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Applied Surface Science 257 (2010) 1390–1394

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

nvestigations of CdS and Ag–CdS nanoparticles by X-ray photoelectronpectroscopy

riya Thakura, Satyawati S. Joshia,∗, K.R. Patil b

Department of Chemistry, University of Pune, Ganeshkhind, Pune 411007, Maharashtra, IndiaNational Chemical Laboratory, Pune 411008, India

r t i c l e i n f o

rticle history:eceived 17 April 2010eceived in revised form 4 August 2010

a b s t r a c t

In the present report, CdS and Ag–CdS nanoparticles were synthesized using cysteine as a capping agent.Surface properties CdS and Ag–CdS nanoparticles were studied by X-ray photoelectron spectroscopy(XPS). XPS study of CdS nanoparticles was carried out as a function of pH and for a refluxed sample at pH

ccepted 8 August 2010vailable online 13 August 2010

eywords:dSg–CdS

11.2. Effect of dopant concentration on surface properties of Ag–CdS nanoparticles was also studied foras prepared samples as well as for annealed sample at 2% doping. Effect of pH, dopant concentration, andeffect of particle size on different sulfur species present in the system was studied. Features of Cd 3d, S2p and Ag 3d core level have been discussed in detail.

© 2010 Elsevier B.V. All rights reserved.

ysteinePS

. Introduction

Electronic and optical properties of semiconductor nanopar-icles are size dependant. When the size of the semiconductorpproaches exciton Bohr radius, its properties start to change asresult of quantum confinement [1–6]. The quantum size effect

rises due to dramatic reduction in a number of electrons [7]. Thisccurs mainly in low band gap materials where the effective massf electron is small. For small semiconductor nanocrystals, bothinear and nonlinear optical properties arise as a result of transi-ions between electron and hole quantum size levels. In additiono optically created electron and hole, there exists the Coulombnteraction, which strongly affects the nanocrystals optical spectra.pecifically, in small nanocrystals, the Coulomb energy becomes amall correction to the quantization energies of electrons and holes.n large nanocrystals, the Coulomb interaction is more importanthan the quantization energies of the electrons and holes.

CdS is wide gap semiconductor (Eg ≈ 2.42 eV) and has beenidely studied by many researchers. Most of the researchers have

tudied optical and structural properties of the CdS nanoparticless it exhibits pronounced quantum size effect [8]. Further, mod-fication of semiconductor surface with metal improves chargeeparation and promotes interfacial charge-transfer processes in

∗ Corresponding author. Tel.: +91 020 25601394x532/569;ax: +91 020 25691728.

E-mail addresses: ssjoshi@chem.unipune.ac.in,sjoshi@chem.unipune.ernet.in (S.S. Joshi).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.08.035

nanocomposite systems. This is a most convenient way to tailorthe properties of photocatalyst and tune the luminescence proper-ties. For the nanoparticles, surface properties become dominant asthe surface to volume ratio increases with decrease in particle size.Photoelectron spectroscopy is the powerful technique to study thesurface of the nanoparticles. It gives an idea about the chemicalshifts, absolute energy and bonding aspect of the material.

There are few reports on the X-ray photoemission studies of CdSnanoparticles [9a–e]. Nanda et al. [9a] and Winkler et al. [9b] havereported detailed investigations of differently sized CdS nanopar-ticles by XPS. In another report of Winkler et al., they have studiedannealing behavior of CdS nanoparticle surfaces [9c]. Nakanishiet al. [9d] have confirmed the layer-by-layer structure of self-assembly of composite thin film of CdS by XPS. Hota et al. [9e]have used the XPS method to characterize the elemental surfacecomposition of CdS–Ag2S core shell nanoparticles.

Recently, we have investigated pH dependent structural phasebehavior of cysteine capped CdS nanoparticles [10a]. Also, inanother study we have reported fluorescence properties of cysteinemediated Ag–CdS nanoparticles at pH 11.2 [10b]. In this paper, wereport further studies of CdS and Ag–CdS nanoparticles by X-rayphotoelectron spectroscopy (XPS). XPS study of CdS nanoparticleswas carried out as a function of pH and for a refluxed sample atpH 11.2. Effect of dopant concentration on surface properties of

Ag–CdS nanoparticles was also studied for as prepared samplesas well as annealed sample at 2% doping. Effect of pH, dopantconcentration, and also effect of particle size on different sulfurspecies present in the system was studied. Features of Cd 3d, S 2pand Ag 3d core level have been discussed in detail. XPS study of

ace Science 257 (2010) 1390–1394 1391

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ysteine capped CdS and Ag–CdS nanoparticles has not yet beeneported.

. Experimental

CdS and Ag–CdS nanoparticles were synthesized as per our ear-ier reports [10]. Typically, CdS and Ag–CdS nanoparticles wereynthesized as follows.

.1. Synthesis of CdS nanoparticles

CdS nanoparticles were synthesized using l-cysteine as a cap-ing agent. 2 mM cadmium perchlorate and 5 mM l-cysteine wereixed together with 50 mL of mill-Q water. The pH of the solu-

ion was adjusted with NaOH under vigorous stirring for effectiveinding of cadmium to capping molecule. This was followed by theddition of sodium sulphide with stirring in an inert atmosphere toake a final concentration of 1 mM. CdS nanoparticles synthesized

t pH 11.2 were refluxed for 24 h.

.2. Synthesis of Ag–CdS nanoparticles

Synthesis of l-cysteine capped CdS nanoparticles was carriedut at pH 11.2 as described in Section 2.1. Ag–CdS nanoparticlesere synthesized by mixing cysteine capped CdS nanoparticles and

ilver ions. Appropriate amount of AgClO4 solution with concen-ration of 10−2 M was added to the CdS solution under stirring. Theamples were prepared with 2 and 4 wt.% silver with respect toadmium.

All doped and undoped samples were brought into the powderorm by drop wise addition of a non-solvent 2-propanol and simul-aneous stirring. The sample prepared at 2% Ag was then annealedt 400 ◦C for 2 h.

Photoelectron spectra were recorded with a V. G. Microtechnit ESCA 3000 spectrometer equipped with AlK� X-ray sourceh� = 1486.6 eV) and a hemispherical electron analyser. The X-rayource was operated at 150 W. The residual pressure in the ion-umped analysis chamber was maintained below 1.0 × 10−9 Torruring data acquisition. The C 1s peak at a binding energy 284.6 eVas taken as an internal standard.

. Results and Discussion

XPS is very sensitive to chemical environment and chemicalomposition. XPS analysis of CdS nanoparticles synthesized at pH.4 and 11.2 and for sample refluxed for 24 h at pH 11.2 was car-ied out. XPS study of Ag–CdS nanoparticles was carried out at 2%nd 4% silver and after annealing at 2% Ag–CdS. XPS signals showedresence of S 2p, Cd 3d, Ag 3d as well as C 1s, O 1s, N 1s core levels.

The increase in particle size was observed with increase in pHrom 7.4 to 11.2. Further, after refluxing larger sized particles werebtained as compared to as prepared sample at pH 11.2. After dop-ng with silver, particle size was found to increase with increase inilver concentration and also for annealed sample [10]. Thus, differ-nt sized particles were formed at different experimental conditioniven below:

Particle size−→CdS nanoparticles : pH 7.4 < pH 11.2 < refluxed samAg–CdS nanoparticles : 2% Ag < 4% Ag < annealed sam

Fig. 1 shows Cd 3d level spectra of CdS nanoparticles. As could beeen in Fig. 1, Cd 3d3/2 and Cd 3d5/2 peaks were observed at 411.1nd 404.4 eV respectively with spin orbit separation of 6.7 eV ingreement with the values reported earlier [9a]. For the nanopar-icles synthesized at pH 7.4 (Fig. 1a), Cd 3d levels peaks are slightly

at pH 11.2at 2% Ag

Fig. 1. XPS spectra of Cd 3d core levels. (a) CdS nanoparticles at pH 7.4, (b) CdSnanoparticles at pH 11.2 and (c) CdS nanoparticles after refluxing for 24 h at pH11.2.

broadened as compared to samples (b) and (c). The peak broad-ening goes on decreasing with increase in size. Such broadeningwith size was observed previously by Nanda et al. [9a]. Further-more, a small hump observed at low binding energy side of Cd 3dspectra is attributed to N 1s coming from amino group of cappingmolecule and has binding energy between 399.2 and 400.3 eV. Fig. 2shows XPS spectra of Cd 3d levels for Ag doped CdS nanoparti-cles. As shown in Fig. 2b, at 4% Ag, Cd 3d spectrum have shifted by0.2 eV to lower binding energy. This is due to the increase in particlesize with dopant concentration. For 2% Ag–CdS after annealing at400 ◦C a shift by about 0.7 eV to lower binding energy was observed(Fig. 2c). This shift after annealing is in agreement with the previ-ous report by Winkler et al. [9c]. The shift observed is due to thesegregation process during annealing.

Fig. 3 shows XPS spectra of S 2p levels of CdS nanoparticles. Asobserved for Cd 3d level spectra, S 2p peak seems to be broadened,for the nanoparticles synthesized at pH 7.4 (Fig. 3a) and this broad-ening considerably decreases for the larger size nanoparticles. S 2ppeaks were observed at 162, 161.1 and 160.9 eV, for the samples(a), (b), and (c) respectively which are attributed to metal sulphide.S 2p peaks shift to the higher binding energy with decrease in parti-cle size. For the smaller size particles, contribution from thiol fromcapping molecule increases due to larger surface to volume ratiomaking possible shift in binding energy. Colvin and Alvisatos haveshown that, the shift in the binding energy with size, ascribed toboth quantum confinement and loss of dielectric solvation energyin smaller particles [11]. Peak fitting results for S 2p level allows

identification of different sulphur species. Spectra were best fitted

by four sets of Gaussian functions with binding energy ranging from160 to 164 eV at pH 7.4 (Fig. 3a). Fitting results are shown in Table 1.Thus, component 1 has been suggested for the sulfur in the bulk ofthe CdS nanoparticles (S–CdS) [9a and b], components 2 and 3 are

1392 P. Thakur et al. / Applied Surface Science 257 (2010) 1390–1394

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Table 1Fitting results of S 2p for CdS nanoparticles.

Sample Peak Position (eV) FWHM

pH 7.4 1 160.8 1.922 162.0 1.813 162.9 1.784 164.0 1.84

pH 11.2 1 160.8 1.972 161.8 1.673 162.9 1.56

ig. 2. XPS spectra of Cd 3d core levels of Ag doped CdS nanoparticles. (a) at 2% Ag,b) at 4% Ag and (c) 2% Ag–CdS nanoparticles after annealing at 400 ◦C.

ssigned to the surface sulfur and sulfur from capping moleculeespectively. The 4th component at binding energy 164.04 eV wasssigned to S–S bond formed between sulfur from capping moleculend surface S atoms [9b and f]. However, for both as prepared andefluxed samples prepared at pH 11.2 three components (compo-ents 1–3) were observed. No S–S bond (component 4) was found

or these samples. Instead, a peak at about 167.6 eV was observed atH 11.2 for as prepared sample, has been assigned to the oxidizedorm of sulphur (Fig. 3b) [9b,12,13]. However, this peak is absentn samples (a) and (c). This indicates that probable oxidation ofdS nanoparticles surface takes place at higher pH. As mentioned

ig. 3. Deconvolated XPS spectra of S 2p core levels. (a) CdS nanoparticles at pH 7.4,b) CdS nanoparticles at pH 11.2 and (c) CdS nanoparticles after refluxing for 24 h atH 11.2.

pH 11.2 after refluxing for 24 h 1 160.6 1.782 161.5 1.953 162.2 1.76

earlier particle size is higher at pH 11.2 than at pH 7.4. As per pre-vious reports by Winkler et al. [9b and c] they have observed thatfor larger sized particles, significant oxidation of nanoparticle sur-face takes place and no species corresponding to oxidized formof sulphur was found for smaller sized particles. Similarly, in thepresent case a peak at 167.6 eV corresponding to oxidized form ofsulphur was observed at pH 11.2 indicating probable oxidation ofCdS nanoparticle at the surface. In our earlier work, we have done IRstudies of cysteine capped CdS nanoparticles in detail to understandthe structure–spectral relationship induced as a function of pH andthe capping effect of l-cysteine on the phase behavior [10a]. Cys-teine is slightly decomposed due to enolization in presence of base.Therefore, at pH 7.4 coverage of the capping molecule is highercompared to pH 11.2. Consequently, at pH 11.2 capping capabil-ity is reduced and hence the sulphur atoms at the surface becomesusceptible to oxidation indicating the possibility of formation ofS–O bond. Conversely, at pH 7.4 sulphur from capping moleculesattached with the unsaturated bonds of surface sulphur formingS–S bond instead of S–O bond.

Further, when the size of nanoparticles increases, due todecreased surface to volume ratio, contribution from surface sulfurdecreases. As shown in Fig. 3b and c relative intensity of compo-

nent 2 goes on decreasing with size. Also, intensity of component3 decreases in samples (b) and (c) as compared to (a) as cappingmolecule slightly decomposes at higher pH.

Fig. 4 shows S 2p spectra for Ag–CdS nanoparticles. Since dopingof CdS nanoparticles was carried out at pH 11.2, we are comparing

Fig. 4. Deconvulated XPS spectra of S 2p core levels of Ag doped CdS nanoparticles.(a) at 2% Ag, (b) at 4% Ag and (c) 2% Ag–CdS nanoparticles after annealing at 400 ◦C.

P. Thakur et al. / Applied Surface Science 257 (2010) 1390–1394 1393

Table 2Fitting results for S 2p for Ag–CdS nanoparticles.

Sample Peak Position (eV) FWHM

2% Ag–CdS 1 160.8 2.012 161.8 1.623 163.0 1.32

4% Ag–CdS 1 160.8 2.032 161.8 1.85

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The observed changes in XPS are found to depend on the pH,dopant concentration, refluxing and annealing of the sample. Incase of CdS nanoparticles, surface properties are affected by pHof the solution. At pH 7.4, cysteine molecules are in the formof Cd2+–cysteine complex while at pH 11.2, cysteine is slightly

3 162.9 1.56

2% Ag–CdS afterannealing

1 160.3 1.862 161.8 1.095

hese results with Fig. 3b. At 2% Ag, (Fig. 4a) no change in bindingnergy was observed compared to undoped CdS (Fig. 4b). It is inter-sting to observe that like undoped CdS, variation in the differentcomponents in the S 2p core level spectra was seen for the first

ime. At 4% Ag, S 2p spectra shift towards lower binding energys particle size increases with dopant concentration. However, noppreciable change in particle size was observed at 2% Ag. XPS of 2%g–CdS nanocrystals was also carried out after annealing at 400 ◦C.2p spectra also shift towards lower binding energy due to the seg-

egation to the nanoparticle surface after annealing. Also, it can beeen that at 2% Ag, S peak is broadened and this broadening goes onecreasing at 4% Ag and for annealed sample. Again, fitting resultshow three components as undoped CdS (Table 2). Intensity of com-onent 2 decreases with increase in dopant concentration. For theigher percentage of silver, contribution of surface sulfur decreasesue to increase in particle size. Further, for annealed sample com-onent 3 has completely disappeared (Fig. 4c). Moreover, relative

ntensity of component has increased and that of component 2 hasecreased. Winkler et al. have shown segregation of Cd atoms andesorption of sulfur species after annealing and XPS spectra exhibit

ntense Cd rich component [9c].Fig. 5 shows XPS spectra of Ag 3d level of 2% Ag–CdS nanoparti-

les before annealing (Fig. 5a) and after annealing (Fig. 5b). As cane seen in Fig. 5, shift in binding energy from 367.8 (Ag+) to 367 eVAg2+) was observed after annealing. This indicates that oxidationf silver takes place during the annealing process. Fig. 6 shows XPSpectra of Ag 3d level of Ag doped CdS nanoparticle after decon-olution. Fitting result showed three components at 2% and 4% Ag.omponent 1 at 367 eV and component 2 at 367.7 eV have beenuggested for AgO and Ag2O respectively [14], while component 3uggests binding between Ag and S (Ag–S) (Table 3). At higher %f silver relative intensity of component 1 has increased and thatf component 2 has decreased. This indicates AgO has increasedith increase in amount of silver. Further, for annealed sample

omponent 2 disappears completely (Fig. 6c). These observationsuggest that, during annealing process, the oxidation of silver might

e taking place forming Ag2+ [10b]. Li et al. [15] and Sant et al.16] have also shown that Ag0 and Ag+ oxidized to Ag2+ duringhe heat treatments respectively. Additionally, it can be observedrom Fig. 6c that a peak due to the component 1 is much broader

able 3itting results for Ag 3d for Ag–CdS nanoparticles.

Sample Peak Position (eV) FWHM

2% Ag–CdS 1 367.0 1.822 367.7 1.923 368.0 1.65

4% Ag–CdS 1 367.0 1.572 367.7 1.953 368.0 1.13

2% Ag–CdS afterannealing

1 366.7 2.353 368.0 1.50

Fig. 5. XPS spectra of Ag 3d core levels of 2% Ag–CdS (a) before annealing, (b) afterannealing.

as compared to Fig. 6a and b. This observation suggests the pres-ence of mixed cations Ag+ and Ag3+ [14]. Thus, during annealingprocess, oxidation of silver may take place to Ag2+ or further itcan be oxidized to Ag3+. This is in accordance with our previ-ous report where from IR studies we have shown that cysteineacts as a tridentate ligand for annealed sample. XPS study con-firms the possibility of silver to bind cysteine in tridentate manner[10].

Fig. 6. Deconvolated XPS spectra of Ag 3d core levels of Ag doped CdS nanoparticles.(a) at 2% Ag, (b) at 4% Ag and (c) 2% Ag–CdS nanoparticles after annealing at 400 ◦C.

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ecomposed. In addition, as discussed in our previous article, CdSanoparticles are chelated in different manner after refluxing atH 11.2 for 24 h. Thus, the modification of surface is responsi-le to change the surface composition, chemical environment, andonsequently the particle size. As observed in XPS spectra of Cdnd S, at pH 7.4 these peaks are broader compared to other peaksFigs. 1 and 3). Also, S peaks are shifted towards higher bindingnergy from (c) to (a) (Fig. 3). Bittencourt et al. have observed ahift in binding energy accompanied by the peak broadening for Auanoparticles with decrease in particle size, which they attributedo the quantum size effect [17]. Zhang et al. attributed similar shifto the initial-state substrate electronic effect, though it is consid-red to be due to the final-state Coulomb charging effect [18,19]. Inhe present case, these changes are ascribed to the change in sur-ace composition and quantum size effect. After doping with silver,d and S peaks shift towards lower binding energy from (a) to (c)Figs. 2 and 4) and broadening of S peaks goes on decreasing froma) to (c) (Fig. 4). For the doped sample, silver can affect the CdSattice. However, from the XRD studies it was shown that silver isccupying interstitial position for as prepared samples. Therefore,imilar to the above-mentioned case, these changes are assignedo the quantum size effect. The shifts observed in Ag peaks mighte attributed to the change in Fermi level after doping. However,e assigned these changes to the change in oxidation state of the

ilver (confirmed from EPR studies). Moreover, after annealing sil-er is diffused into CdS lattice. This can also shift Cd and S peaksowards lower binding energy for the annealed samples. In addi-ion, we have to take into consideration the phase transformationccurring after refluxing and annealing to stable hexagonal phase.s a result of phase transformation, relocation of grain boundariesay occur at the nanoparticle surface. This may be responsible for

he changes observed in XPS.

. Conclusions

Effect of pH and dopant concentration on CdS and Ag–CdSanoparticles respectively have been successfully investigatedy XPS. XPS results are in agreement with our previous stud-

es. Different sulphur species in S 2p core levels were identified

or both doped and undoped samples. For the first time, weave shown that it is possible to study the variation in theifferent S species for doped samples. After annealing silver isxidized, thus cysteine may acts as a tridentate ligand for thennealed sample as shown by XPS. Shifts observed were assigned

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ience 257 (2010) 1390–1394

to the surface composition, quantum size effect, and phase transi-tions.

Acknowledgement

Priya Thakur would like to thank funding agency BARC for thefellowship.

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