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Materials Science and Engineering B 195 (2015) 90–97

Contents lists available at ScienceDirect

Materials Science and Engineering B

jo ur nal home p age: www.elsev ier .com/ locate /mseb

acile synthesis of silver nanoparticles and their application in dyeegradation

iby Josepha, Beena Mathewb,∗

Department of Chemistry, St. George’s College, Aruvithura, Kottayam 686122, Kerala, IndiaSchool of Chemical Sciences, Mahatma Gandhi University, Kottayam 686560, Kerala, India

r t i c l e i n f o

rticle history:eceived 20 October 2014eceived in revised form 13 February 2015ccepted 15 February 2015vailable online 27 February 2015

a b s t r a c t

The present article reports a simple, facile and eco-friendly method based on microwave irradiation for thesynthesis of silver nanoparticles in aqueous medium using starch as stabilizing agent and a new reducingagent namely hexamine. The silver nanoparticles were characterized by UV–vis, FTIR, XRD and HR-TEManalysis. UV–vis spectroscopic studies provided sufficient evidences for the formation of nanoparticles.The role of starch in the synthesis and stabilization of the nanoparticles was obtained from FTIR studies.

eywords:icrowave

ilver nanoparticleexaminetarchethyl orange

The XRD and HR-TEM investigations clearly demonstrated the crystalline nature of the nanoparticles.From the TEM images, the silver nanoparticles were found to be spherical and of nearly uniform sizewith an average diameter of 18.2 ± 0.97 nm. The nanoparticles showed excellent catalytic activity in thedegradation of methyl orange and rhodamine B by NaBH4.

© 2015 Elsevier B.V. All rights reserved.

hodamine B

. Introduction

Synthetic organic dyes are extensively used in various industriesuch as textile, paper, plastic, food, cosmetic and pharmaceutical1]. The discharges from these industries result in substantial envi-onmental pollution. Studies have shown that many of the dyesre carcinogenic, mutagenic and harmful to the environment [2].hese coloured dyes are toxic to aquatic organisms and perturb thequatic life. Various physical, chemical and biological water treat-ent methods have been used for removing the dye wastes. These

nclude methods such as adsorption, membrane filtration, chem-cal oxidation and reduction, photochemical and electrochemicalreatment, anaerobic treatment etc. [3–8]. Since the dye pollu-ants are chemically stable, traditional water treatment methodsre found to be ineffective. They are highly resistant to micro-rganisms and hence the water soluble dyes are not generallyecolourized effectively by conventional biological treatment [9].ecently, nanotechnology has been extended to the area of wasteater treatment [10]. Nanocatalysis has undergone a remarkable

rowth in recent years and seems to be a revolution in the fieldf catalysis. Metallic nanoparticles exhibit physical and chemicalroperties that differ considerably from those of the bulk materials

∗ Corresponding author. Tel.: +91 9447145412; fax: +91 481 2731036.E-mail addresses: sibyjoseph4@gmail.com (S. Joseph), beenamscs@gmail.com

B. Mathew).

ttp://dx.doi.org/10.1016/j.mseb.2015.02.007921-5107/© 2015 Elsevier B.V. All rights reserved.

[11]. This is largely due to their finite size and large surface area tovolume ratio and the reactivity that depends mostly on their size.The size dependent reactivity and large surface area have madethem efficient catalysts [12]. Several researchers have reported theuse of nanocatalysts for the effective removal of dye stuffs [12–15].

Among metal nanoparticles, silver nanoparticles (AgNPs) con-tinue to be interesting in nanotechnology due to their excellentoptical and electronic properties as well as their strong toxicity toa wide range of microorganism. Several synthetic approaches havebeen developed for the synthesis of silver nanoparticles includingchemical [16], photochemical [17], sonochemical [18], radiolytic[19], polyol [20] and biological methods [21]. Most of these pro-duction routes involve the use of toxic chemicals and require harshreaction conditions. Among these, the most popular method for thepreparation of Ag colloids is still the chemical reduction of a silversalt in presence of a stabilizing agent because of its short reactiontime. The synthesis and application of silver nanoparticles requiresthe stabilization of them in suitable stabilizing systems. However,the chemical reagents used as reducing and stabilizing agents inthis method are usually highly toxic and poses huge environmentalhazards which limits their utility.

In recent years, immense amount of research has been car-ried out in the area of nanosynthesis by using non-toxic and

easily available materials with the aim to reduce environmentalthreats. Microwave-assisted synthesis of nanoparticles in aqueousmedium using green chemicals is receiving much considerationin recent times due to the adherence of this method to the three

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increased rapidly with increase in reaction time up to 4 min due tothe continuous formation of silver nanoparticles.

However, further increase in reaction time did not cause anyappreciable change in the intensity of the SPR band. The sharp

S. Joseph, B. Mathew / Materials Scie

rinciples of green nanoparticle synthesis which are (i) the selec-ion of a non-toxic reducing agent, (ii) a cost-effective and easilyenewable stabilizing agent and (iii) an environmentally benignolvent system. Furthermore it has several attractive features overonventional thermal heating methods such as short reactionime, lower energy consumption and better product yield [22].

icrowave irradiation offers rapid and uniform heating of theeaction medium and thus provides homogeneous nucleation androwth conditions for nanoparticles. Many successful reports onicrowave-assisted green synthesis of silver nanoparticles have

een published in recent years [23–26].In this work, we report the microwave-assisted synthesis of sil-

er nanoparticles in aqueous medium using a new reducing agentamely hexamine in presence of starch as stabilizing agent. Hex-mine is a low-cost chemical that finds application in medicinend food industry. It has been observed that when a strong reduc-ng agent like NaBH4 is used, the reaction is very fast and has toe cooled to control the rate of reduction in many cases. More-ver, it is highly toxic and synthesis of larger nanoparticles has beenound to be difficult [27]. On the other hand, when a mild reduc-ng agent like sodium citrate or ascorbic acid is used, the reductioneaction is slow and has to be carried out at elevated temperatureso enhance the rate of reduction. In addition, they usually yieldelatively larger nanoparticles of varying size and shape [28]. Inontrast, hexamine is non-toxic, easy to handle, and yields spher-cal nanoparticles with narrow size distribution at a moderatelyast rate. Since all materials used in this method are environmentriendly, this may be regarded as a green approach for nanoparticleynthesis. In addition, this method is simple, fast and economic. Weave investigated the catalytic utility of the starch stabilized silveranoparticles (AgNP-starch) in the dye degradation reactions byaking the reduction reactions of methyl orange and rhodamine By NaBH4 as model reactions.

. Materials and methods

.1. Materials

Silver nitrate (AgNO3), hexamine, methyl orange, rhodamine and sodium borohydride (NaBH4) were purchased from Merck

ndia Ltd and used without further purification. All aqueous solu-ions were made by using double distilled water.

.2. Methods

.2.1. Synthesis of silver nanoparticles (AgNPs)To 90 mL of aqueous solution containing 0.1 g starch which was

aken in a 250 mL beaker, 10 mL 0.05 M AgNO3 solution was addedn a drop wise manner and was stirred for 15 min. Followed by this,.028 g (0.002 M) hexamine was added and the reaction mixtureas subjected to microwave irradiation for 4 min by placing in aomestic microwave oven (Sharp R-219T (W)) operating at a powerf 800 W and frequency 2450 MHz. The nanoparticle formation wasonitored using UV–vis spectrophotometer by scrutinizing the

eaction mixture after 1, 2, 3 and 4 min of microwave irradiationn the scan range 300–700 nm. Upon microwave irradiation, theolour of the reaction medium changed into yellowish-brown dueo nanoparticle formation.

.2.2. Catalytic reduction of methyl orangeThe catalytic reduction of methyl orange by NaBH4 was stud-

ed as follows. To 2 mL of aqueous methyl orange solution

0.01 × 10−2 M) taken in a quartz cell of 1 cm path length, 0.5 mLecently prepared NaBH4 solution (0.06 M) was added. Then 0.5 mLf AgNP-starch colloidal solution of a definite concentration wasdded to start the reaction. The variation in the concentration of

d Engineering B 195 (2015) 90–97 91

methyl orange with time was monitored using UV–vis spectropho-tometry by following the change in the absorbance of the peak at464 nm. The absorption spectra were recorded in 1 min intervals inthe range of 200–600 nm at ambient temperature.

2.2.3. Catalytic reduction of rhodamine BTo study this reaction, 0.5 mL freshly prepared NaBH4 solution

(0.06 M) was added to 2 mL of rhodamine B solution (0.06 × 10−3 M)contained in a quartz cuvette. Subsequently, 0.5 mL of nanocatalystsolution of a definite concentration was added and UV–vis spectrawere recorded every 1 min in the range of 300–700 nm. The kineticsof the reaction was studied by measuring the change in intensityof the peak at 554 nm with time.

2.2.4. CharacterizationUV–vis spectroscopic studies were carried out on a Shimadzu

UV-2450 spectrophotometer. FTIR spectra were recorded usingPerkin Elmer-400 spectrometer with ATR facility. The sample forXRD measurement was prepared by depositing a thin film of thesample on a microscopic glass slide and the diffraction patternwas recorded on a PANalytic X’PERT-PRO X-ray spectrometer. Highresolution-transmission electron microscopic (HR-TEM) measure-ments were done using a JEOL JEM-2100 microscope.

3. Results and discussion

3.1. Synthesis and UV–vis spectroscopic investigation of silvernanoparticles

UV–vis spectrophotometric analysis is used to follow andconfirm the formation of starch stabilized silver nanoparticles(AgNP-starch). The reduction of Ag+ ions into Ag nanoparticleswas monitored by recording the absorption spectrum of the reac-tion mixture with time in the range of 300–700 nm. The spectraobtained during the synthesis AgNP-starch are depicted in Fig. 1. Itis well known that silver nanoparticles show an absorption band inthe range of 350–450 nm due to surface plasmon vibrations of con-ducting electrons [29]. As is evident, initially no band was observedin the range 350–450 nm. But after 2 min of microwave heating,the colourless solution began to change into yellowish brown anda small absorption band appeared around 409 nm suggesting theformation of silver colloids. The intensity of this absorption band

Fig. 1. UV–vis absorption spectra recorded at 1 min interval during the course ofmicrowave-assisted AgNP-starch synthesis.

92 S. Joseph, B. Mathew / Materials Science and Engineering B 195 (2015) 90–97

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spectrum of starch and starch stabilized silver nanoparticles were

Fig. 2. Schematic representation of the synth

ntense band suggests the formation of monodispersed and spher-cal nanoparticles and the absence of any peak in the range of50–700 nm indicates the lack of nanoparticle aggregation [30,31].he stability of the synthesized AgNP-starch was confirmed by tak-ng its UV–vis absorption spectrum after storage for several months.he absorption spectrum did not show any appreciable change inhe shape as well as wavelength of the SPR band. The SPR bands sensitive to size and morphology of the nanoparticles [31]. Thislearly rules out the possibility of aggregation of the nanoparticlespon storage and hence indicates the stability of the starch cappedilver nanoparticles.

The polymer chain of starch is rich in hydroxyl groups. So theyan easily attract the Ag+ ions towards it through electrostatic inter-ction and play a key role in the reduction process. The silver atomsroduced as a result of reduction by hexamine act as nucleationentres and subsequently the Ag atoms coalesce leading to the for-ation of nanosized metal clusters. Once silver nanoparticles could

ave grown, the OH groups behave as a docile contact for the sil-er nanoparticles and thus protect them from further coalescenceFig. 2). The reaction mixture is warmed up uniformly and instan-aneously during microwave heating which helps in preventing theggregation of the particles. Even though silver is a noble metal, thisethod of nano synthesis seems to be economic. This is because

ll other chemicals used in this method are inexpensive and themount of AgNO3 used is very small which is almost quantitativelyonverted to AgNP.

To establish the role of hexamine as reducing agent in thisynthesis process, a control experiment was performed under theame conditions without using hexamine. Only a faint change wasbserved in the colour of the reaction mixture even after irradi-tion for 4 min. The UV–vis spectra obtained at 1 min intervals inhe control experiment are shown in Fig. 3. As is evident from thegure, we can see only a slight change in the absorbance with time.

hus it follows that starch by itself is unable to reduce Ag+ ions tog by an appreciable extent.

ig. 3. UV–vis absorption spectra recorded at 1 min intervals during the microwaveynthesis of AgNP-starch without using hexamine.

f AgNP-starch under microwave irradiation.

To study the role of starch in the synthesis of stable nanopar-ticles, a control experiment was also conducted under the sameconditions in the absence of starch. Upon microwave irradiation,instead of a yellowish brown coloured solution, a greyish whiteresidue was obtained. Furthermore, the UV–vis spectrum of theresulting solution did not show any absorption in the 300–700 nmregion. Hence it follows that, the silver nanoparticles are not stablein the absence of starch. Hexamine acts only as a reducing agentand is not able to stabilize the nanoparticles. Silver nanoparticlesbecause of their highly reactive nature, coalesce into a residue inthe absence of starch. This clearly demonstrates the role of starchas a capping agent in the synthetic process.

The concentration of silver nitrate used in the feed mixture forthe synthesis of AgNP has largely affected the outcome of nanopar-ticle synthesis. The intensity of the SPR band as well as the size ofsilver nanoparticles were found to increase with increase in AgNO3concentration in the reaction mixture. This may be explained onthe basis of the fact that, as the concentration of AgNO3 in thereaction medium increases, the number of Ag+ ions available forreduction by hexamine also increases. This enhances the chance ofcollision between the silver atoms which in turn affects the growthprocess [32]. As a result, the size of silver nanoparticles increaseswith increase in AgNO3 concentration. It is also possible to synthe-size silver nanoparticles by heating the reaction mixture thermallyinstead of using microwave irradiation. But the synthesis processwas found to be time consuming and yielded larger nanoparticleswith broader size distribution.

3.2. FTIR analysis

Further evidence for the formation and stabilization of sil-ver nanoparticles is obtained from FTIR measurements. The FTIR

recorded in order to identify the functional groups of the biopoly-mer involved in the synthesis and stabilization of nanoparticles.Fig. 4 shows the FTIR spectrum of starch and AgNP-starch. The

Fig. 4. FTIR spectra of (a) starch, and (b) AgNP-starch.

nce and Engineering B 195 (2015) 90–97 93

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S. Joseph, B. Mathew / Materials Scie

ajor peaks present in the spectrum of starch were observedt 3290, 2918, 1350 and 996 cm−1 respectively. The broad peakbserved at about 3290 cm−1 corresponds to O H stretching vibra-ions of hydroxyl group. The peak at 2918 cm−1 could be assignedo stretching vibrations of aliphatic C H groups. The peaks at 1350nd 996 cm−1 can be attributed to C O and C O C stretchingibrations respectively. The spectrum of AgNP-starch showed thebove mentioned peaks at 3300, 2927, 1368, and 1004 cm−1 respec-ively.

Compared with starch, the vibration peaks of AgNP-starch arehifted to higher wave numbers. This observation clearly confirmshe successful capping of silver nanoparticles by starch. More-ver, a new peak was observed at 1740 cm−1 in the spectrum ofgNP-starch. This peak is assigned to C O stretching vibrations ofarbonyl groups which are formed from the oxidation of hydroxylroups. This shows the involvement of O H groups of starch alongith hexamine in the reduction of Ag+ ions into Ag atoms. Further

vidence for the effective capping of the silver nanoparticles by thetarch molecules is obtained from the HR-TEM images (Fig. 6).

.3. XRD study

X-ray diffraction studies were conducted to obtain informationbout the crystalline nature of nanoparticles. The XRD pattern ofgNP-starch (Fig. 5) clearly shows four diffraction peaks at 38.30◦,4.47◦, 64.86◦, and 77.98◦ which can be indexed respectively to the1 1 1), (2 0 0), (2 2 0), and (3 1 1) crystallographic planes of fcc silveranoparticles. The intensity of the peak due to the reflection from1 1 1) Bragg plane is higher than that of other planes due to thereferential adsorption of Ag atoms on (1 1 1) plane during crys-

al growth. This diffraction pattern is in good agreement with thetandard diffraction pattern of JCPDS file no. 04-0783. Thus X-rayiffraction studies undoubtedly demonstrate the crystalline naturef the synthesized silver nanoparticles.

ig. 6. (a–c) TEM images of AgNP-starch at different magnifications, (d) the correspondinlectron diffraction (SAED) pattern.

Fig. 5. XRD pattern of AgNP-starch.

3.4. High resolution-transmission electron microscopy (HR-TEM)studies

The size and morphology of the synthesized silver nanoparti-cles were examined using transmission electron microscopy (TEM)analysis. The typical TEM images of AgNP-starch are given in Fig. 6.The images show that the nanoparticles synthesized using starchas stabilizing agent are almost spherical and of nearly uniform size.The histogram representing the size distribution of the particles(Fig. 6(d)) obtained by analyzing the images support this observa-tion. The average size of the particles is found to be 18.2 ± 0.97 nmand the size of the nanoparticles varies between 16 and 21 nm.

Almost 75% of the nanoparticles have size between 17 and 19 nm,19% in the range of 19–20 nm and a very small percentage of thenanoparticles have size above and below the range of 17–20 nm.The representative high resolution TEM (HR-TEM) image (Fig. 6(e))

g particle size histogram, (e) high resolution TEM image, and (f) the selected area

94 S. Joseph, B. Mathew / Materials Science and Engineering B 195 (2015) 90–97

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Fig. 7. Chemical structure of methyl orange.

ith clear lattice fringes indicates the high crystallinity of nanopar-icles. The spacing between the lattice planes was measured as.34 A. This distance is in good agreement with the lattice spacingf (1 1 1) planes of fcc silver. The selected area electron diffractionSAED) pattern (Fig. 6(f)) once again confirms the crystalline naturef the nanoparticles. The bright circular spots observed in the elec-ron diffraction pattern corresponds to (1 1 1), (2 0 0), (2 2 0), and3 1 1) reflection planes.

.5. Catalytic degradation of methyl orange

The reduction of organic dyes by excess of NaBH4 is widely usedor evaluating the catalytic efficiency of metal nanoparticles pro-ided their reduction reaction is thermodynamically favourable butot kinetically and their major spectral bands do not overlap withhe SPR band of metal nanoparticles [33]. Methyl orange (MO) isn organic azo dye which is commonly used as an indicator in vari-us analytical fields (Fig. 7). Since methyl orange generates severalnvironmental and health problems, its degradation and removals of very interest.

The aqueous solution of methyl orange is orange red in colour.he UV–vis spectrum of aqueous solution shows strong absorptionst 464 nm and 264 nm. The �max at 464 nm is due to the absorptionf N N group. Since the �max of methyl orange is well separatedrom the surface plasmon absorption of silver nanoparticles, thisatalytic reaction can be easily followed spectrophotometrically.

The reduction of methyl orange by NaBH4 in the absence ofgNP-starch catalyst is insignificantly slow. This is evident from thebservation that the intensity of �max at 464 nm remains virtuallynchanged for several hours when a blank experiment is per-ormed without AgNP catalyst. Thus this reaction is not kineticallyavourable in the absence of the catalyst and could not be realizednly with NaBH4. But the degradation reaction started immediatelypon the addition of the catalyst. This is apparent from the fadingf the orange colour of the reaction medium as well as the decreasen intensity of the peak at 464 nm. The reduction of methyl orangey NaBH4 results in the reduction of azo group ( N N ) of methylrange thereby yielding the corresponding amino compounds. The

inetics of AgNP-starch catalyzed degradation of methyl orangey NaBH4 was studied spectrophotometrically by monitoring thehange in intensity of the absorption peak at 464 nm. Fig. 8 showshe UV–vis spectra recorded at 1 min intervals for the reduction

ig. 8. UV–vis absorption spectra for the reduction of methyl orange by NaBH4

atalyzed by 0.015 mg/mL AgNP-starch at 24 ◦C recorded at 1 min intervals.

Fig. 9. Schematic diagram showing the catalytic degradation of organic dyes byAgNP-starch.

of MO catalyzed by AgNP-starch of concentration 0.015 mg/mL at24 ◦C.

As soon as the catalyst was added, the absorbance at 464 nmdecreased continuously with time. This was accompanied by theconcomitant appearance and growth of a new peak at 250 nm.This peak appeared during the course of the reaction is attributedto the NH2 group absorptions of the product molecules. Withthe decrease in intensity at 464 nm, the intensity of the productpeak was found to increase with passage of time. The reaction wascompleted in 10 min as was evident from almost zero absorptionat 464 nm. After the completion of the reaction, a weak absorp-tion band was observed at 398 nm. This is believed to be the SPRband of nano silver catalyst. At intermediate stages of the reac-tion, this weak band is obscure as this is covered by the strongabsorption band of methyl orange which extends over the rangeof 350–600 nm. But during the catalytic process, the SPR band ofAgNP undergoes a blue shift from 409 nm to 398 nm. This providesa clear evidence for the electron relay process occurring during thereduction reaction. Electron transfer play the crucial role in thedegradation of methyl orange. The large difference in redox poten-tial between the donor borohydride ion and the acceptor methylorange hinders the electron transfer between them [34,35]. Thusthe reduction of methyl orange by NaBH4 in the absence of thenanocatalyst is thermodynamically favourable and not kinetically.AgNP catalyst provides an alternative path of low activation energyfor the reaction and hence reduces the kinetic barrier thereby mak-ing it thermodynamically as well as kinetically favourable [36].Upon addition of the catalyst, the borohydride ions which donatesthe electrons and methyl orange which accepts the electrons getsadsorbed on the surface of the nanocatalyst. The starch moleculeswhich cap the nanoparticles help in bringing the reactants closerto the catalyst surface. The reduction occurs by the transfer of elec-trons from borohydride ions to methyl orange. Silver nanoparticleshelp this electron shuttling process by relaying electrons from thedonor to the acceptor (Fig. 9). The blue shift observed in the positionof the SPR band of AgNP is attributed to the surface modificationthat occurs to the nanoparticles due to the electron relay process[37] and also to the interaction between the absorption peak ofmethyl orange and SPR band of AgNP which overlap one another.

In this reaction, the concentration of NaBH4 was adjusted toexceed largely that of methyl orange and the catalyst. As the con-centration of NaBH4 is very high, it remains practically constantduring the reaction. Therefore pseudo-first order kinetics with

respect to methyl orange could be used in this case to evaluate thekinetics of the reaction. The reaction kinetics can be representedusing the equation ln [A]/[A0] = −kt, where k is pseudo-first orderrate constant, t is the reaction time, [A0] is the concentration of

S. Joseph, B. Mathew / Materials Science and Engineering B 195 (2015) 90–97 95

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Fig. 11. Plot of rate constant (k) against concentration of catalyst for the reductionof methyl orange at 24 ◦C.

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ig. 10. Plots of ln [A] against time for the reduction of methyl orange usingarying concentrations of AgNP-starch at 24 ◦C. Conditions: [MO] = 0.01 × 10−2 M,NaBH4] = 0.06 M.

ethyl orange at time t = 0 and [A] is the concentration at time ‘t’hich can be obtained from the absorbance of the peak at 464 nm.

o study the effect of nanocatalyst concentration upon rate of theeaction, the degradation reaction was carried out using varyingoncentration of AgNP-starch, keeping other parameters constant.

The relationship between ln [A] and time for the reduction ofethyl orange using different concentration of the nanocatalyst

re given in Fig. 10. A linear relationship was obtained in all casesonfirming the pseudo-first order nature of the reaction. The firstrder rate constants can be evaluated directly from the slope ofhese plots. A small induction time was observed for all the reac-ions studied when carried out under air and this was found toecrease with increase in amount of the catalyst. The inductionime may be caused by the reduction of O2 present in the reac-ion medium which takes place faster than the reduction of methylrange and also may be the time required for the NaBH4 moleculeso eliminate oxides from the catalyst surface and thus to activatehe catalyst [37–40].

The first order rate constant obtained from the slope of ln [A]ersus time plot for different catalysts and the corresponding corre-ation coefficients are given in Table 1. It is clear from the table that,ll the reactions are very fast. They strictly adhere to pseudo-firstrder kinetics as is evident from the values of correlation coeffi-ient.

With increase in the concentration of silver nanoparticle cata-yst in the reaction system, a rapid increase in rate and hence rateonstant of the reactions was observed (Fig. 11). This is attributedo the fact that as the concentration of AgNP in the reaction mediumncreases, the catalyst surface available for adsorption of the reac-ants also increases. This in turn enhances the speed of degradationrocess.

.6. Catalytic degradation of rhodamine B

The catalytic performance of AgNPs was also evaluated usinghe reduction reaction of rhodamine B by NaBH4. Rhodamine BRhB) is a fluorescent dye belonging to the family of xanthenes

able 1seudo-first order rate constants for the reduction of methyl orange catalyzed by AgNP-s

Conc. of AgNP (mg/mL) Reaction time (min)

0.015 10

0.025 08

0.035 06

0.050 04

Fig. 12. Chemical structure of rhodamine B.

(Fig. 12). Rhodamine dyes are widely used as fluorescent probesowing to their high absorption coefficient and broad fluorescence inthe visible region of electromagnetic spectrum. Due to their excel-lent photostability and photophysical properties, they are used aslaser dyes and for imaging in living cells [41]. Moreover, it is widelyused in textile industry and forms an important dye pollutant.

This reaction is selected for catalytic study because the redoxpotential of Ag nanoparticles is in between those of RhB (−0.48 V)and NaBH4 (−1.33 V). So AgNPs can act as electron transfer agentsand relay electrons from the donor NaBH4 to the acceptor RhBdye. The aqueous solution of rhodamine B is pink red in colour.The UV–vis absorption spectrum of rhodamine B exhibits strongabsorptions in the range of 200–700 nm with �max at 554 nm(Fig. 13). Thus the �max of RhB is well separated from the SPRoscillations of AgNP-starch and this makes the kinetic study easy.

The degradation reaction of RhB can be kinetically followed byexamining the absorbance value of the peak at 554 nm as a functionof time. The UV–vis absorption spectra recorded during the reduc-tion reaction of RhB by NaBH4 in presence of AgNP-starch catalystof concentration 0.04 mg/mL at 24 ◦C is given in Fig. 14. The succes-

sive spectra were measured at an interval of 1 min. The reaction wasvery insignificant in the absence of AgNP catalyst. But the reductionof RhB started immediately upon the addition of the catalyst as evi-denced by the gradual disappearance of the pink red colour of RhB.

tarch.

k × 10−3 (s−1) Correlation coefficient (R2)

2.22 0.99754.04 0.99716.22 0.99348.28 0.9886

96 S. Joseph, B. Mathew / Materials Science and Engineering B 195 (2015) 90–97

TiatootIwd

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Fig. 13. UV–vis absorption spectrum of rhodamine B.

he reactions was completed in 9 min. The peak observed at 406 nms characteristic of the surface plasmon oscillations of Ag nanocat-lyst. This peak has undergone a blue shift in its position from 409o 406 nm. But this shift is less than that observed in case of methylrange degradation. This may be attributed to the fact that the �max

f RhB is detached from the SPR absorption of AgNP catalyst andhis reduces the possibility of interaction between these two peaks.nitially, a small induction time was observed in this case too. This

as found to decrease with increase in temperature and catalystosage.

For the evaluation of the catalytic rate, pseudo-first order kinet-cs with respect to the concentration of RhB is assumed in this caselso since the concentration of NaBH4 used essentially exceeds thatf RhB. The pseudo-first order plot obtained for the above reactiony measuring the absorbance value at 554 nm as a function of time

s depicted in Fig. 15.A linear plot with correlation coefficient (R2) value of 0.9942 was

btained and the value of pseudo-first order rate constant obtainedrom the slope of the above plot was found to be 2.99 × 10−3 s−1.he mechanism of this reaction also involves an electron transferrocess. With respect to AgNP, BH4

− ion is nucleophilic and RhB islectrophilic in nature. Right after the adsorption of both BH4

− ionsnd RhB on the catalyst surface, the AgNPs transfer electrons fromhe donor BH4

− ions to the acceptor RhB molecules. As a result, theink red coloured rhodamine B is reduced to leuco rhodamine.

In order to study the reusability of the AgNP catalyst in theye degradation reactions, the nanocatalyst was recovered fromhe reaction mixture at the end of the reaction by centrifugation

nd was dispersed in double distilled water to remove any speciesdhering to their surface. The rate of degradation of both methylrange and rhodamine B was found to be more or less the sameor the first and second cycle but decreased from the third cycle

ig. 14. Successive UV–vis spectra measured at 1 min intervals during the catalyticegradation of rhodamine B using 0.04 mg/mL AgNP-starch at 24 ◦C.

[[[[[

Fig. 15. Plot of ln [A] versus time for the NaBH4 reduction of rhodamine B using0.04 mg/mL AgNP-starch at 24 ◦C.

onwards. This observation is attributed to the deactivation of theactive reaction sites on the catalyst surface due to repeated adsorp-tion of the reactants [39].

4. Conclusions

In the present study, we have reported a simple, one pot, andeconomic method for the green synthesis of silver nanoparticles inaqueous medium. Microwave assisted synthesis using hexamineas the reducing agent and starch as the stabilizer appears to be aneffective method for the large scale production of uniform sizedsilver nanoparticles. The nanoparticles were found to be stablein aqueous medium under ambient conditions even after storagefor several months. The nanoparticles were characterized usingUV–vis, FTIR, XRD, and HR-TEM analysis. This new method fornanosynthesis produced well dispersed and highly crystalline sil-ver nanoparticles with an average diameter of 18.2 ± 0.97 nm. Thecatalytic ability of the synthesized nanoparticles was investigatedusing the degradation reactions of methyl orange and rhodamineB by NaBH4. The catalyst showed outstanding catalytic activityin these dye degradation reactions. The reactions were very fastand followed pseudo-first order kinetics. Thus this new syntheticmethod affords a very efficient route for the generation of highlystable spherical silver nanoparticles which find application as effec-tive catalyst for the degradation of organic dyes from variousindustrial effluents.

Acknowledgment

The financial assistance provided by UGC (under FIP Scheme),Government of India is gratefully acknowledged.

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