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Materials Letters 61 (20

Synthesis of silver chloride nanocrystal on silk fibers

Pranut Potiyaraj a, Panittamat Kumlangdudsana a, Stephan T. Dubas b,⁎

a Materials Science Department and Textile Institute, Chulalongkorn University, Bangkok, Thailandb Metallurgy and Materials Science Research Institute, Chulalongkorn University, Bangkok, Thailand

Received 21 July 2006; accepted 18 September 2006Available online 5 October 2006

Abstract

The synthesis of silver chloride nanocrystals on silk fiber is reported. The growth of the nanocrystal was achieved by sequential dipping of the silkfibers in alternating solution of either silver nitrate or sodium chloride followed by a rinse step. The negative charge present at the surface of the fiberscan immobilize silver ions which react in the next sodium chloride bath to form an initiator seed of silver chloride. The repeated alternating dipping ofthe fiber in the different solutions leads to the growth of AgCl nanocrystals. After 20 alternating dipping steps, the formation of cubic AgClnanocrystal was observed by SEM, which revealed the presence of 100 nm cubes on the silk fiber. The crystallographic nature of the AgCl crystalswas confirmed by X-ray diffraction which confirmed the presence of characteristics diffraction peak of AgCl and a cell size of 5.549 Å. The resultingfiber coated with nano-AgCl crystals could be used as a photocatalyst in water splitting applications or as an antibacterial agent.© 2006 Elsevier B.V. All rights reserved.

Keywords: Silver chloride; Nanocrystals; Synthesis

1. Introduction

The synthesis and physicochemical characterization of silverhalides have interested researchers and engineers for decadesbecause of their particular properties [1–4]. This interest hasbeen fuelled by the fact that these materials were found to besemiconductors and excellent ionic conductors due to thehopping on silver ions on Frenkel defects. However, they aremostly known for their behavior under exposure to light, whichwas the basis for the development of photography. Recently,researchers' interest for silver halides has been renewed whenCalzaferri et al. reported that thin silver chloride layers onSnO2-coated glass plates could catalyze the splitting of water inthe presence of a small excess of Ag+ ions in aqueous solutionunder near-UV illumination [5,6]. Silver chloride is also a wellknown antibacterial agent which is commonly used in hospitalas catheter coating, for wound dressing applications and is alsoused mixed with PMMA in bone cement application [7,8].

Among the methods used for the synthesis of nanocrystal, thereverse micelle system is commonly cited [9]. Extremely smallsize crystals can be synthesized in these water pools, which act asnanoreactor. Although powerful in terms of synthesis, the

⁎ Corresponding author. Tel.: +66 2 218 4234; fax: +66 2 611 7586.E-mail address: Stephan.d@chula.ac.th (S.T. Dubas).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.09.039

micelle-based methods are not easily applicable for the laterdeposition of the nanoparticles onto a substrate. The efficiencyprovided by the high surface/volume ratio of the nanoparticlescan be further enhanced by using a substrate having a highsurface to volume ratio as well such as fibrous or micro-porousmaterials. In our approach, the principle of the synthesis of silverchloride nanocrystal is achieved by sequential dipping of a silkfiber into alternating bath of silver nitrate and sodium chloride.This new method combines the high efficiency of nanocrystalswith high surface ration from the micro-size of the silk fiberssubstrate. The formation and growth of the crystal wasconfirmed by scanning electron microscopy and the AgClcrystallographic nature of the nanoparticles confirmed by X-raydiffraction. Our experiment suggests that it would be possible tofabricate these nanocrystals on other type of fibers includingconducting organic or inorganic fibers for water splittingapplication. In other applications, the antibacterial activity ofAgCl could lead to the preparations of antibacterial fabric.

2. Chemicals and methods

2.1. Chemicals

The silk fibers (multifilament fibers: no twist) were donatedby Chul Thai Silk Company Limited, Petchaboon, Thailand.

Fig. 1. Steps describing the preparation of the silver chloride nanocrystal bysequential dipping in silver nitrate and sodium chloride.

2465P. Potiyaraj et al. / Materials Letters 61 (2007) 2464–2466

Silver nitrate (AgNO3) analytical grade was purchased fromMallinckrodt Thailand. The sodium chloride was purchasedfrom Carloerba, Thailand, and the pH of the solution wasadjusted to pH 9 with diluted sodium hydroxide. In allexperiments, double distilled water was used and all reagentswere used as received without any further purification.

2.2. AgCl crystal growth

The silk fibers were wrapped around a rectangular aluminumholder, 2.5×3.5 cm2 and spun in various solutions using a smalldc motor. A home-built robotic platform, accommodating eight100 ml beakers, was programmed to successively expose thesilk fibers to either silver nitrate or sodium chloride solutionsfollowed by three water rinses of 1 min each. The number ofdipping steps was fixed to 20 meaning that the fiber was dipped10 times in silver nitrate and 10 times in sodium chloridesolutions. At the end of the deposition process the samples wereallowed to dry overnight and wrapped on a plastic holder beforefurther characterization.

2.3. Scanning electron analysis and X-ray diffraction

Prior to SEM analysis, samples were cut from the preparedsilk fiber coated with silver chloride nanocrystals, mounted onthe sample holder with double sided tape and coated with a thinlayer of evaporated gold. The images were taken using a JEOL

Fig. 2. Scanning electron microscopy image of the AgCl nanocrystals formed at the sdipping in AgNO3 and NaCl solutions followed by a rinse in water.

scanning electron microscope, JSM-5410LV using an acceler-ating voltage of 15 kV. For the X-ray diffraction a long sampleof 4 m was wrapped onto a hollow holder and placed in theinstrument. The X-ray diffractometer used was a Bruker AXSModel D8 discover.

3. Results and discussion

The silver chloride nanocrystals were synthesized by sequentialdipping of a chemically cleaned silk fiber in alternating bath of sodiumchloride and silver nitrate followed by a rinse step (Fig. 1). The methoddescribed here for the nanocrystal formation takes advantage of theelectrostatic interaction between silk and the silver ions and much careshould be given to avoid cross contamination from the silver AgNO3

and the NaCl solution which would lead to the formation of AgClprecipitate. To avoid this cross contamination 1 mM AgNO3 and 1 mMNaCl solutions were used. The dipping step in each AgNO3 or NaClsolutions was fixed at 1 min followed by three rinses in pure water of1 min each. The surface of a the silk fiber, when dipped in a mildalkaline solution is known to become negatively charged due to thedeprotonation of the carboxylic group present at the fibers' surface.The induced electrostatic charge on the fiber will tend to attract andimmobilize the silver ion present in the solution. The rinse step insuresthat no excess silver nitrate is brought into contact with the followingNaCl solution. The dipping step in the NaCl solution will allow theformation of the AgCl complex and initiate the formation of new AgClcrystal. The repetition of these dipping steps leads to the growth ofAgCl crystals at the surface of the fibers, which were later confirmed tobe in nanoscale.

To confirm the formation of nanocrystal at the surface of the silkfiber, the samples were analyzed with a scanning electron microscope.The analysis revealed the presence of cubical crystals on the fibersurface, which appear to have a size of a several tenths of nanometers ascan be seen on Fig. 2. The cubes appear to have a broad size dis-tribution, which suggests that new crystals are being generated aftereach dipping steps. The larger crystals were initiated early in thedipping process and grew larger with each dipping in silver nitrate andsodium chloride solutions.

The crystallographic nature of the cubic structure as seen by theanalysis of the fiber with SEM was further investigated by X-raydiffraction. Fig. 3 shows the spectrum of a freshly prepared sample andthe characteristic peaks corresponding to the AgCl crystals, which wasidentified as Chlrorargyrite. The 5 major peaks found at 27.83, 32.25,

urface of the silk fiber. The AgCl nanocrystals were synthesized by 20 alternate

Fig. 3. X-ray diffraction pattern of the silver chloride nanocrystals at the surfaceof the silk fibers. The pattern indicate a cubic face centered with a cell dimensionof 5.549 Å.

2466 P. Potiyaraj et al. / Materials Letters 61 (2007) 2464–2466

46.27, 54.87 and 57.58° on the 2 theta scale correspond respectively tothe (1,1,1), (2,0,0), (2,2,0), (3,1,1) and (2,2,2) crystal planes. Thespacing of the crystallographic plane of the cubic face centeredstructure was found to correspond to a cell size of 5.549 Å which againis characteristic if AgCl crystal. The first peak at 20° corresponds to thesilk substrate and is located at the same position than in the blank silksample. This final evidence confirmed that the crystals formed at thesurface of the silk fibers correspond to AgCl nanocrystals.

4. Conclusion

In conclusion we have demonstrated the possible synthesisof silver chloride nanocrystals at the surface of silk fibers. Thesynthesis is achieved by sequential dipping in AgNO3 and NaCl

solution and the resulting AgCl crystals could be used as anantibacterial agent or if assembled on conducting fibers, couldbe used in water splitting application. This technique could beextended to the step growth of other type of crystals such assemiconductor quantum dots of ZnS or CdSe. The sequentialdipping might further allow the control of the crystallographicstructure and impurity content of a crystal by dipping the coatedfiber in various solutions.

Acknowledgement

This work was supported by the National Science andTechnology (NSTDA) of Thailand under grant number NN-B-22-m29-10-49-43.

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