of thin films and ZnO powder. Firstly, diffraction
peaks of thin films have lower intensity and higher FWHM (Fully Width at Half
Maximum) compared to powder; secondly, the films are predominantly (002)
oriented, and finally films are crystallized (wurtzite)
at comparatively higher temperature as compared to powder that has been
prepared from chemical producers. Increase of annealing temperature affected
the intensity of (002) peak which was the highest at 550°C. The ZnO
films grow with a (002) orientation is kinetically preferred, which in turn
likely reflects the fact that the highest density of Zn atoms is found along
the (002) plane [15].Figure 3
XRD pattern of the ZnO thin films on microscope glass slide by five spin-coatings (5 layers), preheated at 275°C for
10 minutes and postheated for 80 minutes at different temperature.Figure4 shows the XRD pattern of the ZnO thin film on
microscope glass for various layers by spin-coating (2, 4, 6, 8, and 10 layers),
preheated at 275°C for 10 minutes and postheated for 80 minutes at 550°C.Figure 4
XRD pattern of the ZnO thin films on microscope glass with different layers (2, 4, 6, 8, and 10 layers), preheated
at 275°C for 10 minutes and postheated for 80 minutes at 550°C.Figures5(a)–5(c) show
the scanning electron microscopy (SEM) image of the nanostructure ZnO thin
films at various temperatures, and
Figure 5(d) shows atomic force microscopy (AFM) image of ZnO thin film at 450°C  that prepared by spin coating method on a
glass substrate. The average grain size of nanostructure ZnO thin films
was near 40 nm, and the crystal size increased with increasing the annealing
temperature. As shown in Figures 5(a)–5(d), thin films
of ZnO prepared by spin coating reveal the formation of a porous granular surface.((a)–(c)) SEM images of the ZnO thin film at different temperature
((a) = 350°C, (b) = 450°C, and (c) = 550°C), (d) AFM image ZnO thin film prepared at 450°C.
(a)(b)(c)(d)Optical
transmittance of ZnO thin films with 5 times spin coating, preheated at 275°C
for 10 minutes and postheated at various temperatures, 350°C, 450°C,
and 550°C, is shown in Figure 6. The absorption at about 370 nm
corresponds to an electronic transition beyond the bandgap, 3.2 eV of the wurtzite crystalline phase of ZnO.Figure 6
Optical transmittance of ZnO thin films, postheated at various temperatures (a = 350°C, b = 450°C, and c = 550°C).The crystalline size of ZnO in the films was calculated by Scherrer’s formulaD = 0.9λ/βCosθ, where D is the grain size, λ (1.548 Å) is the wavelength of X-ray radiation used, β is the full width at half maximum (FWHM) of the diffraction
peak, and θ is the Bragg diffraction angle of the XRD peak. The average
crystalline size of ZnO in the films annealed at 350°C, 450°C, and 550°C was about 16, 23 and 25 nm, respectively. The average
crystalline size of ZnO in the films with various times of coating annealed at 550°C is shown in Table 1. It was observed that the c-axis orientation improves, and the grain size
increases as indicated by an increase in intensity of the (002) peak and the decrease in the FWHM (Fully Width at Half Maximum) high with the increase of annealed temperature. An average grain size was about 25 nm.Table 1
The grain size (crystalline size) of ZnO thin films with various times of
coating, postheated at 550°C.
Samples of thin film2θFWHM =βD (nm)2-layer34.4840.35023.94-layer34.4710.34224.76-layer34.7430.35823.58-layer34.4430.33525.110-layer34.4620.33225.1


### 3.2. Photocatalytic Activity

Evaluation of the films as potential photocatalyst for water pollutant purification was based on the degradation of a model azo dye, X6G. The results showed that nanostructure ZnO
thin film and UV light (365 nm) had a negligible effect when they were used on their own after 240
minutes. Figure7 shows the degradation of 10 ppm X6G under
UV irradiation (365 nm) using ZnO thin film. Glass plate supported ZnO thin films with different layers (2, 4, 5, 6, 8, and 10 layers) were tested for degradation of the X6G dye for 80 minutes (see Figure 8).Figure 7
UV-visible spectra of X6G (initial concentration of X6G = 0.013 mM) in deionized
water after photocatalytic degradation under UV irradiation on ZnO film with 5 layers.Figure 8
X6G concentration (ppm) (initial concentration of X6G = 0.013 mM) after photocatalytic degradation under UV irradiation for 80 minutes on ZnO films with
various layers.Thin films with 5 layers were reused five times and did not show much catalytic activity change (see Figure9).Figure 9
X6G concentration (ppm) (initial concentration of X6G = 0.013 mM) after photocatalytic
degradation under UV irradiation for 80 minutes on ZnO film with 5-layer six-cycle reuse.


## 3.1. Stability of Sol

Sol was prepared by altering the ratio of Zn(Ac)2·2H2O : MEA. In this work, the best sol was obtained with a Zn(Ac)2·2H2O : MEA molar  ratio 1 : 1 in isopropanol. Increasing the water content and decreasing the time of reflux may change the sol to gel immediately. MEA was added to keep the sol solutions stable and clear for a long period (more than 30 days). There
are many factors affecting the crystallization behavior of the films such as
substrate, the time and temperature of reflux, molar ratio of starting material,
and composition of stabilizers such as MEA. In this work, sol was prepared at
60°C for 60 minutes until a clear homogeneous solution appeared. The XRD pattern of the ZnO thin films on microscope glass slide by five spin-coating (5-layer) preheated at 275°C for 10 minutes and postheated for 80 minutes at different temperature, 350°C, 450°C,
and 550°C, was shown in Figure 3. There are three distinct
differences between the XRD of thin films and ZnO powder. Firstly, diffraction
peaks of thin films have lower intensity and higher FWHM (Fully Width at Half
Maximum) compared to powder; secondly, the films are predominantly (002)
oriented, and finally films are crystallized (wurtzite)
at comparatively higher temperature as compared to powder that has been
prepared from chemical producers. Increase of annealing temperature affected
the intensity of (002) peak which was the highest at 550°C. The ZnO
films grow with a (002) orientation is kinetically preferred, which in turn
likely reflects the fact that the highest density of Zn atoms is found along
the (002) plane [15].Figure 3
XRD pattern of the ZnO thin films on microscope glass slide by five spin-coatings (5 layers), preheated at 275°C for
10 minutes and postheated for 80 minutes at different temperature.Figure4 shows the XRD pattern of the ZnO thin film on
microscope glass for various layers by spin-coating (2, 4, 6, 8, and 10 layers),
preheated at 275°C for 10 minutes and postheated for 80 minutes at 550°C.Figure 4
XRD pattern of the ZnO thin films on microscope glass with different layers (2, 4, 6, 8, and 10 layers), preheated
at 275°C for 10 minutes and postheated for 80 minutes at 550°C.Figures5(a)–5(c) show
the scanning electron microscopy (SEM) image of the nanostructure ZnO thin
films at various temperatures, and
Figure 5(d) shows atomic force microscopy (AFM) image of ZnO thin film at 450°C  that prepared by spin coating method on a
glass substrate. The average grain size of nanostructure ZnO thin films
was near 40 nm, and the crystal size increased with increasing the annealing
temperature. As shown in Figures 5(a)–5(d), thin films
of ZnO prepared by spin coating reveal the formation of a porous granular surface.((a)–(c)) SEM images of the ZnO thin film at different temperature
((a) = 350°C, (b) = 450°C, and (c) = 550°C), (d) AFM image ZnO thin film prepared at 450°C.
(a)(b)(c)(d)Optical
transmittance of ZnO thin films with 5 times spin coating, preheated at 275°C
for 10 minutes and postheated at various temperatures, 350°C, 450°C,
and 550°C, is shown in Figure 6. The absorption at about 370 nm
corresponds to an electronic transition beyond the bandgap, 3.2 eV of the wurtzite crystalline phase of ZnO.Figure 6
Optical transmittance of ZnO thin films, postheated at various temperatures (a = 350°C, b = 450°C, and c = 550°C).The crystalline size of ZnO in the films was calculated by Scherrer’s formulaD = 0.9λ/βCosθ, where D is the grain size, λ (1.548 Å) is the wavelength of X-ray radiation used, β is the full width at half maximum (FWHM) of the diffraction
peak, and θ is the Bragg diffraction angle of the XRD peak. The average
crystalline size of ZnO in the films annealed at 350°C, 450°C, and 550°C was about 16, 23 and 25 nm, respectively. The average
crystalline size of ZnO in the films with various times of coating annealed at 550°C is shown in Table 1. It was observed that the c-axis orientation improves, and the grain size
increases as indicated by an increase in intensity of the (002) peak and the decrease in the FWHM (Fully Width at Half Maximum) high with the increase of annealed temperature. An average grain size was about 25 nm.Table 1
The grain size (crystalline size) of ZnO thin films with various times of
coating, postheated at 550°C.
Samples of thin film2θFWHM =βD (nm)2-layer34.4840.35023.94-layer34.4710.34224.76-layer34.7430.35823.58-layer34.4430.33525.110-layer34.4620.33225.1


## 3.2. Photocatalytic Activity

Evaluation of the films as potential photocatalyst for water pollutant purification was based on the degradation of a model azo dye, X6G. The results showed that nanostructure ZnO
thin film and UV light (365 nm) had a negligible effect when they were used on their own after 240
minutes. Figure7 shows the degradation of 10 ppm X6G under
UV irradiation (365 nm) using ZnO thin film. Glass plate supported ZnO thin films with different layers (2, 4, 5, 6, 8, and 10 layers) were tested for degradation of the X6G dye for 80 minutes (see Figure 8).Figure 7
UV-visible spectra of X6G (initial concentration of X6G = 0.013 mM) in deionized
water after photocatalytic degradation under UV irradiation on ZnO film with 5 layers.Figure 8
X6G concentration (ppm) (initial concentration of X6G = 0.013 mM) after photocatalytic degradation under UV irradiation for 80 minutes on ZnO films with
various layers.Thin films with 5 layers were reused five times and did not show much catalytic activity change (see Figure9).Figure 9
X6G concentration (ppm) (initial concentration of X6G = 0.013 mM) after photocatalytic
degradation under UV irradiation for 80 minutes on ZnO film with 5-layer six-cycle reuse.


## 4. Conclusion

Zinc acetate and MEA have proved to be suitable compounds to obtain high quality pure coatings.
These precursor compounds and the proper annealing conditions have proved to be
suitable to produce stable guest oxide nanocrystalline size. The thin films show
good photocatalytic activity for degradation of X6G dye solution.



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*Source: 356765-2008-04-01.xml*