Facile One-Pot Synthesis of Metal-Semiconductor Hybrid Nanocrystals via ChemicalTransformation: The Case of Cu-CuxS Heterodimers and Hetero-Oligomers

Yonglin Liu* and Angela R. Hight WalkerOptical Technology DiVision, Physics Laboratory, National Institute of Standards and Technology,Gaithersburg, Maryland 20899-8443

ReceiVed: September 16, 2009; ReVised Manuscript ReceiVed: January 18, 2010

Metal-semiconductor hybrid nanocrystals (HNCs) hold great potential in the fields of photocatalysis,biomedical imaging, and anticancer nanomedicine. However, their chemical synthetic procedures remaincomplex and challenging. Here, we apply chemical transformation to convert metal nanostructures intometal-semiconductor HNCs following a facile one-pot polyol synthetic procedure. As case studies, two typesof Cu-CuxS HNCs, heterodimers and hetero-oligomers, are produced from Cu nanocrystals using an alkanethiolas both surfactants and the sulfur source. This protocol utilizes Cu nanocrystals’ high reactivity towardsulfidization for heterogeneous growth of copper sulfide over the remaining Cu nanocrystals. We find thatunder H2/argon, such a polyol process leads to an exclusive formation of heterodimers, while under argon,hetero-oligomers are produced predominantly. The morphological evolution upon further chemical transforma-tion of both heterodimers and hetero-oligomers, including the formation of hollow nanostructures, has alsobeen investigated. A thorough physicochemical characterization of the as-prepared HNCs is achieved throughmultiple transmission electron microscopy methods, combined with scanning electron microscopy, X-raydiffraction, and UV-vis spectroscopy.

IntroductionEver since the successful synthesis of single-material colloidal

nanocrystals (NCs) with controlled sizes and shapes, there hasbeen increased attention in the development of hybrid nano-crystals (HNCs).1,2 In each HNC, two or more NC domains ofdifferent materials are integrated into one nanoparticle throughepitaxial interfaces, usually in a controlled fashion. In additionto multifunctionality of every enclosed material, HNCs canprovide novel properties beyond each individual component.Thus, HNCs offer a higher technological potential comparedwith conventional NCs. In metal-semiconductor HNCs, forinstance, significant improvements of optical3,4 or electric5,6

properties have been reported. Consequently, metal-semi-conductor HNCs show great promise in the fields of photoca-talysis,7,8 biomedical imaging,9 and anticancer nanomedicine.10

The nanostructure of metal-semiconductor HNCs can vary fromcore-shell to asymmetrical heterodimers or hetero-oligomers.When there is no significant difference in lattice constantsbetween the metal and semiconductor materials, or when onecan lower the interfacial energies of NCs by controlling thereaction parameters, the formation of core-shell or onion-likestructural HNCs is often found.4,11-15 Utilizing more complexsynthetic approaches, anisotropic HNCs of heterodimers orhetero-oligomers with peanut-like,16 dumbbell-like,17 and nano-rod morphologies and its derivatives8,18-22 have been pro-duced. In terms of materials, most previously reported colloidalmetal-semiconductor HNCs include diverse combinations ofAu,6,7,11,12,17,18,23-32 Ag,17,26,27,31-34 Cu,31,32 Pt,8,21 Zn,35 Co,14,22,36

FePt16,37 and other metal alloy-metal chalcogenides includingCdSe7,8,18,19,23 and other metal selenides,17,27 CdS6,11,14,16,21,22,24,37

and other metal sulfides,11,17,27 TiO2,6,12,28,33,36 ZnO,31,32,35 SnO2,30

Cu2O,25 and other semiconductors.29,34 Cu NCs are of specificinterest due to their potential applications in various fields

including catalysts,38 electronic devices,39 and nonlinear opticalmaterials.40 In addition, the NCs of copper sulfide, known asp-type broad band semiconductors with a band gap of 1.2-1.5eV, have been extensively investigated in solar cells41 and otherapplications.42 Heterostructures containing Cu or copper sulfide,nevertheless, were fabricated in just a few cases such asCu-ZnO nanojunctions by Mulvaney and co-workers,31 andAu-CuS core-shell nanostructures by Wang and co-workers.11

Copper-indium sulfide NCs were also produced with the shapesof acorn, bottle, and larva by Hygon and co-workers,43 match-stick, teardrop, and pencil by Tang and co-workers,44 andbiphasic rods by Cui and co-workers.45 Additionally, Alivisatosand co-workers recently reported the formation of CdS-Cu2Sbinary nanorods during cation exchange.46 To our knowledge,however, HNCs containing both Cu and copper sulfide(Cu-CuxS HNCs) have not been reported until this work.

The most common strategies for the solution-based synthesisof metal-semiconductor HNCs follow a seed-mediated protocol.This generally includes the sequential growth of a secondmaterial over the presynthesized NC, or the diffusion of onematerial, usually a metal, into the other. These syntheticprocedures are complex and challenging, usually requiring atleast two steps under strictly controlled conditions. Only alimited number of one-pot reaction procedures exist in theliterature.15,37 Chemical transformation provides a simple pathto produce inorganic nanostructures.47 Despite numerous re-search approaches toward the synthesis of metal-semiconductorHNCs, there has been no report on the production of nonhollow,anisotropic metal-semiconductor HNCs via chemical transfor-mation. Herein, we demonstrate our approach for the wet-chemistry synthesis of Cu-CuxS HNCs following a facile one-pot polyol procedure. By taking advantage of their highreactivity during chemical transformation toward sulfidization,presynthesized Cu NCs are used as both templates and reactantsfor heterogeneous growth of copper sulfide over the remaining* To whom correspondence should be addressed, yonglin.liu@nist.gov.

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10.1021/jp908981n 2010 American Chemical SocietyPublished on Web 02/23/2010

Cu NCs. Similar synthetic protocols were reported by Xia’sgroup47 and others,35,48 and hollow nanostructures were usuallyproduced.49 Additionally, H2 gas has served as the reducingagent in the synthesis of metallic NCs in previous reports.50

Xia and co-workers further demonstrated that H2 gas plays akey role in the formation of silver nanocubes during a polyolprocess. It both accelerates the reduction rate and generates nitricacid, which presumably selectively dissolves twinned silverseeds.51 We found, in this case, that one H2-assisted polyolprocess can lead to the exclusive formation of nonhollowCu-CuxS heterodimers, while the very same process, in theabsence of H2, produces mostly hetero-oligomers. The advan-tages of this approach over preceding methods include a one-pot facile reaction, and fewer reaction parameters being requiredfor HNCs’ size control, since the partial consumption of CuNCs leads to the formation of Cu-CuxS HNCs.

We synthesized Cu NCs (See Experimental Section fordetails) by reducing copper acetylacetonate (Cu(acac)2) with 1,2-hexadecanediol (HDD) in a solution of 1-adamantanecarboxylicacid (ACA) and 1-hexadecylamine (HDA) in diphenyl ether(DPE) heated to 180 °C under either a mixture of H2/argon (H2,7%) or argon, following a similar synthetic protocol of platinum(Pt) NCs by Yang et al.52 with some modifications. Cu NCs,characterized as a dark purple solution, were obtained after 3 hunder H2/argon or 12 h under argon, indicating that H2 hassignificantly increased the speed of the reduction reaction.Without further purification or separation, 1-dodecanethiol(DDT), as a sulfur source, is added to the DPE solution at 180°C under either H2/argon or argon, resulting in the formationof Cu-CuxS heterodimers or hetero-oligomers, respectively.

Experimental Section

Chemicals. Copper acetylacetonate (Cu(acac)2, 99.99%), goldchloride trihydrite (99.9%), 1-dodecanethiol (DDT, 99%), 1-adamantanecarboxylic acid (ACA, 99%), 1,2-hexadecanediol(HDD, 90%), 1-hexadecylamine (HDA, 90%), and diphenylether (DPE, 99%) were purchased from Aldrich. All chemicalsand reagents were used as received.

Synthesis of Cu-Cu2S Heterodimers. The preparation ofCu-Cu2S heterodimers involves an initial synthesis of Cu NCsand a subsequent partial sulfidization of the resulted Cu NCs.The synthesis of copper NCs follows the previously reportedprocedures of Pt NCs with some modifications.52 In a typicalsynthesis, Cu(acac)2 (65 mg, 0.246 mmol), ACA (270 mg, 0.964mmol), HDD (1.6 g, 6.191 mmol), and HDA (2.0 g, 8.283mmol) were put in a 25 mL three-neck flask combined withDPE (10 mL) as a solvent with a condenser in a fume hood. AH2/argon (H2, 7%) purge system was used during the entirereaction. The solution under vigorously stirring was heated andkept at 100 °C for 2-3 h. The observed color of the reactionmixture was blue and characterized as Cu2+. The reactionmixture temperature was then rapidly raised at a rate of 8 °Cmin-1 and kept at 180 °C for 2-3 h. During this period, thecolor of the resulting solution was changed from blue tocolorless, and subsequently to dark purple after 3-4 h. Thisobservation indicates the formation of Cu NCs. For the purposeof characterization and comparison, 1 mL of colloidal Cususpension was collected with a long needle glass syringe anddiluted into 5 mL chloroform followed by centrifugation andredispersion into chloroform. To achieve Cu-Cu2S het-erodimers, DDT (0.5 mL, 2.087 mmol) was slowly injected intothe remaining solution at 180 °C, and the color of the solutionslowly turned to green after 1 h. For the purpose of character-ization, 0.3 mL of the solution was taken and diluted into 0.66

mL of chloroform. The resulting solution was centrifuged at14000 rpm for 5 min, and the supernatant discarded. Theprecipitate was redispersed in chloroform and centrifuged againat 14000 rpm to remove the excess reagent.

Synthesis of Cu-Cu1.81S Hetero-Oligomers. The preparationof Cu-Cu2S hetero-oligomers followed the same strategy andprocedure as above. However, instead of H2/argon purge, a standardair-free technique under an argon purge was applied. In a typicalsynthesis, Cu(acac)2 (65 mg, 0.246 mmol), ACA (270 mg, 0.964mmol), HDD (1.6 g, 6.191 mmol), and HDA (2.0 g, 8.283 mmol)were placed in a 25 mL three-neck flask and combined with DPE(10 mL) as a solvent with a condenser in a fume hood. The solutionunder vigorously stirring was heated and kept at 100 °C for around1 h. The reaction mixture temperature was then rapidly raised at arate of 8 °C min-1 and kept at 180 °C for about 10-12 h. Theresulting solution turned dark purple, indicating the formation ofCu NCs. To obtain hetero-oligomers, DDT (0.5 mL, 2.087 mmol)was slowly injected into the remaining solution at 180 °C. Thecolor of the solution slowly turned to blue after 1 h. The resultingsolution was centrifuged at 14000 rpm for 5 min, and thesupernatant was discarded. The precipitate was redispersed intochloroform and centrifuged again at 14000 rpm to remove theexcess reagent.

Characterization. Samples for TEM characterization wereprepared by adding several drops of NC solutions in chloroformonto 300-mesh copper or nickel grids with carbon support film(no. 01753 or no. 01800, Ted Pella). Those for SEM wereprepared by adding a couple drops of NC solutions inchloroform onto silica wafers. All HRTEM images were takenfrom JEOL 2100F field-emission and 2100 LaB6 TEMs at 200kV, while SEM images were taken on Hitachi SU-70 Schottkyfield emission gun scanning electron microscope. Scanning TEM(STEM) images and energy-dispersive X-ray spectroscopy(EDX) spectra were acquired on a JEOL 2100F field-emissionTEM operated at 200 kV. The crystal structures of the sampleswere determined from powder X-ray diffraction (XRD) obtainedwith a Bruker C2 Discovery system equipped with a Cu KR1

radiation (λ ) 1.540598 Å). UV-vis absorption spectra wererecorded at room temperature with a Shimadzu (BioSpec-1601)UV-vis spectrometer.

Results and Discussion

XRD Characterization of HNCs. The composition andcrystal phase of the as-synthesized products were first examinedby powder X-ray diffraction (XRD). Figure 1a presents the XRDpattern of Cu-CuxS heterodimers formed 1 h after the reactionof Cu NCs with DDT at 180 °C under H2/argon (H2, 7%). TheXRD peaks at 43.2°, 50.4°, and 74.1° can be assigned as (111),(200), and (220) reflection lines of Cu with a face centered cubic(fcc) crystalline structure, in good agreement with the diffractionstandard of Cu (JCPDS 04-0836), while the peaks at 37.5°,39.7°, 45.8°, and 47.8° are consistent with a hexagonal Cu2Sstructure (JCPDS 84-0208). Figure 1b shows the XRD patternof Cu-CuxS hetero-oligomers obtained 1 h after the reactionof Cu NCs with DDT at 180 °C under argon purge. The twosets of peaks at 43.3°, 50.5°, 74.1°, and 31.7°, 32.6°, 37.6°,45.4°, 45.8° can be indexed as fcc structured Cu (JCPDS 04-0836) and tetragonal structured Cu1.81S (JCPDS 41-0959),respectively. The XRD characterization of Cu-Cu2S het-erodimer and Cu-Cu1.81S hetero-oligomer HNCs is consistentwith our XPS analysis.

Synthesis and Characterization of Cu-Cu2S HeterodimerHNCs. Figure 2a shows the representative transmission electronmicrograph (TEM) image of the as-prepared Cu NCs under H2/

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argon before the addition of DDT. The Cu NCs are polycrys-talline with an average size of (40.8 ( 12.2) nm (obtained fromcounting ∼100 nanoparticles). The lattice spacing of 0.210 nmmeasured in the high-resolution TEM (HRTEM) images (inset,Figure 2a) is consistent with the distance of (111) lattice spacingin Cu, indicating the cubic and multiply twinned crystallinenature of these NCs. The TEM image of the nearly monodis-

perse Cu-Cu2S heterodimers, formed 1 h after the addition ofDDT into the colloidal solution of Cu NCs, is displayed inFigure 2b. Each heterodimer HNC is comprised of two joineddomains. The darker domain (more electron dense) with ahemispherical morphology corresponds to the Cu portion, whilethe lighter domain with the hexagonal shape is due to hexagonalCu2S based on the XRD experiments (Figure 1), and energydispersive X-ray spectroscopy (EDX) (Figure S1 in the Sup-porting Information). Additionally, scanning transmission elec-tron microscopy (STEM), in conjunction with EDX line scanprofiling along the long axis of heterodimers (insets, Figure 2band Figure S2 in the Supporting Information), reveals that thedark side of the dimer is composed of Cu and the lighter domainis mainly composed of Cu and S with an approximate atomicratio of 2:1. The two domains of heterodimers can be clearlydistinguished using dark-field TEM contrast imaging shown inFigure 2c, where the bright sections are due to Cu, while thegray regions are assigned to Cu2S. The vast majority of as-prepared NCs exhibit asymmetrical dimeric structures. However,a few NCs exhibit uniform gray color throughout the entiredomains and are likely due to free Cu2S NCs. These free Cu2SNCs were most likely formed from the reaction between DDTand a fraction of small Cu NCs (e5 nm), which are known fortheir fast sulfidization reactions.53

The statistic analysis of ∼100 heterodimer HNCs shows thatthe average dimensions of the Cu portions is (23.4 ( 6.7) nm alongwith the long axis of dimers (Figure 2b, X direction), and is(34.7 ( 10.8) nm along the direction perpendicular to the aboveaxis (Figure 2b, Y direction). Thus, compared with the originalspherical Cu NCs (Figure 2a), the dimension of the Cu portionwith a hemispherical shape in each heterodimer, obtained after 1 hof reaction, has been reduced ∼43% in the dimer long axisdirection. The dimension perpendicular to the long axis shows only∼ 15% reduction, indicating that the portion nearest to the Cu2Ssection of each Cu NC is preferentially consumed to produce Cu2S.Typical HRTEM images of as-prepared heterodimers are shownin panels d and e of Figure 2. The measured lattice spacing of0.220 nm is consistent with the distance of (102) lattice spacing inCu2S. The Cu2S portions are single crystalline, while the Cu regionsoften show multiple lattice orientations, indicating the multiplytwinned nature of Cu NCs. This is further supported by thecalculated fast Fourier transform (FFT) of the selected areascorresponding to the Cu and Cu2S portions (insets, Figure 2e).

The nature of the reaction, in which the partial consumptionof Cu NCs leads to the formation of heterodimers, enables usto easily control the sizes and shapes of Cu and Cu2S portionsby simply managing the reaction time. For instance, TEMimages of several typical heterodimers, obtained ∼2.5 h afterthe addition of DDT, are shown in Figure 2f. At 2.5 h, the Cu2Sportions of most heterodimer HNCs have developed symmetricalhexagonal shapes. The average size of Cu domains is 17.3 (5.9 nm (by counting ∼100 NCs), showing ∼58% size reductionfrom the initial Cu NCs (Figure 2a). Interestingly, the Cuportions appear to be perfectly spherical compared with thoseobtained after 1 h (Figure 2b). This suggests that during theperiod of 1 and 2.5 h after the addition of DDT the sulfidizationreaction preferentially takes place along the Y direction (Figure2f), perpendicular to the dimers long axis.

To better understand the formation and evolution mechanismsof heterodimers, TEM images of the NCs, produced at 20 min,50 min, 2.5 h, and 5.5 h after the addition of DDT, are presentedin Figure 3. Surprisingly, at 20 min, the as-prepared NCs showcore-shell structures with a Cu2S layer covering each Cu NCthoroughly without any obvious spatial preference. After 50 min,

Figure 1. X-ray diffraction patterns of Cu-CuxS (a) heterodimer and(b) hetero-oligomer HNCs obtained 1 h after the addition of 1-dode-canethiol (DDT). Heterodimers (a) are composed of hexagonal chal-cocite-structured Cu2S (JCPDS 84-0208) and face centered cubic (fcc)structured Cu (JCPDS 04-0836), while hetero-oligomers (b) consist oftetragonal structured Cu1.81S (JCPDS No. 41-0959) and fcc structuredCu.

Figure 2. TEM images of (a) presynthesized Cu NCs (inset, enlargedimage of one multiply twinned NC) and (b) Cu-Cu2S heterodimerHNCs obtained 1 h after the addition of DDT into the solution of CuNCs (inset, STEM image and the corresponding EDX spectra followinga line scanning profile of one single heterodimer). (c) Dark-field TEMimage of Cu-Cu2S heterodimer HNCs obtained 1 h after the additionof DDT into the solution of Cu NCs, and the bright regions correspondto Cu, and the gray regions correspond to Cu2S. HRTEM images ofCu-Cu2S heterodimer HNCs obtained (d and e) 1 h and (f) 2.5 h afterthe addition of DDT into the solution of Cu NCs (inset of (e), thecalculated fast Fourier transform (FFT) of Cu2S (top) and Cu (right)portions). All the reactions were performed under H2/argon (H2, 7%).The scale bars represent 20 nm.

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however, Cu2S portions are formed overwhelmingly on one sideof Cu NCs. H2 gas has been used as a reducing agent for metallicNCs synthesis during the previous polyol processes.51 Similarly,in this reaction, we propose that H2 gas has dual reductionfunctions. First, it helps reduce Cu2+ into Cu NCs as evidencedby the much faster formation of Cu NCs under H2/argon (∼4h) than under argon (∼12 h). Second, it appears to reduce someof the Cu2S portions formed initially to reproduce Cu NCs (eq1), as previously reported.54 This has been further proven bythe coexistence of the surface plasmon resonance (SPR) bandof Cu NCs in addition to the heterodimers’ band, as discussedlater. Furthermore, we found that a continuous H2/argon purgeis necessary for the formation of monodisperse heterodimers.For instance, after they were produced under H2/argon, thesubsequent reaction of Cu NCs with DDT under argon produces∼20% of hetero-oligomers along with heterodimers (Figure S3in the Supporting Information). While the reaction of Cu NCs,which were presynthesized under argon, with DDT under H2/argon leads to the exclusive formation of hetero-oligomers(Figure S4 in the Supporting Information).

Synthesis and Characterization of Cu-Cu1.81S Hetero-Oligomer HNCs. Under argon, Cu- Cu1.81S hetero-oligomersare produced instead of heterodimers. Figure 4 presents TEMimages showing the growth of multiple copper sulfide NCs oneach individual Cu NC to form hetero-oligomers, obtained 1 h(Figure 4, panels a, b, c, and e) or 2.5 h (Figure 4d) after theaddition of DDT. From the analysis of TEM images of over200 NCs, the relative yields of morphologies formed were foundto be 34% symmetrical, 30% asymmetrical oligomer, and 7%core-shell and the remaining were NCs with random, aniso-tropic morphologies. Some typical symmetrical hetero-oligomersare shown in Figure 4c. The composition of hetero-oligomerHNCs has been identified as multiple tetragonal Cu1.81S NCs(the lighter colored branches) attached to face center cubic (fcc)Cu cores (the dark colored center), based on XRD (Figure 1)and EDX (Figure S5 in the Supporting Information) experi-ments. Both Cu cores and each attached Cu1.81S NC in these

symmetrical oligomers are single-crystalline, based on theappearance of continuous lattice fringes oriented in the samedirection across each domain as well as the FFT image (inset,Figure 4, panels c and e). This observation represents asignificant difference from the asymmetrical hetero-oligomersor core-shell like nanostructures where Cu portions appear tobe multiply twinned crystalline (Figure S6 of the SupportingInformation).

Additionally, TEM images of Cu NCs before the addition ofDDT, shown in Figure S7 of the Supporting Information,indicate that there are ∼30% of single-crystalline Cu NCs,consistent with that of symmetrical hetero-oligomers.

HRTEM images are further investigated in order to show thedetailed junction structures between the fcc Cu core and themultiple tetragonal Cu1.81S NCs in the symmetrical hetero-oligomers (Figure 4e). The measured d spacing analysis on thebasis of the HRTEM image shown in Figure 4e indicates thatthe Cu1.81S NC epitaxially attaches to the (200) plane of the Cucore through its (227) plane. The lattice mismatch along thisdirection can be calculated as ((0.185 - 0.182)/0.185 ×100%) ) 1.6%. Such a small mismatch leads to a smoothepitaxial growth of tetragonal Cu1.81S on fcc Cu without anyobvious dislocation.

In addition, the morphological evolution of the symmetricalhetero-oligomers was investigated using time-dependent TEMstudies (Figure 5). At 20 min after the addition of DDT, weobserve that certain surface facets of Cu NCs preferentiallysulfidize to produce Cu1.81S portions sticking to the remainingCu NCs. This leads to the formation of symmetrical Cu-Cu1.81Shetero-oligomers. The dimensions of Cu1.81S portions increasewhile those of the central Cu domains diminish upon further

Figure 3. TEM images of (a) core-shell like Cu-Cu2S HNCsobtained 20 min after the addition of 1-dodecanethiol (DDT) andCu-Cu2S heterodimer HNCs obtained (b) 50 min and (c) 2.5 h afterthe addition of DDT. (d) SEM images of Cu2S NCs in polygon shapeswith one surface hole (marked with arrows) obtained 5 h after theaddition of DDT (inset, HRTEM image of a single NC). All thereactions were performed under H2/agron (H2, 7%). The scale barsrepresent 20 nm.

H2 + Cu2S f 2Cu + H2S (1)

Figure 4. (a) TEM image and (b) dark-field TEM image of Cu-Cu1.81Shetero-oligomer HNCs obtained 1 h after the addition of DDT into thesolution of Cu NCs under argon, and the bright regions correspond toCu, while the gray regions correspond to Cu1.81S (inset, STEM imageand the corresponding EDX spectra following a line scanning profileof one single hetero-oligomer). HRTEM images of several typicalhetero-oligomer HNCs obtained (c) 1 h and (d) 2 h after the additionof DDT into the solution of Cu NCs under argon (inset, FFT of Cuportion indicating single crystalline). (e) HRTEM image of the selectedarea of one hetero-oligomer (marked as a red square in panel c). Allunlabeled scale bars represent 20 nm.

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chemical transformation. Notice that in some cases the Cu1.81Sportions of hetero-oligomers can overlap to form core-shelllike nanostructures (Figure 5c).

The Formation of Hollow NCs. Ultimately, heterodimersevolve into polygon-shape NCs with surface holes (Figure 3d),while hetero-oligomers evolve into nanocages (Figure 5d). Bothtypes of NCs have been characterized as hexagonal Cu2S onthe basis of XRD data (Figure S8 in the Supporting Information).Notice that there is a phase change from tetragonal Cu1.81S tohexagonal Cu2S during hetero-oligomer evolution. This is likelydue to the greater stability of the Cu2S phase at 180 °C, asreported previously.44 The formation of empty spaces on bothsurface and interior can by explained with a mechanismanalogous to the Kirkendall effect,55 in which the fast flow ofCu toward and through the sulfide shell results in supersaturationof vacancies, leading to the formation of a single hole in each

NC. Similar strategies have been applied to produce hollownanostructures.56 To the best of our knowledge, however, thesynthesis of NCs with one single surface vacancy has not beenpreviously reported.

Optical Characterization of Cu-CuxS HNCs. The reactionevolution of both heterodimers (Figure 6, panels a and c) andhetero-oligomers (Figure 6, panels b and d) can also bemonitored by UV-vis spectra. Before DDT is added, bothspectra exhibit an absorption band centered at ∼580 nm, atypical SPR band for copper NCs greater than 4 nm.53,57 Afterthe addition of DDT, the morphological changes of HNCsdescribed above are reflected in the optical properties ofheterodimers and hereo-oligomers, resulting in solutions gradu-ally changing color from purple through green to yellow (Figure6c), and from purple to blue, light blue, yellow, and finally gray(Figure 6d), respectively. The corresponding UV-vis spectra

Figure 5. TEM images of Cu-Cu1.81S hetero-oligomer HNCs obtained (a) 20 min, (b) 1 h, and (c) 2.5 h after the addition of DDT into the solutionof Cu NCs, and (d) TEM images of Cu2S nanocages obtained 24 h after the addition of DDT into the solution of Cu NCs (inset of (d), SEM imageof one Cu2S nanocage showing smooth surfaces in the absence of any surface hole). All unlabeled scale bars represent 20 nm.

Figure 6. Room-temperature UV-vis absorption spectra (top) and respective digital photographs (bottom) of Cu-Cu2S heterodimer HNCs (a andc) forming solutions monitored at (i) 0, (ii) 1, (iii) 1.5, (iv) 5, and (v) 24 h after the addition of DDT under H2/argon (H2, 7%) and Cu-Cu1.81Shetero-oligomer HNCs (b and d) forming solutions at (i) 0, (ii) 1, (iii) 2, (iv) 4, and (v) 24 h after the addition of DDT under argon.

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of hetero-oligomers show that the maximum of the SPR bandred shifts from 580 to 616 nm after 4 h, and broadens (Figure6b), similar to those of core-shell metal-semiconductorsystems11,30 and metal-Fe3O4 systems.2 The UV-vis spectraof heterodimers (Figure 6a), however, demonstrate not only asimilar SPR band’s red shift from 580 to 642 nm after 1.5 h,consistent with the previously reported heterodimer systems,2

but also a remaining SPR band at 580 nm characteristic of CuNCs. The remaining SPR band at 580 nm indicates thereproduction of Cu from Cu2S reduced by H2, further confirmingour TEM observation (Figure 3). The decreasing relativeintensity of the band at 580 nm after 1.5 h is consistent withthe observed diminishing sizes of Cu portions (Figure 3). Thedisappearance of the same SPR band after 5 h for heterodimersand after 24 h for hetero-oligomers showing the completeconsumption of Cu NCs confirms again that H2 gas hassignificantly increased the reaction rate of the heterodimersystems. Eventually, heterodimers evolve into hexagon-phaseCu2S NCs with surface holes, evidenced by a SPR band at 450nm. This represents a larger blue shift from 1022 nm observedin bulk phase58 than previously reported Cu2S NCs,59 indicatinga stronger quantum size effect. Hexagon-phase Cu2S bearingpolygon shapes with hollow interiors are formed upon furtherconversion of hetero-oligomers, whose SPR band has beenobserved at 480 nm, consistent with previously reported data.60

Additionally, we are unable to detect any other UV-vis peaksthrough 2000 nm for both HNCs. We also failed to detect anyphotoluminescence of these two HNCs, and it is likely that thephotoluminescence of CuxS has been completely quenched dueto the close Cu-CuxS contact, which promotes charge separationacross the interface between CuxS and Cu.

The Sulfur Source. The slow releasing of sulfur during thethermolysis of DDT is necessary for the formation of Cu-CuxSHNCs. The control experiment of adding elemental sulfurinstead of DDT to the solution of presynthesized Cu NCs ledto the instantaneous formation of copper sulfide NCs (FigureS10 in the Supporting Information), indicating the much fasterreaction between Cu NCs and elemental sulfur. This observationis consistent with the previous report by Shen and co-workers.53

Additionally, DDT is also a surface protecting molecule(surfactant) to Cu NCs, and the tight bonding between S andCu presumably reduces the speed of the sulfidization reaction.

The Formation Mechanism and The Role of H2. As a well-known reducing agent, H2 plays a critical role in the formationof Cu-Cu2S heterodimers. Considering the positive standardreduction potential (E0 ) 0.34 V) for Cu2+/Cu, H2 gas firstfacilitates in reducing Cu(aca)2 into Cu NCs, as evinced by themuch faster formation reaction of Cu NCs under H2/argon(∼4 h) than under argon (∼12 h). Moreover, the majority ofCu NCs produced under H2/argon are polycrystalline withmultiple-twinned nanostructures (Figure 1a). This represents asignificant difference from those obtained under argon, where30% single crystalline Cu NCs are obtained (Figure S7 in theSupporting Information). After the addition of DDT underH2/argon, the surface layer of Cu NCs starts to react with sulfur,and highly defective and amorphous copper sulfide layersinitially form on these multiple-twinned Cu NCs, eventuallyleading to the formation of core-shell like HNCs (Figure 3a).Upon further reaction under H2/argon, the amorphous coppersulfide layers appear to crystallize, leading to the formation ofasymmetrical Cu-Cu2S heterodimer HNCs (Figure 3b). Thedetailed mechanism is currently under further investigation, andthe formation of Cu-Cu2S heterodimers from core-shell likestructures is likely due to crystalline lattice mismatch, which

makes complete coating of Cu NCs unfavorable and results indimeric Cu-Cu2S nanoparticles. Similar mechanisms have beenproposed for the formation of other HNCs.61,62 During crystal-lization, a gradual strain at the Cu/Cu2S junction regionintensifies owing to the poor lattice matching between thesetwo materials. As shown in Figure S11 in the SupportingInformation, lattices of both Cu and Cu2S can be resolved in asingle dimer HNC. The lattices are indexed to be Cu (111) witha spacing of 0.200 nm and Cu2S (103) with a lattice spacing of0.186 nm, leading a large mismatch of ∼7.5%. Over time, theshell coalesces and segregates into a discrete Cu2S domain,attaching at one side to the remaining Cu region. The formationof heterodimer HNCs has greatly relieved the large junctiontension at core-shell nanostructures due to the reduced inter-facial area. It has been reported that such processes (fromamorphous to crystal) usually require a significant heat treatment(>280 °C).61 The continuous flow of H2 gas in the experimentalsetup appears to speed up the process by reducing some of theCu2S layers into Cu (eq 1). This indicates that H2 gas has playeda critical role during the formation of heterodimer HNCs byassisting coalescence crystallization.

Sulfidation of metal NCs such as Cd has been well studiedby Alivisatos and co-workers.48,63 They demonstrate that hollowCdS NCs form only if and when the self-diffusion is larger forCd than for S, while the growth is controlled by the ions self-diffusion. In other cases where the growth is not controlled bythe ion diffusion, but rather by the reaction rate at each interface,solid (or asymmetrical sometimes) CdS NCs are obtained whenthe probability of reaction is higher at the Cd/CdS than at theCdS/solution interface. We can also explain the formation ofCu-Cu2S heterodimer and Cu-Cu1.81S hetero-oligomer HNCsusing similar mechanisms. For instance, under H2/argon, thegrowth is controlled by the reaction rate at the Cu/Cu2S interface,as evinced by the anisotropic shape of the Cu portion inheterodimers (Figure 2b-e), showing that Cu near the Cu/Cu2Sinterface is preferentially consumed to produce Cu2S. Therefore,Cu-Cu2S heterodimer HNCs form as intermediates, eventuallyleading to the formation of Cu2S solid NCs (Figure 3d) after5 h. This is a kinetic-controlled process. Under argon, the ionsself-diffusion appears to control the growth, with the formationof Cu-Cu1.81S hetero-oligomer and core-shell HNCs asintermediates, resulting in the thermodynamically favoredproducts of Cu2S nanocages (Figure 5d) after 24 h.

Conclusion

In summary, both Cu-Cu2S heterodimers and Cu-Cu1.81Shetero-oligomers have been synthesized via a simple one-potpolyol process. To our knowledge, this represents the firstcolloidal synthesis of nonhollow, metal-semiconductor HNCsvia chemical transformation. We believe such a syntheticstrategy can be readily extended to the fabrication of othersimilar metal-semiconductor HNC systems. Though the detailsof the formation mechanism are still elusive and under furtherinvestigation, we demonstrate that H2 gas plays a key role inthe transformation of heterodimers from core-shell like nano-structures, and it presumably reproduces Cu by reducing someof the Cu2S portions. Under argon, chemical transformationspreferentially take place on certain surface facets of Cu NCs toproduce mostly hetero-oligomers. Heterodimers and hetero-oligomers further evolve into polygon-shape NCs with surfaceholes and nanocages, respectively, likely due to a phenomenonsimilar to the Kirkendall effect. Finally, these results willfacilitate the potential application of metal-semiconductor HNCsin fuel and solar cells and in medical imaging.

Metal-Semiconductor Hybrid Nanocrystals J. Phys. Chem. C, Vol. 114, No. 10, 2010 4269

Acknowledgment. Y.L. acknowledges the support of Na-tional Research Council (NRC, NIH(NIBIB)/NIST) researchassociate fellowship. Both authors acknowledge the MarylandNanoCenter and its NispLab (The NispLab is supported in partby the NSF as a MRSEC Shared Experimental Facility), andthe University of Maryland X-ray crystallographic center.Certain commercial equipment, instruments, or materials areidentified in this paper to foster understanding and does notimply recommendation or endorsement by NIST, nor does itimply that the materials or equipment identified are necessarilythe best available for the purpose.

Supporting Information Available: EDX spectra ofCu-Cu2S heterodimers and Cu-Cu1.81S hetero-oligomers, TEMimages of hybrid nanocrystals obtained under different condi-tions, XRD patterns of Cu2S as final products upon furtherchemical transformation of heterodimers and hetero-oligomers,histograms of Cu nanocrystals and Cu portions in heterodimers,TEM images of Cu2S nanocrystals obtained by adding elementalsulfur into the solution of Cu nanocrystals, and the HR-TEMimage of one Cu-Cu2S heterodimer showing the interface ofCu and Cu2S. This material is available free of charge via theInternet at http://pubs.acs.org.

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