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 You 1, Chin-Bin Lin 1, Yang-Min Liang 2, Chi-Wang Li 2 * and Hui-Chung Hsueh 1 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: chiwang@mail.tku.edu.tw 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 Ag 0 ), i.e., photosensitivity or self-sensitization, highly active hydroxyl radicals are produced by oxidation of OH or H 2 O 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

10 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. 2. Experimental Section 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 AgNO 3 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 AgNO 3 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 AgNO 3(aq) to liquid state initiating precipitation reaction to form AgCl. After pre-determined reaction time (10 min, 12 hrs, and 24 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 AgNO 3 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 cm 2 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. 2.2 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.

A Novel Method for Preparation of Silver Chloride Thin Films 11 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 (CuK ) by copper target, scanning angle 2 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 m 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 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 AgNO 3 surface. When NaCl solution (~20 C)wasaddedontothe frozen AgNO 3(aq) surface, a thin liquid layer of AgNO 3 was formed from melting of frozen AgNO 3(aq) and precipitation of AgCl was started at the interface. The surface of frozen AgNO 3(aq) provides a temporary sediment surface as AgCl crystal grows through heterogeneous nucleation at interface between AgNO 3 solid and AgNO 3 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. AgNO 3 = 8.4 M.

12 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 m. 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 m, 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 AgNO 3 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 AgNO 3 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 AgNO 3 concentration on the morphology of SCTF at the reaction time of 24 hrs. The SEM images of the top surface under various AgNO 3 concentrations are shown in Figure 4 (T1~T4). Compared with the top surface of SCTF made by AgNO 3 of 8.4 M (Figure 4 (T1)), the morphology of films made at lower AgNO 3 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 AgNO 3 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 bottom 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. 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. 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 before 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 AgNO 3 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.

A Novel Method for Preparation of Silver Chloride Thin Films 15 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. Acknowledgement This work was supported by the National Science Council, Taiwan. Reference [1] Li, Y. and Ding, Y., Porous AgCl/Ag Nanocomposites with Enhanced Visible Light Photocatalytic Properties, J. Phys. Chem. C, Vol. 114, No. 7, pp. 3175 3179 (2010). doi: 10.1021/jp911678q [2] Gu, S., et al., Preparation and Characterization of Visible Light-Driven AgCl/PPy Photocatalyst, J. Alloys Compd., Vol. 509, No. 18, pp. 5677 5682 (2011). doi: 10.1016/j.jallcom.2011.02.121 [3] Choi, M., et al., Plasmonic Photocatalytic System Using Silver Chloride/Silver Nanostructures under Visible Light, J. Colloid Interf. Sci., Vol. 341, No. 1, pp. 83 87 (2010). doi: 10.1016/j.jcis.2009.09.037 [4] Glaus, S. and Calzaferri, G., Silver Chloride Clusters and Surface States, J. Phys. Chem. B, Vol. 103, No. 27, pp. 5622 5630 (1999). doi: 10.1021/jp990701m [5] Liu, Y., et al., Photocatalytic Degradation of Direct Sky Blue 5B Using AgCl as a Photocatalyst in an Aqueous, in 5th International Conference on Bioinformatics and Biomedical Engineering, Wuhan, China (2011). doi: 10.1109/icbbe.2011.5781068 [6] Calzaferri, G., et al., Quantum-Sized Silver, Silver Chloride and Silver Sulfide Clusters, J. Imaging Sci. Technol., Vol. 45, No. 4, pp. 331 339 (2001). [7] Schürch, D., et al., The Silver Chloride Photoanode in Photoelectrochemical Water Splitting, J. Phys. Chem. B, Vol. 106, No. 49, pp. 12764 12775 (2002). doi: 10. 1021/jp0265081 [8] Wang, P., et al., Ag@AgCl: a Highly Efficient and Stable Photocatalyst Active under Visible Light, Angew. Chem. Int. Ed. Engl., Vol. 47, No. 41, pp. 7931 7933 (2008). doi: 10.1002/anie.200802483 [9] Wang, P., et al., Synthesis of Highly Efficient Ag@AgCl Plasmonic Photocatalysts with Various Structures, Chem.- Eur. J., Vol. 16, No. 2, pp. 538 544 (2010). doi: 10.1002/chem.200901954 [10] Ma, B., et al., Highly Stable and Efficient Ag/AgCl Core-Shell Sphere: Controllable Synthesis, Characterization, and Photocatalytic Application, Appl. Catal.

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