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Journal of Luminescence 128 (2008) 1635–1640

www.elsevier.com/locate/jlumin

Photoluminescence from colloidal silver nanoparticles

Aiping Zhanga,�, Jinzhi Zhanga, Yan Fangb

aCollege of Sciences, Experimental Center of Basic Sciences, North China University of Technology, Beijing 100144, PR ChinabBeijing Key Lab for Nano-photonics and Nano-structure, Capital Normal University, Beijing 100037, PR China

Received 3 August 2007; received in revised form 10 March 2008; accepted 13 March 2008

Available online 25 March 2008

Abstract

Highly luminescent singles of Ag sol with gradual changes were detected when adjusting the granularity and concentration of particles.

It can be deduced that these Ag sols, composed of a large amount of silver nanoparticles and clusters, may have their surface energy

bands alterable, which might be caused by the interactions between particles. A model that describes the shift of energy band is proposed,

and it can be understood as the hybridization of elementary plasmons when interactions occur between particles. Besides, both

hybridization and absorption-rescattering mechanisms were proposed to explain the changeable phenomena of photoluminescence with

different concentrations.

r 2008 Elsevier B.V. All rights reserved.

Keywords: Photoluminescence; Silver nanoparticles; Hybridization

1. Introduction

The physical properties of noble metal nanoparticleshave been widely studied for several years, in parallel to theefforts that have been devoted for the understanding oftheir elaboration and their nature [1–5]. Besides theextensive use for biological labeling [6–8] and their abilityto promote surface enhanced spectral phenomena [9–11], itwas discovered that noble metal nanoparticles displayintense light emission and it is expected to yield newinsights into the practical applications in optical devicesand in biology. In the case of silver, it was reported thatneutral nanoparticles emit light in rare gas matrix atcryogenic temperature under photoactivation or electro-activation, and this photoluminescence was attributed to spto sp-like transitions, analogous to intraband transitions inbulk silver [12–14]. Besides, some observations had beenattributed to photoreduction processes at the interfacebetween metallic silver and silver oxide [15,16], and otherinvestigations carried out in solid-state materials assignedthe luminescence to be from charged silver congeries[17,18]. However, the exact mechanism about how the

e front matter r 2008 Elsevier B.V. All rights reserved.

min.2008.03.014

ing author. Tel./fax: +86 10 88803271.

ess: ncutalex@126.com (A. Zhang).

influencing factors of photoluminescence work are stillunclear up to now, which always makes the repetition andexplanation of some spectral experiments difficult.Luminescence from noble metal nanoparticles from

experiments has received only a limited amount ofattention, because most of the emission processes alwayshave very low efficiency [19,20] and have not proved to beimportant technologically nowadays [21–23]. In a simplemodel of a semiconductor, a transition across the energygap between the valence band and the conduction band willleave too much energy for a phonon to dissipate, and thusit will be radiative if it is allowed. In noble metals, theconventional point on the rarity of luminescence is alsoquite simple: noble metals do not have band gaps, and thenonradiative decay can proceed all the way back down tothe ground state, making luminescence exceedingly im-probable [22,23]. Complementary to bulk metal, nanopar-ticles have large surface, are a more complex system, andtheir luminescence often reflects the interaction that affectselectron–hole recombination [24–26]. In most cases, theinvolvement of silver clusters in the vicinity of the surfacein the luminescence process is hypothesized to be im-portant. But it is still unclear what mechanisms andstructural or environmental factors are responsible for theemission signals, though these must be considered very

ARTICLE IN PRESSA. Zhang et al. / Journal of Luminescence 128 (2008) 1635–16401636

important to understand the properties of noble metalparticles.

This study discusses some of these problems byinvestigating the photoluminescence of Ag sol with aregulated change of their granularity and concentration. Ofparticular interest is the interaction between nanoparticles,for which the spectral changes are interpreted in terms ofhybridization on the surface that leads to the formation ofnew surface energy bands. Besides, the changes ofintensities and emission centers with their concentrationsreduced step-down proved that two emission mechanismsmay exist in Ag sol when considering photoluminescence.

Hybrid energy band SIBE1

2. Experimental

2.1. Materials

Silver nitrate (AgNO3, A.R.) and sodium citrate(Na3C6H5O7, A.R.) from Beijing Chemical Company wereused as received without further purification. All the otherchemicals used in the experiments were analytical gradereagents, and deionized water was used for solutionpreparation.

Primaryenergy band

Primaryenergy band

Hybrid energy band SIBE2

Fig. 2. An energy-level diagram describing the hybridization of surface

energy bands resulting from the interaction between primary energy

bands. The newly formed energy bands are called superamolecular

interface energy bands (SIBE1, SIBE2).

2.2. Preparation of Ag sols

The Ag sol was prepared according to a modifiedprocedure developed by Lee and Meisel [27]. A 90mgAgNO3 sample was dissolved in 500mL deionized waterand the solution was heated to boiling. In all, 9mL of a 1%solution of Na3C6H5O7 was added drop by drop to theboiling solution with vigorous stirring, and then thesolution was held at boiling for a further 10min with

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nsity

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nsity

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.)

Wav300 400 500

Fig. 1. The emission (lex ¼ 220 nm) and absorption (i

constant stirring. Finally, a gray Ag sol was obtained,which was found to be stable for several weeks. Centri-fugal separation of Ag sol was done by applying aTDL-50B centrifugal separator. After the centrifugalseparation at 1800 r/m for 10min, the solution wasseparated into five average layers from top to bottomartificially.

2.3. Apparatus and measurements

The transmission electron micrograph (TEM) imageswere taken with a H-600 TEM detector (Hitachi) afterplacing several drops of Ag sol on Ni–Cu grid. Thephotoluminescence was recorded on a Fluorolog-3 spectro-meter (Jobin Yvon) and UV–vis absorption on a UV-2401PC (Shimadzu) UV–visible spectrometer. All these

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Wavelength/nm

elength/nm600 700 800 900 1000

600 800

nset figure) spectra of fresh-prepared gray Ag sol.

ARTICLE IN PRESSA. Zhang et al. / Journal of Luminescence 128 (2008) 1635–1640 1637

spectral measurements were carried out at room tempera-ture on a spectrophotometric 1� 1 cm quartz curette cell,and the emission spectra were received with the excitedwavelength fixed at 220 nm, recorded from 220 to 900 nm.

Hybrid states SIBE2

increase of the number of interacting particles

energy

Hybrid states SIBE1

Fig. 3. The changes of hybrid energy bands with the increase in the

number of particles.

Fig. 4. The TEM images of silvernanoparticles of different layers after centrifug

bottom after being centrifugalized at 1800 r/m for 10min.

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Fig. 5. The emission and absorption (inset figure) spectra of different layers of

separately from top to bottom after being centrifugalized at 1800 r/m for 10m

3. Results and discussion

Gray Ag sol, prepared by a classical chemical deoxidiza-tion method, showed a characteristic plasmon resonanceabsorption band centered at 410 nm (as can be seen frominset of Fig. 1) and consisted of mostly globularitynanoparticles, in diameters of 20–80 nm from TEM.Fig. 1 presented the photoluminescence of fresh-preparedgray Ag sol, which exhibited a sharp and strong peak near330 nm and a broadened band between 500–700 nm.It is known that a nanoparticle, whose diameter is much

smaller than the wavelength of light, will respond as adipole in an optical field. With this dipole limit, theabsorption and emission of nanoparticles should primarilycoherent with their surface excited energy bands or thesurface active sites [28,29]. Several mechanisms have beenproposed to explain the luminescence of metallic nanopar-ticles, most of which indicate the photoelectron at thesurface energy states absorbs light at its plasmon resonancefrequency strongly, then part of the absorption energytransfers into heat energy (for example, vibration collision)

alization. (a)–(e) represent the first to the fifth layer separately from top to

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elength/nm600 700 800 900

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Ag sol after centrifugalization. (a)–(e) represent the first to the fifth layer

in.

ARTICLE IN PRESSA. Zhang et al. / Journal of Luminescence 128 (2008) 1635–16401638

and part radiates in light near the wavelength of absorptionwith its emission intensity greatly owing to this process ofabsorption rescattering [9,20–22]. However, the resultsfrom our experiments, as can be seen from Fig. 1, showed adifferent phenomenon: the emission near the resonanceabsorption wavelength (410 nm) is so weak that it can betotally neglected in whole photoluminescence spectrum;meanwhile, another two strong bands staying apart at bothsides of 410 nm were observed. The strong emission bandthat approximately occurred at 330 nm should be assignedto neither ligand-to-metal charge transfer nor metal-to-ligand charge transfer and can probably be assigned to theAg–Ag interactions, which have been found by otherauthors [5,30–33]. Therefore, a viewpoint of hybridizationwas deduced to explain the unusual photoluminescence inour case.

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nsity

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Fig. 6. The emission spectra of different concentrations of Ag sols. From (A) t

adding into isopyknic deionized water.

There are strong molecular forces (including induction,dispersion and tropism forces) between metallic nanopar-ticles [32,33]. By means of those forces, the hybrid energyband of nanoparticles may alternate from primary energybands to form new superamolecular interface energy bands(SIEBs) with one turned to a lower energy and the other toa higher one, as can be depicted in Fig. 2. The shift ofplasmon energy bands will depend on the coupling strengthand the energy gap between primary ones. Besides,considering the photoelectrons for formation of SIEBsare not in ground state only, but follow the Boltzmanndistribution law, the SIEBs will become broader andbroader with the increase of interactions caused by theconcentration rais of nanoparticles, which is schematicallydepicted in Fig. 3. From the hybridization assumption, itwas described that the primary plasmons of metallic

550 660

velength/nm0 600 700 800 900 1000

o (I), the concentration was gradually reduced to half of its near former by

ARTICLE IN PRESSA. Zhang et al. / Journal of Luminescence 128 (2008) 1635–1640 1639

nanoparticles will be changed by the interactions betweenthese free ones, leading to mixing, splitting, and shifts ofthe plasmon energies, and this hybridization assumptioncan be used to describe the variations of the surface energybands of metallic nanoparticles sensitive to their dispersi-bility and granularity.

The size of silver nanoparticles after the centrifugalseparation procedure was measured according to thestatistical analysis of a large number (100–150) of particlesfrom TEM (as shown in Fig. 4). The average diameters ofparticles were measured as 35, 47, 52, 68 and 76 nm,respectively, corresponding to the first to the fifth layerfrom top to bottom. The UV–vis absorption spectra (insetof Fig. 5) of these five sols showed a gradual red shift of themaximum absorption wavelength with the increasing ofgrain size, which agrees with the classical plasmonresonance absorption theory of metallic colloidal particles.The ratio of surface atoms can increase along with thedecrease of particle diameter [34], and the interactionsbetween particles may become stronger, leading to astronger hybridization effect and a bigger energy gapbetween SIEBs. As a result, two emission bands assigned toSIEBs may have a synchronous shift to red and blueindividually, which can be considered to both a predictionand a good explanation for our experiments. As seen fromFig. 5, with the decrease of grain size, the 330 nm emissionbands had a little blue-shift with its intensity increased,and, at the same time, the 520 nm band shifted to the long-wavelength with its intensity decreased gradually.

Fig. 6 showed the photoluminescence of gray Ag sol withits concentration decreased step down by adding deionizedwater. It is clear that two bands centered at 330 and 550 nmwere observed separately with different profile andintensity at high concentration (Fig. 6A, B). When theconcentration of sol decreased gradually, these two bandsshifted closer and closer and finally combined to be a newband centered near 410 nm (Fig. 6E), and all thesubsequent emission spectra presented a fixed center at410 nm with similar profile. In this whole process, theemission intensity increased sharply at first and then felldown step by step. These regular changes can be wellexplained in the presence of hybridization assumption, i.e.,the 330 and 550 nm bands can be assigned to the SIEBs andtheir intensities may be intensively influenced by thedecrease of interactions between particles due to thedecrease of concentration, leading to the decrease ofemission intensity. In addition, the position of the newlyappeared emission band at 410 nm is much close to theplasmon resonance absorption of gray Ag sol, and thisband represented a similar decreased trend as the plasmonabsorption when the concentration was reduced; thus, this410 nm emission band was assumed to be from theabsorption-rescattering mechanism. Therefore, it indicatesthat two distinct emission mechanisms may be existing inthe whole concentration experiments and a strong hybri-dization effect may determine the photoluminescence of Agsol at high concentration, while at low concentration the

absorption-rescattering mechanism may be more appro-priate to explain the changes of photoluminescence.

4. Conclusion

In conclusion, highly photoactivated luminescences ofAg sol with changeable phenomena for different concen-tration and granularity were detected. A model describingthe changes of photoluminescence is proposed, which canbe understood as a hybridization assumption betweenprimary plasmon energy bands, and in the presence of thisassumption the spectral changes observed, including bothdifferent grain sizes and different concentrations, can bewell interpreted. Besides, the integrated use of the newlydeduced hybridization and the traditional absorption-rescattering mechanisms can afford a clear and self-consistent explanation when considering the spectralchanges of Ag sol with different concentrations.

Acknowledgments

The authors are grateful for the support of this researchby the National Natural Science Foundation of China andNatural Science Foundation of Beijing.

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