CHAPTER 1

Mechanistic Insights into theBrust–Schiffrin Synthesis ofOrganochalcogenolate-Stabilized Metal Nanoparticles

YUAN GAO, YANGWEI LIU, YING LI, OKSANA ZALUZHNA ANDYUYE J. TONG*

Department of Chemistry, Georgetown University, 37th & O Streets, NW,Washington, DC 20057, USA*Email: yyt@georgetown.edu

1.1 IntroductionMetal nanoparticles (NPs) made of tens, hundreds, or thousands of atomscan have tunable chemical and physical properties as a function of NP size(number of atoms), elemental composition, and/or chemical environment(ligand-stabilized, matrix-embedded, or structurally-encaged). These NPs areartificial atoms1–7 and novel building blocks for new materials that holdnovel physicochemical properties as compared to the existing (atomic/mo-lecular) materials. It is expected that these novel materials will enablewidespread technological breakthroughs in the not too distant future, forinstance in molecular and/or nano-electronics and clean energy gener-ation.8,9 Within this broad context, organoligands, particularly organothio-late-stabilized metal (mainly Au) NPs, have been subjected to intensive

RSC Smart Materials No. 7Functional Nanometer-Sized Clusters of Transition Metals: Synthesis, Properties andApplicationsEdited by Wei Chen and Shaowei Chenr The Royal Society of Chemistry 2014Published by the Royal Society of Chemistry, www.rsc.org

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research over the last two decades due to their potential applications innano-optics,10 nano-electronics,11 (bio)sensing12 and medicinal science(theranostics).13

The first step towards any practical applications of metal NPs is the syn-thesis of these metal NPs, preferably air-stable and of homogeneous sizedistribution and known chemical composition. Among many syntheticmethods, the Brust–Schiffrin two-phase method (BSM) synthesis worked outby Brust, Schiffrin, and company in 1994,14 including its late variants, isdefinitively the most widely employed synthetic approach to make o5 nmorgano-ligand-stabilized metal NPs. Briefly, a typical BSM consists of threesteps: Step 1, metal ions are phase transferred (PT-ed) from an aqueous to anorganic phase (usually toluene or benzene) with a PT reagent (usually tet-raoctylammonium bromide (TOAB), i.e. R4NBr, R¼C8H17). Step 2, orga-nochalcogen-containing ligand (usually RSH) is added to the separatedorganic phase during which AuIII cations can be reduced to AuI cations. Step3, metal ions residing in the separated organic phase are reduced into M0 bya reducing reagent like NaBH4 during which organochalcogenolate-pro-tected metal NPs are formed.

Despite the prevailing use of the BSM in the synthesis of sub-5 nm metal(mainly Au) NPs (according to Thomson Reuters’ Web of Knowledge, theoriginal paper14 has accumulated a current number of citations as high as3755, and counting), mechanistic details of the BSM synthesis have beensketchy until very recently.15–18 A long-held belief concerning the metalprecursor in the synthesis of metal NPs, probably due to earlier papers byWhetten et al.,19,20 has been that the metal-thiolate polymer, [AuISR]n, is themetal ion precursor of metal NPs. However, a recent paper by Goulet andLennox18 has shown that the metal–TOA1 complex, [TOA][AuIBr2], can alsobe the major metal ion precursor. Our ensuing studies have not only con-firmed the results of Goulet and Lennox, but also proposed that the BSMsynthesis is an inverse micelle based approach based on their proton NMRresults and showed via Raman spectroscopic study that the Au–S bond doesnot form until the formation of Au NPs.15 In this chapter, we will review anddiscuss in various degrees of detail the relevant chemistry involved,particularly the role of encapsulated water, in the BSM synthesis of alkyl-chalcogenolate-stabilized metal NPs unravelled after the paper of Gouletand Lennox18 and highlight the similarity and difference when ligandscontaining different chalcogen elements (S, Se, or Te) are used as the startingsource of the NP-stabilizing agents.

1.2 Phase Transfer of Metal Ions: Formation ofInverse Micelle Encapsulated Water

1.2.1 Proton NMR Evidence of Encapsulated Water

The experimental evidence of possible encapsulated water by TOAB inan organic phase came first from the observation of a large down-field shift

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(B2 ppm, due largely to the appearance of hydrogen bonding among thewater molecules that strongly suggests the formation of water aggregates) ofthe water proton peak in C6D6 containing dissolved TOAB (0.03 mmol ofTOAB in 0.8 mL C6D6) as compared to that of pure C6D6 after both beingmixed with 0.105 mL Milli-Q water (18.2 MO) and the extra water layer beingthen removed, as shown in Figure 1.1. The water peak at 2.43 ppm was alsoreported in the Goulet and Lennox paper.18

More convincing and detailed evidence of the inverse micelle formation isshown in Figure 1.2, proton NMR spectra of a series of samples prepared bymixing various amounts of TOAB with 0.8 mL C6D6 and 0.210 mL Milli-Qwater and then separating the undissolved water. The two clearly dis-tinguishable regimes enable the critical micelle concentration (CMC) to bedetermined:21 the intersection of the two dashed lines gives the CMC ofTOAB in C6D6¼ 7.5 mM, which is about 4 to 5 times smaller than the TOABconcentrations generally used in a typical BSM synthesis of metal NPs. Thatis, under the normal condition of the BSM synthesis, inverse micelles en-closed by TOAB are formed.

Unlike other well-known inverse micelle systems, such as sodium2-ethylhexylsulfosuccinate (AOT), whose size can be readily varied by chan-ging water/surfactant ratio,22 our proton NMR data show that the inversemicelles of TOAB can be saturated even with a very small amount of water, asshown by the proton NMR spectra in Figure 1.3A and the correspondingnormalized peak integrals in Figure 1.3B.16 The samples for the spectrain Figure 1.3A were prepared with the same amount of TOAB (0.03 mmol) in

3 2 1 0d (ppm)

NM

R A

mpl

itude

(a)

(b)

2.43 ppm

0.44 ppm

encap. H2O

H2O in C6D6

Figure 1.1 Proton NMR spectra of (a) water in pure C6D6 and (b) in C6D6 withdissolved TOAB. Both samples were prepared using the same volume ofC6D6 (0.8 mL) and mixed with the same volume of water (0.105 mL)before being separated. The amount of TOAB was 0.03 mmol in (b).Modified from ref. 15. Copyright 2011, American Chemical Society.

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0.8 mL C6D6 but mixed with different amounts of water before the organicphase being separated for the NMR measurements. Interestingly, theamount of encapsulated water remained constant even if the water amountvaried from 0.0425 mL to 0.21 mL, so did the peak positions of the en-capsulated water and –CH2N1.

With the information in Figure 1.3, we can estimate the average size of theinverse micelles. According to Figure 1.3B, the average number of protonsfrom the H2O in Samples (a–d) is 5.11 per TOAB. After subtracting thenumber of protons from the H2O saturated in C6D6 (i.e. 1.63 per TOAB

3.0

2.5

2.0

1.5

1.0

0.5

d (p

pm)

80006000400020000

1/[TOAB], M–1

Critical Micelle Concentration (CMC):CTOAB = 7.5 mM

Figure 1.2 The proton peak shift of water in C6D6 solution of TOAB as a functionof the TOAB concentration. The two clearly distinguishable variationregimes indicate the formation of inverse micelles.

Figure 1.3 The proton NMR spectra (A) and the corresponding normalized peakintegrals (B) of a series of samples whose preparation parameters areexplained in (B).Modified from ref. 16. Copyright 2011, American Chemical Society.

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(Sample (f)), the number of protons from the H2O encapsulated in the in-verse micelles formed by TOAB is 3.48, which means that the ratio of theencapsulated H2O to TOAB is 1.74. Assuming that one molecule of TOAB issurrounded by one molecule of H2O, 57% of the encapsulated H2O might belocated in the outer layer of the inverse micelle core. Together with the factthat the H2O volume per molecule is 0.03 nm3 at 20 1C (density of H2O:0.998 g mL�1), the diameter of the inverse micelle core, i.e. the part of theH2O encapsulated in the inverse micelle of TOAB, is B2.5 nm, which is atthe small end of known inverse micelles.22

1.2.2 Phase Transfer of Metal Ions: Formation of MetalComplex

The first step of the BSM synthesis is the phase transfer (PT) of metal ionsfrom the initial aqueous solution to the organic phase using TOAB as PTagent. The proton spectra in Figure 1.4A were obtained after the PT of dif-ferent amounts of Au ions into the organic phase.16 The correspondingnormalized proton peak integrals and sample preparation parameters arecollected in Figure 1.4B. In addition to constant C6D6 volume (0.8 mL) andamount of TOAB (0.03 mmol), the volumes of the aqueous solutions werekept the same as used for Figure 1.3. Thus, the corresponding TOAB : Auratios were 1 : 1, 2 : 1, 3 : 1, and 5 : 1 for spectra (g) to (j) respectively. Forcomparison, the 1H NMR spectra of the synthesized [TOA][AuBr4] complex(0.0125 mmol) in 0.8 mL of C6D6 with (0.09 mL) and without H2O mixing areshown in spectra (k) and (l) respectively.

As can be seen in Figure 1.4A, the introduction of metal species not onlycaused clear variation of peak positions of the encapsulated water and a-

Figure 1.4 The proton NMR spectra (A) and the corresponding normalized protonpeak integrals and sample preparation parameters (B).Modified from ref. 16. Copyright 2011, American Chemical Society.

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proton in –CH2N1, but also the amount of encapsulated water. The pair(H2O/–CH2N1) peak values are now down-field shifted as the Au contentdecreases: 0.70/2.97, 2.34/3.18, 2.63/3.22, and 2.69/3.26 ppm for TOAB : Auratios of 1 : 1, 2 : 1, 3 : 1, and 5 : 1, respectively. Notice that the a-1H peakposition of –CH2N1 in the TOAB : Au¼ 1 : 1 case [Figure 1.4(g)] is the same asthat of the synthesized [TOA][AuBr4] complex [Figure 1.4(k), (l)]. This indi-cates strongly that one [TOA]1 cation was associated with one [AuIIIX4]�

anion with X¼Br� or/and Cl�. As the TOAB : Au ratio increases, whichcorresponds to a decrease of Au content because a constant TOAB amountwas used, more and more [TOA]1 cations can be associated with halogenanions (e.g., Br� or Cl�) other than the AuIII complex. However, the fact thatonly one a-1H peak of –CH2N1 was observed in all cases is a sign of fastexchange between the AuIII complex units and the halogen anions in theinverse micelles.

Now taking 2.97 ppm [Figure 1.4(g), (k), (l)] and 3.34 ppm [Figure 1.3(a) to(d)] as the a-1H peak positions of the –CH2N1 in two extremities, i.e., all AuIII

units vs. all halogen anions, we calculated the expected peak position usingthe fast-exchange model d(a-1H)¼ 2.97yþ 3.34(1� y) (in ppm) where y is thefraction of the AuIII units among the total anions (i.e., [AuX4]� and X�). Wefound 3.16, 3.22, and 3.27 ppm for TOAB : Au ratios of 2 : 1, 3 : 1, and 5 : 1,respectively. These should be compared with the experimentally observedvalues: 3.18, 3.22, and 3.26 ppm [Figure 1.4(h) to (j)]. That these two sets ofvalues are in excellent agreement indicates strongly that the partitioning ofthe AuIII units and the halogen anions in a given inverse micelle follows thenominal stoichiometry.

Figure 1.5 presents the 1H NMR spectra of samples containing Ag ions inthe organic phase after the PT. The samples were prepared with 0.8 mL C6D6,

d (ppm)

a – CH2N+

4 3 2 1

NM

R A

mpl

itude

2.16

2.02

2.06

1.993.25

3.28

3.28

3.30

en-H2O

(a)

(b)

(c)

(d)

Figure 1.5 Proton NMR spectra of reaction intermediate C6D6 solutions in the1st step of BSM synthesis of Ag NPs as a function of TOAB : Ag ratio:(a) 3 : 1, (b) 3.5 : 1, (c) 4 : 1, and (d) 5 : 1.

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0.03 mmol of TOAB, 0.4 mL aqueous solution of AgNO3 of different con-centrations. After the PT of Ag ions, the organic phase was collected andused for NMR measurements. As can be seen from the figure, the peakpositions of water are very similar to those in Figure 1.4A, indicating theformation of inverse micelles. Moreover, the same trend in down-field shiftand increase of water amount were observed as the Ag content decreases inthe order TOAB : Ag¼ 3 : 1, 3.5 : 1, 4 : 1, and 5 : 1. The peak integrals for theencapsulated water (normalized by setting that of a-1H of –CH2N1 to 8) are2.7, 2.9, 3.4, and 3.7 respectively, which are also close to those found in theAu case (see Figure 1.4B).

We show in Figure 1.6 the 1H NMR spectra of the organic phase after PT ofPdII (from PdCl2) or PtVI (from H2PtCl6). The samples were again preparedwith constant C6D6 volume (0.8 mL) and constant TOAB amount (0.03 mmol)but various metal content: TOAB : Pd or Pt¼ 1 : 1, 2 : 1, 3 : 1, and 5 : 1 forspectra (a) to (d) respectively. Interestingly, as the metal content decreases,we observe again the down-field shift and amplitude increase of the waterpeak for both PdII and PtVI samples, as those observed for the Au and Agsamples in Figures 1.4A and 1.5, respectively, also suggesting the formationof inverse micelles. However, the trend of variation for the a-1H peak of –CH2N1 in the PdII case (left panel in Figure 1.6) is opposite to those of thethree other metals, for which the chemical reason is still unclear but prob-ably has to do with the difference in the type of complex structure formedwith TOA1. Notwithstanding such difference, the water aggregates behavein a remarkably similar fashion that is related directly to the formation ofinverse micelles, as clearly alluded to by the results shown in Figure 1.6.

Figure 1.6 Proton NMR spectra of reaction intermediate C6D6 solutions in the1st step of the BSM synthesis of Pd (A) and Pt (B) NPs as a function ofTOAB : Pd(Pt) ratio: (a) 1 : 1, (b) 2 : 1, (c) 3 : 1, and (d) 4 : 1.

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1.3 Addition of Ligand: Reduction of Metal Complexor Formation of Polymeric Species

1.3.1 Alkylthiols RSH

The second step in a typical BSM synthesis is to add ligand to the organicphase that contains PT-ed metal ions. This is the step where divergenceexists in the literature as to what precursor species become the PT-ed AuIII

ions. Earlier work by Whetten asserted that the AuIII ions were reduced to AuI

and formed [AuIRS]n polymeric species.19,20 This assertion has been widelyaccepted, cited, or/and assumed as the Au precursor species in the BSMsynthesis until the Goulet and Lennox paper18 in which [TOA][AuIX4] and[TOA][AuIX2] complexes were shown to be the relevant metal ion precursors,which also applies to Ag and Cu. Our work confirms this important discoveryby showing that no metal–sulfur bond is formed during this step, as pre-sented in Figure 1.7.15

It is expected that a Au–S bond will form when RSH self-assembles on a Ausurface or polymeric [AuSR]n species are formed. This is indeed what we haveobserved, as shown in Figure 1.7 by the presence of a Au–SR vibrational bandat 327 cm�1 in the Raman spectra (c) and (d) for C12SH self-assembling on arough Au surface and for the synthesized [AuSR]n-like polymer respectively.

30002500200015001000500

Raman Shift (cm–1)

Cou

nts

(a)

(c)

(b)

(e)(d)

(f)

(g)

2568

(h)991

RS-H

209

525

327

RS-SR

Au-SR

Au-Br

Figure 1.7 Raman spectra of (a) dodecanethiol, (b) didodecyl disulfide, (c) dode-canethiol self-assembled on a rough Au electrode, (d) synthesized[AuSR]n-like polymer, (e) the concentrated C6H6 layer of HAuCl4 and3 equiv. of TOAB after the addition of 2 equiv. of dodecanethiol, (f)synthesized [TOA][AuBr2] complex, (g) TOAB, and (h) C6H6.Modified from ref. 15. Copyright 2011, American Chemical Society.

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However, no such vibrational band was observed when C12SH was added toan organic phase of TOAB that contained PT-ed Au ions obtained in the firststep of a BSM synthesis (see spectrum (e) in Figure 1.7), indicating no for-mation of the Au–S bond that is expected for the presence of polymeric[AuSR]n species. The appearance of a Au–Br band at 209 cm�1 in spectrum (e)is direct experimental evidence confirming the exchange of Br� from TOABwith Cl� in the original (AuCl4)�.

Figure 1.8 shows the Raman spectra of species formed in the 2nd step ofthe BSM synthesis of Ag (A) and Cu (B) NPs and of some reference ma-terials.15 Again, no Ag–S or Cu–S vibrational band was observed among thespecies formed in the 2nd step of the BSM synthesis: spectrum (c) inFigure 1.8A and B. No RS–SR but RS–H observed in the Ag case indicates thatno reduction of AgI took place. On the other hand, a strong RS–SR band wasobserved in the Cu case, illustrating that the added C12SH reduced CuII toCuI without forming a Cu–S bond.

30002500200015001000500Raman Shift (cm–1)

Cou

nts

Cu–Cl

RS–SR: 525 cm–1

Cu–RS: 326 cm–1

Ag–RS: 323 cm–1

RS–H: 2568 cm–1

RS–H

(a)(b)

(c)(d)

(a)

(b)

(c)(d)

(e)

(A) Ag

(B) Cu

Figure 1.8 Raman spectra of species formed in the 2nd step of the BSM synthesis ofAg (A) and Cu (B) NPs. (A): (a) synthesized [AgSR]n polymer, (b)concentrated organic phase of TOAB containing PT-ed AgI ions, (c)concentrated organic phase after adding dodecanethiol to the pre-concentrated (b) sample, and (d) TOAB. (B): (a) synthesized [CuISR]n

polymer, (b) synthesized [TOA]2[CuX4], (c) concentrated toluene solu-tion of [TOA]2[CuX4] plus 3 equiv. of C12SH, (d) CuCl2 �H2O, and (e)CuX2. (X: Cl or Br).Modified from ref. 15. Copyright 2011, American Chemical Society.

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Figure 1.9A presents the proton NMR spectra of the organic phase ob-tained in the 2nd step of the BSM synthesis for samples (g) to (k) shown inFigure 1.4A, together with those of reference disulfide (spectrum (r)) andthiol (spectrum (s)).16 The formation of disulfide (peak at 2.58 ppm) in allsamples indicates that the added thiol reduced AuIII to AuI ions, which wasalso observed by Goulet and Lennox previously.18 The appearance of evenmore down-field shifted water peaks evidences the acidification of the en-capsulated water by the reaction

[TOA][AuIIIX4]þ 2RSH-[TOA][AuIX2]þRSSRþ 2HX (1.1)

in which acidic protons were generated. That is, the existing inverse micelleencapsulated water or/and organic-solvent-dissolved water provided ahydrophilic receiving medium for the reaction-generated protons, enablingReaction (1.1) to proceed forward readily.

Figure 1.10 compares the proton NMR spectra of the reaction intermediateC6D6 solutions in the 2nd step of the BSM synthesis of Pd (a) and Pt (b) NPs.As can be seen, the added C12SH did not react with PT-ed PdII ions but re-duced PT-ed PtIV ions. As in the case of Au, the latter led to the formation ofdisulfide:

[TOA]2[PtIVX6]þ 2RSH-[TOA]2[PtIIX4]þRSSRþ 2HX (1.2)

The large down-field shift of the encapsulated water peak (5 ppm) was theresult of Reaction (1.2).

Figure 1.9 (A) Proton NMR spectra of samples (g) to (k) shown in Figure 1.4A afterthe addition of C12SH and of reference disulfide (r) and thiol (s). (B) Thecorresponding normalized proton peak integrals and sample prepar-ation parameters.Modified from ref. 16. Copyright 2011, American Chemical Society.

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1.3.2 Dialkyl Diselenide RSe–SeR

When using Se- and Te- (vide infra) containing organo-ligands as metal NP-stabilizers, one has to use air-stable diselenides or ditellurides rather thanair-instable selenols or tellurols. Earlier work by Ulman et al.23 showed thatAu NPs synthesized with one-phase (1p) BSM had better quality than thosesynthesized by two-phase BSM. Our research has shown that this is highlylikely related to the fact that only one dominant single metal precursorspecies exists for the 1p BSM synthesis while multiple metal precursorspecies co-exist in the 2p BSM synthesis.

Figure 1.11 presents the 1H and 13C NMR spectra of the reaction inter-mediate solution (a)/(c) and the starting dioctyl-diselenide (C8Se)2 (b)/(d) ofthe 1p BSM synthesis in which the THF solution of (C8Se)2 and aqueoussolution of HAuCl4 were directly mixed and no PT agent was used.24 The 1Hpeak of 1H2CSe– at 2.75 ppm in the starting ligand [Figure 1.11(b)] dis-appeared in the 1p intermediate [Figure 1.11(a)], which indicates that the Se–Se bond of the starting ligand was broken, i.e., the reaction between (C8Se)2

and HAuCl4 occurred. This also corroborates with the large 13C shift forC1/C2 of the reaction intermediate solution [Figure 1.11(c)] vs. those ofthe starting ligand [Figure 1.11(d)]. Most importantly, the eight 13C peaks inFigure 1.11(c) imply that the 1p intermediate consists dominantly of a singletype of metal complex.

Figure 1.12 shows the corresponding Raman spectra of the starting ligand(a), the reaction intermediate solution (b), and aqueous solution of

5 4 3 2 1d (ppm)

NM

R A

mpl

itude

(a) Pd

(b) Pt

(c)

(d)

2.58 ppm–S–S-C1H2–

HS-C1H2–2.17 ppm

Figure 1.10 Proton NMR spectra of the reaction intermediate C6D6 solutions of (a)Pd (PdCl2þ 2TOABþ 3C12SH) and (b) Pt (H2PtCl6þ 5TOABþ 3C12SH)samples. (c) and (d) are reference spectra for (C12S)2 and C12SHrespectively.

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HAuCl4.24 The RSe–SeR vibrational band at 287 cm�1 observed in thestarting ligand [spectrum (a)] largely disappeared in spectrum (b) of thereaction intermediate solution, indicating the majority of Se–Se bonds werebroken, in agreement with the NMR results discussed above. This is further

Figure 1.11 1H (a and b) and 13C NMR (c and d) spectra of the 1p intermediate (aand c) and the pure (C8Se)2 (b and d) respectively. The arrows indicatethe assignments of the respective peaks.Modified from ref. 24. Copyright 2012, The Royal Society ofChemistry.

Figure 1.12 Raman spectra of (a) dioctyl diselenide, (b) the concentrated 1pintermediate after removing THF, i.e., 1 equiv. of HAuCl4 and 0.5equiv. of dioctyl diselenide, and (c) aqueous HAuCl4 solution.From ref. 24. Copyright 2012, The Royal Society of Chemistry.

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corroborated by the disappearance of the characteristic bands of startingHAuCl4. Moreover, aided by DFT calculations,24 the band 142 cm�1 wasassigned to Au–Se vibration. Additionally, XPS measurements showed thatthe oxidation state of Se in the reaction intermediate solution was 2þ in-stead of the starting �1 and that of Au was �2. Thus, by combining theabove NMR, Raman, and XPS results altogether, it was concluded that thereaction between HAuCl and (C8Se) did occur and produced dominantly onetype of metal complex, highly likely in the form of Cl2AuSe(C8)Cl2.

1H NMR (A) and Raman (B) spectra of the reaction intermediate solutionsobtained in the 2nd step of the 2p BSM synthesis with different TOAB : Auratios but the same Au : Se (1 : 1) ratio are presented in Figure 1.13.24 Both 1HNMR and Raman of the simply mixed C6D6 solution of TOAB and (C8Se)2

(spectrum (a) in Figure 1.13A and B) proved that no reaction took placebetween the two species, although the slight blue-shift of the RSe–SeRband25 (from 287 to 292 cm�1) does indicate a certain degree of interactionbetween them. However, reaction occurred when the AuIII ions were added,but was not as fast as observed in the 1p BSM synthesis, as indicated by the

Figure 1.13 (A) 1H NMR spectra of (a) 3TOABþ 0.5(C8Se)2, (b) 1TOABþ1HAuCl4þ 0.5(C8Se)2, (c) 1.25TOABþ 1HAuCl4þ 0.5(C8Se)2, (d) 2TOABþ1HAuCl4þ 0.5(C8Se)2, and (e) 3TOABþ 1HAuCl4þ 0.5(C8Se)2 in C6D6.(B) Raman spectra of (a0) (C8Se)2 and of (a)–(e) as those in A.Modified from ref. 24. Copyright 2012, The Royal Society of Chemistry.

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persisting proton peak of _C1H2N1 at 2.73 ppm in Figure 1.13A and the RSe–SeR vibrational band at 292 cm�1 in Figure 1.13B. As TOAB : Au ratio in-creased, spectral features (2.73 ppm peak in (A) and 287 cm�1 band in (B))related to the RSe–SeR bond decreased and eventually disappeared com-pletely (from (a) to (e)), implying that the majority of RSe–SeR bonds werebroken.

The observation of the hydronium ions (H3O1), as indicated inFigure 1.13A, is intriguing because a simple phase transfer of AuIII ions doesnot generate observable hydronium ions. Although the subsequent additionof thiol does, where the hydrogen in the –SH group acts like a reductant,(C8Se)2 does not have such an obvious proton source. Thus, hydrolysis ofencapsulated water, such as Se–Seþ 2H2O-2SeOHþH1, might have hap-pened. We speculate that the band at 660 cm�1 is related to a hydrogen-bonding-like Se–H2O interaction26 before the Se–Se bond breaking and theband at 642 cm�1 to a Se–OH like bond after the Se–Se bond breaking. Onthe other hand, the appearance of the further down-field shifted water whoseamount increases as the TOAB : Au ratio decreases suggests the existence ofinverse micelles and that the degree of the hydrolysis reaction depended onthe amount of encapsulated water: the larger the latter is, the higherthe former will be. The available XPS data for the 3TOAB : 1Au (e) and2TOAB : 1 Au (d) samples indicate that the oxidation states of Au in these two2p samples (84.2 eV and 84.3 eV respectively) were basically the same asthat of Au in the 1p synthesis (84.25 eV) but those of the Se (57.98 eV and58.05 eV) were different from the 1p synthesis (58.4 eV).27 That is, the smallerbinding energies in the former indicate that the charge screening of Se washigher than that in the latter, which is consistent with the proposed Se–OHbonding.

Both the 1H NMR (Figure 1.13A) and Raman (Figure 1.13B) data stronglysuggest that more than one species exist as the 2p intermediates. The ap-pearance of encapsulated H3O1 as shown in Figure 1.13A suggests that hy-drolysis of water had happened and likely led to species such asX2AuSe(C8)(OH)X (X¼Br or Cl) as one of the intermediates whose exactcompositions still need to be further studied. One important differencebetween the 1p and 2p BSM syntheses, however, is that the results shown inFigures 1.11 and 1.12 were highly reproducible but not those in Figure 1.13which appeared to be highly dependent on many hard-to-control factors.

1.3.3 Dialkyl Ditelluride RTe–TeR

Among organochalcogenolate-stabilized metal NPs, organotellurolate-metalNPs are the least investigated system. Figure 1.14 presents the proton NMRspectra of (a) the intermediate reaction C6D6 solution obtained in the 2nd

step of the BSM synthesis of Au NPs using (C12Te)2, (b) that obtained in the1st step, (c) pure (C12Te)2 solution, and (d) pure C12SH solution. The firstobservation is that, in great contrast to using thiol (d), there was no acidicH1 formation when (C12Te)2 was used (a): the peak position of the

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encapsulated water remained almost unchanged as compared to theorganic phase right after PT (b), although broadened significantly. Instead,a peak assignable to RTeX3 appeared (a). Also, little unreacted (C12Te)2

is left.28

Raman measurements28 corroborate well with the NMR ones. Figure 1.15compares the Raman spectrum (a) of the sample (a) in Figure 1.14 with thatof (b) pure (C12Te)2, (c) the synthesized [TOA][AuIBr2] complex, and (d) TOAB.Several observations can be made here. First, disappearance of the RTe–TeRband at 194 cm�1 in the reaction intermediate indicates that the reactionbetween the added (C12Te)2 and the PT-ed [TOA][AuIIIX4] complex broke theTe–Te bond. Second, the appearance of the 209 cm�1 band15,29 in (a) that canbe reasonably assigned to the Au–Br2 band in the synthesized [TOA][AuIBr2]complex (c) suggests that the initial AuIII ions were reduced to AuI ions.Third, that no Au–Te vibrational band (B190 cm�1) was observed impliesthat no Au–Te bond was formed during the reaction. Fourth, assisted by theDFT calculations, the 280 cm�1 band can be assigned to RTe–X3.

Moreover, the available XPS data showed that the oxidation state of Auin the reaction intermediate [sample a in Figure 1.14(a) and 1.15(a)] was þ1and that of Te was þ2. Therefore, summarizing the above discussion,we propose the following stoichiometric reaction for the reduction of AuIII

to AuI by the ditelluride, whose oxidation state changes accordingly from�1 to þ2:

[TOA][AuIIIX4]þ 3(RTe)2-[TOA][AuIX2]þ 3RTeX3 (1.3)

Figure 1.14 Proton NMR spectra (in C6D6) of (a) the intermediate solution of theAu of (3TOABþHAuCl4þ 0.33(C12Te)2), (b) the organic phase after PT(3TOABþHAuCl4), (c) (C12Te)2, and (d) the solution formed by add-ition of 2 equiv. of C12SH to the solution in (b). The concentration ofeach chemical was kept the same in all samples.Modified from ref. 28. Copyright 2012, American Chemical Society.

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On the other hand, the ditelluride does not react with PT-ed Ag complexif added to the separated Ag-containing organic phase.

1.4 Critical Role of WaterAs alluded to in the previous discussion, the inverse micelle encapsulatedwater offers a hydrophilic micro-environment as a proton accepting reactionmedium, enabling Reaction (1) to proceed. Such an important role wasfurther confirmed by using the synthesized, anhydrous metal complex as thestarting material. We have observed that the reduction of the synthesized[TOA][AuBr4] complex to [TOA][AuBr2] by thiols in an anhydrous organic(toluene or benzene) solvent was extremely slow but happened almost in-stantaneously in the presence of H2O.16

It has been observed that if the inverse micelle encapsulated water is themain form of water existing in the BSM synthesis, then addition of thiol doesnot lead to the formation of Au–S bonds, i.e., formation of polymeric [AuSR]n

species. However, if more water beyond saturating the inverse micellesexists, then addition of thiol can lead to the formation of polymeric [AuSR]n

species providing that the thiol : Au ratio is larger than 2. For instance, if thewater layer of the original aqueous solution of Au salt was not discarded afterthe PT of Au ions to the organic phase and to which thiol was added, thenstirring would lead to the formation of a white cloudy material that sitsbetween the aqueous and organic layers. Such a white cloudy materialgave solution Raman spectrum (c) in Figure 1.16 in which those of the

Figure 1.15 Raman spectra of (a) dried intermediate of the Au system(3TOABþHAuCl4þ 0.33(C12Te)2), (b) (C12Te)2, (c) [TOA][AuBr2], and(d) TOAB.Modified from ref. 28. Copyright 2012, American Chemical Society.

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self-assembled thiol on a rough Au surface (a) and the synthesized polymeric[AuSR]n species (b) are also presented for comparison.16

That Figure 1.16(c) is almost identical to Figure 1.16(b) suggests stronglythat the white cloudy material was the polymeric [AuSR]n species. On theother hand, stirring the organic phase obtained in the 2nd step of a BSMsynthesis in the air long enough can also lead to the formation of a smallamount of polymeric [AuSR]n species in addition to forming disulfide, asindicated by Figure 1.16d. Summarily, Figure 1.17 presents a general metalNP formation mechanism15 in a BSM synthesis (A) and reaction conditions16

that lead to different, complex vs. polymer, reaction routes.It has also been discovered that the encapsulated water is also essential for

S–S or Se–Se bond breaking in a BSM synthesis if the former is used as thesource of stabilizing ligand. This is illustrated by the proton spectra of thereaction intermediate solutions with and without encapsulated water pre-sented in Figure 1.18.30 For both (C12S)2 and (C12Se)2, the pre-existence ofencapsulated water enabled S–S and Se–Se bond breaking to happen, asindicated by the appearance of the acidified water peaks in Figure 1.18(a)and (b) for disulfide and in Figure 1.18(d) and (e) for diselenide with TOAB :Au ratio¼ 3 : 1 and 2 : 1 respectively. On the other hand, the presence ofremaining disulfide as suggested by the residual peak at 2.57 ppm suggeststhat diselenide was more reactive than disulfide. Without the presence of

Figure 1.16 Raman spectra of (a) self-assembled C12SH on a rough Au surface, (b)synthesized [AuSR]n, (c) white cloudy material formed after C12SH wasadded to a Au ion-PT-ed solution that retained the original aqueouslayer of Au salt solution, and (d) separated organic phase after add-ition of C12SH and being stirred for 24 h. (a) and (b) are the samespectra as Figure 1.7(c) and (d) but are reproduced here to facilitatethe comparison and discussion.Modified from ref. 16. Copyright 2011, American Chemical Society.

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Figure 1.17 (A) A general Au NP formation mechanism, and (B) complex vs.polymeric Au ion intermediates in a BSM synthesis.Modified from ref. 15 and 16. Copyright 2011, American ChemicalSociety.

Figure 1.18 Proton NMR spectra of reaction intermediate solutions with andwithout encapsulated water for didodecane disulfide and didodecanediselenide.Modified from ref. 16 and 30. Copyright 2012, American ChemicalSociety.

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inverse micelle encapsulated water, no S–S and Se–Se bond breaking tookplace, as confirmed by spectra in Figure 1.18(c) and (f) where no acidifiedwater peaks but those of disulfide (2.57 ppm) and diselenide (2.78 ppm) areobserved.

Since no viable proton sources other than water existed in the reactionsolutions and no vibrational evidence of Au–S15,31 and Au–Se24 bond for-mation was observed, the appearance of acidic protons as described abovestrongly implies that the overall reaction may involve hydrolysis, most likelyin the form:

[TOA][AuIIIBr4]þ 12 RX�XRþH2O-[TOA][AuIIBr3]þRXOHþHBr (1.4)

where X stands for either S or Se. The presence of paramagnetic AuII ions wasalluded to, although not definitively, by the preliminary electrochemistryand UV characterizations that at least ascertained that the species in thereaction solutions were neither AuIII nor AuI.

1.5 Reduction by NaBH4: Formation of Zero-ValenceMetal Nanoparticles

1.5.1 Normal Reduction Sequence

The last step of a normal BSM synthesis is to reduce the metal ions to thezero valence state, usually by NaBH4, and form metal NPs.14 The exactchemical state of the metal ion precursors and the composition of the re-action intermediate solutions before this final stage are critically importantin determining how much control, and therefore how good the quality of themetal NPs, can be achieved. Figure 1.19 compares the TEM images of threeAu NP samples made with the same starting (0.025 mmol) [TOA][AuIBr2]complex in a mixture of 10 mL toluene and 0.21 mL water but slightly dif-ferent ligand compositions before the addition of aqueous solution ofNaBH4: (a) 3C12SH, (b) a mixture of 1C12SH and 1(C12S)2, and (c) 1.5(C12S)2.17

Figure 1.19 TEM images with corresponding size distribution histograms of theAu NPs formed from a mixture of [TOA][AuIBr2] toluene solution andwater with (a) 3 equiv. of C12SH, (b) a mixture of 1 equiv. of C12SH and1 equiv. of (C12S)2, (c) 1.5 equiv. of (C12S)2. All three samples had thesame S : Au ratio¼ 3 : 1.Modified from ref. 17. Copyright 2011, The Royal Society ofChemistry.

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Since AuI was the starting metal ion, the addition of thiol would not lead toReaction (1) and neither would it form thiolate polymer. As can be seen, amixture of different ligands, which is usually the case in a normal BSMsynthesis of thiolate-stabilized Au NPs, degrades the quality of the Au NPsformed. If a single type of ligand is used, thiol would be a better choice thandisulfide.

In Section 1.3.2, we discussed the chemical state of the species existing inthe reaction intermediate solutions of the 1p vs. 2p BSM synthesis of orga-noselenolate-stabilized Au NPs. It was concluded that a single dominantmetal precursor species existed in the reaction intermediate solution of the1p BSM synthesis but multiple species co-existed in that of the 2p BSMsynthesis. The TEM images of the final Au NPs made by the 1p (a) and 2p (b)BSM synthesis are presented in Figure 1.20.24 As can be clearly seen, theformer generated much better quality Au NPs than the latter did.

In Section 1.4, we discussed the enabling effect of the inverse micelleencapsulated water to break the S–S or Se–Se bond that led to differentchemical states of the reaction solutions before the addition of NaBH4.Figure 1.2130 presents the TEM images of the Au NPs formed after addingNaBH4 to reaction solutions that gave the proton NMR spectra shown inFigure 1.18 (a), (c), (d) and (g). The contrast in quality of Au NPs made withand without encapsulated water is quite impressive: from 1.9� 0.3 nm to3.6� 1.6 nm for disulfide and from 2.4� 0.8 nm to 5.8� 2.1 nm fordiselenide.

1.5.2 Reversed Reduction Sequence

In addition to the identification of [TOA][AuIIIBr4] and [TOA][AuIBr2] com-plexes as the major intermediate species in the BSM synthesis by Goulet andLennox,18 we have demonstrated, as discussed in the previous sections, thatthe Au–S bond, and metal–ligand bond in general, does not form until themetal ions are reduced to zero valence atoms and forming nascent metal

Figure 1.20 TEM images of octaneselenolate-stabilized Au NPs synthesized by (a)one-phase and (b) two-phase BSM syntheses.Modified from ref. 24. Copyright 2012, The Royal Society ofChemistry.

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clusters, as illustrated in Figure 1.17A. This discovery led us to experimentreversing the order of adding stabilizing ligand and reductant NaBH4 to thereaction solution,15 i.e., adding NaBH4 first then followed by adding theligand. Indeed, narrowly distributed thiolate-stabilized Au NPs could bemade by using the reversed BSM synthesis and the size could be controlledby the time interval between the addition of the reductant and the ligandeven with the same S : Au ratio, as shown in Figure 1.22.15 Notice that in the

Figure 1.21 TEM images and corresponding size distribution histograms of AuNPs obtained by adding aqueous solution of 10 equiv. of NaBH4 to thereaction intermediate solutions that gave the proton NMR spectrashown in Figure 1.18 (a), (c), (d), and (e) for image (a), (b), (c), and (d)respectively.Modified from ref. 30. Copyright 2012, American Chemical Society.

Figure 1.22 TEM images with size distribution histograms of the dodecanethio-late-protected Au NPs obtained through the reversed BSM synthesiswith stirring time interval between adding reductant (NaBH4) and theligand (C12SH) of (a) 10 s, (b) 10 min, and (c) 24 h. The same S : Auratio¼ 3 : 1 was used in all three syntheses.Modified from ref. 15. Copyright 2011, American Chemical Society.

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normal BSM synthesis, S : Au ratio is used to control the NP size. Thus, thesame S : Au ratio would lead to the same NP size.

One unique feature of the reversed BSM synthesis is that it enables sub-2nm selenolate- or tellurolate-stabilized Au NPs to be synthesized as dem-onstrated by Figure 1.23,15 which had never been achieved previously. In thesyntheses that led to the formation of those Au NPs, the starting source forAu ions was the synthesized [TOA][AuIBr2] complex and dioctyl diselenideor didodecyl ditelluride as stabilizing ligand. 10 s lapsed between addingthe reductant and the ligand. It is also worth noting that the reversed BSMapplies equally well to synthesizing small Ag and Cu NPs.

1.6 Conclusions and Future OutlookIn this chapter, we have discussed in rather detailed fashion metal NP for-mation chemistry, particularly the chemical state of the intermediate speciesleading to the final formation of metal NPs, in light of the recent advancedmechanistic understanding of the popular BSM synthesis. The most salientpoint of the latter is that the BSM synthesis is basically an inverse micelleprocess in which encapsulated water plays a critical role in determining thedetails of metal NP formation chemistry. For instance, it provides a hydro-philic proton accepting reaction-facilitating medium if thiol is used as thesource of stabilizing ligand (Reaction (1.1) and (1.2)). In the case of disulfideor diselenide, it also acts as a bond breaking enabler (Reaction (1.4)). On theother hand, what critical role the encapsulated water would play in the caseof ditellurite (Reaction (1.3)) is still not clear.

It is important to point out that the inverse micelles formed by TOAB maybehave differently from the traditionally well-studied inverse micelles suchas AOT in that the wall of TOAB inverse micelles may not be as closelypacked, therefore as rigid, as that of AOT-like inverse micelles because of thefour equal-length, therefore space-occupying branches, particularly forshorter ones. Moreover, it has been observed that the TOAB inverse micelles

Figure 1.23 TEM images and the corresponding size distribution histograms of (a)octylselenolate- and (b) dodecyltellurolate-stabilized Au NPs.Modified from ref. 15. Copyright 2011, American Chemical Society.

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do not have an easily measurable size range that can be varied because theyonly need a tiny amount of water to get saturated. Still, their physico-chemical properties warrant further investigations due to the prevailing useof the BSM synthesis in making sub-5 nm metal NPs. Overall, it is expectedthat the chemistry detailed in this chapter will be helpful to any furtherattempts at improving synthesis of metal NPs of better quality and withbetter control of properties.

AcknowledgementsThe research in the Tong Lab at Georgetown University is financially sup-ported by NSF (CHE-0923910 and CHE-0456848) and by DOE-BES (DE-FG02–07ER15895).

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