Polyol Mediated Solvothermal Synthesis and Characterization of CuIn(1−x)GaxS2Nanocrystals

Polyol Mediated Solvothermal Synthesis and Characterization of CuIn _(12x) Ga _x S ₂ Nanocrystals

Mohamed Benchikhi^1,2• Rachida El Ouatib^1• Lahcen Er-Rakho^1• Bernard Durand²

Received: 9 March 2016 / Accepted: 22 July 2016 ÓSpringer Science+Business Media New York 2016

Abstract

Gallium-substituted copper indium sulfide (CuIn

_(1-x)

Ga

_x

S

₂

) nanoparticles have been synthesized by a convenient solvothermal method without usage of surfac- tants or toxic reductants such as hydrazine. Thiourea, sodium hydroxide, CuCl

₂2H2

O, InCl

₃

and GaCl

₃

were used as starting materials and ethylene glycol as solvent and reducing agent. The reactions were performed at 200

°C for 16 h. Effect of sodium hydroxide on the reac-

tion products is analyzed. The powders are mainly char- acterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, BET surface area mea- surements and UV–Vis absorption spectroscopy. The results show that the gallium is successfully incorporated into the chalcopyrite crystal structure. The homogeneous powders obtained are constituted of nanoparticles with sizes in the range 20–30 nm and exhibit a specific surface area close to 65 m

²

/g. Further, the possible mechanism for the formation of CuIn

_(1-x)

Ga

_x

S

₂

nanocrystals is explained.

The optical band gap energies of the nanoparticles were in the range 1.48–1.75 eV.

Keywords

CuIn

_(1-x)

Ga

_x

S

₂

Nanocrystals Solvothermal synthesis Solar energy materials

1 Introduction

In recent years, the I–III–VI

₂

ternary chalcopyrite com- pounds have attracted much attention because of their excellent structural, chemical, optical, electronic, and electrical properties [1–3]. The chalcopyrite structure of I-III-VI

₂

is similar to the zinc blende structure in which Zn atoms are replaced alternatively by monovalent and triva- lent cations [3]. The band-gap energies of metal chalco- genide semiconductors are considerably smaller than those of their oxide analogues. The system of copper chalcopy- rites CuMX

₂

(M

=

Al, Ga, In, and X

=

S, Se) covers a wide range of band-gap energies from 1.04 eV in CuInSe

2

up to 2.7 eV in CuAlS

₂

, therefore covering a large part of the visible spectrum [4]. Among them, the ternary com- pounds CuInSe

2

and CuInS

2

have attracted considerable attention over the past decades due to their high absorption coefficients in visible or NIR range of the spectrum. The direct bandgap energy observed in these materials make them candidates for application in solar energy conversion and light-emitting diodes [4–8]. Insertion of Ga in CuIn(S,Se)

₂

with content between 25 and 35 at % is of great interest because it raised up the energy conversion efficiency of solar cells based on the CuIn(S,Se)

₂

absorber materials. It was shown that the defect concentration in CuIn

_(1-x)

Ga

_x

Se

₂

is dependent upon the gallium content [9,

10]. Hanna et al. [10] showed that the defect density in

CuIn

_(1-x)

Ga

_x

Se

₂

is minimum at x

=

0.3. Solar cells based on CuIn

_(1-x)

Ga

_x

Se

₂

have reached conversion efficiency as high as 20 %, which is the highest value so far achieved for any polycrystalline thin film solar cell [11]. However, these materials are harmful due to the toxicity of selenium.

Copper indium sulfide (CuInS

₂

) is a good candidate material because of its high chemical and thermal stability and low toxicity [12]. Single-junction Cu(In,Ga)S

₂

solar

& Mohamed Benchikhi

Benchikhi_mohamed@yahoo.fr

1 Laboratoire de Physico-Chimie Des Mate´riaux Inorganiques, Faculte´ Des Sciences Ain Chock, Universite´ Hassan II Casablanca, Casablanca, Morocco

2 Institut Carnot CIRIMAT, CNRS Universite´ de Toulouse, 118 Route de Narbonne, 31062 Toulouse Cedex 9, France DOI 10.1007/s10904-016-0423-6

cells have demonstrated nearly 13 % solar energy con- version efficiency [13]. However, it is far below the theo- retical limit of about 30 % efficiency under terrestrial conditions [14].

Copper indium sulfide has traditionally been prepared by physical methods, such as laser removal [15], sput- tering [5], electrodeposition [16], single source evapora- tion [17], coevaporation from elemental sources [18], and sulfurization of metallic precursors [5]. However, most of these methods usually require sophisticated instrumenta- tion and some of them use toxic gases such as H

₂

S. For these reasons, various chemical methods have been investigated to prepare CuInS

₂

, such as molten salts [19], ion layer gas reaction (ILGAR) [20], solvothermal [21], molecular single-source precursors [22], spray [23], chemical bath deposition [24], and aerosol-assisted chemical vapor deposition (AACVD) [25]. Until now, there are only a few reports on the synthesis of CuIn

_(1-x)

Ga

_x

S

₂

nanoparticles by wet chemical routes [26,

27].

In this work, CuIn

_(1-x)

Ga

_x

S

₂

(x

B

0.3) nanoparticles were prepared by a simple solvothermal method without any surfactants or toxic reductants such as hydrazine. The structural, morphological, and optical properties of the samples were investigated.

2 Experimental Procedure

2.1 Synthesis of CuIn_(12x)Ga_xS₂Nanocrystals

The starting materials were copper chloride CuCl

₂2H2

O (Sigma Aldrich), indium chloride InCl

₃

(Sigma Aldrich), gallium chloride GaCl

₃

(Sigma Aldrich), thiourea CS(NH

₂

)

₂

(Acros Organics) and sodium hydroxide NaOH (Sigma Aldrich). All the chemical reagents were analytical grade and used as purchased without further purification.

CuIn

_(1-x)

Ga

_x

S

₂

(x

B

0.30) nanocrystals were synthe- sized by a simple solvothermal method. At first, stoichio- metric amounts of metal chlorides were dissolved in ethylene glycol (abbrev. EG) at room temperature. Sec- ondly, thiourea (abbrev. Tu) was added to the solution in the molar ratio Tu/Cu

=

5 and stirred for 15 min. After that, Solid NaOH (abbrev. OH) was added to the reaction mixture in the molar ratio OH/Cu

=

5 and stirred until complete dissolution. Then the resulting mixture was transferred into a Teflon-line stainless steel autoclave.

Lastly the autoclave was sealed and statically heated at 200

°C for 16 h. After cooling at room temperature natu-

rally, the sulfides were extracted by centrifugation. The black powders obtained were then washed several times with deionized water, absolute ethanol and finally dried at room temperature. The same experiment was repeated with

the molar ratio Tu/Cu

=

3 and without adding sodium hydroxide.

2.2 Characterization of CuIn_(12x)Ga_xS₂ Nanocrystals

The crystalline structure was investigated by X-ray diffraction analysis using a D4 Endeavor X-ray diffrac- tometer (CuKa

=

0.154056 nm, Bruker AXS, Karlsruhe, Germany) from 20° to 60°. Raman spectroscopy was per- formed using a dispersive laser spectrophotometer (Jobin–

Yvon Labram HR 800) at room temperature with an excitation wavelength of 633 nm. The microstructure and chemical compositions of samples were analyzed using scanning and transmission electron microscopies (SEM, Jeol JSM 6400 provided with energy dispersion X-EDS;

HRTEM, Jeol JEM 2100F). Specific surface areas were determined using a BET (Micrometrics Flowsorb II 2300).

Optical properties were investigated using UV–Vis spec- troscopy (UV-1601). Cuvettes used for the experiments are made of Suprasil quartz from Hellma Analytics. In order to avoid artifacts, the cuvettes contribution to the absorption has been systematically subtracted from the raw data.

3 Results and Discussion

Preliminary experiments were performed without any sodium hydroxide addition. In this condition, the formation of a unique ternary chalcopyrite CuInS

₂

structure by solvothermal reaction of metal chlorides with thiourea in ethylene glycol was observed at 200

°C for 16 h.

The XRD patterns of CuInS

₂

powders synthesized by reaction of metal chlorides with thiourea in ethylene glycol in the absence and presence of sodium hydroxide are given in Fig.

1. In both cases, XRD patterns show three major

diffraction peaks at 27.82°, 46.38°, and 55.03°, which can be indexed to the (112) (024)/(220), and (116)/(312) planes of chalcopyrite phase CuInS

₂

(JCPDS no. 047-1372), respectively. As shown in Fig.

1, when NaOH was added to

the reaction mixture, the diffraction peaks become very broad. The broadening of X-ray diffraction peaks indicates that the crystallite sizes are in the nanometer range. The average crystallite sizes (D) were calculated from XRD powder pattern according to the Scherrer equation (Eq.

1),

D

¼ ðKkÞ=bðcoshÞ ð1Þ

where K is the shape factor of the average crystallite,

k

is the

wavelength for the Cu Ka radiation,

b

is the full width at

half-maximum of the diffraction line and

h

is the Bragg’s

angle. The average crystallite sizes were 12 and 7 nm for the

powders synthesized in the absence and presence of sodium

hydroxide in the reaction mixture, respectively.

To investigate the morphological properties of CuInS

₂

powders, TEM micrographs were performed (Figs.

2

,

3).

For sample synthesized in the absence of sodium hydrox- ide, TEM image (Fig.

2) shows very thin platelet-like

particles with a size of 0.2–1

lm, whereas, the powder

prepared in the presence of sodium hydroxide was consti- tuted of agglomerates of more or less spherical elementary grains with sizes in the range 20–30 nm (Fig.

3). These

nanoparticles exhibit a specific surface area close to 65 m

²

/ g.

XRD patterns of CuIn

_(1-x)

Ga

_x

S

₂

(x

=

0, 0.25 and 0.3) samples prepared in ethylene glycol in the presence of

sodium hydroxide are given in Fig.

4. All XRD peaks were

indexed to the tetragonal copper indium disulfide, accord- ing to standard JCPDS 47-1372. No impurities, such as binary sulfides or oxides, were detected by XRD analysis.

It is noticed that the diffraction peaks shift slowly to the higher diffraction angle with increasing the gallium con- tent. This result indicates that our products are single phase and isostructural with CuInS

₂

. Lattice parameters (a and

Fig. 1 XRD patterns of the CuInS₂ nanoparticles prepared in

ethylene glycol in the absence a and the presence b of sodium hydroxide indexed to the tetragonal chalcopyrite crystal structure. The standard pattern of CuInS₂(JCPDS file no. 47-1372) is provided at the bottom of this figure

Fig. 2 TEM micrographs of the CuInS₂nanoparticles prepared by the solvothermal reaction of CuCl₂2H2O, InCl₃ with thiourea in ethylene glycol without adding sodium hydroxide

Fig. 3 TEM micrographs of the CuInS₂nanoparticles prepared by the solvothermal reaction of CuCl₂2H₂O, InCl₃ with thiourea in ethylene glycol in the presence of sodium hydroxide. Insert: High magnification image

Fig. 4 XRD patterns of the CuIn_(1-x)Ga_xS₂ nanoparticles with ax=0,bx=0.25 andcx=0.3. The standard pattern of CuInS₂ (JCPDS file no. 47-1372) is provided at the bottom of this figure. All samples were prepared by solvothermal reaction of metal chlorides with thiourea in ethylene glycol in the presence of sodium hydroxide

c) were determined for each composition according to

equation Eq. (2) [28]:

l=d

² ¼ ðh²þ

k

²Þ=ða²þ

l

²=c²Þ ð2Þ

where d is the d-spacing (interplanar distance), a and c represent the lengths of the unit cell edges; h, k, and l are Miller indices. The lattice parameters decrease with the increase of Ga content (x) in the CuIn

_(1-x)

Ga

_x

S

₂

(x

B

0.3) nanocrystals (Table

1). This result confirms that the solid

solution CuIn

_(1-x)

Ga

_x

S

₂

is of substitution in which indium atoms are substituted by smaller atoms of gallium.

The crystallite size (calculated according to the Scherrer equation from the broadening of X-ray diffraction peaks) seems to decrease slightly with increasing Ga content in the CuIn

_(1-x)

Ga

_x

S

₂

(x

B

0.3) series (Table

1).

In addition to the XRD, Raman investigation was per- formed to fully characterize the samples (Fig.

5). The

spectrum of non-doped CuInS

₂

powder (Fig.

5a) exhibits

two peaks around 296 cm

^-1

and 334 cm

^-1

. Similar Raman study results have been reported for CuInS

₂

[29,

30]. The

295 cm

^-1

peak is induced by the A

₁

mode, which repre- sents the vibration of S anions in the x–y plane, while cations remain at rest [31]. The Raman peak near 334 cm

^-1

can be attributed to the E

_LO

/B

₂

modes of the CuInS

₂

structure. These modes include displacements of copper and indium atoms [32,

33]. For CuIn_(1-x)

Ga

_x

S

₂

(x

=

0.25 and 0.3) samples (Fig.

5b and c), the same phase

was identified, indicating that our products have the chal- copyrite structure. The frequency of the main peak (A

₁

) increases by increasing the inclusion rate of gallium. This agrees well with the results reported by Hossain et al. [30].

No other characteristic peaks from impurities, such as Cu

_2-x

S (267 and 474 cm

^-1

), In

₂

S

₃

(244 and 367 cm

^-1

), Ga

₂

S

₃

(471 and 437 cm

^-1

), are observed in the Raman spectra [34–37].

The chemical composition of samples was checked by elemental analyses. The results are summarized in Table

2, which indicate that the Ga/(In ?

Ga) ratio is in agreement with the intended stoichiometry. The excess of sulfur content is due to the presence of thiourea surface species.

In order to improve chemical homogeneity of the reac- tion mixture, we have used CuCl

₂

instead of CuCl. It is

well known that the reaction of CuCl

₂

with ethylene glycol (T

[

190

°C) leads to the formation of copper metal

[38,

39]. According to Carroll et al. [38], the reduction of

CuCl

₂

by ethylene glycol involves the crystallization of a Cu-glycolate intermediate phase. The same reaction in the presence of thiourea in reaction mixture leads to the for- mation of copper (I) sulfides [40,

41]. Since the coordi-

nation ability of thiourea is stronger than that of ethylene glycol, therefore Cu–EG complexes were substituted with Cu–Tu complexes. In our process, a possible reaction

Table 1 Lattice parameters and average crystallite size of the samples CuIn_(1-x)Ga_xS₂as obtained from XRD data

X Crystallite size (nm) Optical bandgap (eV) Lattice parameters A(A˚) C(A˚)

0 7.0 1.48 5.532 (2) 11.166 (3)

0.25 6.8 1.67 5.505 (7) 11.066 (6)

0.3 6.6 1.75 5.478 (6) 10.984 (5)

The optical bandgaps measured for these samples using UV–Vis absorption spectra are also shown Fig. 5 Raman spectra of CuIn_(1-x)Ga_xS₂nanoparticles withax=0, bx=0.25 andcx=0.3

Table 2 Chemical compositions of CuIn_(1-x)Ga_xS₂ nanoparticles from EDS/SEM analysis

X Cu In Ga S

0 0.960 1 0 2.18

0.25 0.974 0.75 0.237 2.15

0.3 0.969 0.7 0.290 2.13

These formulae are normalized using the expected indium content

mechanism is described as following. When thiourea was added to reaction mixture containing the metal salts, the blue solution (due to Cu

^2?

) immediately becomes brown.

This indicates the disappearance of free Cu

^2?

ions and the formation of metal-thiourea complexes in the reaction mixture [42]. At the solvothermal temperature, the pre- formed metal-thiourea complexes decompose to release dissociative metal ions. Meanwhile, thiourea and ethylene glycol can reduce Cu

^2?

to Cu

^?

ion [43,

44]. Then, thiourea

reacts with the sodium hydroxide or water introduced by the solvent and hydrated metal salt (CuCl

₂2H2

O) and produce S

^2-

[45,

46]. Finally, Cu^?

, Ga

^3?

and In

^3?

react with S

^2-

to form CuIn

_(1-x)

Ga

_x

S

₂

. The sodium hydroxide facilitates the decomposition of the thiourea under the effect of temperature and produced progressively S

^2-

in the reaction mixture. The various reactions involved in this process are given in Eqs. (3)–(9):

Cu

^2þ þð

n

þ

1

Þ

Tu

! ½Cu^IðTuÞ^þ þ

1=2 Tu

ð Þ^2þ₂ ð3Þ

2 Cu

^2þ þ

2 CH

₂ðOHÞ

CH

₂ðOHÞ

!

CH

3

CO OCCH

3 þ

2 Cu

^2þþ

2H

^þ þ

2H

2

O

ð4Þ

Tu

þ

OH

$

SH

þ

H

2

O

þ

CH

2

N

2 ð5Þ

Tu

þ

2 H

₂

O

$

H

₂

S

þ

2 NH

_3þ

CO

₂ ð6Þ

H

2

S

$

SH

þ

H

^þ ð7Þ

SH

$

S

þ

H

^þ ð8Þ

Cu

^þ þ ð1xÞIn^3þ þ

xGa

^3þ þ

2S

² !

CuIn

_ð1xÞ

Ga

x

S

2: ð9Þ

The value of the bandgap determines the part of the solar spectrum that could be theoretically absorbed by a semiconductor. In order to determine the band-gap energy of CuIn

_(1-x)

Ga

_x

S

₂

nanoparticles, the optical absorption was measured. It is well known that Cu(In,Ga)S

₂

are direct band-gap semiconductors [3]. The band-gap Eg of the semiconductor with a direct transition is determined using the following equation [47]:

ðahtÞ ¼

A

ðht

Eg

Þ¹⁼² ð10Þ

where

a

is the absorption coefficient (cm

^-1

), hm the energy of the incident photons (eV) and A a constant. The band gap energy (Eg) was determined by the intersection of the linear part of the curve (ahm)

²

versus hm (Fig.

6). As shown

in Table

1, the band gap value increases with the increas-

ing Ga content. These values are in the range reported in the literature [48,

49]. A decrease of absorbance with

increasing Ga content is observed (inset Fig.

6). This

agrees well with the results reported by Vahidshad et al.

[49]. The detailed mechanism to explain the Ga effect on the decrease of the absorbance is not yet clearly understood.

4 Conclusions

In this paper, we reported a simple solvothermal method for the synthesis of nanocrystalline CuIn

(1-x)

Ga

x

S

2

at 200

°C for 16 h. Ethylene glycol is used as the sol-

vent, reducing agent and stabilizer. The powders obtained are constituted of primary crystallites with sizes in the range 20–30 nm. The key to producing these nanoparticles is to use a copper (II) salt, sodium hydroxide and a large excess of sulfur. The results confirm that the Ga

^3?

ions are successfully incorporated into the CuInS

₂

structure. The optical band gaps of the CuIn

_x

Ga

_(1-x)

S

₂

nanoparticles increase with increasing Ga content. This method could be used for the preparation of other ternary or quaternary chalcogenide compounds.

Acknowledgments This work was supported by two French- Moroccan projects: Volubilis Partenariat Hubert Curien (PHC no:

MA 09 205) and Projet de Recherches Convention Internationale du CNRS (CNRS-CNRST no: w 22572).

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