112_cluster Size Effect of Gold Nano Particles From XPS

CLUSTER SIZE EFFECT OBSERVED FOR GOLDNANOPARTICLES SYNTHESIZED BY SOL-GEL TECHNIQUEAS STUDIED BY X-RAY PHOTOELECTRON SPECTROSCOPY

S. Shukla and S. Seal*Advanced Materials Processing Analysis Center, Department of Mechanical, Materials and

Aerospace Engineering, University of Central Florida, Orlando, Florida

(Received August 23, 1999)(Accepted November 21, 1999)

Abstract—Gold nanoparticles have been synthesized by sol-gel technique. AFM analysis shows the presence of# 12–14 nm and 40–60 nm sized particles for H2S/not heated and H2S/heated samples respectively. XPS analysis ofH2S/not heated sample reveals that the core-level Au 4f7/2 B.E. is shifted by10.3 eV with increase in the FWHMby 0.2 eV relative to the respective bulk values of 84.0 eV and 1.1 eV. The results are interpreted in terms of thechanges in the electronic structure due to finite cluster size and creation of1ve charge over the surface of clusterduring the photoemission process itself. The electronic structure of Au nanoparticles (or clusters) produced by presentsol-gel technique is compared with that of Au clusters deposited by evaporation method described in the literature.©2000 Acta Metallurgica Inc.

Introduction

The physicochemical properties as well as magnetic and optical properties of nanometer sized materialsare very unique and are different from their bulk counterparts (1). Besides, the requirement for higherfunctionality, increased memory density, and higher speed are driving the need for smaller dimensionsin various microelectronic industries. Nanomaterials, thus, hold promising future as precursors forbuilding components of chips (2), for the development of sensors (3), for generating very small activeelements in magnetic recording (4), and also as functional biological materials (5).

Work is under progress in our laboratory to synthesize (by sol-gel method) and characterize sulfidenanoparticles of group IB elements of the periodic table (6,7). Synthesis and the characterization of CuSnanoparticles of size 2–5 nm have already been reported (6).

The present article discusses the synthesis of gold nanoparticles by sol-gel method and theirsubsequent morphological and chemical characterization by Atomic Force Microscopy (AFM) andX-ray photoelectron spectroscopy (XPS). XPS is a powerful technique to probe the electronic structureof very small clusters of atoms (8–22).

XPS analysis of these Au nanoparticles showed that the core-level 4f electrons in gold atoms, withinthe nanoparticles synthesized by present sol-gel technique, are either tightly or loosely bound to thenucleus than those in the bulk metal depending upon the processing conditions. Detail analysis revealedsome interesting factors causing these changes in the electronic structure of nano-sized gold particles.Similar experimental observations have been reported earlier for variety of noble- Au, (8–16) Ag,(13,17–19) Cu, (13,23) and transition- Pt, (20,21) Pd, (19,20,22,23) Ni, (23) supported metal clusters.

*To whom correspondence should be addressed.

Pergamon

NanoStructured Materials, Vol. 11, No. 8, pp. 1181–1193, 1999Elsevier Science Ltd

Copyright © 2000 Acta Metallurgica Inc.Printed in the USA. All rights reserved.

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1181

The remarkable difference between these experiments and the present work is that almost all earlierinvestigations involved examining the clusters produced by evaporation technique whereas, in thepresent work, the Au nanoparticles are produced by chemical synthesis method. Hence, the objectiveof the present article is to report and discuss these findings.

Experimental

Gold nanoparticles were produced by organometallic synthesis (modified sol-gel) followed by gasdiffusion and are deposited on silica based substrates. The procedure for the preparation of nanopar-ticles is already described elsewhere (6). Figure 1 shows the block diagram of various processing stepsinvolved (here referred as method-I). In short, 0.5 g of HAuCl4 z 3H2O and 0.29 g of silica sol weredissolved in 100 ml of methanol containing 0.5 g of hydroxypropyl cellulose (HPC). The preparationof silica sol and the importance of HPC are already discussed elsewhere (6).

Gold powder was also prepared by the process (here referred as method–II) involving generation ofH2S gas, by the reaction between potassium sulfide and hydrochloric acid, diluted with distilled water,

K2S1 2HCl 3 2KCl 1 H2S1

and passing this gas into the solution of HAuCl4 z 3H2O dissolved in methanol.

Figure 1. Block diagram showing different processing steps involved in the synthesis of gold nanoparticles by sol-gelmethod.

CHARACTERIZATION OF GOLD NANOPARTICLES1182 Vol. 11, No. 8

The precursor film, in method-I, was formed on the glass plate by a dip drying method at roomtemperature. The SiO2 plates were dipped (once) in the solution for 1–2 minutes and were dried for 3–4minutes before H2S exposure, (here referred as no H2S/not heated sample).

The nano-sized gold/gold sulfide particles dispersed film was obtained by exposing the precursorfilm to H2S gas (here referred as H2S/not heated sample) for 2 to 5 seconds which results in followingreaction:

2HAuCl4 1 3H2S 3 Au2S3 1 8HCl1

However, gold sulfide is found to decompose partially to pure gold via following reaction:

Au2S3 3 2Au 1 3S

Heating the film in a vacuum furnace (13 1024 Torr) at 150–180 °C for 1h (hereafter referred asH2S/heated sample) subsequently followed the gas exposure.

The particle size analysis for H2S/not heated and H2S/heated samples was then carried out by DIAtomic Force Microscopy (AFM) while the surface chemistry of nanoparticles deposited on SiO2

substrate was studied by 5400 PHI ESCA (XPS) spectrometer having a base pressure of 10210 torrusing MgKa X-radiation (1235.6 eV) at a power of 350 watts. Both survey and high resolution narrowspectra were recorded with an electron pass energy of 44.75 and 35.75 eV to achieve the maximumspectral resolution. The binding energy (B.E.) of the Au 4f7/2 at 84.06 0.1 eV was used to calibratethe B.E. scale of the spectrometer. Any charging shifts produced by the samples were carefully removedby using a B.E. scale referred to C(1s) B.E. of the hydrocarbon part of the adventitious carbon line at284.6 eV (24). A method described by Sherwood was adopted to remove non-linear backgrounds fromthe spectra (25). Non-linear least square curve fitting was performed using a Gaussian/Lorentzian peakshape (25,26) after the background removal.

Results

The morphology of the Au nanoparticles produced by organometallic synthesis are further studied byAFM. Figures 2(a) and 2(b) represent AFM photographs of H2S/not heated and H2S/heated samplesrespectively. After H2S exposure, the surface of the film appears to be quite smooth; Figure 2(a), evenat nanometer level, indicating the presence of#12–14 nanometer sized particles. Precise particle sizedetermination is, however, difficult due to very small size of the particles, beyond the resolution limitof the instrument. Heating the film at;150–180 °C for;1h has a drastic effect on the filmmorphology. The surface, as indicated by the Figure 2(b), appears to be quite rough after heating,revealing the growth of the particles from#12–14 nm to 40–60 nm. Moreover, the particles are seento form agglomerates/clusters at some places and are identified in the Figure 2(b) by circles. Theboundaries in between these particles appear to be straight, as indicated by the black lines which, webelieve, suggest the faceted nature of the particles.

Broad scan XPS analysis within the B.E. range of 0–1100 eV for the organometallic film undergoingvarious processing steps is presented in the Figure 3.

Au was found to be present near the surface of the film throughout the process. Prior to H2Sexposure, Figure 3(a), the additional presence of Cl was detected on the surface and is expected sinceHAuCl4 was used as a precursor.

Exposing this film to H2S gas results in sulfur pick up at the surface of the film, Figure 3(b),indicating the formation of gold sulfide through the reaction mentioned earlier. In fact, as discussedlater, the gold sulfide formed is found partly to decompose to gold. The formation of gold (and not gold

CHARACTERIZATION OF GOLD NANOPARTICLES 1183Vol. 11, No. 8

Figure 2. Atomic force micrographs of (a) H2S/not heated and (b) H2S/heated samples. In (a) particle size is#12–14 nm. andin (b) the particle size is;40–60 nm. The ‘circles’ enclose agglomerated gold nanoparticles while the ‘lines’ indicate the straightinterface boundaries between the particles indicating their faceted nature.


sulfide) nanoparticles through the direct reduction of gold chloride to gold by H2S gas has been reportedearlier by Yang et. al. (27).

Heating the film at 150–180 °C for 1h completely eliminates S from the surface, Figure 3(c),indicating that the decomposition of any residual gold sulfide to pure gold is further aided by the heattreatment.

Powder synthesized by method-II shows the prominence presence of gold and sulfur on the surface,Figure 3(d).

Narrow and high resolution XPS of Au (4f) within the B.E. range of 80–100 eV revealed interestingspectral features related to the surface of nanoparticles, occurring during different processing steps, andare presented in the Figure 4.

First, in Figure 4(a), immediately after H2S gas exposure, two peaks can be identified and are labeledas I and II. The first peak (I) corresponds to lower B.E. side and is that of pure Au (B.E.5 84.3 eV,FWHM 5 1.3 eV) and the peak corresponding to higher B.E. side (II) is of Au2S3 (B.E. 5 86.0 eV,FWHM 5 1.35 eV) (7). In fact, the peak-II is chemically shifted at 85.7 eV and further shift of10.3eV is caused by the “cluster size effect” as discussed later. Scaini et. al. reported the B.E. of 85.1 eVfor gold in Au2S when they studied the reaction of aqueous Au11 sulfide species with pyrite (28). Therelatively higher B.E. of gold, peak-I in the Figure 4(a), hence indicates the higher oxidation state ofgold i.e. Au31, and the formation of Au2S3. This is also in agreement with Yang et. al. who discussedthe formation of Au2S3, rather than Au2S, by the present sol-gel technique (27).

The Au 4f7/2 peak corresponding to the bulk Au was found to be at 84.0 eV (FWHM5 1.1 eV),Figure 4(b), and is also indicated in the Figure 4 by a vertical line. Thus, the core-level B.E. of electronsin gold nanoparticles of size# 12–14 nm is shifted by10.3 eV with an increase in the FWHM by10.2eV with respect to the B.E. and FWHM of core-electrons in the bulk gold. This strongly suggests the

Figure 3. XPS survey spectra for Au nanoparticles produced by sol-gel synthesis, method-I: (a) no H2S/not heated sample, (b)H2S/not heated sample, (c) H2S/heated sample, and (d) Au powder produced by method-II.


occurrence of “Cluster Shift” (and not a chemical shift) phenomena in the gold nanoparticles synthe-sized by sol-gel method. This point will be discussed in detail in the next section and an attempt willbe made to correlate the observed particle size, B.E. shift, and FWHM by comparison with the literaturedata.

Secondly, heating the film at high temperature results in the elimination of the peak-II, Figure 5(a).This suggests the complete decomposition of gold sulfide to gold, at least at the surface of the particles,as detailed by XPS. In addition to this, the peak-I in Figure 5(a) is observed to be shifted at a lower B.E.side (B.E.5 83.2 eV) with a marginal increase in FWHM (1.5 eV) relative to that of peak-I in theFigures 4(a) and 4(b). Thus, heating the film at 150–180 °C for 1h has caused the particles todecompose; but further growth and agglomerates have been observed. This has been reflected in adecrease in the core-electron B.E.

From the Figure 5(b), no presence of gold sulfide (peak-II) is detected near the surface of the goldpowder particles, although the Figure 3(d) does reveal a very small presence of sulfur. It is suggestedthat the high power of X-rays might have caused further decomposition of Au2S3, which has a greatertendency to decompose to gold (27). The Au 4f7/2 B.E. corresponding to powder particles is also foundto be shifted to a lower B.E. side relative to bulk value and appears at 83.3 eV (FWHM5 1.3 eV). Thus,the core electron B.E.s are almost identical for Au powder and coarse and agglomerated Au nanopar-ticles.

Figure 4. XPS Au (4f) spectra for (a) Au nanoparticles (H2S/not heated sample) produced by method-I and (b) solid pure Au.The peak –I corresponds to pure Au and peak-II corresponds to Au2S3.


Discussion

The most important findings made in the present work is a positive upward shift of X-ray photoemissioncore-level Au (4f) spectra of Au nanoparticles of size#12–14 nm. The shift is towards higher B.E. sideby 0.36 0.1 eV accompanied by a line broadening of the Au (4f) spectrum relative to that of bulk Au.This shift is not a chemical shift and hence draws more attention.

In general, the measured B.E. of an electron in the jth orbital of an atom (A), EmA(j) , in a particularchemical environment (say 1) is expressed as (29):

EmA(j)~1! 5 Em

F~N 2 1! 2 EmI~N! (1)

Where, EF (N-1) 5 energy of an atom, A, with (N-1) electrons in a frozen state, after the photoemissionprocess, and EI (N) 5 energy of an atom, A, with N electrons in the initial state.

This orbital B.E. is highly sensitive to the chemical surrounding of an atom, and hence, results in a“chemical shift” for the same atom in two different environments, as given by the following equation(29):

DEmA(j) 5 Em

A(j)~2! 2 EmA(j)~1! (2)

In the present investigation, the peak-II (related to Au2S3), in the Figure (4a), is found to be chemicallyshifted from the peak-I (related to pure Au nanoparticles),DEm

Au(4f), by 10.7 eV due to differentchemical surroundings of Au atoms in Au2S3 and pure solid gold.

Figure 5. XPS Au (4f) spectra for (a) Au nanoparticles (H2S/heated sample) produced by method-I and (b) Au powder producedby method-II.


Interestingly, the peak-I is itself shifted from the bulk value of 84.0 eV by10.3 eV. This shift is,however, nothing but a “cluster size effect” (and not a chemical shift) observed in many transition aswell as noble metal clusters when examined by XPS (8–22).

All of the earlier investigations (8–22) related to the cluster shift effect were devoted to understandthe physical phenomena causing this type of shift. The clusters of controlled sizes studied in almost allof these investigations were produced by evaporation technique. Hence, the cluster size effect observedhere with Au nanoparticles synthesized by sol-gel technique is really striking, and hence, one can nowroughly calculate the particle size variation in the nanomaterials knowing the core-level binding energyshift.

The (1ve) B.E. shifts observed in the supported metallic clusters are in fact exactly opposite innature relative to (–ve) shifts observed for the surface atoms of bulk metal (30,31). Citrin et. al. (30)studied the core-level Au (4f) spectrum as a function of take off angle and observed that the core–levelsfor surface atoms shift by 0.4 eV towards lower B.E. side (i.e. –ve shift) relative to the bulk value of84.0 eV. They also observed –ve shifts in the valence band spectrum of surface atoms in gold.According to the authors, reduction in the co-ordination number for surface atoms results in thelocalization of the valence d-electrons which are further localized by decrease in the degree of s3 dhybridization (32). This not only results in narrowing of the valence d-band but also reduces the energygap between the fermi level and the centroid of the d-band. The valence d-band, hence shifts towardsthe fermi level as fermi level is often fixed by the bulk. This is then reflected in the shift of core-electronB.E. of surface atoms to the lower side.

In a cluster, the ratio of total number of surface to bulk atoms is very high. Hence, decrease in thecluster size should reduce the B.E. of the core electrons. However, as observed in the present work andby others for Au (8–16) the core-level (and valence level, too) shifts are generally1ve relative to thebulk value and increase with decrease in the cluster size. Similar observations have also been reportedfor the number of other systems such as Ag, (13,17–19) Pt, (20,21) Pd, (19,20,22,23). The mechanismswhich overcome the surface effect and produce a net1ve B.E. shift in the core as well as valence bandspectra are now well recognized as initial and final state effects (14). The former is related to theelectronic structure of clusters, while the latter is related to the1ve charge left on the surface of thecluster during the photoemission process.

According to Mason, (14) the1ve B.E. shifts observed in the transition and noble metals can beattributed to the reduction in the repulsive force between d-electrons and core electrons as d-electroncount decreases with decrease in the cluster size (33). Moreover, the photoemission from the clusterresults in the creation of a core-hole which, if not effectively screened, may result in the additional1veB.E. shift in the photoemission. However, this latter phenomena is highly dependent upon the clustersize (16,36) and the conductive nature of the substrate (14,36).

The relative importance of initial and final state effects (14), described above, is still a matter ofcontroversy for1ve B.E. shifts observed in the supported Au clusters. For example, Wertheim et. al.(15) studied the core-level shifts in Au clusters supported on vitreous carbon substrate and attributed the1ve B.E. shifts to the final state effect. Liang et. al. (9) observed similar shifts in Au film (also clusters)deposited on Al2O3 and ascribed it to the initial state effect. According to DiCenzo et. al. (16), similarshifts in Au clusters supported onto amorphous carbon substrate are the effect of both, initial and finalstate effects. It is interesting to note that all these investigations involved non-conducting substrates.

As the changes in the electronic structure of clusters are inevitable as cluster size changes (14),irrespective of the substrate material means that the initial state effect will always have its owncontribution, although varying, to the B.E. shift. Mason strongly argued that for weakly interactingsubstrates and host metals, the energy shifts in the photoemission are the effect of initial state propertiesonly (14). He compared XPS and XAS shifts for Cu supported on C, as reported by two differentinvestigators (34,35) and found that both shifts are identical. As the final state in XPS and XAS are


nearly the same, it was suggested that complete screening occurs after photoemission process forclusters supported on weakly interacting substrates (at least in the case of Cu-C system). From thiscomparison, it was also concluded that the final state effect has no contribution to the observed B.E.shifts for weak interacting substrate like C. However, large difference between XPS and XAS shifts wasnoticeable for Pd-C system (0.85 eV) which was neglected on the basis of experimental error. We, onthe other hand, believe that the above difference suggests incomplete screening of1ve charge for Pdclusters supported on weak interacting carbon substrate. The complete screening of the charge for theclusters supported on other poor conductive substrates such as SiO2 and Al2O3 is also seems to behighly unlikely.

The 1ve charge left on the surface of the cluster due to photoemission process can be neutralizedby local, external and metallic screening processes (36). Hence, the contribution to the total B.E. shiftby the final state effect is dependent upon the total degree of freedom available for neutralizing thischarge. Hence, it is necessary to examine at least qualitatively, which of these screening mechanismsare likely to be operative in the present work where Au nanoparticles are formed on silica substratewhich produce a total B.E. shift of10.3 eV relative to the bulk value.

First, Mason (14) reported Au 4f7/2 B.E. as a function of metal coverages for Au supported oncarbon, SiO2 and Al2O3 (Figure 7). From that figure, we note that the B.E. shift of10.3 eV, as observedin the present investigation (sol-gel method), corresponds to a narrow coverage range of 1–23 1015

atoms/cm2 (evaporation technique). In a study, Mason (14) further reported cluster size of approxi-mately 70–90 atoms within this coverage range for Au supported on carbon. DiCenzo et. al. (16) hasshown that for Au clusters, supported on the carbon support, minimum of 150 atoms are required topossess metallic properties. This shows that, the clusters corresponding to the coverage range of 1–231015 atoms/cm2 are likely to have non-metallic character.

Further, Oberli et. al. (13) reported the cluster size of 2.5 nm for the coverage of 1.13 1015

atoms/cm2, when they examined the behavior of Au clusters deposited on amorphous carbon. Since, thiscluster size is comparable with the predicted size of Au nanoparticles (H2S/not heated sample) producedby sol-gel technique, as discussed later on, and also as observed experimentally by AFM (#12 nm,present investigation) and TEM (3 nm, by Yang et. al. (27)). This makes us to believe that the Aunanoparticles produced just after H2S treatment are possibly non-metallic in nature, and hence, haveminimum metallic screening effect.

Second, literature studies reported a drastic reduction in the B.E. of Au 4f7/2 relative to the clustersize within the coverage range of 1014 to 1–23 1015 atoms/cm2 (14). It is to be noted that within thisrange, B.E. shifts were independent of semi (C) and nonmetallic substrates (Al2O3 and SiO2). The slopeof this variation can be determined from the graph (Figure 7 in ref. (14)) and it is found to be equal to24310216 eV.cm2/atom. Similar variation of B.E. shifts as a function of coverage has been reportedby Wertheim et. al. (36) for Sn deposited on carbon substrate within the coverage range of 1014 to1–23 1015 atoms/cm2. We estimated the approximate slope of this line and it is found to be equal to-8310216 eV.cm2/atom, almost comparable with that obtained for Au. Hence, the variation in the B.E.as a function of coverage within the coverage range of 1014 to 1–23 1015 atoms/cm2 appears to be acharacteristic feature of noble and transition metals.

Interestingly, Wertheim et. al. (36) reported the complete elimination of B.E. shift (although withsmall –ve shift) for Sn deposited on Metglass within the coverage range of 1014 to 1–2 3 1015

atoms/cm2. This clearly indicates that for the narrow range of 1–23 1015 atoms/cm2 (considered to bethe upper end of above coverage range), the final state effect do have its own contribution to the B.E.shift, which is unaffected for semi and non-metallic substrates and is eliminated completely for metallicsubstrates. This shows that the external screening by the substrate is ineffective at the coverage of1–2 3 1015 atoms/cm2 when the Au clusters are supported on SiO2 substrate.


Thus, it appears in the present investigation, that the1ve charge created by the photoemissionprocess is screened only by local screening process and not by external (substrate) or metallic screeningprocesses. Hence, as the degree of freedom for neutralizing the1ve charge are reduced, we believe thatthe total B.E. shift of10.3 eV observed here for Au nanoparticles (H2S/not heated sample) supportedon SiO2 substrate is the effect of both initial and final state phenomenon, in accordance with DiCenzoet. al. (16).

A plot of FWHM for Au 4f7/2 is illustrated in the Figure 6 as function of B.E. by collecting the datafrom the literature (8,9,16) and subsequently comparing with our data. The solid line drawn representsthe best-fit curve obtained by regression analysis. The FWHM is found to increase with an increase inthe B.E. and follows an empirical relationship of the form:

FWHM ~in eV! 5 0.6768 * B.E. ~in eV! 2 55.774 (3)

This equation predicts a FWHM of 1.28 eV for the B.E. of 84.3 eV. (This data point is indicated by anopen circle in Figure 6). This value is in well agreement with the experimental value of 1.3 eV obtainedfor H2S/not heated sample. It is to be noted that the above equation is obtained by using the datareported for Au clusters produced by evaporation technique while our data corresponds to the sol-gelderived Au nanoparticles and are in well agreement with each other.

Attempt is also made here to estimate the cluster size from the observed B.E. shift of10.3 eV. InFigure 7, the B.E. shift of Au 4f7/2 is plotted as a function of cluster size for Au by collecting the datafrom the literature (12,13). It shows that below a cluster size range of 1.5 to 2.5 nm, the B.E. shiftincreases rapidly with decrease in the cluster size while above this range it changes marginally withincrease in the cluster size. It is well known that when cluster shifts are caused by the coulombinteraction between photoelectron and1vely charged cluster, the B.E. shift (DB.E.) is related to thecluster size as (15)

DB.E., e2/r (4)

Figure 6. A plot of FWHM (eV) vs B.E. (eV) for core-level Au 4f(7/2). The data is obtained from the literature (8,9,17). Theopen circle indicates the position of the data point (B.E.5 84.3 eV and FWHM5 1.3eV) obtained for H2S/not heated sample.‘f’ - indicates the position of data point (B.E.5 85.8 eV and FWHM5 1.5 eV) obtained for H2S/heated sample, in the presentinvestigation.


where r is the cluster radius. This equation shows that the product of cluster radius andDB.E. is aconstant. Hence, the nature of graph in the Figure 7 is understandable. We observe that for the B.E. shiftof 10.3 eV, this graph predicts the cluster size of;2–3 nm. AFM measurements (Figure 2(a)) indicatedthe particle size of#12 nm, however, precise measurement could not be made and needed TEMobservations in future. TEM examination by Yang et. al., however, did show the presence of 3 nm sizedAu particles synthesized by present sol-gel technique (27). They also reported very narrow sizedistribution for these particles. Seal et. al. (6) reported CuS nanoparticles of size 2–5 nm produced bythis technique.

As discussed earlier, the heating of the film at 150–180 °C for 1h resulted in the growth andagglomeration of the particles followed by –ve B.E. shift relative to the bulk value. As reported by manyinvestigators for Au (9,10,12,14,16) and also for other noble and transition metals (13,19,22,36), withincrease in the cluster size, B.E. shift decreases. Moreover, non-metallic clusters become metallic andexhibit metallic screening (i.e. screening by conduction electrons) beyond a particular cluster size. Afterthis stage, the B.E. doesn’t change drastically and approaches the bulk value with further increase in thecluster size (16,36).

Here, for Au powder and heated sample, the shifts are becoming –ve relative to bulk value. The sizesof the Au powder and nanoparticles (in H2S/heated sample: 40–60 nm) are, in fact, large enough toexhibit bulk behavior. Hence, it appears that something else is happening apart from the particle sizeeffect, which may be common for both heated sample and powder. The –ve B.E. shifts of this kind arereported for Au, and also for W; but only under specific conditions of testing (14,30,31).

The –ve B.E. shifts (relative to bulk value) are generally ascribed either to the interacting substrate(14) or to the surface effect (30,31). In the present investigation, Au nanoparticles are formed on SiO2

substrate which is non-interacting with respect to Au (4f) band (14). Citrin et. al. reported –ve B.E. shiftof 0.4 eV from the surface atoms of Au obtained from angle resolved XPS measurements (30). Theangle resolved experiments are, however, not conducted in the present investigation and the -ve B.E.shifts, obtained for H2S/heated and powder Au specimens, are also somewhat larger than the reportedvalue.

Figure 7. A plot of B.E. shift (eV) vs cluster size (nm) for Au clusters produced by evaporation technique. The data is obtainedfrom the literature (13,14). The graph predicts the cluster (particle) size of;2–3 nm corresponding to the B.E. shift of10.3 eVobserved for the H2S/not heated sample. ‘M’ –indicates that 12 nm particle size corresponds to B.E. shift of;1.5–2.0 eV.


It is to be noted that for H2S/heated sample, not only the B.E. is shifted to –ve side but also theFWHM is increased from 1.1 (for bulk) and 1.3 eV (H2S/not heated sample) to 1.5 eV. This is inagreement with Duc et. al. (31) who reported a –ve shift of 0.3 eV with increase in the FWHM forsurface atoms of W relative to that of the bulk atoms. This comparison, on the other hand, supports thehypothesis that the observed –ve B.E. shifts, in the present investigation, may be a surface phenomena.

From AFM photographs, we observe the straight interface boundaries in H2S/heated sample, whichindicate the faceted nature of the particles as mentioned earlier. Through TEM analysis of CuS and Aunanoparticles, both Seal et. al.(6) and Yang et. al. (27) also claimed these particles to be faceted byobserving abrupt changes in their boundaries. Hence, it appears here that the random orientation of thesefacets (which may form the surface of nanoparticles) relative to X-radiation, during photoemissionprocess, may produce an effect similar to the angle resolved experiments, giving rise to –ve B.E. shiftsand increase in the FWHM of core-level Au (4f) spectrum. This might happen if the structure is eitherorder/disorder or in form agglomerates.

No other direct experimental support for this hypothesis, other than above observations, can bepresented at this stage. More investigation in this direction is in progress.

In summary, the observations made on the changes in the core-level B.E. and FWHM, for Aunanoparticles produced by sol-gel synthesis, relative to those for bulk, appear to follow the same generaltrend exhibited by Au clusters produced by evaporation technique.

Conclusions

(1) Au nanoparticles of size#12–14 nm (H2S/not heated) and 40–60 nm (H2S/heated) have beensuccessfully synthesized by the present sol-gel method.

(2) Average particle size of 2–3 nm is estimated for the H2S/not heated sample from the observedcore-level B.E. shifts and is comparable with our and reported experimental results.

(3) The core-level Au 4f7/2 shift of10.3 eV, relative to the bulk value of 84.0 eV, observed forH2S/not heated sample is attributed to the initial as well as final state effects.

(4) –ve core-level B.E. shift accompanied by peak broadening, relative to the bulk values, as observedfor H2S/heated and Au powder sample may be a result of faceted nature of the particles.

(5) The differences in the electronic structure of atoms in Au nanoparticles (or clusters) synthesized bysol-gel technique (present article) and that of Au clusters produced by the evaporation technique(reported in the literature), as studied with the help of XPS, appear to be similar in nature.

Acknowledgments

The authors thank Prof. V. Desai, University of Central Florida, Orlando, for the encouragement givenfor publishing the paper. The authors also thank AMPAC graduate student support. The authors alsoacknowledge the use of UCF-CIRENT Materials Characterization Facility.

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112_cluster Size Effect of Gold Nano Particles From XPS

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