Synthesis of band-gap tunable Cu–In–S ternary nanocrystals in aqueoussolution{

Meina Wang,a Xiangyou Liu,b Chuanbao Cao*a and Cui Shia

Received 5th January 2012, Accepted 5th January 2012

DOI: 10.1039/c2ra00034b

Cu–In–S ternary nanocrystals (NCs), with an average size of less

than 10 nm, were synthesized in an aqueous solution containing

bovine serum albumin (BSA). X-Ray powder diffraction (XRD)

and selected-area electron diffraction (SAED) analyses showed

that these NCs featured a roquesite structure. The composition of

the NCs could be adjusted by controlling the molar ratio of the

starting Cu/In precursors in the reaction solution, which led to a

tunable band gap ranging from 1.48 eV to 2.30 eV. Cytotoxicity

testing showed that the BSA-stabilized Cu–In–S NCs had little

effect on the cell viability, which suggested that they are user-

friendly and environmentally benign. With low cost, minimal

energy input and environmental impact, this simple approach

shows great potential for industrial applications.

Introduction

The increasingly severe global energy and environmental crises call

for new eco-friendly and low-cost materials with high performance in

energy harvesting, storage and conversion.1–4 In this context, ternary

and quaternary semiconductor NCs, especially CuInS2 (CIS) and

related materials, have attracted much attention recently due to their

numerous advantages.5–11 First of all, bulk CIS is a direct band-gap

semiconductor with a 1.5 eV band gap energy, which is close to the

best band gap for solar cells.6 Secondly, CIS NCs are free from

poisonous heavy metal ions (e.g. Cd, Pd, Hg), which suggests that

they are environmentally benign and suitable for bio-applications

(e.g. near-infrared bio-imaging).12,13 Last but not least, the band gap

of CIS NCs can be readily tuned by tailoring their composition or

size.14 Such a tunable band gap is of particular importance in the

fabrication of optoelectronic devices.15–17 For these reasons, CIS

NCs have been considered as one of the most promising materials

for photovoltaic applications.

Many synthesis approaches (e.g. solvothermal and hydrothermal

routes)18–21 have been developed recently to prepare CIS NCs which

are generally achieved in organic solvents with the addition of

noxious reagents (e.g. dodecanethiol).10,11 In this case, a negative

environmental impact is inevitable. Meanwhile, the synthesis

processes are often under harsh conditions (e.g. high vacuum and

high temperature).6–11,20,22 This undoubtedly increases the cost and

energy input, and the production is hardly scaled up to manufacture

large amounts of CIS particles on an industrial scale. Herein we

reported a facile and low-cost approach to synthesize band-gap

tunable CuxInyS0.5x+1.5y NCs in aqueous solution under mild

conditions (e.g. non-vacuum and room temperature). The synthe-

sized CuxInyS0.5x+1.5y NCs, stabilized by BSA, possessed roquesite

structure and exhibited little toxicity. The band gaps of the

CuxInyS0.5x+1.5y NCs could be readily tuned by varying the molar

ratio of Cu/In precursors. All the advantages ensure that this

synthesis approach has great potential for industrial applications.

In the aqueous synthesis of CuxInyS0.5x+1.5y NCs, appropriate

capping agents or stabilizing agents are needed to prevent the

aggregation of the particles and to produce small sized particles with

a narrow size distribution and uniform shape. Significantly, there has

been a keen interest recently in exploring natural biological

macromolecules (e.g. proteins and polysaccharides) as the stabilizing

agents, among which BSA attracts a great deal of attention.23–28

BSA has a strong affinity toward nanoparticles due to there being

plenty of groups (e.g. –SH and –NH2) on its side chains. It can avoid

the aggregation of nanoparticles and improve their colloidal stability

in aqueous solution.27,28 Moreover, it confers excellent biocompat-

ibility on the nanoparticles, which lays substantial foundation for

their bio-applications.24 For these reasons, BSA was selected as the

stabilizing agent in this study. In addition, an appropriate sulfur

source was also critical for the successful synthesis of Cu–In–S NCs.

In a pre-experiment, both thioacetamide (TAA) and Na2S were

tested as the sulfur source, and TAA was finally selected as it could

slowly release S22 into the reaction solution which avoided the quick

growth and aggregation of the CuxInyS0.5x+1.5y particles;26 thereby

small sized (even less than 10 nm) and nearly dispersed nanoparticles

were obtained.

We first synthesized CuxInyS0.5x+1.5y NCs with a starting molar

ratio of Cu/In precursors of 1 : 1. The synthesized NCs were then

characterized with transmission electron microscopy (TEM) and

high-resolution TEM (HRTEM), etc. As shown in Fig. 1(a) and (b),

nearly dispersed NCs of irregular shape were obtained. The average

size of the particles was statistically measured to be 7.1 ¡ 2.5 nm.

The lattice fringes had an interplanar spacing of 0.271 nm (Fig. 1(c)),

matching well with the (200) interplanar spacing of the roquesite-type

aResearch Center of Materials Science, Beijing Institute of Technology,Beijing, 100081, China. E-mail: cbcao@bit.edu.cn;Fax: +86-10-6891 3792; Tel: +86-10-6891 2001bDalian Institute of Chemical Physics, Chinese Academy of Sciences,Dalian, 116023, China{ Electronic supplementary information (ESI) available: Fig. S1 and TableS1. See DOI: 10.1039/c2ra00034b

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nanocrystal structure (see the following analyses of the XRD results).

To analyze the chemical composition of the NCs, energy dispersive

X-ray spectroscopy (EDS) experiments were performed which

definitely confirmed the existence of Cu, In and S elements in the

synthesized NCs (Fig. 2). It was estimated from the peak intensity in

Fig. 2 that the approximate atomic ratio of the three elements

approached the stoichiometric ratio of CuInS2. Additionally,

inductively coupled plasma-atomic emission spectrometer (ICP-

AES) analyses also confirmed that the relative molar ratio of

Cu/In in the as-synthesized NCs was close to 1 : 1 (Table S1 in the

ESI{). Subsequently, XRD was employed to further characterize the

crystalline structure of the nanoparticles. As shown in Fig. 3(a), a

typical diffraction pattern corresponding to the tetragonal roquesite

structure (JCPDS card No. 15-0681) was observed. The phase purity

was also confirmed from Fig. 3(a). Taken together, these results gave

us direct evidence that CIS NCs were successfully synthesized. In

addition, SAED also verified the roquesite-type crystal structure of

the CIS NCs. As shown in the inset of Fig. 1(a), three distinct

diffraction rings, highly consistent with the diffractions of the (112),

(220) and (312) planes of the roquesite-type nanocrystal structure

could be distinguished unambiguously, and two more relatively weak

rings indexed to the diffractions of the (316) and (228) planes could

also be discerned. Significantly, it was reported that different capping

agents could result in different crystalline structures. For example,

zinc blende CIS NCs were obtained when oleic acid was chosen as

the capping agent, whereas wurtzite CIS NCs were yielded under the

same conditions when the oleic acid was replaced by dodecanethiol.20

In the current work, for the first time BSA was chosen as the capping

agent in CIS synthesis, which gave rise to a roquesite-type structure.

A tunable band gap is required for photovoltaic materials to

maximize their solar absorption, make full use of the energy of

photons and improve their energy conversion efficiencies.15–17

Tuning the band gaps of ternary Cu–In–S NCs by changing their

size or compositions in organic solution has been reported

recently.14,17 However, it has never been achieved in the aqueous

phase due to the difficulty of precisely controlling the composition

or size of the crystals in aqueous synthesis. To address this issue,

we synthesized various CuxInyS0.5x+1.5y NCs with different molar

ratios of Cu/In by adjusting the starting amounts of Cu and In

precursors in the reaction solution. TEM images revealed that the

Fig. 2 EDS spectrum of the CIS NCs. Inset shows the results of

quantitative elemental analysis.

Fig. 1 Representative TEM (a, b) and HRTEM (c) images of the

synthesized CIS NCs. The insets in (a) and (b) are SAED and the size

distribution histogram of the CIS NCs, respectively.

Fig. 3 (a) XRD patterns of the CuxInyS0.5x+1.5y NCs synthesized with varying molar ratios (1 : 3, 1 : 1 and 3 : 1) of Cu/In precursors. Reference pattern of

roquesite is shown at the bottom. (b) Expanded view of the (112) peaks in (a) showing the peak shift. The dotted lines indicate the peak positions of the

corresponding samples.

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shape of the CuxInyS0.5x+1.5y NCs was still irregular and their size

was comparable with CIS (see Fig. S1 in the ESI{), which indicated

that the changes in composition of the Cu–In–S NCs had little

influence on the particle shape and size. EDS and ICP-AES analyses

showed that the composition of these NCs varied with the starting

molar ratio of Cu/In precursors (Table 1 and Table S1 in the ESI{).

Generally, a higher ratio of Cu/In precursors led to a higher Cu

content in the final NCs. However, it was worth noting that a high

ratio of Cu/In precursors of 3 : 1 only gave rise to the NCs with a

Cu/In ratio of y1.5 : 1. It seemed that Cu atoms were difficult to

incorporate into the NCs, which was contrary to the cases in organic

synthesis of the Cu–In–S ternary NCs.20 It was speculated that,

at a higher Cu concentration, the reduction of Cu2+ to Cu+ was

insufficient in aqueous solution at room temperature. The exact

reason for this result is currently under research. As shown in

Fig. 3(a), all the CuxInyS0.5x+1.5y NCs exhibited three representative

diffraction peaks at around 2h = 28u, 47u and 55u, which

corresponded to the (112), (220) and (312) planes of the roquesite-

type nanocrystal structure, respectively. Thus it was concluded that,

although the ratio of Cu/In varied, all the CuxInyS0.5x+1.5y NCs

possessed a similar structure, which was possibly attributed to the

similar ionic radius of Cu+ (0.74 A) and In3+ (0.76 A).22 In addition,

a slight shift for the (112) peak at around 2h = 28u could be clearly

observed from the expanded view in Fig. 3(b). Specifically, the (112)

diffraction peak of the samples shifted toward higher angles with the

increase of the Cu/In ratio. This trend is similar to a previous

report.22 Such a slight shift in the diffraction peak was possibly

attributed to the decrease in the unit-cell dimensions with the

incorporation of more Cu atoms which had a smaller atomic size

than In.

Subsequently, the optical properties of the CuxInyS0.5x+1.5y NCs

were investigated. As shown in Fig. 4(a), a characteristic absorption

peak at 280 nm (the absorption feature of BSA) was observed for all

the samples, which confirmed the existence of BSA. In addition, it

was clearly shown that, with the increase of Cu content, the

absorption onsets of the CuxInyS0.5x+1.5y NCs were red-shifted from

y560 nm to y870 nm (Fig. 4(b)). These distinct absorption onsets

were used to estimate the optical band gaps of the samples according

to an approach described previously (Fig. 4(a) inset).16 The results

showed that the band gaps calculated for CuxInyS0.5x+1.5y NCs, with

a starting Cu/In ratio of 3 : 1, 1 : 1 and 1 : 3, are 1.48 eV, 1.93 eV

and 2.30 eV, respectively. Obviously, the band gap increased with the

decrease of the molar ratio of Cu/In. Overall, these results suggested

that a tunable band gap could be achieved through adjusting the

initial molar ratio of Cu/In in the reaction solution.

Finally, we tested the biocompatibility of the BSA-stabilized

CuxInyS0.5x+1.5y NCs. Even though the CuxInyS0.5x+1.5y NCs are not

imperatively applied in living systems, it is still necessary to test their

potential hazards to the organisms since direct contact or inhalation

of these particles by the researchers/users during the fabrication/

applications cannot be completely avoided. To this end, the

cytotoxicity test was performed. As shown in Fig. 5, at low Cu

concentrations (,10 mM), the particles in the experimental and

control groups both showed good biocompatibility. When the Cu

concentration was over 10 mM, a significant dose-dependent decrease

of the cell viability in the control was observed. However, in the

experimental groups, more than 60% of the cells still survived after

they were treated with 125 mM Cu–In–S NCs. We have mentioned

above that BSA could stabilize and disperse the particles. Herein the

cytotoxicity results further emphasized the importance of BSA which

could obviously improve the biocompatibility of the nanomaterials.

It should be mentioned that Cd-containing quantum dots (e.g. CdS,

CdSe and CdTe) under similar conditions were much more toxic.29

For example, 20 mM CdTe quantum dots (with an average size of

6 nm) could result in a y50% decrease of the viability of the human

hepatoma cell line HepG2 cells after incubation for 48 h.30 However,

20 mM CIS NCs only caused 28% decrease under the same

conditions (Fig. 5). Moreover, it was also concluded from Fig. 5 that

adjusting the composition of the NCs did not influence the cell

viability greatly and all the BSA-stabilized CuxInyS0.5x+1.5y NCs

showed good biocompatibility.

Table 1 The compositions of the synthesized roquesite CuxInyS0.5x+1.5y

NCs

Formulaa

Atomic composition (%)b

FormulabCu In S

Cu3.0In1.0S3.0 29.28 19.56 51.16 Cu1.5In1.0S2.6

Cu1.0In1.0S2.0 25.86 26.72 47.42 Cu1.0In1.0S1.8

Cu1.0In3.0S5.0 10.96 39.34 49.70 Cu1.0In3.6S4.5

a Calculated from the molar ratios of Cu/In precursors used.b Determined by EDS.

Fig. 4 (a) UV-Vis-NIR absorption spectra of the CuxInyS0.5x+1.5y NCs synthesized with varying molar ratios (1 : 3, 1 : 1 and 3 : 1) of Cu/In precursors. Inset

shows the plots of (ahn)2 versus photon energy (hn) of corresponding samples. (b) Expanded view of the region between 500 nm and 900 nm in (a) which shows

the different absorption onsets of the samples.

2668 | RSC Adv., 2012, 2, 2666–2670 This journal is � The Royal Society of Chemistry 2012

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Conclusions

Sub-10 nm and irregularly-shaped Cu–In–S ternary NCs with

roquesite-type structure were successfully synthesized in aqueous

solution. The composition of the NCs could be adjusted by

controlling the molar ratio of the starting Cu/In precursors in the

reaction solution. XRD and SAED confirmed the tetragonal

roquesite structure and phase purity of the CuxInyS0.5x+1.5y NCs.

Optical absorption of the NCs showed distinct onsets fromy560 nm

to y870 nm with the increase of Cu content, which corresponded to

the tunable band gaps ranging from 2.30 eV to 1.48 eV. To the best

of our knowledge, this is the first report to synthesize Cu–In–S

ternary NCs with tunable band gaps in aqueous solution. In

addition, these BSA-stabilized NCs showed little cytotoxicity,

suggesting that they are user-friendly and environmentally benign.

It is believed that such a simple synthesis approach is also adapted to

synthesize other ternary and even quaternary semiconductor NCs

(e.g. Cu2ZnSnS4). Further study to gain insights of the formation

mechanisms of the BSA-stabilized ternary NCs and to expand the

applications of the presented approach is currently underway.

Experimental section

Materials

Copper(II) sulfate pentahydrate (CuSO4?5H2O, 99.99%), indium(III)

chloride (InCl3, 99.99%) and TAA (C2H5NS, ¢99.0%) were all

purchased from Aladdin reagent company. BSA (purity ¢99%) was

purchased from Amresco. All the other chemicals were commercial

available and used without further purification.

Synthesis of CIS NCs

In a typical process, BSA (60 mg) was dissolved in 45 mL phosphate

buffered saline (PBS buffer, pH 7.4) to form a clear solution. Then

CuSO4?5H2O (125 mmol) and InCl3 (125 mmol) were added and the

solution was kept stirring for 12 h at room temperature.

Subsequently 5 mL TAA aqueous solution (50 mM) was added

dropwise. After stirring for another 24 h at room temperature, a

brown solution was obtained. This solution was kept static for 72 h

at 4 uC, then centrifuged at 4 uC for 20 min. The sediment was

discarded, and the supernatant was dialyzed in order to remove the

free ions. Finally the CIS solution was concentrated by lyophiliza-

tion. Other CuxInyS0.5x+1.5y NCs were also synthesized according to

the procedures, only except that the starting amounts of Cu and In

precursors varied. In order to study the influence of BSA on the

formation of CIS NCs as well as on the biocompatibility of the

particles, a control experiment was performed according to the same

procedures as described above except that no BSA was added.

Characterization of CIS NCs

For TEM and HRTEM observation, the sample solution was

dropped onto a nickel grid covered with a thin carbon film, then

dried in air at room temperature. TEM images and SAED were

obtained on an FEI Tecnai G2 F20 microscope operating at an

accelerating voltage of 200 kV. HRTEM images were taken on

an FEI Tecnai G2 F30 S-TWIN microscope operating at an

accelerating voltage of 300 kV. EDS spectra were acquired using a

Hitachi S-4800 scanning electron microscope equipped with a Bruker

AXS XFlash detector 4010. Besides EDS, ICP-AES (Teledyne

Leeman Labs) was also employed to analyze the nanocrystal

composition. The UV-Vis-NIR spectra of the sample solutions were

recorded with a Hitachi U-4100 spectrophotometer. XRD measure-

ments were performed on a PANalytical X’Pert PRO X-ray

diffractometer with Cu-Ka radiation (l = 0.15406 nm) at 40 kV

and 40 mA.

Cell culture and cytotoxicity analysis

HepG2 cells (ATCC, Manassas, VA, USA) were cultured in

Dulbecco’s modified Eagle’s medium (DMEM, GIBCO,

Invitrogen) supplemented with 10% (v/v) newborn calf serum

(Invitrogen) and 1% penicillin–streptomycin (Beyotime, China) in a

humidified incubator at 37 uC with 5% CO2.

A Cell Counting Kit-8 (CCK-8) assay kit (Beyotime, China) was

employed to evaluate the toxicity of the samples according to the

manufacturer’s instructions. Briefly, the cells were seeded into a 96-

well plate (Corning, USA) and grown to a density of 104 cells/well.

Then serial dilutions of the samples were added and co-incubated

with the cells for 48 h. Subsequently, CCK-8 solution (20 mL/well)

was added and the plate was further incubated for 30 min. The

absorbance of each well at 450 nm was finally measured using a

microplate reader (Infinite M200, Tecan).

Acknowledgements

This work was financially supported by the National Natural

Science Foundation of China (Grant No. 50972017 and No.

20471007).

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