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Nanoscale copper sulfide hollow spheres with phase-engineered composition:covellite (CuS), digenite (Cu1.8S), chalcocite (Cu2S)†

Peter Leidinger,a Radian Popescu,b Dagmar Gerthsen,b Heinrich L€unsdorfc and Claus Feldmann*a

Received 19th January 2011, Accepted 22nd March 2011

DOI: 10.1039/c1nr10076a

Covellite (CuS), digenite (Cu1.8S) and chalcocite (Cu2S) are prepared as nanoscaled hollow spheres by

reaction at the liquid-to-liquid phase boundary of a w/o-microemulsion. According to electron

microscopy (SEM, STEM, TEM, HRTEM) the hollow spheres exhibit an outer diameter of 32–36 nm,

a wall thickness of 8–12 nm and an inner cavity of 8–16 nm in diameter. The phase composition is

determined based on HRTEM, electron-energy loss spectroscopy, X-ray powder diffraction and

thermal analysis. In face of the advanced morphology of the hollow spheres, precise control of its phase

composition is nevertheless possible by adjusting the experimental conditions (i.e. type and

concentration of the copper precursor, concentration of ammonia inside of the micelle). Such phase-

engineering of nanoscale hollow spheres is firstly observed and might allow adjusting even further

compositions/structures as well as tailoring of phase-specific properties in the future.

1. Introduction

The system copper–sulfur exhibits at least nine different CuxS-

phases with different compositions and crystal structures. This

includes villamaninite (CuS2), covellite (CuS), yarrowite

(Cu1.12S), spionkopite (Cu1.39S), geerite (Cu1.6S), anilite

(Cu1.75S), digenite (Cu1.8S), djurleite (Cu1.96S) and chalcocite

(Cu2S).1–4As different as the phase composition is as different are

the material properties. Thus, copper sulfides are known as p-

type semiconductors (e.g. Cu2S),4 superionic conductors (e.g.

Cu2�xS), thermo- or photoelectric transformers (e.g. Cu1.8S) and

high-temperature thermistors (e.g. Cu1.8S).1 Covellite interest-

ingly represents the first example of a natural mineral showing

superconductivity (Tc: 1.6 K).3 Selective preparation of all these

copper sulfide phases with their specific properties requires

a precise adjustment of the relevant experimental conditions.

Applying state-of-the-art chemical synthesis (e.g. hydro-/sol-

vothermal synthesis, solid state reactions) even for bulk materials

such phase control is not straightforward.5–7 When aiming at

nanoscaled hollow spheres with an advanced morphology (i.e.

inner cavity of 5–50 nm, wall thickness of 2–20 nm), huge specific

surfaces (100–500 m2 g�1) and a synthesis that already requires

elaborate experimental conditions,8 a precisely adjusted

aInstitut f€ur Anorganische Chemie, Karlsruhe Institute of Technology(KIT), Engesserstraße 15, D-76131 Karlsruhe, Germany. E-mail: claus.feldmann@kit.edu; Fax: +49-(0)721-6084892bLaboratorium f€ur Elektronenmikroskopie, Karlsruhe Institute ofTechnology (KIT), Engesserstraße 7, D-76131 Karlsruhe, GermanycDepartment of Vaccinology and Applied Microbiology, Helmholtz Centerfor Infection Research, Inhoffenstrasse 7, D-38124 Braunschweig,Germany

† Electronic supplementary information (ESI) available. See DOI:10.1039/c1nr10076a

2544 | Nanoscale, 2011, 3, 2544–2551

generation of the one or other phase composition can be

expected to be even more challenging.

Altogether, the system copper–sulfur with its variety of phase

compositions is an excellent playground to monitor the precise-

ness of advanced hollow sphere synthesis. In fact, CuxS hollow

spheres have been already described, but are basically limited to

the covellite phase (CuS). Here, solvothermal methods,9 hydro-

thermal synthesis,10 liquid-droplet approaches,11 hard-template

methods12 and sonication methods13 have been applied.

Furthermore, Xu et al. have reported on mesoscaled covellite

and chalcocite hollow spheres that were gained via a chemical

conversion route.14 Finally, polydisperse hollow spheres of Cu2S,

Cu1.8S and Cu1.75S with outer diameters ranging from 2–4 mm

have been realized by Nan et al. via ethylene-glycol-mediated

solvothermal synthesis.15 In sum, all experimental work pre-

sented till now is limited to polycrystalline hollow spheres with

outer diameters that most often largely exceed the nanoregime (i.

e. d < 100 nm).

Based on a microemulsion approach we have already shown

a wide flexibility towards the chemical composition of nanoscale

hollow spheres. Thus, the synthesis includes metals (e.g. Au,

Ag)16,17 and oxides/hydroxides (e.g. g-AlO(OH), La(OH)3, ZnO,

SnO2).18,19 Typically, these hollow spheres exhibit outer diame-

ters of 10–50 nm, a wall thickness of 2–20 nm and an inner cavity

of 2–40 nm. Most recently, we could also show that a precise

adjustment of outer diameter and inner cavity size is possible via

the microemulsion approach.20 Based on this high adaptability

regarding the composition and the size of the hollow spheres, the

microemulsion approach exhibits certain advantages as

compared to the much more common hard-template techniques,

viz. is the precipitation of a shell on a given hard template (e.g.

SiO2, Au, Bi, Ag, CdSe, polymer lattices) as a first step, and the

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removal of this template (e.g. by thermal burnout or chemical

dissolution) out of the shell to yield the final hollow sphere.21,22

To further elucidate the potential of the microemulsion

approach, we here address a precise control of the phase

composition in the copper–sulfur system. To the best of our

knowledge such phase-engineering has not been shown for

nanoscale hollow spheres at all.

2. Experimental section

2.1 Materials and synthesis

All experimental work was performed in vacuum or under

nitrogen utilizing Schlenk-techniques or glove-boxes. Toluene

was distilled over sodium wires; all further chemicals were

applied as received.

Chlorotris(triphenylphosphine)-copper(I) (CTTPPC). CTTPPC

was prepared via a modified synthesis as described elsewhere.23,24

Accordingly, CTTPPC was prepared by addition of triphenyl-

phosphine (40 g, 153 mmol) to CuCl2$2H2O (5.86 g, 34 mmol) in

ethanol (800 ml). This solution was refluxed for 6 hours while

excess PPh3 initiated a reduction of Cu2+ to Cu+. The white

remnant was washed twice with ethanol and dried in vacuum

(yield: 30 g, 83%). The purity of the as-prepared CTTPPC was

validated by XRD and FT-IR. The product was slightly sensitive

to daylight and therefore stored in the dark.

Standard microemulsion (SME). All hollow spheres were

prepared via a modified microemulsion technique, which in the

following is named as a ‘‘standard microemulsion’’ SME(x) with

x indicating the concentration of aqueous ammonia inside of the

microemulsion. Such SME(x) consisted of 50 ml of toluene as the

non-polar phase, 0.5 mmol of thiourea (TU) in 2 ml of an x wt%

aqueous ammonia solution as the polar phase, 1.82 g of cetyl-

trimethylammonium bromide (CTAB) as a surfactant and 5 ml

of n-hexanol as a co-surfactant. After 30 min of vigorous stirring

at 35 �C (heating via oil bath), the micellar system became

transparent, indicating the presence of a stable w/o-

microemulsion.

Covellite (CuS) hollow spheres. To prepare CuS hollow spheres

80.4 mg of bis(cyclohexanebutyrate)-copper(II) (CHBC, 0.2

mmol) were dissolved in 20 ml of toluene, applying an ultrasonic

treatment for 1 min (standard ultrasonic bath). The resulting

green solution was added to an SME(6.25), which instanta-

neously changed its color to deep blue. Afterwards, the temper-

ature of the oil-bath was increased under moderate stirring to

60 �C. During the heating process the colour of the system

changed from deep blue (with green, orange and brown as

intermediate colors) to black, indicating the formation of

covellite. When cooled to room temperature, 20 ml of diethylene

glycol (DEG) were added to initiate a phase separation.25 To this

concern, the complete solution was filled into a 100 ml graduated

cylinder. After some minutes the black precipitate was trans-

ferred to the upper non-polar toluene phase. Light scattering

with a greenish colour indicated the presence of the nanoparticles

in the toluene phase. Thereafter, the upper phase was diluted

with ethanol and centrifuged. The remnant was washed three

This journal is ª The Royal Society of Chemistry 2011

times with ethanol and dried in an oven at 60 �C (yield: 7 mg).

Note that a significant excess of thiourea was applied here, so

that a [Cu2+]/[S2�] ratio of 0.4 results.

Digenite (Cu1.8S) hollow spheres. To gain Cu1.8S hollow

spheres a slurry of 442 mg CTTPPC (0.5 mmol) in 20 ml of

toluene was treated for 1 min in a standard ultrasonic bath.

Subsequently, the slurry was added to an SME(10.0). With the

addition of the whitish CTTPPC-slurry to the microemulsion,

the complete system became transparent within some seconds.

Heating and washing was performed as described for the CuS

hollow spheres (yield: 22 mg). Here, a [Cu+]/[S2�] ratio of 1.0 was

introduced, but nevertheless reproducibly led to the composition

Cu1.8S.

Chalcocite (Cu2S) hollow spheres. To obtain Cu2S hollow

spheres a slurry of 885 mg CTTPPC (1.0 mmol) in 20 ml of

toluene was treated for 1 min in a standard ultrasonic bath.

Subsequently, the slurry was added to an SME(6.25). With the

addition of the whitish CTTPPC-slurry to the microemulsion the

complete system became transparent within some seconds.

Thereafter, the system was heated and washed as described

above. Cu2S hollow spheres were obtained with a yield of 32 mg.

Note that a [Cu+]/[S2�] ratio of 2 was applied here in accordance

with the stoichiometry of the compound.

Trioctylphosphine oxide (TOPO) stabilized CuxS hollow

spheres. TOPO-stabilized hollow spheres were prepared imme-

diately after formation of the relevant CuxS hollow spheres as

follows: 1 g TOPO was added to the suspensions at 60 �C under

vigorous stirring, followed by vigorous stirring for 30 min at

80 �C. Stabilized hollow spheres were collected by centrifugation

and washing with toluene and ethanol. The remnant was redis-

persed in an ethanol–toluene mixture (1 : 1). The resulting

suspensions were colloidally stable for months.

2.2 Materials characterisation and analytical tools

Scanning electron microscopy (SEM). SEM was conducted on

a Zeiss Supra 40 VP, using an acceleration voltage of up to 30 kV

and a working distance of 4 mm. SEM samples were made with

the as-prepared powder samples on silica plates, followed by Pt-

sputtering.

Scanning transmission electron microscopy (STEM). STEM

was conducted on a Zeiss Supra 40 VP, too, using an acceleration

voltage up to 30 kV and a working distance of 4 mm. STEM

samples were prepared by 3 days vacuum exsiccation of droplets

of TOPO-stabilized CuxS hollow spheres in an equimolar

ethanol–toluene mixture over P4O10 on holey carbon-film

copper-grids at room temperature. Mean diameters of the hollow

spheres were deduced by statistical evaluation of at least 200

particles. Note that systematically different diameters obtained

by SEM and STEM imaging are related to different electron-

detection modes and interaction volumes of the primary elec-

trons with the sample. While SEM images are based on back-

scattered secondary electrons that were detected by an Everhart-

Thornley detector, only transmitted electrons are collected in the

STEM mode of operation.

Nanoscale, 2011, 3, 2544–2551 | 2545

Table 1 Experimental details to precisely adjust the composition ofCuxS hollow spheresa

Experimental parameterCovellite Digenite Chalcocite(CuS) (Cu1.8S) (Cu2S)

Concentration of ammonia 6.25 mol% 10.00 mol% 6.25 mol%Type of copper precursorb CHBC CTTPPC CTTPPCAmount of copperprecursor

0.20 mmol 0.50 mmol 1.00 mmol

Amount of thiourea 0.50 mmol 0.50 mmol 0.50 mmol

a All microemulsions were constituted of 50 ml of toluene as the non-polar phase, 2 ml of aqueous ammonia solution as the polar phase,1.82 g of cetyltrimethylammonium bromide (CTAB) as a surfactantand 5 ml of n-hexanol as a co-surfactant. b CHBC: bis(cyclohexanebutyrate)-copper(II); CTTPPC: chlorotris(triphenyl-phosphine)-copper(I).

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Transmission electron microscopy (TEM). TEM was per-

formed with a Philips CM 200 FEG/ST microscope, operated at

200 kV. TEM analysis was carried out with the identical sample

as used for STEM.

Electron energy loss spectroscopy. Electron energy loss spec-

troscopy was done on CuxS hollow spheres that were dried from

the alcohol solute onto 30 nm carbon foils supported by retic-

ulum foils. Wide range parallel EELS (WR-PEELS) was recor-

ded for 220 s on the whole of three integration cycles with an

integrated energy-filter TEM (LIBRA120plus, Zeiss) at an

emission current of 1–2 mA, a spectrum magnification of �100,

an illumination aperture of 0.80 mrad and a 90 mm objective

aperture. Spectra were background corrected by power-law.

Electron spectroscopic imaging (ESI) was performed at the same

aperture settings and an energy-slit width of 10 eV, background

correction was performed, according to the two-window-power-

law method.

X-Ray powder diffraction (XRD). XRD was carried out with

a Stoe Stadi-P diffractometer using Ge-monochromatized

Cu-Ka1 radiation.

Dynamic light scattering (DLS). DLS was performed with

a Nanosizer ZS fromMalvern Instruments (equipped with a He–

Ne laser, detection via non-invasive back-scattering at an angle

of 173�, 256 detector channels). Analysis was carried out with

TOPO-stabilized hollow spheres after resuspension in 1 : 1

mixture of ethanol and toluene.

Thermogravimetry (TG). TG was performed with a Netzsch

STA 409C instrument, applying a-Al2O3 as a crucible material as

well as a reference sample. For sample preparation, first of all,

the as-prepared hollow spheres were pre-dried (150 �C, 60 min) in

order to remove water and ethanol absorbed on the particles

surface. The samples were heated in air up to 1000 �C with

a heating rate of 5 K min�1.

3. Results and discussion

3.1 Strategy of microemulsion-based synthesis

All the nanoscale CuxS hollow spheres presented here were

prepared via microemulsion techniques. To this concern, a w/o-

(water-in-oil) microemulsion was established with toluene as the

non-polar oil-phase, cetyltrimethylammonium bromide (CTAB)

as surfactant, n-hexanol as co-surfactant, and finally aqueous

ammonia as the polar phase. Moreover, thiourea was added to

the polar phase as a reactant and S2� source (Table 1). In

concrete, a water-to-surfactant ratio u ¼ [H2O]/[CTAB] ¼ 11

turned out as optimal to reproducibly prepare uniform hollow

sphere. According to dynamic light scattering, this results in

a hydrodynamic diameter of 6.7 nm of the initial micelles. While

these conditions and concentrations were exactly identical

(therefore denoted as standard microemulsion SME(x) contain-

ing x wt% of aqueous ammonia), the essential difference to

precisely control the formation of covellite (CuS), digenite

(Cu1.8S) and chalcocite (Cu2S) is related to three aspects: the

concentration of ammonia as well as the type and concentration

2546 | Nanoscale, 2011, 3, 2544–2551

of the copper precursor. In detail an SME(x) was used with

x ¼ 6.25 to gain covellite or chalcocite and with x ¼ 10.0 to gain

digenite. As the copper precursor, bis(cyclohexanebutyrate)-

copper(II) (CHBC) was introduced to prepare chalcocite (CuS);

chlorotris(triphenylphosphine)-copper(I) (CTTPPC) was used to

gain digenite (Cu1.8S) and chalcocite (Cu2S) (Table 1).

As soon as the microemulsion had been established at

a temperature of 35 �C, the relevant copper precursor was addedto the non-polar oil-phase. Thereafter, the reaction was initiated

by heating the system to 60 �C. In order to gain nanoscaled

hollow spheres, the formation of CuxS now has to occur at the

water-to-oil phase boundary of the micellar system. The reaction

can be followed by a considerable color change in a course of

blue or light green to brown and deep black (Fig. 1). After about

20 min the reaction was finished. In sum, a slow, diffusion-

controlled reaction at the liquid-to-liquid phase boundary of the

micellar system turned out as crucial to realize hollow spheres

and to suppress a formation of massive CuxS particles.16–20 To

this concern, the slow decomposition of the copper complexes

(i.e. CHBC, CTTPPC) and thiourea, accompanied by a slow

release of the actual reactants (i.e. Cu+/2+, S2�) can be regarded as

a prerequisite.

With the experimental conditions described here, covellite

(CuS), digenite (Cu1.8S) and chalcocite (Cu2S) are obtained as

pure phases (cf. 3.2) with highly uniform size and shape. In

addition there are hints to a formation of additional CuxS

phases such as geerite (Cu1.6S). The differentiation of some

CuxS phases is however difficult due to their very similar

diffraction pattern. The conditions were moreover selected in

order to gain hollow spheres with a uniform size and a narrow

size distribution. Applying other temperatures or concentrations

led to less uniform or even massive nanoparticles. Finally, the

example La(OH)3 has already shown that the outer

diameter and the inner cavity size of the hollow spheres can be

modified in certain limits by changing the water-to-surfactant

ratio.19,20 This opportunity may persist for CuxS hollow spheres,

too.

3.2 Particle size, morphology and materials composition

Subsequent to destabilizing the micellar system and washing via

sequential resuspension/centrifugation in/from ethanol, electron

This journal is ª The Royal Society of Chemistry 2011

Fig. 1 Typical colour change during the microemulsion-based synthesis of covellite hollow spheres (A) as well as of chalcocite and digenite hollow

spheres (B) when heating the system to 60 �C.

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microscopy was performed to elucidate size, size distribution and

morphology of the obtained deep black powder samples. Since

most of the CTAB as the surfactant of the microemulsion and

stabilizer of the particles was removed by the washing procedure,

the nanoparticles are now significantly agglomerated. Overview

SEM images nevertheless point to the presence of a large number

of nanoparticles exhibiting a uniform size (i.e. 35–40 nm) and

shape (Fig. 2). To gain non-agglomerated samples for trans-

mission electron microscopy, the as-prepared hollow spheres

were stabilized with trioctylphosphine oxide (TOPO) and redis-

persed in an equimolar mixture of ethanol and toluene. Dynamic

light scattering of these TOPO-stabilized hollow spheres indi-

cates the absence of any significant agglomeration (cf. Fig. S1†).

In detail, hydrodynamic diameters of 37 nm (CuS), 38 nm

(Cu1.8S) and 40 nm (Cu2S) were obtained. These values are in

good agreement to the diameters deduced from electron

microscopy.

Slow evaporation of the TOPO-stabilized suspensions on

a holey-carbon copper grid allows preparing suitable samples for

STEM/HRTEM analysis (Fig. 3). Thus, STEM evidences a mean

diameter of 35 nm (CuS), 34 nm (Cu1.8S) and 36 nm (Cu2S) as

well as the presence of hollow spheres exhibiting an inner cavity.

All these data were deduced from statistical evaluation of at least

Fig. 2 Representative overview SEM images of CuxS hollow spheres: (a) pel

field STEM image of TOPO-stabilized hollow spheres on holey-carbon copp

This journal is ª The Royal Society of Chemistry 2011

200 hollow spheres (Table 2). These results are confirmed by

HRTEM with mean diameters of 34 nm (CuS), 32 nm (Cu1.8S)

and 35 nm (Cu2S). Furthermore, the wall thickness is observed

with diameters of 8 nm (CuS), 12 nm (Cu1.8S) and 12 nm (Cu2S).

Consequently, the inner cavity is about 8–16 nm in size. HRTEM

images, furthermore, indicate the sphere wall of all CuxS hollow

spheres as highly crystalline (Fig. 3). The observed lattice fringes

with d-values of 1.9(2) �A (CuS), 3.3(2) �A (Cu1.8S) and 3.0(2) �A

(Cu2S) correspond well with reference data of the relevant bulk

copper sulfide phases (CuS/covellite: (110) with 1.90 �A; Cu1.8S/

digenite: (101) with 3.39 �A; Cu2S/chalcocite-high: (101) with

3.05 �A).26–28

Electron-energy loss spectroscopy (EELS) was used to verify

the elemental composition of single CuxS hollow spheres (Fig. 4).

The element mapping clearly validates the hollow sphere struc-

ture to consist of copper and sulfur (Fig. 4b and c). The signifi-

cance of the analytical method and the obtained data, however,

are not sufficient to reliably differentiate between CuS, Cu1.8S

and Cu2S regarding the Cu : S ratio or the relevant oxidation

states. Equal elemental maps obtained via electron spectroscopic

imaging (ESI) show different signal intensities because of the

different L23-edge characteristics and the different electron-

energy loss levels (i.e. S-L23 with DEmax ¼ 200 eV versus Cu-L23

let of powder sample, (b) powder samples on silicon plate and (c) bright-

er grid.

Nanoscale, 2011, 3, 2544–2551 | 2547

Fig. 3 Overview TEM (left) and HRTEM (right) images of TOPO-

stabilized (a) covellite (CuS), (b) digenite (Cu1.8S) and (c) chalcocite

(Cu2S) hollow spheres.

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with DEmax ¼ 944.5 eV).29 S-L23 ELNES nevertheless shows

different features relative to Fe(III)SO4 and CdS reference spectra

(Fig. 4d).

To finally prove the crystallinity and the composition of the

different CuxS hollow spheres, X-ray powder diffraction analysis

was involved (Fig. 5). Based on the position and intensity of the

Bragg peaks, the three phases covellite, digenite and chalcocite

can be reliably determined and differentiated. The width of the

Bragg peaks as well as the absence of those peaks with minor

Table 2 Mean diameter of the as-prepared CuxS hollow spheres asindicated by electron microscopy and crystallite size as deduced fromXRD pattern via the Scherrer equation (standard deviation in brackets).

Diameter/nm Wallthickness/nm

Crystallitesize/nmSEM STEM TEM

Covellite (CuS) 40(5) 35(4) 34(6) 8(2) 3–6Digenite (Cu1.8S) 40(5) 34(5) 32(4) 12(2) 3–4Chalcocite(Cu2S)

40(5) 36(4) 35(4) 12(2) 7–12

2548 | Nanoscale, 2011, 3, 2544–2551

intensity is attributed to the low crystallite size—viz. is related to

the limited thickness of the sphere wall. Note furthermore that

a high-pressure modification is observed in the case of chalcocite

(Cu2S). This finding can be ascribed to a certain internal pressure

of the highly crystalline, but strongly curved sphere wall and has

been already described, for instance, for SnO2 hollow spheres or

carbon onions.30–32 In the latter case the internal pressure was

proven to exceed 38 GPa and led to a phase transition of graphite

to diamond. For the CuxS hollow spheres the observed shift of all

Bragg peaks in comparison to the relevant reference pattern

furthermore indicates a certain distortion of the crystalline

lattice. Finally, the crystallite size can be deduced via Scherrer’s

equation (Table 2). While considering the significance of this

estimation, the obtained values of 2.8–6.1 nm (CuS), 3.3–4.3 nm

(Cu1.8S) and 7.2–11.5 nm (Cu2S) indeed reflect the thickness of

the crystalline sphere wall.

In addition to HRTEM and XRD, thermal decomposition of

the CuxS hollow spheres in air points to their different compo-

sition, too (cf. Fig. S2†). Prior to thermal analysis all as-prepared

hollow spheres were first pre-dried (150 �C, 60 min) in order to

remove all water inside of the cavity of the hollow spheres (�2 to

7 wt%). Thereafter, thermogravimetry in air shows a clear

difference between covellite on the one hand and digenite and

chalcocite on the other hand. Most characteristic is a significant

increase in weight that is only observed for digenite (+31.8%) and

chalcocite (+34.7%) at 250–400 �C. Moreover, a weight loss is

observed for all phase compositions at 650–800 �C (i.e. CuS:

�37.2%, Cu1.8S: �47.2%, Cu2S: �46.5%) with CuO as the final

remnant. In general, the thermal behaviour follows to what is

reported for the bulk compounds.33,34 A quantification, however,

is difficult since a variety of intermediate oxidation products have

been observed even for the bulk. This includes Cu2O, CuO,

CuO$CuSO4, CuSO4, SO2 and SO3.33,34 In the case of the hollow

spheres, a quantification of the thermal decomposition is even

more difficult due to the thermal decomposition of excess thio-

urea as well as due to minor amounts of residual surfactants (cf.

Fig. S2†).

3.3 Mechanism of formation of CuxS hollow spheres

While the successful synthesis of CuxS hollow spheres with three

different phase compositions and structures is reliably proven

based on the above characterization, finally, the mechanism of

formation is discussed more detailed (Fig. 6). Hence, a closer

look has to be taken at the relevant equilibria occurring inside of

the aqueous micelle—with ammonia and thiourea dissolved

herein. Especially, the base reaction of ammonia (eqn (1)) and

the hydrolysis of thiourea (eqn (2a) (ref. 35) and (2b) (ref. 36 and

37)) are of certain relevance:

NH3 + H2O 4 NH4+ + OH� (1)

(NH2)2CS + 2OH� 4 2NH3 + S2� + CO2 (2a)

(NH2)2CS 4 NH3 + HSCN 4 NH4+ + SCN� (2b)

To gain covellite (CuS), bis(cyclohexanebutyrate)-copper(II)

(CHBC) was introduced as a precursor. Cu2+ oxidizes part of the

S2� anions—stemming from the thermally induced hydrolysis of

This journal is ª The Royal Society of Chemistry 2011

Fig. 4 Electron-energy loss spectroscopy (EELS) of CuxS hollow spheres: (a) cluster of hollow spheres adsorbed to a carbon-foil which was used for

WR-PEELS registration; (b and c) ESI-maps of Cu-L23 and S-L23 show the elemental distribution merged with the hollow sphere motive; (d) WR-

PEELS of CuxS hollow spheres (green), Fe(I)SO4 (blue) as well as CdS (red) as reference data.29 Elemental energy edges are indicated; the boxed area

shows the S-L23 ELNES range.

Fig. 5 X-Ray powder diffraction pattern of as-prepared (a) covellite

(reference: ICDD-No. 1074-1234, covellite CuS), (b) digenite (reference:

ICDD-No. 47-1748, digenite Cu1.8S) and (c) chalcocite (reference:

ICDD-No. 1089-2670, chalcocite-high Cu2S) hollow spheres.

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thiourea (eqn (2a))—under formation of (S2)2� and respective

reduction of Cu2+ to Cu+. Under such conditions the complex

composition of covellite is obtained according to (eqn (3)):

3Cu2+ + 3S2� / (Cu+)2Cu2+S2�(S2)

2� (3)

After addition of the green CHBC solution in toluene to the

colorless SME, a color change to intense blue occurred imme-

diately (Fig. 1A). This characteristic blue colour points to the

presence of a copper(II)-tetramine complex. Its formation,

however, seems to be restricted to the liquid-to-liquid phase

boundary of the micelles. A significant diffusion of Cu2+ into the

polar water phase, on the other hand, can be precluded since any

coexistence of Cu2+ and CTAB in water would otherwise lead to

a precipitation of an insoluble Cu2+-surfactant complex—which

is not observed. When heating the micellar system to 60 �C, itscolour changed via green to brown to black, indicating the

formation of copper sulfide. A characteristic pale greenish

shade—especially when looking at the cone of the scattered

light—already hints to the presence of covellite. Chalcocite and

digenite, in contrast, show a brownish-red shade of scattered

light cone.

In the case of digenite (Cu1.8S) and chalcocite (Cu2S) chlorotris

(triphenylphosphine)-copper(I) (CTTPPC) was used as the

copper precursor. According to literature, the formation of

a Cu+–thiourea complex is to be expected here at the liquid-to-

liquid phase boundary of the micelle (Fig. 6). Such complexes are

well-known and indicated by their yellow-greenish colour.38,39

Nanoscale, 2011, 3, 2544–2551 | 2549

Fig. 6 Scheme illustrating the proposed mechanism for the formation of CuxS hollow spheres with phase-engineered composition at the water-to-

toluene phase boundary of a micelle.

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Such colour is indeed observed here subsequent to the addition of

CTTPPC—and is in contrast to the deep blue color as observed

for the synthesis of CuS (Fig. 1B). With the thermally induced

decomposition of thiourea (eqn (2a)), the reaction of Cu+ and S2�

directly describes the formation of chalcocite (Cu2S). Such

behaviour is reproducibly observed if stoichiometric amounts of

CTTPPC were used. Addition of ammonia turned out as crucial

in order to initiate the reaction. By decreasing the amount of

CTTPPC, a more or less randomized precipitation of chalcocite,

digenite, geerite and covellite was observed. Moreover, a struc-

turally non-identified black copper compound was then

frequently obtained as an intermediate (cf. Fig. S3†). FT-IR

spectra of the intermediate exhibit a strong SCN�-related

vibration. In accordance with (eqn (2b)), this finding points to

a formation of copper(I)-thiocyanate (CuSCN)—a compound

that has not been described till now.

As the concentration of ammonia already plays a key-role with

regard to the formation of chalcocite, it is again relevant to gain

digenite (Cu1.8S) hollow spheres. Thus, the synthesis of phase-

pure chalcocite was successful based on an SME(6.25). With

regard to digenite, a further increase of the ammonia concen-

tration and pH-level to an SME(10.0) is necessary. Under these

conditions, partial oxidation of Cu+ to Cu2+ occurred and

selectively resulted in the formation of digenite (Cu1.8S). Alto-

gether, the type and concentration of the copper precursor as

well as the amount of ammonia influence the Cu+/Cu2+ ratio and

thereby the resulting phase composition. In accordance with the

above consideration and equilibria, adjusting the Cu+/Cu2+ ratio

only by mixing the copper precursors CTTPPC and CHBC was

not successful.

4. Conclusions

In sum, nanoscale hollow spheres with three different CuxS

compositions and structures are prepared via a microemulsion

approach. Namely this is covellite (CuS), digenite (Cu1.8S) and

2550 | Nanoscale, 2011, 3, 2544–2551

chalcocite (Cu2S) with an outer diameter of 32–36 nm, a wall

thickness of 8–12 nm and an inner cavity of 8–16 nm in diameter.

Surprisingly, the different CuxS phases are accessible with an

identical strategy of synthesis. In contrast to previous investi-

gations that report on the synthesis of polycrystalline CuxS

hollow spheres with outer diameters most often exceeding the

nanoregime (i.e. diameter >100 nm), the hollow spheres pre-

sented here exhibit a single-crystalline sphere wall and a diameter

below 50 nm. Reproducible adjustment of the CuxS phases is

possible via precise control of the experimental conditions,

including the type and concentration of the copper precursor as

well as the amount of ammonia. Such phase-engineering is first

realized for hollow spheres with an advanced morphology and

a large surface.

Based on these results additional CuxS phases may be

obtainable. A precise adjustment of the phase composition

furthermore may allow tailoring phase-specific properties and

applications—such as superionic conductors, thermo- or

photoelectric transformers, high-temperature thermistors or

efficient catalysts. In addition to CuxS hollow spheres, this may

also hold for other compounds with hollow sphere morphology.

Acknowledgements

P.L., R.P., D.G. and C.F. are grateful to the Center for Func-

tional Nanostructures (CFN) of the Deutsche For-

schungsgemeinschaft (DFG) at the Karlsruhe Institute of

Technology (KIT) for financial support.

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