7/30/2019 Synthesis and photoresponse of novel Cu2.pdf

1/3

Synthesis and photoresponse of novel Cu2CdSnS4 semiconductor nanorods

Yong Cui, Gang Wang* and Daocheng Pan*

Received 1st April 2012, Accepted 18th May 2012

DOI: 10.1039/c2jm32034g

Novel semiconductor Cu2CdSnS4 nanorods with a wurtzite structure

have been successfully synthesized and characterized in detail. The

suitable band gap of 1.4 eV and photoresponse property of

Cu2CdSnS4 nanorods indicate that they have a high potential

application in low-cost thin film solar cells.

Recently, stoichiometric Cu2(MII)(MIV)(S,Se)4 (MII Mn, Fe, Co,Ni, Zn, Cd, Hg; MIV Si, Ge, Sn) chalcogenide semiconductor

nanocrystals have obtained great interest because of their potential

application in thin film solar cells for structure analogues to

CuIn(Ga)Se2.1 These multiple component semiconductors possess

suitable band gap (1.11.5 eV) and high absorption coefficient (104 to

105 cm1) and do not contain low abundance elements, such as

indium and gallium. Recently, the high power conversion efficiencies

of solar cells using Cu2ZnSnS(Se)4 and Cu2ZnSn(Ge)S(Se)4 nano-

crystals as high as 7.2% and 6.8% have been reported by Agrawals

group,2,3 but the synthesis of other similar multiple component

nanocrystals is still a challenging task. To our best knowledge, only

chalcogenide Cu2CoSnS4, Cu2FeSnS4, and Cu2CdSnSe4 nano-

crystals have been synthesized46 and there are no reports of thesenanocrystals with a one-dimensional (1D) structure but Cu2ZnSnS4nanorods.7 It should be noted that the nanocrystal shape can strongly

influence their optical and electrical properties, and one-dimensional

nanorods have been observed to exhibit unusual optical properties

and excellent charge transport performance, for instance CdSe

nanorods showed higher efficiency than CdSe nanoparticles in CdSe

P3HT hybrid solar cells, because the nanorods provide a directed

path for electrical transport.8 Therefore, the synthesis of 1D semi-

conductor nanorods is critically important to develop next-genera-

tion, low-cost, and high-performance solar cells. Here, we adopted

a solvent thermal approach to prepare wurtzite Cu2CdSnS4 nanorods

with a suitable band gap of 1.4 eV, which indicates that they have

a high potential application in the field of low-cost thin film solarcells.

It is well known that Cu2CdSnS4 (CCTS) compound usually

crystallizes in a stannite (space group: I42m) or kesterite (space

group: I4) structure in the literature, and all of the metal ions have

a fixed position in the unit cells.9 Recently, Ibaa~nez and coworkers

have synthesized the sphere-like Cu2CdSnSe4 nanocrystals with

a tetragonal chalcopyrite structure.10 However, the surface energy

difference between various facets of chalcopyrite structure is insig-

nificant. Thus it is difficult to synthesize 1D chalcopyrite structure

nanorods. For wurtzite semiconductor nanocrystals, the difference of

surface energies is large, therefore it is quite possible to synthesize 1D

nanorods with a wurtzite structrue.11 More recently, wurtzite

CuIn(Ga)S2 and Cu2ZnSnS4 nanorods have been successfully

synthesized through controlling the release speed of sulfur source

under relatively high reaction temperature (240290 C).7,12 Here,

a solvothermal approach was utilized to synthesize wutzite

Cu2CdSnS4 nanorods under mild experimental conditions.13 We

choose hexylamine as solvent, carbon disulfide as sulfur source, and

3-mercaptopopionic acid (MPA) as a capping agent. The stoichio-

metric wurtzite Cu2CdSnS4 nanorods were nucleated at low

temperature (140 C) and then grown at high temperature (180 C).

The detailed experimental procedure and synthetic conditions are

provided in the ESI.

X-ray diffraction (XRD) was utilized to analyze the crystal struc-

ture ofCu2CdSnS4 nanocrystals. Fig. 1 shows theXRD pattern of as-

synthesized Cu2CdSnS4 nanocrystals with a wurtzite structure. It is

found that our diffraction pattern does not match those reported for

bulk Cu2CdSnS4 in the literature9 and those standard patterns in

JCPDS card database (stannite phase, PDF no. 29-0537). Therefore,

Fig. 1 The experimental and simulated powder XRD patterns of

wurtzite Cu2CdSnS4 nanorods.

State Key Laboratory of Rare Earth Resource Utilization, ChangchunInstitute of Applied Chemistry, Chinese Academy of Sciences, 5625Renmin Street, Changchun 130022, China. E-mail: wsu@ciac.jl.cn; pan@ciac.jl.cn

Electronic supplementary information (ESI) available: Detailedsynthesis of nanocrystals; unit cells, crystal data, EDS spectra,chemical compositions and size distribution. See DOI:10.1039/c2jm32034g

This journal is The Royal Society of Chemistry 2012 J. Mater. Chem., 2012, 22, 1247112473 | 12471

Dynamic Article LinksC

7/30/2019 Synthesis and photoresponse of novel Cu2.pdf

2/3

we simulated an XRD pattern for wurtzite Cu2CdSnS4. As can be

seen in Fig. 1, the experimental pattern matches well with the simu-

lated one by comparison between the experimental and simulated

XRD peak positions in Table S1, indicating that CCTS nano-

crystals possess a wurtzite structure. The major diffraction peaks

centered at 2q 26.47, 27.89, 29.98, 38.90, 46.73, 50.59, 55.29 can be

indexed to the (100), (002), (101), (102), (110), (103), and (112) crystal

planes of the wurtzite crystal structure, respectively. Fig. S1 shows

the unit cell of wurtzite Cu2CdSnS4. While wurtzite CCTS nano-crystals can be viewed as the deviation of wurtzite ZnS. In this

structure, sulfur anions are hexagonally close packed, and Cu+, Cd2+,

and Sn4+ ions randomly occupy Zn2+ position, and their occupancy

possibilities are 1/2, 1/4 and 1/4, respectively. Additionally, no other

impure phases, such as stannite or kesterite structure, could be

observed in our XRD pattern, signifying that a pure wurtzite struc-

ture is obtained. During the synthesis process, 3-mercaptopopionic

acid plays a key role in the formation of wurtzite CCTS nanorods

since MPA has a strong bonding strength with precursors, which

leaves the majority of precursors for the growth of nanorods.

Transmission electron microscopy (TEM) was used to characterize

the morphologies of the Cu2CdSnS4 nanorods. As shown in Fig. 2a,

the TEM image reveals that CCTS nanocrystals have a rod-likestructure. The average diameter and length of CCTS nanorods are

about 5.7 nm and 26 nm, respectively, and the aspect ratio (length/

diameter) of CCTS nanorods was about 4.7. In addition, the high

resolution TEM image in Fig. 2b confirms the high crystallinity of

CCTS nanorods. Selected area electron diffraction (SAED) can

further confirm the crystal structure of these Cu2CdSnS4 nanorods.

The SAED image in Fig. 2c clearly reveals that the as-synthesized

CCTS nanorods possess a wurtzite structure instead of traditional

stannite, chalcopyrite, or kesterite structure. Note that a low nucle-

ation temperature is very important for the formation of CCTS

nanorods, because a low nucleation temperature results in less critical

nuclei and leaves the majority of precursors for the growth of longer

nanorods. In theinitial period of reaction, the nuclei were formed and

then extended anisotropically in the (002) facet direction in the

following reaction time. Due to sufficiently different binding affinity

of the capping ligand on different crystal facets and difference of the

surface energy, unequal growth rates in different directions result indifferent morphologies, such as semiconductor nanorods and nano-

wires which were grown along the c-axis via preferential passivation

of facets perpendicular to the growth axis. This mechanism is similar

to the formation mechanism of wurtzite ZnSe,14 CdS,15 and CdSe16

nanorods or nanowires.

To confirm the chemical composition of Cu2CdSnS4 nanorods,

energy dispersive X-ray spectroscopy was applied to analyze the

elemental composition. Approximately 10 different points on the

nanocrystal film were detected. As shown in Fig. S2, the molar ratio

of Cu/Cd/Sn/S is close to 2 : 1 : 1 : 4, which is consistent with the

stoichiometric composition of Cu2CdSnS4.The valence states of Cu, Cd, Sn, S in CCTS nanocrystals were

determined by X-ray photoelectron spectroscopy (XPS). Two peaksof Cu2p, Cd3d, and Sn3d, located at 931.2 eV and 951.1 eV, 404.8 eV

and 411.4 eV, 485.9 eV and 494.2 eV, suggest that the valence states

of Cu, Cd, and Sn ions in the nanocrystals are +1, +2, and +4,

respectively (Fig. 3). The two peaks of sulfur located at 161.3 eV and

162.5 eV are assigned to S2p with a valence of2.

Fig. 4 shows the UV-vis-NIR absorption spectrum of CCTS

nanorods. Note that Cux

S nanocrystals do not exist in our sample

because of theabsence of the absorption in therangeof 10001400nm

resulting from Cux

S nanocrystals.17,18 Inset of Fig. 4 displays the abs2

vs.eVcurveforthenanocrystals; thebandgap isapproximately1.4eV,

which is in good agreement with the literature value of 1.37 eV.19 This

band gap value of CCTS nanorods indicates that CCTS nanocrystals

have a high potential application in thin film solar cells.To evaluate the potential applicability of wurtzite CCTS nanorods

as a photovoltaic material, the IV curves of CCTS thin film have

Fig. 2 Low resolution (a) and high resolution (b) TEM as well as SAED

(c) images of wurtzite Cu2CdSnS4 nanorods.

Fig. 3 The X-ray photoelectron spectroscopy (XPS) spectra of as-

synthesized Cu2CdSnS4 nanorods; (a) Cu2p; (b) Cd3d; (c) Sn3d; (d) S2p.

12472 | J. Mater. Chem., 2012, 22, 1247112473 This journal is The Royal Society of Chemistry 2012

View Article Online

http://dx.doi.org/10.1039/c2jm32034g

7/30/2019 Synthesis and photoresponse of novel Cu2.pdf

3/3

been measured in the dark and under illumination of the solar

simulator. A smooth CCTS film was obtained by spin casting

concentrated nanocrystal solution and then post-annealed at 400 C

on a ceramic hot plate for a few minutes in the glove-box. After the

removal of ligands, electrical conductivity of CCTS film was signifi-

cantly improved. The sheet resistance of CCTS film with a thickness

of 730 nm after post-treatment is approximately 16 000 U ,1,

which is close to the sheet resistance of a semiconductor material,confirming that the CCTS nanorod thin film possesses semi-

conductor property. The IVcurves, shown in Fig. 5, for the CCTS

nanorod thin film, exhibit photoresponse property. Thecurrent of the

CCTS nanorods at 5 V changes from 28 mA in the dark to 62 mA

under AM 1.5 G irradiation (100 mW cm2), two times higher, and

the Ilight/Idark ratio ofthe Cu2CdSnS4 nanorod film is close to those of

Cu2ZnSnS4 nanocrystal film.17 This result demonstrates that CCTS

nanorods can be potentially applied as an absorbing layer material in

thin film solar cells.

In conclusion, a solvothermal approach has been successfully

utilized to synthesize wurtzite Cu2CdSnS4 nanorods for the first time.

The absorption spectrum demonstrates that Cu2CdSnS4 nanorods

have an optical band gap of 1.4 eV. The observed photoresponse

from the spin casted film of the nanorods indicated that the earth-

abundant Cu2CdSnS4 nanorods are promising semiconductor

material for low-cost thin film solar cells.

This work was supported by the National Natural Science

Foundation of China (Grant no. 21071142; 51172229), the Fund forCreative Research Groups (Grant no. 20921002), the Natural Science

Foundation for Young Scientists of Jilin Province (20100105).

Notes and references

1 D. B. Mitzi, O. Gunawan, T. K. Todorov, K. Wang and S. Guha, Sol.Energy Mater. Sol. Cells, 2011, 6, 1421.

2 Q. J. Guo, G. M. Ford, W. C. Yang, B. C. Walker, E. A. Stach,H. W. Hillhouse and R. Agrawal, J. Am. Chem. Soc., 2010, 132,17384.

3 G. M. Ford, Q. J. Guo, R. Agrawal and H. W. Willhouse, Chem.Mater., 2011, 23, 2626.

4 (a) J. Y. Chane-Ching, A. Gillorin, O. Zaberca, A. Balocchi andX. Marie, Chem. Commun., 2011, 47, 5229; (b) A. Gillorin,A. Balocchi, X. Marie, P. Dufour and J. Y. Chane-Ching, J. Mater.Chem., 2011, 21, 5615.

5 C. Yan, C. Huang, J. Yang, F. Y. Liu, J. Liu, Y. Q. Lai, J. Li andY. X. Liu, Chem. Commun., 2012, 48, 2603.

6 F. J. Fan, B. Yu, Y. X. Wang, Y. L. Zhu, X. J. Liu, S. H. Yu andZ. F. Ren, J. Am. Chem. Soc., 2011, 133, 15910.

7 A. Singh, H. Geaney, F. Laffir and K. M. Ryan, J. Am. Chem. Soc.,2012, 134, 2910.

8 W. U. Huynh, J. J. Dittmer and A. P. Alivisatos, Science, 2002, 295,2425.

9 (a) W. Schafer and R. Nitsche, Mater. Res. Bull., 1974, 9, 645; (b)M. Himmrich and H. Haeuseler, Spectrochim. Acta, Part A, 1991,47, 933; (c) K. Ito and T. Nawazawa, Jpn. J. Appl. Phys., 1988, 27,2094; (d) S. Wagner and P. M. Bridebaugh, J. Cryst. Growth, 1977,39, 151; (e) H. Matsushita, T. Maeda, A. Katsui and T. Takizawa,J. Cryst. Growth, 2000, 208, 416.

10 M. Ibaa~nez, D. Cadavid, R. Zamani, N. Garc

a-Castello

a,V. Izquierdo-Roca, W. H. Li, A. Fairbrother, J. D. Prades,

A. Shavel, J. Arbiol, A. Pearez-Rodrguez, J. R. Morante andA. Cabot, Chem. Mater., 2012, 24, 562.

11 (a) P. D. Cozzoli, T. Pellegrino and L. Manna, Chem. Soc. Rev., 2006,35, 1195; (b) L. Manna, D. J. Milliron, A. Meisel, E. C. Scher andA. P. Alivisatos, Nat. Mater., 2003, 2, 382; (c) S. Kudela,L. Carbone, M. F. Casula, R. Cingolani, A. Falqui, E. Snoeck,W. J. Parak and L. Manna, Nano Lett., 2005, 5, 45; (d) X. T. Lu,Z. B. Zhuang, Q. Peng and Y. D. Li, CrystEngComm, 2011, 13, 4039.

12 Y. H. Wang, X. Y. Zhang, N. Z. Bao, B. P. Lin and A. Gupta, J. Am.Chem. Soc., 2011, 133, 11072.

13 (a) L. J. Chen, J. D. Liao, Y. J. Chuang and Y. S. Fu, J. Am. Chem.Soc., 2011, 133, 3704; (b) J. Yang, C. Xue, S. H. Yu, J. H. Zeng andY. T. Qian., Angew. Chem., Int. Ed., 2002, 41, 4697; (c) H. C. Jiang,P. C. Dai, Z. Y. Feng, W. L. Fan and J. H. Zhan, J. Mater. Chem.,2012, 22, 7502.

14 A. B. Panda, S. Acharya and S. Efrima, Adv. Mater., 2005, 17,2471.

15 D. V. Talapin, J. H. Nelson, E. V. Shevchenko, S. Aloni, B. Sadtlerand A. P. Alivisatos, Nano Lett., 2007, 7, 2951.

16 L. Manna, E. C. Scher and A. P. Alivisatos, J. Am. Chem. Soc., 2000,122, 12700.

17 X. T. Lu, Z. B. Zhuang, Q. Peng and Y. D. Li, Chem. Commun., 2011,47, 3141.

18 R. G. Xie, M. Rutherford and X. G. Peng, J. Am. Chem. Soc., 2009,131, 5691.

19 H. Matsushita, T. Ichikawa and A. Katsui, J. Mater. Sci., 2005, 40,2003.

Fig. 4 UV-vis-NIR absorption spectrum of wurtzite Cu2CdSnS4nanorods. The inset displays a plot of (abs2) vs. hv for the Cu2CdSnS4nanorods with the band gap estimated at 1.4 eV.

Fig. 5 The currentvoltage (IV) curves of the Cu2CdSnS4 film: in the

dark (black) and under simulated solar light illumination (red).

This journal is The Royal Society of Chemistry 2012 J. Mater. Chem., 2012, 22, 1247112473 | 12473

View Article Online

http://dx.doi.org/10.1039/c2jm32034g