10332 Chem. Commun., 2011, 47, 10332–10334 This journal is c The Royal Society of Chemistry 2011

Cite this: Chem. Commun., 2011, 47, 10332–10334

Morphology evolution of Cu2�xS nanoparticles: from spheres to

dodecahedronsw

Wenhua Li,aAlexey Shavel,

bRoger Guzman,

cJavier Rubio-Garcia,

aCristina Flox,

a

Jiandong Fan,bDoris Cadavid,

aMaria Ibanez,

bJordi Arbiol,

cdJoan Ramon Morante

aband

Andreu Cabot*ab

Received 25th June 2011, Accepted 9th August 2011

DOI: 10.1039/c1cc13803k

An oriented attachment and growth mechanism allows an accurate

control of the size and morphology of Cu2�xS nanocrystals, from

spheres and disks to tetradecahedrons and dodecahedrons. The

synthesis conditions and the growth mechanism are detailed here.

The ability to control the nanocrystals composition, size, shape,

crystal phase and surface planes allows tuning their optical

and optoelectronic properties and their activity and selectivity

towards specific catalytic and photocatalytic reactions. The

control at the nanoscale of the composition and morphology

of copper chalcogenides is an especially interesting case, because

of their stoichiometry-dependent properties and their ample

range of applications. However, the rich phase diagrams of

copper chalcogenides make their growth mechanisms also

especially challenging to uncover and control. The equilibrium

phase diagram of copper sulfide exhibits at least seven phases:

(i) monoclinic low-chalcocite Cu2S, (ii) hexagonal high-chalcocite

Cu2S, (iii) monoclinic djurleite Cu1.96S, (iv) hexagonal digenite

Cu1.8S, (v) monoclinic roxbyite Cu1.78S, (vi) orthorhombic anilite

Cu1.75S, (vii) hexagonal covellite CuS.1 Such variety of crystallo-

graphic phases has allowed the preparation of nanoparticles

with different morphologies and compositions: (i) spherical

chalcocite2 and djurleite3 nanoparticles; (ii) chalcocite nano-

wires;4 (iii) chalcocite,5,6 roxbyite7,8 and covellite8,9 nanodisks;

(iv) anilite hollow nanocages;10 (v) djurleite and digenite irregular

nanocrystals.9 The goal of the present work is to provide the

mechanisms to extend the control of the Cu2�xS nanoparticles

morphology to a wider range: from spherical to tetradecahedral

and dodecahedral geometries (Scheme 1). Herein we reveal the

synthetic routes and discuss the growth mechanisms allowing

tuning of the nanoparticle morphology in such extended range.

Cu2�xS nanoparticles were obtained from the reaction of

copper chloride with di-tert-butyl disulfide (TBDS) in a heated

oleylamine solution (OLA). In a typical preparation, 0.0852 g

of CuCl2�2H2O (0.5 mmol, 99.99%, Aldrich) and 12 g of

OLA (70%, Aldrich) were introduced inside a four-neck flask

and heated to 200 1C under argon flow. The yellowish

transparent solution produced was maintained at 200 1C for

an additional hour for purification, i.e. to remove oxygen,

water and other low-boiling point impurities. Afterwards, the

temperature was set to 180 1C and 1 ml of TBDS (5 mmol,

97%, Aldrich) was injected through a septum. The mixture

was maintained at the reaction temperature for up to 1 hour to

allow the nanoparticles growth. Finally, the flask was rapidly

cooled down to room temperature. Detailed information on

the particular synthesis conditions is included in the ESI.wThe reaction of copper chloride with an excess of TBDS

initially yields Cu2�xS spherical nanoparticles (Fig. 1A). Due

to their low crystallinity, their phase could not be unambiguously

identified. An X-ray diffraction pattern could be matched with

that of chalcocite (Cu2S), as previously assigned,2 but also with

that of roxbyite (Cu1.78S). The determination of the chemical

composition of the nanocrystals by spectroscopy techniques

was particularly imprecise because of the large concentration

of sulfur and copper complexes which remained unreacted and

were extremely difficult to remove. The slow nanoparticle

growth rates obtained by the present route at relatively low

temperatures (r200 1C) and precursor concentrations (0.05 M)

allowed following their gradual morphology evolution. Spherical

nanocrystals evolved into circular nanodisks (Fig. 1B) at early

reaction times. The sphere to disk transition could be delayed

and even suppressed by introducing thiols in the reaction

mixture. With the reaction time, circular nanodisks became

Scheme 1 Schematic representation of the Cu2�xS morphologies

obtained.

a Catalonia Institute for Energy Research, IREC, Jardı de les Donesde Negre, 1, Planta 2, 08930, Sant Adria del Besos, Barcelona,Spain. E-mail: acabot@irec.cat; Fax: +34 933563802;Tel: +34 933562615

bDepartament Electronica, Universitat de Barcelona, 08028, Spainc Institut de Ciencia de Materials de Barcelona, ICMAB-CSIC,Campus de la UAB, Bellaterra, 08193, Spain

d Institucio Catalana de Recerca i Estudis Avancats, ICREA,Barcelona, 08010, Spain

w Electronic supplementary information (ESI) available: Additionalmaterials characterization and Cu2�xS electrocatalytic performance inall-vanadium redox flow batteries. See DOI: 10.1039/c1cc13803k

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This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 10332–10334 10333

faceted in 6 equivalent directions, resulting in hexagonal nano-

disks (Fig. 1C and D). The improved crystallinity of the nano-

disks allowed the identification of their crystal phase as roxbyite

(Cu1.78S). For thick enough nanodisks, the faceting of their

lateral surface was also thermodynamically favored, and the

nanodisks became thin tetradecahedrons. Longer reaction times

and higher temperatures increased the nanodisks crystallinity

and diameter, but only modified slightly their thickness and not

perceptibly their crystallographic phase and composition.

A change of scenario was obtained when modifying the Cu

concentration in solution. In the presence of relatively elevated

Cu concentrations (0.1–1 M), high densities of thin nanoplates

were initially formed. Probably driven by dipole–dipole inter-

actions,6 thin nanoplates assembled face-to-face into dimers,

trimers or quadrumers, depending on their concentration

(Fig. 2A and B). The relatively low growth rate of the present

system allowed the formed assemblies to continue growing in a

still rather concentrated solution. In such assemblies, the

crystal growth took place preferentially in between the pilled

nanoplates, fusing them together into single nanoparticles

(Fig. 2C and D). With the reaction time, these polycrystalline

nanoparticles restructured into single-crystal domains. Such

nanocrystals gradually became faceted, adopting a tetrahedral

shape (Fig. 2E, F and 3). At increasingly higher precursor

concentrations, larger assemblies were initially formed and

thus, more elongated tetradecahedrons were finally obtained

(Fig. 3C and D). In the limit, large enough assemblies resulted

in dodecahedrons (Fig. 3E and F).

Due to the low nucleation rate of the present synthetic route,

the relatively high monomer concentration remaining in solution

after nucleation ensured amoderately slow but continuous growth

of the nanoparticle. Such slow growth rates allowed an accurate

size control of the prepared nanocrystals, as shown in Fig. 4.

An increase of the copper precursor concentrations not

only influenced the particle size and morphology, but also

promoted a higher incorporation of this element into the

nanocrystal structure. This increase of the copper uptake by the

nanoparticles translated into a change of the crystal phase,

from the monoclinic roxbyite Cu1.78S, identified in the faceted

nanodisks obtained at relatively low copper concentrations, to

the still monoclinic djurleite Cu1.96S observed in the tetrahedral

and dodecahedral nanoparticles obtained at precursor con-

centrations above 0.1 M.

Fig. 1 TEM (A)–(C) and SEM (D) images of Cu2�xS nanoparticles

with various morphologies: (A) spherical; (B) circular nanodisks; (C)

and (D) hexagonal nanodisks. Scale bars = 200 nm.

Fig. 2 TEM images showing the morphology evolution of Cu2�xS

nanoparticles with the reaction time; from nanoplates assembled in dimers

and quadrumers to tetradecahedral nanocrystals. (A)–(C) [Cu] = 0.1 M;

(D)–(F) [Cu] = 0.2 M. All figures have the same scale bar = 100 nm.

Fig. 3 TEM (left) and SEM (right) images of Cu1.96S nanoparticles:

(A) and (B) small tetradecahedrons; (C) and (D) elongated tetra-

decahedrons; (E) and (F) dodecahedrons. Scale bars = 200 nm.

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10334 Chem. Commun., 2011, 47, 10332–10334 This journal is c The Royal Society of Chemistry 2011

HRTEM analysis of the thin tetradecahedrons showed them

to be strongly faceted along the h100i top and bottom planes

and having h111i and h120i lateral facets (ESIw and Fig. 5).

Tetradecahedron elongation took place along the h100i direction.In the limit, dodecahedrons were obtained when the elongation

of the tetradecahedrons in the h100i direction resulted in the

suppression of the h100i facets (Fig. 5).The performance of Cu2�xS nanocrystals as a cathode in

all-vanadium redox flow batteries (VRB) was also tested. The

efficiency of this type of battery is usually limited by the rate and

potential of the [VO]2+/[VO2]+ cathodic reaction. Cu2�xS nano-

crystals were thoroughly purified, deposited on a substrate and

fixed by Nafion. Their electrochemical activity was characterized

in an inert atmosphere by means of cyclic voltammetry. Their

characteristics were compared with those obtained for a

polyacrylonitrile-derived graphite felt, which is a material widely

used as a VRB cathode. Lower oxidation potentials were system-

atically obtained for Cu2�xS nanocrystals when compared to the

PAN-based graphite felt (ESIw). At a 2 mV s�1 scan rate, the

oxidation potential was found at 0.72 V for the PAN-based

graphite felt and at 0.44 V for the electrode containing Cu2�xS

nanocrystals. This result denotes faster electrocatalytic kinetics

of the oxidation process for the electrode containing Cu2�xS

nanocrystals. The difference of potential between the oxidation

and reduction peaks was 0.36 V for the PAN-based graphite felt

and 0.10 V for the Cu2�xS electrode. Furthermore, the ratio

between the currents at the oxidation and reduction peaks for

Cu2�xS nanocrystals was close to unity and changed moderately

with the scan rate. These experimental results pointed towards a

significant performance improvement in terms of reversibility of

the [VO]2+/[VO2]+ redox process with the use of Cu2�xS nano-

crystals. However, the currents measured with Cu2�xS cathodes

were lower than those obtained with PAN-based graphite felts.

Thus, a further optimization of the density and dispersion of the

nanocrystals and their surface conditioning are still required.

In conclusion, by tuning the precursor concentration and

reaction conditions, Cu2�xS nanoparticles with different morpho-

logies were obtained. In particular, tetradecahedrons and

dodecahedrons were synthesized at relatively high precursor

concentrations by means of an oriented attachment and

growth mechanism involving the assembly of nanoplates into

dimers, trimers, quadrumers and even larger assemblies, and

their recrystallization into faceted single-crystal nanoparticles.

In terms of reversibility, Cu2�xS nanocrystals showed promising

electrocatalytic performance as cathodes in all-vanadium redox

flow batteries.

This work was supported by the Spanish MICINN projects

MAT2008-05779, MAT2008-03400-E/MAT, ENE2008-03277-E/

CON, MAT2010-15138, MAT-2010-21510, CDS2009-00050 and

CSD2009-00013 and by Generalitat de Catalunya 2009-SGR-770

and XaRMAE

Notes and references

1 (a) D. J. Chakrabarti and D. E. Laughlin, Bull. AlloyPhase Diagrams, 1983, 4, 254; (b) W. G. Mumme, G. J. Sparrowand G. S. Walker, Mineral. Mag., 1988, 52, 323.

2 S. Li, H. Wang, W. Xu, H. Si, X. Tao, S. Lou, Z. Du and L. S. Li,J. Colloid Interface Sci., 2009, 330, 483.

3 W. Han, L. Yi, N. Zhao, A. Tang, M. Gao and Z. Tang, J. Am.Chem. Soc., 2008, 130, 13152.

4 (a) L. Chen, Y.-B. Chen and L.-M. Wu, J. Am. Chem. Soc., 2004,126, 16334; (b) Z. Liu, D. Xu, J. Liang, J. Shen, S. Zhang andY. Qian, J. Phys. Chem. B, 2005, 109, 10699.

5 (a) Y.-B. Chen, L. Chen and L.-M. Wu, Chem.–Eur. J., 2008,14, 11069; (b) X.-S. Du, Z.-Z. Yu, Z. Dasari, J. Ma, Y.-Z. Mengand Y.-W. Mai, Chem. Mater., 2006, 18, 5156; (c) M. B. SigmanJr., A. Ghezelbash, T. Hanrath, A. E. Saunders, F. Lee andB. A. Korgel, J. Am. Chem. Soc., 2003, 125, 16050.

6 Z. Zhuang, Q. Peng, B. Zhang and Y. Li, J. Am. Chem. Soc., 2008,130, 10482.

7 W. P. Lim, C. T. Wong, S. L. Ang, H. Y. Low and W. S. Chin,Chem. Mater., 2006, 18, 6170.

8 H. Zhang, Y. Zhang, J. Yu and D. Yang, J. Phys. Chem. C, 2008,112, 13390.

9 (a) A. Ghezelbash and B. A. Korgel, Langmuir, 2005, 21, 9451;(b) W. Du, X. Qian, X. Ma, Q. Gong, H. Cao and J. Yin,Chem.–Eur. J., 2007, 13, 3241.

10 Y. Zhao, H. Pan, Y. Lou, X. Qiu, J. Zhu and C. Burda, J. Am.Chem. Soc., 2009, 131, 4253.

Fig. 4 TEM images of dodecahedral nanocrystals obtained after

different reaction times from 10 to 60 min. Longest dimension: (A)

72 � 4 nm; (B) 110 � 5 nm; (C) 138 � 5 nm; (D) 155 � 5 nm. All scale

bars are identical = 200 nm.

Fig. 5 Scaled (1 : 5) 3D atomic models of Cu1.96S Djurleite nanoparticles

with different morphologies, from hexagonal nanodisks to dodecahedrons.

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