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COMMUNICATION View Article OnlineView Journal | View Issue

Key Laboratory of Materials Physics, Institute of Solid State Physics, and Key

Laboratory of New Thin Film Solar Cells, Chinese Academy of Sciences, Hefei

230031, China. E-mail: slji@issp.ac.cn, chye@issp.ac.cn; Fax: +86 551 65591434;

Tel: 86 551 65595629

† Electronic supplementary information (ESI) available: Experimental descriptionfor synthesis and characterization of Cu3SbS3 nanocrystals, experimental setupfor the synthesis of Cu3SbS3 nanocrystals, Raman spectra and EDS spectrum. SeeDOI: 10.1039/c3ce41861h

CrystEngComm, 201This journal is © The Royal Society of Chemistry 2013

Cite this: CrystEngComm, 2013, 15,10431

Received 16th September 2013,Accepted 17th October 2013

DOI: 10.1039/c3ce41861h

www.rsc.org/crystengcomm

Synthesis, characterization, and surface-enhancedRaman scattering of near infrared absorbingCu3SbS3 nanocrystals†

Xiaodong Qiu, Shulin Ji,* Chao Chen, Guangqiang Liu and Changhui Ye*

Ternary semiconductor nanocrystals from the I–V–VI family have

potential applications in many fields. In this work, we report on

the synthesis of well-dispersed Cu3SbS3 nanocrystals with an

average diameter of 23.0 ± 4.9 nm. The as-synthesized Cu3SbS3nanocrystals are of high phase purity and stable against phase

separation when heating up to 400 °C in an inert atmosphere or

left at room atmosphere for 80 days. The optical band gap of

the nanocrystals is calculated as ~1.87 eV. The nanocrystal film

shows strong near infrared absorption and is sensitive towards

detecting trace amounts of Rhodamine 6G by using surface-

enhanced Raman scattering. These observations imply the huge

potentials of Cu3SbS3 nanocrystals in photothermal, photovoltaic,

and sensing applications.

Semiconductor nanocrystals (NCs) are interesting researchtargets because of the tunability of their optical propertieswith the size, the high extinction coefficient, and the largebuilt-in dipole moment facilitating the charge carrier separa-tion.1–3 Methods have been developed to synthesize semicon-ductor NCs, and many fascinating properties have beenreported in recent years.4–12 Ternary semiconductors of theI–V–VI family have attracted tremendous research attentionin recent years owing to their widespread applications suchas in optoelectronics, light-emitting devices, and thermo-electric converters.13–15 Among these semiconductors, Cu3SbS3is especially interesting because it is important in electro-opticdevices and thermoelectric application.16,17 Despite the impor-tance of this material, so far, there have been only limitedreports for the synthesis of Cu3SbS3 nanomaterials.18,19 Cu3SbS3

usually appears as a tetrahedrite phase (cubic) or a skinneritephase (monoclinic). Cu3SbS3 nanomaterials reported in theliterature are mostly in the cubic phase. Nanomaterials ofother Cu–Sb–S family members, such as chalcostibite CuSbS2(orthorhombic) and famatinite Cu3SbS4 (tetragonal), havebeen synthesized by other researchers.20,21 However, to thebest of our knowledge, there have been no previous reportson the synthesis of Cu3SbS3 NCs with a narrow size dis-persion. Therefore, synthesis of monodispersed Cu3SbS3 NCswith high phase purity is essential to explore more significantproperties and extend the applications.

In recent years, semiconductors that demonstrate nearinfrared absorption have stimulated dramatic research zealfor the potential applications in photothermal and near infrareddetection.22–24 In addition, some researchers have alsodevoted efforts to explore non-metal surface-enhanced Ramanscattering (SERS) substrates for the purpose of detecting thetrace amount of target molecules such as Rhodamine 6G(R6G).25–27 There have been no previous reports on the appli-cation of Cu3SbS3 for these applications.

In this work, we have synthesized cubic phased Cu3SbS3NCs with high phase purity and narrow size dispersion by ahot-injection method. The NCs are stable against oxidation atroom temperature and phase separation at 400 °C in an inertatmosphere. In addition, the NCs show a strong near infraredabsorption and could act as an excellent surface-enhancedRaman scattering (SERS) substrate.

Cu3SbS3 NCs were synthesized by using a hot-injectionmethod. The details for the synthesis and characterizationsare included in ESI.† The experimental setup is shown inFig. S1.† The phase purity of the NCs has been examined byXRD measurements. We can see from Fig. 1A that all thediffraction peaks can be indexed to the cubic phase of Cu3SbS3(JCPDS card no. 75-1574), matching well with the standardpattern. There are no diffraction peaks from the Cu2S or Sbimpurity phase in the pattern. Even when we increased theproduction yield to gram scale, we still obtained Cu3SbS3 NCs

3, 15, 10431–10434 | 10431

Fig. 1 XRD patterns of the products. (A) Cu3SbS3 NCs of different production

scales. (B) Cu3SbS3 NCs after being exposed to the air for 80 days. (C) Cu3SbS3 NCs

after being heated at 400 °C for 5 min.

Fig. 2 XRD patterns of the products synthesized at different conditions. (A) Precursors

were added in together. (B) OLA was used as the sole solvent. (C) Sb and S precursors

were added after Cu precursor.

Fig. 3 (A) TEM image of Cu3SbS3 NCs. (B) HRTEM image of a Cu3SbS3 NC. Inset is

the SAED pattern taken along the [1̄11] zone axis. (C) Photograph of the large-scale

synthesis of Cu3SbS3 NCs. (D) Diameter distribution statistics of Cu3SbS3 NCs.

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with high phase purity (Fig. 1A). Cu3SbS3 NCs were exposedto the air for 80 days to investigate the stability. From theXRD pattern in Fig. 1B, no diffraction peaks from the Cu2S orSb impurity phase appeared, indicating the excellent stabilityof Cu3SbS3 NCs. It is well-known that copper chalcogenideNCs are prone to phase separation upon thermal treatment atan elevated temperature because of the high equilibriumvapor pressure of chalcogen elements and the large diffusionconstant of Cu ions. In order to check the thermal stability ofCu3SbS3 NCs, we have carried out temperature-dependentXRD measurements. As shown in Fig. 1C, when the tempera-ture increased from room temperature to 400 °C, no impurityphase appeared and all the peaks shifted to lower 2θ. As weknow, the lattice constant usually increases with the tempera-ture, which is the so-called thermal expansion. Since thelattice constant increases, the interplanar distance d increases.According to the Bragg equation, the diffraction peaks shouldshift to lower angles. Raman spectroscopy measurements ofthe Cu3SbS3 NCs in powder and film forms all show one wellcharacterized scattering peak at ~354 cm−1 and a shoulder at325 cm−1 (Fig. S2†). These peaks correspond to the phononvibration modes of [SbS3] units in the Cu3SbS3 lattice cells,supporting the good crystallinity of the NCs.28,29

The phase purity of Cu3SbS3 NCs is subject to the influenceof many experimental parameters. When CuCl, SbCl3, and Swere added into the flask together and heated at 240 °C, Cu2Simpurity appeared (Fig. 2A), which is probably because Cu+

is more active than Sb3+, and S2− preferred to react with Cu+

to form Cu2S impurity. If OLA was used as the sole solvent,Sb impurity was produced (Fig. 2B), which is probably becauseOLA is more reductive than a mixture of OLA and GLY andSb3+ tended to transform into Sb. When OLA and GLY wereused with Sb3+ and S being added after Cu+ reacting for90 min, monoclinic Cu3SbS3 impurity was observed (Fig. 2C).Therefore, controlling the experimental parameters is ratherimportant to obtain Cu3SbS3 NCs of high phase purity. The

10432 | CrystEngComm, 2013, 15, 10431–10434

formation processes of Cu3SbS3 NCs will be investigated infuture work.

The photograph in Fig. 3C exhibits the as-synthesized pro-duct in gram scale. Cu3SbS3 NCs with an average diameter of23.0 ± 4.9 nm have been synthesized (Fig. 3D), and shown inFig. 3A and B. From the HRTEM image in Fig. 3B, the NC iswell crystallized. The lattice fringes with a spacing of 0.73 nmin the image could be readily indexed to the {110} family. TheSAED pattern (inset in Fig. 3B) taken along the <1̄11> zone

This journal is © The Royal Society of Chemistry 2013

Fig. 5 SEM images of a Cu3SbS3 NC film. (A)–(C) Top view of the film at different

magnifications. (D) Cross-sectional view of the film with a thickness of 1.8 μm.

Fig. 6 (A) Optical absorption spectrum of Cu3SbS3 NCs. (B) The Tauc plot.

Fig. 7 CV curves (A) and the energy band positions of the NC films (B).

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axis shows the well-defined hexagonal arrangement of thediffraction spots, agreeing well with the HRTEM image. OneCu3SbS3 NC imaged along the <01̄1> zone axis is shown inthe HRTEM image in Fig. S3.† The lattice fringes with spacingof 0.73 and 0.52 nm correspond to {110} and {200} families.EDS was used to confirm the average composition of the NCs(Fig. S4†). The elemental mapping and the EDS analysis showthat the element distribution in the NCs is homogeneous andthe atomic ratio of Cu : Sb : S is 3.06 : 1.00 : 3.22, close to the stoi-chiometric composition of Cu3SbS3. XPS has been employedto examine the valence state of the elements in the product.As shown in Fig. 4, the valence states of Cu, Sb, and S ionsare +1, +3, and −2, respectively. The satellite peaks for Sb 3dmight originate from the slight oxidation of Sb at the surfaceof Cu3SbS3 NCs. The O 1s peaks to some extent overlap withthe Sb 3d peak and correspond to the Sb–O bonding oxygenand the adsorbed oxygen, respectively, at lower and higherbinding energy.30 The composition determined from XPS isalso close to the stoichiometric composition of Cu3SbS3, agreeingwell to the EDS result.

Cu3SbS3 NCs have been spin-coated on Si substrates toform densely packed films (Fig. 5). The films are smooth andhave no cracks or pinholes on the surface. We have measuredthe optical absorption spectrum of the Cu3SbS3 NC films(Fig. 6). It is striking that intense near infrared absorption ofCu3SbS3 NC film was observed. The spectrum consists of twoparts, the visible light and the near infrared range. The directband gap energy of ~1.87 eV determined by using Tauc plottingis consistent with the data reported by Maiello et al. for theCu3SbS3 phase instead of Cu12Sb4S13 with a smaller bandgap.31 According to van Embden et al.,32 the near infraredabsorption is due to the midband gap states, although thenature of the midband gap states here is still not well under-stood and requires further research work. We do notice that thenear infrared absorption seems common to copper-containingsemiconductors, such as CuxS, CuxSe, and CuxTe.

33

Fig. 4 XPS spectra of Cu3SbS3 NCs. (A) The full scan spectrum. (B) Cu 2p spectrum.

(C) Sb 3d together with O 1s spectrum. (D) S 2p spectrum.

This journal is © The Royal Society of Chemistry 2013

We have measured the CV curves and determined the bandposition of Cu3SbS3 NCs. As shown in Fig. 7A, an anodic and acathodic peak appears in the CV curve, respectively, at 2.03and 0.15 eV relative to a reversible hydrogen electrode (RHE).Accordingly, the band position could be calculated relative toa normal hydrogen electrode (NHE) and is plotted in Fig. 7B.The band gap energy of 1.88 eV is close to that determined byoptical measurement within the experimental error.

Finally, we found that Cu3SbS3 NC films could be used asan effective SERS substrate. A piece of film was immersed into1 × 10−6 M R6G aqueous solutions for 30 min. The piece wasthen rinsed thoroughly with deionized water and finally driedin N2. Obvious SERS signals could be distinctly observed (Fig. 8).To exclude the effect of Si substrate, we have also carried outa control experiment on bare Si substrate with a R6G concen-tration of 10−5 M. It exhibits no Raman signals even with multi-plying the curve intensity by 100 times. When R6G with aconcentration of 1 ppm was dropped on Cu3SbS3 NC films,

CrystEngComm, 2013, 15, 10431–10434 | 10433

Fig. 8 SERS spectra of R6G molecules adsorbed on Cu3SbS3 NC films.

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the SERS signals could be distinctly observed (Fig. 8). Evenwhen the concentration of R6G was decreased to 10−7 M, wecould still observe some of the SERS signals from R6G. Theseobservations indicate that we can detect trace amounts oforganic pollutants using Cu3SbS3 NC film as an effectiveSERS substrate.

So far, however, the exact origin of the near infrared absorp-tion and the enhanced SERS performance of Cu3SbS3 NC filmshave not been clearly known. From our XPS data, it seems thatthe NCs do not contain a detectable amount of Cu2+ which wasassumed to contribute to the near infrared absorption eitherthrough localized plasmon resonance23 or midband defects.31

Therefore, further work is needed to clarify this point.In summary, we have synthesized Cu3SbS3 NCs with high

phase purity and narrow size dispersion. The NCs exhibitsuperior stability against phase separation both in air at roomtemperature and in an inert atmosphere at a higher tempera-ture. Cu3SbS3 NC films show strong near infrared absorption,and could act as an effective SERS substrate towards the sensi-tive detection of trace amounts of organic pollutants such asR6G in water. These findings indicate that Cu3SbS3 NCs havesignificant potential applications in photothermal, photovoltaic,and SERS detection fields.

Acknowledgements

This work was supported by National Basic Research Programof China (973 Program, grant no. 2011CB302103), NationalNatural Science Foundation of China (grant no. 11074255 and11274308), and the Hundred Talent Program of the ChineseAcademy of Sciences.

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