A Novel Method for Preparation of Silver Chloride Thin Films

Journal of Applied Science and Engineering, Vol. 18, No. 1, pp. 9-16 (2015)
DOI: 10.6180/jase.2015.18.1.02
A Novel Method for Preparation of
Silver Chloride Thin Films
Chang-Ching You1, Chin-Bin Lin1, Yang-Min Liang2,
Chi-Wang Li2* and Hui-Chung Hsueh1
1
Department of Mechanical and Electro-Mechanical Engineering, Tamkang University,
Tamsui, Taiwan 251, R.O.C.
2
Department of Water Resources and Environmental Engineering, Tamkang University,
Tamsui, Taiwan 251, R.O.C.
Abstract
Silver chloride thin film (SCTF) with high specific surface area was synthesized through
precipitation reaction by adding sodium chloride solution on top of frozen silver nitrate solution.
Effects of precipitation time and silver nitrate concentration on the morphology of SCTFs were
investigated. SEM images show that small crystal AgCl grains forming rod-like structure appeared on
the bottom surface of the SCTF. After exposure of SCTFs to UV light, clusters of silver atoms were
formed on the surface of SCTF as indicated by XRD analysis. Six SCTFs were attached to interior
wall of the photo-reactor using double sided adhesive tape for investigation of photocatalytic
property of the films. Almost completed decolorization of orange II dye by SCTF can be achieved
under UV light illumination within two hours. SCTFs made under different initial silver nitrate
solution concentrations of 8.4 M, 4.2 M and 3.6 M have little impact on dye decolorization efficiency.
The photocatalytic degradation of orange II dye by SCTF under visible light illumination is less
efficient than that under UV light. Around 31% of color can be removed after two hours.
Key Words: Precipitation, Silver Chloride Thin Film, Photocatalytic, Dye, Photosensitivity
1. Introduction
Silver chloride (AgCl) is a semiconductor material
with band gap of around 3.1~3.3 ev [1,2]. Upon irradiated by near UV light, electrons on the valence band would
jump onto the conduction band, producing electron and
hole pairs [3]. Because of its unique photosensitivity
property, irradiation of AgCl by UV light would produce silver and chlorine atoms. A small amount of silver
atom clusters will adsorb on the surface of silver chloride, causing photoactivity of AgCl film to extend from
UV into the visible light region. It is known as selfsensitization or photosensitivity phenomenon [4,5]. Selfsensitization would not only lower the band gaps of semiconductor, but also help the electrons transit from the
*Corresponding author. E-mail: [email protected]
valence band to the conduction band. The transition
turns on the photocatalytic reaction and enhances the utilization rate of broadband light energy [6,7]. Recently,
Wang et al. [8,9] found that the plasmonic photocatalyst, Ag@AgCl, which are AgCl particles with silver
nanoparticles formed on their surface, is efficient and
stable under visible light because of the strong visible
light absorption of silver atoms [10].
Along with photographic process (Ag+ + e- ® Ag0),
i.e., photosensitivity or self-sensitization, highly active
hydroxyl radicals are produced by oxidation of OH- or
H2O by these photo-generated holes [2]. Because OH
radicals have high oxidation capacity with oxidation
potential of 2.8 V (only after Fluorine) [11], they can facilitate degradation of organic matters and sterilization
of environmental microorganisms.
Several methods have been proposed to make AgCl
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Chang-Ching You et al.
particles, including sol-gel [12] and chemical bath deposition [13]. It is desired to make AgCl film instead of
particles since recovery/separation processes are needed
for application of AgCl particles as photocatalyst. Song
and Liu [14] proposed a method to make silver chloride
film through fixation of AgCl particles onto glass substrate using PVC in Tetrahydrofuran solution. This method
has a drawback that silver chloride particles cannot be
completely exposed on the surface, thus the efficiency of
sterilization or dye degradation is not very high. In this
paper, we have proposed a novel method for making silver chloride thin film with high surface area. The photocatalytic property of the prepared SCTF was demonstrated
through photo-degradation of an azo dye, orange II.
hrs), residual NaCl solution on the top of SCTF was discarded, and the film was rinsed with DI water for 5 times.
After the Teflon film on the bottom of the PVC tube was
removed to drain the un-reacted AgNO3 solution, the bottom of SCTF was rinsed with DI water for 5 times. The
film was dried in oven at 100 °C for 8 hours. The finished
products have apparent area of 2.54 cm2 and weight of
about 0.036 g. Because the band gap of AgCl is within
3.1 ev~3.3 ev corresponding to light wavelength of 375
nm~400 nm, the whole preparation process was done under yellow light to prevent photolysis reaction.
2.1 Chemical and Materials
SCTFs were fabricated through precipitation reaction between sodium chloride solution and silver nitrate
solution which was first frozen using liquid nitrogen. The
detail procedures are as follows: A PVC tube (with I.D.
of 1.8 cm and length of 3 cm) was sealed with Teflon
film on one end (see Figure 1). A 0.2-mL of known concentration of AgNO3 solution (8.3 M, 6.3 M, 4.2 M, or
2.1 M) was added into the tube. A 6061 aluminum plate
was first immersed in liquid nitrogen, and brought contact to the closed end of the PVC tube to freeze AgNO3
solution into solid state. Then, a 3-mL of NaCl solution
(5.4 M) was added into the PVC tube. The contact of
NaCl solution melts solid state of AgNO3(aq) to liquid
state initiating precipitation reaction to form AgCl. After pre-determined reaction time (10 min, 12 hrs, and 24
2.2 Experimental Methods
The photocatalytic property of prepared SCTF was
studied by testing the photocatalytic degradation of dye
which was prepared by dissolving orange II azo dye
(Sigma) in deionized water (DI) to the concentration of
30 mg/L. Experimental setup for photocatalytic degradation of dyes is shown in Figure 2. A 150-mL glass
beaker was used as the reactor and was placed inside a
circulating constant temperature water bath to maintain
temperature at 25 °C. In each test, a 100-mL of orange
II azo dye with concentration of 30 mg/L was mixed at
constant stirring speed of 325 rpm. Six SCTFs were
fixed onto the inner reactor wall using double sided PET
tape. Both UV and visible lamps with power of 9 W were
employed for the photocatalytic degradation tests. The
UV lamp (Actinic BL PL-S 9W/10/2P, Philips) has characteristic light wavelength of 365 nm and the visible
light (PL-S 9W/865/2P, Philips) has three characteristic
wavelengths of 435 nm, 545 nm, and 612 nm. The whole
system was cover in a black-box to avoid interference
from other light source.
Figure 1. Setup for preparation of silver chloride film.
Figure 2. Experimental setup for photocatalytic degradation.
2. Experimental Section
A Novel Method for Preparation of Silver Chloride Thin Films
2.3 Analytical Methods
A scanning electron microscope (HITACHI, S-2600H)
was employed to observe crystalline morphology of top,
bottom, and cross-section of SCTF which was gold-plated
for 30 seconds with Ion Sputter (HITACHI, E-1010) before observation. The crystalline phase of SCTF was analyzed using a X-ray diffractometer (BRUKER, D8A)
which was operated at incident light with wavelength
1.54056 Å (CuKa) by copper target, scanning angle 2 q
of from 10° to 90°, and scanning rate of 0.1° s-1. Degradation of orange II azo dye was calculated by measuring
ADMI values of samples before and after reaction followed the standard method [15]. Samples were filtered
through 0.45 mm filter membrane before measurement.
3. Results and Discussion
3.1 Effect of Precipitation Time on Morphology of
SCTF Films
To understand the progress of precipitation process,
SCTFs formed at reaction time of 10 min, 12 hrs and
11
24 hrs were observed using SEM. The surface crystalline morphology of the top, bottom and cross-section of
SCTFs were shown in Figure 3. The top surface is the
surface having contact with NaCl solution, and the bottom surface is that having contact with frozen AgNO3
surface. When NaCl solution (~20 °C) was added on to the
frozen AgNO3(aq) surface, a thin liquid layer of AgNO3
was formed from melting of frozen AgNO3(aq) and precipitation of AgCl was started at the interface. The surface of frozen AgNO3(aq) provides a temporary sediment
surface as AgCl crystal grows through heterogeneous
nucleation at interface between AgNO3 solid and AgNO3
liquid layer. The stable nuclei will continue to grow at
the interface from grains into a continuous film by migration between the silver chloride grains.
Figure 3 (T1) shows the top surface of SCTF formed
at reaction time of 10 min, indicating that newly formed
silver chloride grains are small and the film has many
small holes. As solution of sodium chloride contacting
the melting solution of silver nitrate, AgCl is rapidly precipitated and grains of silver chloride are relatively large
Figure 3. SEM images of (T) top, (B) bottom, and (C) cross-section surface of the precipitated SCTFs prepared at reaction time of
(1) 10 min; (2) 12 hrs, and (3) 24 hrs. AgNO3 = 8.4 M.
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Chang-Ching You et al.
due to very high concentration of both solutions [16]. At
this moment, they are quickly piled into continuous thin
films. Many small holes are formed due to different orientation of AgCl grains stacked together.
Figure 3 (B1) shows the microstructure of bottom
surface of SCTF which has the similar small grains as
those on the top surface. In addition, some rod-like structure formed by piling up of many tiny particles can also be
found on the bottom surface. After the continuous thin
film is formed, it will be a layer of obstruction for collisions between chloride ions and silver ions. At the bottom
surface of SCTF, chloride ions can only diffuse through
the tiny gaps/holes of films (such as grain boundary), and
then it causes the decrease of ion concentration. At the
same time, solid silver nitrate solution slowly melts into
liquid. Coupled with the super-cooling effect and low
aqueous solubility of silver nitrate solution under extremely low temperature, relatively small grains of silver
chloride are generated, and are incubated on the bottom
surface of thin film as seeds. At this time, small particles
of AgCl are precipitated continuously, and piled into rod
structure at the bottom surface by priority directions. Figure 3 (C1) shows the cross-section of the film with lots of
tiny holes. The thickness of film is around 30 mm. In this
picture, bottom surface of SCTF is the one facing up.
With the reaction time of 12 hours (Figure 3 (T2)),
the most of the small holes on the top surface are disappeared and the grains have become larger and denser, indicating the small holes were filled by grain growth. In
addition, the surface has many irregular stripes which are
the contraction trace left by diffusion during grain growth
process. The bottom surface of the film is shown in Figure 3 (B2). Apparently, more rod structure could be found,
and the size of grains has become larger. As shown in
Figure 3 (C2), the film growths thicker at around 100
mm, and the holes have been filled gradually became
smaller and fewer. In addition, the bottom surface with
rod structure can be observed.
At reaction time of 24 hours, the size of grain on the
top surface did not grow further (see Figure 3 (T3)). It
is because that the film is densified over time and the
contact between NaCl and AgNO3 is less likely, making
precipitation of silver chloride more difficult. On the bottom surface (Figure 3 (B3)), the grains on the rod structure have become more compact. As indicated in Figure
3 (C3), the film is slightly thickened and the holes are
almost disappeared.
Form the observation above, AgCl grains of the bottom surface are smaller and more abundant, piling into
rod structure. The surface area of bottom surface is apparently higher than that of top surface. In order to achieve
the best photolytic effect, it is the bottom surface used,
i.e., espoused to dye solution and light, for the subsequent photocatalytic degradation of dyes.
3.2 Effect of AgNO3 Concentration on Morphology
of SCTFs
It is well known that size of grains in precipitation
reaction is affected by the concentration of solution where
size of grains increases with increasing concentration
[16]. It is desirable to have smaller grain size to achieve
higher specific surface area. In this study, concentrations
of silver nitrate solution ranging from 8.4 to 2.1 M were
tested to explore the effect of AgNO3 concentration on
the morphology of SCTF at the reaction time of 24 hrs.
The SEM images of the top surface under various
AgNO3 concentrations are shown in Figure 4 (T1~T4).
Compared with the top surface of SCTF made by AgNO3
of 8.4 M (Figure 4 (T1)), the morphology of films made
at lower AgNO3 concentration of 6.3 M (Figure 4 (T2))
shows a very different crystal structure. The tiny rod
structure is composed of small grains, and boundary between crystals is not obvious. When the concentration
of silver nitrate solution is lowered to 4.2 M (see Figure
4 (T3)), the rod structure is less obvious. With 2.1 M of
silver nitrate solution, the rod structure (Figure 4 (T4))
appears again, and more apparent than those in the condition of 6.3 M.
Figure 4 (B1)~(B4) show morphology of the bottom
surfaces made of various concentrations of silver nitrate
solution. The same rod structure can be observed under
silver nitrate concentrations ranging from 8.4 to 4.2 M.
The film made under silver nitrate concentration of 6.3
M has smaller and less grains than those under 8.4 M
condition. Besides, the boundary of rod structure is not
obvious under 6.3 M condition. It looks like stacking of
grains into rod structure is not completed due to lower silver nitrate concentration. However, when the concentration of silver nitrate is further lowered to 4.2 M, rod structure can be seen clearly again and the amount of grains
A Novel Method for Preparation of Silver Chloride Thin Films
13
Figure 4. SEM images of (T) top, (B) bottom, and (C) cross-section surface of the precipitated SCTFs prepared with AgNO3 concentration of (1) 8.4 M; (2) 6.3 M; (3) 4.2 M, and (4) 2.1 M. Reaction time of 24 hrs.
on the rod structure becomes more. As shown in Figure
4 (B3), the microstructure is similar with the one made
under 8.4 M condition, although the grain size under 4.2
M condition is smaller than that under 8.4 M condition.
Some of grains proliferated with each other resulting in
that the links between grains are much denser, and the
grain boundaries become blurs. When the concentration
of silver nitrate solution is lowered to 2.1 M, the rod structure disappears and is replaced with many tiny grains
piled on the surface (as shown in Figure 4 (B4)). Comparison of the size of grains under 2.1 M with those made
under 6.3 M and 4.3 M, the grain size is relatively larger and less denser. Clearly, no crystal precipitated on
the bottom surface, and those grains attached to the bot-
tom surface are formed through heterogeneous nucleation. From the cross section view shown in Figure 4 (C1)~
(C4), thickness of the films are not much difference.
3.3 X-ray Diffraction Pattern of the SCTF
As indicted above, irradiation of AgCl with UV light
would produce silver and chlorine atoms due to its unique photosensitivity property. Silver atoms produced will
adsorb on the surface of silver chloride, causing photoactivity of AgCl to extend from UV into the visible light
region. The phenomenon is known as self-sensitization
[4,5]. To explore the photosensitivity property of film
produced using the proposed novel method, SCTF before and after UV irradiation was analyzed using an X-ray
14
Chang-Ching You et al.
3.4 Photocatalytic Property of SCTF for
Degradation of Orange II Dye
To explore the photocatalytic property of SCTF, six
SCTFs were fixed onto the inner wall of reactor using
double sided PET tape for degradation experiments of
orange II dye with concentration of 30 mg/L. Blank tests
including direct photolysis, i.e., system without SCTF,
under UV and visible light, and adsorption test, i.e., system with SCTFs but without light, were performed be-
fore photocatalytic experiment. No dye lost due to direct
photolysis and adsorption during two hours of reaction
time (data not shown).
Figure 6(A) shows almost complete degradation of
orange II dye within 2 hours of reaction under UV light irradiation. Efficiencies of orange II dye degradation using
SCTFs made under silver nitrate concentrations of 8.4,
6.3, and 4.2 M are the same, while those using films made
under 2.1 M is less effective. The difference in dye degradation efficiency might be related to the crystal structure
produced. As stated in the previous section, the rod structure disappears and is replaced with many tiny grains
piled on the surface at this concentration at the silver nitrate concentration of 2.1 M. As the result, the grain size
is relatively larger and less dense with less surface area.
Consideration of the cost of silver nitrate, the cost of
making SCTFs can be reduced with less concentrated
silver nitrate of 4.2 M without being compromised its
photocatalytic property.
As indicted above, after irradiation of AgCl with UV
light and formation of silver atoms on the surface of sil-
Figure 5. XRD patterns of SCTFs (A) before UV irradiation
and (B) after UV irradiation. Film prepared with
AgNO3 of 8.4 M and reaction time of 24 hrs.
Figure 6. The residual color of orange II dye photo-degraded
under (A) UV light and (B) visible light with SCTFs
prepared using different concentrations of silver
nitrate.
diffractometer (BRUKER, D8A). Figure 5(A) shows the
X-ray diffraction pattern of the SCTF before exposure of
UV light. Compared with JCPDS databases, diffraction
peaks of film at (111), (200), (220), (311) and (222) crystal plane match perfectly with those of AgCl crystal. After UV irradiation (Figure 5 (B)), in addition to the characteristic peaks of AgCl crystal, diffraction peaks of silver atoms at (111), (200), (220) and (311) crystal plane
can also be observed. The result confirms that SCTF produced using the novel method also possess photosensitivity property.
A Novel Method for Preparation of Silver Chloride Thin Films
ver chloride will cause photoactivity of SCTF to extend
from UV into the visible light region, i.e., self-sensitization phenomenon. To test the photocatalytic property of
SCTF after self-sensitization, SCTFs were first exposed
to UV light for 10 minutes before the visible light photocatalytic experiments. The color of SCTF changed from
white to dark gray because of the silver clusters produced. Under the irradiation of visible light, the degradation of orange II dye is less effective with degradation efficiency around 10 to 30% depending on silver nitration concentration. As indicated in Figure 6 (B), SCTFs
made under 4.2 M has the best efficiency for dye degradation. It is consistent to the smallest particles of rodlike structure on the bottom surface of SCTFs obtained at
this concentration according to microstructure observation above. Therefore, it can be speculated that the higher
surface area of films made under 4.2 M of silver nitrate
concentration is the underline reason for the best photodegradation of dye observed.
4. Conclusions
In this study, SCTFs were successfully fabricated
through precipitation reaction of sodium chloride solution with solid state of frozen silver nitrate solution. The
progress of precipitation process was explored and observed using SEM for SCTFs formed at different reaction time and silver nitrate concentrations. The photocatalytic and self-sensitization phenomenon of SCTFs
produced were studied for the degradation of orange II
dye under UV light and visible light. Microstructure observed by the SEM shows that small crystal grains on the
bottom surface of the silver chloride film are stacked into
rod-like structure which provides high specific surface
area. Besides, the silver chloride crystal grain of the rodlike structure is smaller when the film was prepared by
the concentration 4.2 M of silver nitrate solution. XRD
analysis proves that UV illumination of SCTFs produces
silver atoms clusters. The photocatalytic degradation of
orange II dye by SCTFs under UV light illumination is
very effective with completed decolorization within two
hours. Under visible light illumination, the photocatalytic degradation of orange II dye by SCTFs is less effective with the best decolorization of around 31% after
two hours.
15
Acknowledgement
This work was supported by the National Science
Council, Taiwan.
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Manuscript Received: Nov. 6, 2014
Accepted: Feb. 3, 2015