Solvothermal synthesis and optical limiting properties of carbon nanotube-based hybrids containing ternary chalcogenides

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Solvothermal synthesis and optical limiting properties of carbon nanotube-based hybrids containing ternary chalcogenides Huixia Wu a,*, Dandan Liu a, Haoqiang Zhang a, Chenyang Wei b, Bo Zeng a, Jianlin Shi b, Shiping Yang a a

The Key Laboratory of Resource Chemistry of Ministry of Education, College of Life and Environmental Science, Shanghai Normal University, Shanghai 200234, People’s Republic of China b State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China

A R T I C L E I N F O

A B S T R A C T

Article history:

Two or three types of semiconductor nanoparticles (NPs) have been deposited on the side-

Received 18 February 2012

walls of multi-wall carbon nanotubes (MWCNTs) by a solvothermal treatment of a mixture

Accepted 7 June 2012

containing poly(sodium 4-styrenesulfonate) (PSS) wrapped MWCNTs, metal chloride (CuCl2

Available online 13 June 2012

and SnCl2), and thiourea. Changing the ratio of Cu2+ to Sn2+ alters the composition of the resultant MWCNT–PSS–NP hybrids. Under Cu/Sn ratios of 3:1 and 2:1, MWCNTs can be simultaneously decorated with Cu2S and Cu3SnS4 NPs. When the ratio is reduced to 1:1, Cu3SnS4, Cu2S and SnO2 NPs would be formed at the same time. Further decreasing the ratio results in the formation of Cu2SnS3 and SnO2 instead of Cu3SnS4 and Cu2S. Openaperture z-scan measurements have been carried out on three typical MWCNT–PSS–NP samples to study their optical limiting (OL) properties. The addition of semiconductor NPs can improve the OL performance of MWCNTs, and the composition of the NPs has a significant effect on the OL behavior of the hybrids.  2012 Elsevier Ltd. All rights reserved.

1.

Introduction

Carbon nanotubes (CNTs) have shown strong application potential in many fields since they were discovered because of their unique structure and properties, including high surface area, unusual optical and electronic properties, high mechanical strength, and excellent chemical and thermal stability [1,2]. To improve the performance of CNTs for potential applications, there has been widespread interest in fabricating the novel one-dimensional CNT-nanoparticle (NP) hybrid materials [3]. Up to now, various organic and inorganic NPs have been assembled on CNTs by different techniques [3,4].

Semiconductor NPs have also attracted much attention due to their unique size-tunable chemical and physical properties [5–7]. The attachment of semiconductor NPs to CNTs can produce novel hybrids that combine the excellent properties of these two functional nanoscale materials. Various semiconductor NPs such as SnO2 [8], TiO2 [9], CdSe [10], ZnO [10], and CdS [11] have been bound to the surfaces of CNTs. However, there are few papers in the literature concerning the loading of ternary chalcogenide semiconductor NPs on CNTs. As an important category of I–IV–VI ternary chalcogenides, the Cu–Sn–S system has shown great potential to be used as nonlinear optical (NLO) materials and small or

* Corresponding author: Fax: +86 21 64322511. E-mail address: [email protected] (H. Wu). 0008-6223/$ - see front matter  2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.06.011

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mid-band-gap semiconductors due to their outstanding optical, thermal and mechanical properties [12–15]. Cu3SnS4 and Cu2SnS3 are two important Cu-Sn-S ternary semiconductor compounds, and they have attracted considerable interest in recent years [12,13,16]. In this paper, we are concerned about the simultaneous growth of Cu3SnS4 (or Cu2SnS3) NPs and two important semiconductor NPs, namely Cu2S and SnO2, on the sidewalls of multi-wall carbon nanotubes (MWCNTs). Cu2S is a p-type semiconductor, while SnO2 is an n-type semiconductor. On account of their unique properties, these two materials have been widely investigated for various potential applications [17–20]. The solvothermal pathway has been proved to be a helpful technique to prepare nanocrystalline materials under mild conditions without organometallic or toxic precursors [21,22]. Herein, by the solvothermal treatment of a mixture of poly(sodium 4-styrenesulfonate) (PSS) wrapped MWCNTs, metal (Cu2+ and Sn2+) chloride, and thiourea, a series of MWCNT–PSS–NP hybrids with two (Cu2S and Cu3SnS4, or Cu2SnS3 and SnO2) or three types (Cu3SnS4, Cu2S and SnO2) of semiconductor NPs loaded on CNTs have been synthesized. By tuning the ratio of Cu2+ to Sn2+, the composition of NPs can be controlled. The resultant MWCNT–PSS–NP hybrids have been extensively characterized by a variety of microscopy and spectroscopic techniques. Since CNTs and many types of semiconductor NPs have attractive NLO properties [23], the open-aperture z-scan technique was used to study the NLO properties and optical limiting (OL) effect of three typical MWCNT–PSS–NP hybrids.

2.

Experimental

2.1.

Chemicals and materials

MWCNTs with 60–80 nm of outer diameter and 10–15 lm of average length, kindly provided by Shenzhen Nanotech Port Co. Ltd., were purified according to the literature method [24]. CuCl2Æ2H2O, SnCl2Æ2H2O and PSS (Mw 70,000) were purchased from Aldrich. Thiourea and other reagents were obtained from Sinopharm Chemical Reagent Co., Ltd. All chemical reagents were of analytical grade and used as received.

2.2.

Preparation of MWCNT–PSS–NP hybrids

Purified MWCNTs (30 mg) were introduced into an ethanol solution of PSS (0.2 wt.%, 30 mL), followed by further sonication for 2 h. After excess PSS was removed, the obtained PSSwrapped MWCNTs (MWCNT–PSS) were dispersed in ethanol (30 mL) to form stable colloid. CuCl2Æ2H2O and SnCl2Æ2H2O (total metal ions: 1.2 mmol; molar ratio of Cu2+ to Sn2+: 3:1, 2:1, 1:1, 1:2, 1:3) was added to the above MWCNT–PSS suspension, and the mixture was further stirred for 15 min at room temperature to ensure the full adsorption of metal ions on the surface of CNTs. Then the freshly prepared ethanol solution of thiourea (30 mL, 0.04 mol L1) was added dropwise under vigorous stirring. Afterwards, the solution was sealed in a Teflon-lined autoclave (100 mL) and maintained at 200 C for 12 h. The resultant hybrids were collected by centrifugation,

rinsed with ethanol and H2O, and dried under a vacuum at 50 C.

2.3.

Characterization

Zeta-potentials were measured by a Malvern Zetasizer NanoZS90 instrument. The Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet Avatar 370 FT-IR spectrophotometer using KBr pellets. X-ray diffraction (XRD) patterns were determined by a Rigaku DMAX 2000 diffractometer equipped with Cu/Ka radiation (k = 0.15405 nm) (40 kV, 40 mA). Scanning electron microscope (SEM) analyses and energy dispersive X-ray spectroscopy (EDS) measurements were performed on a Hitachi S4800 field emission SEM. Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) data were acquired with a JEOL JEM-2100 high-resolution transmission electron microscope (HR-TEM). X-ray photoelectron spectroscopy (XPS) experiments were carried out on an RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Al Ka radiation (hm = 1486.6 eV). The ultraviolet–visible-near infrared (UV–vis-NIR) absorption spectra were obtained with a UV-3600 spectrophotometer (Shimadzu).

2.4.

Optical limiting behavior measurements

The z-scan technique was used to study the NLO and OL properties of the samples. During the measurement, the sample was gradually moved through the focus of a lens (along the z-axis) and the transmittance of the sample as a function of sample position z was measured [25]. Z-scan experiments were performed using a Nd:YAG laser system operating at the wavelength of 532 nm with a repetition rate of 10 Hz. The laser pulse width was 40 ps, and the energy of a single pulse was 30.7 lJ. The focal lengths of both focusing and collection lens were 7.5 cm. The sample solution was put in a quartz cell with 1 mm path length, and the measurement was carried out at room temperature.

3.

Results and discussion

3.1.

Characterization of MWCNT–PSS–NP hybrids

MWCNTs can be easily wrapped with PSS through the hydrophobic interaction between the surface graphitic layers of CNTs and the hydrophobic segments of PSS [26]. Noncovalent sidewall functionalization of CNTs with negatively charged PSS is propitious to tether metal ion precursors for subsequent chemical deposition of inorganic NPs onto CNTs. The zeta potential of purified MWCNTs dispersed in water was determined to be 12.5 mV. After PSS was coated on CNTs, the surface potential of MWCNT–PSS decreased obviously to 19.0 mV. The modification of PSS on the surface of CNTs was further confirmed by the FT-IR spectra, as indicated in Fig. 1. In the spectrum of MWCNT–PSS, the aliphatic C–H stretch of the polymer backbones is observed at 2924 and 2840 cm1, and the absorption at 1603 cm1 is attributed to the aromatic C–H stretch. The absorption peaks of O@S@O stretch appear at 1182 and 1128 cm1 [27].

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b Transmittance

2924 2840 1603

a

4000

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1128

1182

3000

2500

2000

1500

1000

500

Wavenumber/cm-1 Fig. 1 – FT-IR spectra of PSS (a) and MWCNT–PSS (b).

Cu2S Cu3SnS4

a

Cu2SnS3 SnO2

Intensity (CPS)

b

MWCNT

c d e 10

20

30

40

50

60

70

80

2θ (degree) Fig. 2 – XRD patterns of MWCNT–PSS–NP-1 (a), MWCNT– PSS–NP-2 (b), MWCNT–PSS–NP-3 (c), MWCNT–PSS–NP-4 (d), and MWCNT–PSS–NP-5 (e).

Fig. 2 presents the XRD patterns of the as-prepared MWCNT–PSS–NP hybrids. The detailed quantity ratio, compositions, and phase structures of the MWCNT–PSS–NP hybrids are listed in Table 1. Clearly, the composition of the obtained hybrid materials changes with the ratio of Cu2+ to Sn2+. For the case of 3:1 and 2:1, the XRD data reveal the presence of tetragonal Cu3SnS4 (JCPDS card No. 33–0501) and cubic Cu2S (JCPDS card No. 53–0522) in both the as-prepared MWCNT– PSS–NP-1 and MWCNT–PSS–NP-2 hybrids. The additional diffraction peak at 2h = 26.3 can be assigned to (0 0 2) reflection

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of MWCNTs [28]. When the Cu/Sn ratio is decreased to 1:1, a new phase of tetragonal SnO2 (JCPDS card No. 41-1445) is detected in addition to tetragonal Cu3SnS4 and cubic Cu2S. Further changing the ratio to 1:2 and 1:3, the characteristic diffraction signals from tetragonal Cu3SnS4 and cubic Cu2S disappear, while the diffraction peaks of triclinic Cu2SnS3 (JCPDS card No. 35-0684) can be observed. The tetragonal SnO2 phase still exists in MWCNT–PSS–NP-4 and MWCNT– PSS–NP-5, and the intensity of its diffraction peaks increases with the decrease of Cu/Sn ratio. The direct evidence for the deposition of semiconductor NPs on MWCNTs was given by SEM observations. The SEM images of MWCNT–PSS–NP hybrids prepared at different Cu/ Sn ratio are shown in Fig. 3, which reveals that the surface of MWCNTs is uniformly coated with NPs. Elemental analysis by EDS measurements indicates the presence of Cu, Sn, S, O and C in the as-prepared hybrids. The atomic ratios of Cu to Sn in MWCNT–PSS–NP-1, MWCNT–PSS–NP-2, MWCNT–PSS– NP-3, MWCNT–PSS–NP-4, and MWCNT–PSS–NP-5 are estimated to be 3.2:1.0, 2.1:1.0, 1.1:1.0, 1.0:2.3, and 1.0:3.2, respectively, very close to the initial Cu/Sn ratio. The TEM images and SAED patterns of MWCNT–PSS–NP-1, MWCNT–PSS–NP-3, and MWCNT–PSS–NP-4 were studied to further investigate the structure and composition of the MWCNT–PSS–NP hybrids (Fig. 4). It can be seen from the TEM images of these three samples that the majority of MWCNT surface has been decorated with nanocrystals, and the structure of nanotube remains unchanged. The adhesion of nanocrystals to the MWCNT substrate is very strong, as no detached NP can be observed on the copper grids. The sizes of the particles in the samples range from 4 to 8 nm. The SAED patterns of the samples show well-defined rings and spots, indicating that the NPs on MWCNTs are polycrystalline. The signals of tetragonal Cu3SnS4 and cubic Cu2S structures are distinguishable in the SAED pattern of MWCNT–PSS–NP-1. The TEM images and SAED pattern of MWCNT–PSS–NP-2 are shown in Fig. S1, which reveals that the MWCNTs have been uniformly coated with the tetragonal Cu3SnS4 and cubic Cu2S NPs. For MWCNT–PSS–NP-3, only the diffuse diffraction rings of tetragonal SnO2 and tetragonal Cu3SnS4 are detected, while the signals of cubic Cu2S could not be identified. The SAED pattern of MWCNT–PSS–NP-4 further proves the existence of triclinic Cu2SnS3 and tetragonal SnO2 on the surface of MWNTs. The HR-TEM image of MWCNT–PSS–NP-1 presented in Fig. 4b shows the lattice spacings of 0.28, 0.31 and 0.34 nm, which should be attributed to the d values of the (2 0 0) plane of Cu2S, (1 1 2) plane of Cu3SnS4, and the interlayer distance of MWCNT, respectively. The HR-TEM micrograph shown in Fig. 4d reveals that MWCNT–PSS–NP-3 is consisted of the graphitized walls, Cu3SnS4, Cu2S and SnO2 NPs. For

Table 1 – The detailed quantity ratio, compositions, and phase structures of the MWCNT–PSS–NP hybrids. Cu/Sn ratios 3:1 2:1 1:1 1:2 1:3

Samples

Compositions and phase structures of NPs

MWCNT–PSS–NP-1 MWCNT–PSS–NP-2 MWCNT–PSS–NP-3 MWCNT–PSS–NP-4 MWCNT–PSS–NP-5

Tetragonal Cu3SnS4 and cubic Cu2S Tetragonal Cu3SnS4 and cubic Cu2S Tetragonal Cu3SnS4, cubic Cu2S, and tetragonal SnO2 Triclinic Cu2SnS3 and tetragonal SnO2 Triclinic Cu2SnS3 and tetragonal SnO2

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Fig. 3 – SEM images of MWCNT–PSS–NP-1 (a), MWCNT–PSS–NP-2 (b), MWCNT–PSS–NP-3 (c), MWCNT–PSS–NP-4 (d), and MWCNT–PSS–NP-5 (e).

Fig. 4 – TEM and HR-TEM images of MWCNT–PSS–NP-1 (a and b), MWCNT–PSS–NP-3 (c and d), and MWCNT–PSS–NP-4 (e and f). The inset in (a, c and e): SAED patterns of the hybrids.

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MWCNT–PSS–NP-4, the lattice fringes of SnO2 and Cu2SnS3 are fairly visible, as indicated in Fig. 4f. The above SAED and HR-TEM data of MWCNT–PSS–NP-1, MWCNT–PSS–NP-3, and MWCNT–PSS–NP-4 agree well with their corresponding XRD data. To study the valence states of Cu–Sn–S ternary sulfide compounds in the hybrids, the XPS spectra of MWCNT–PSS– NP-2 and MWCNT–PSS–NP-5 were determined. The XPS survey spectra of both samples show the signals of C1s, O1s, S2p, Cu2p and Sn3d, revealing the presence of C, O, S, Cu and Sn elements. In the Cu2p core level spectrum of MWCNT–PSS–NP-2 (Fig. 5a), the main and satellite peaks of Cu2p3/2 and Cu2p1/2 can be observed. The XPS core level spectrum in the Cu2p3/2 energy region can be deconvoluted into two peaks. The peak at 932.6 eV is related to Cu+, while the signal at 934.7 eV is due to Cu2+ [29]. By further analyzing the XPS result shown in Fig. 5a, the atomic ratio of Cu+ to Cu2+ is estimated to be 2.5, which is higher than the stoichiometric value of Cu3SnS4 because of the presence of Cu2S NPs. As for MWCNT–PSS–NP-5, the binding energies of Cu2p3/2 and Cu2p1/2 locate at 932.5 and 952.3 eV (Fig. 5c), respectively, in agreement with the literature values of Cu (I) [30]. Besides, there is no shake-up satellite at about 942 eV attributed to Cu2+, indicating that only Cu+ exists in this sample [31]. XPS analyses also reveal the normal valence states of Sn and S atoms in MWCNT–PSS–NP-2 and MWCNT–PSS–NP-5.

(a)

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The Sn3d core level spectra of both samples (Fig. 5b and d) illustrate that the observed binding energies of Sn3d5/2 and Sn3d3/2 are close to those reported for Sn4+ [13,32]. Moreover, the Sn3d5/2 satellite peak characteristic of Sn2+, which is usually centered at about 485.7 eV [13], is absent. In the case of sulfur, the binding energies, appearing at about 162.5 eV in the XPS core level spectra of S2p for both samples, correspond to the reduction (2) state of S atoms [33]. According to the XPS data, it can be concluded that the ternary sulfide in MWCNT-PSS-NP-2 is Cu3SnS4 with normal valence states of 2þ 4þ 2 Cuþ 2 Cu Sn S4 , whereas Cu2SnS3 with normal valence 4þ 2 states of Cuþ 2 Sn S3 exists in MWCNT–PSS–NP-5. The UV–vis-NIR spectra of purified MWCNTs, MWCNT– PSS–NP-1, MWCNT–PSS–NP-2, MWCNT–PSS–NP-3, MWCNT– PSS–NP-4, and MWCNT–PSS–NP-5 are shown in Fig. 6. Each sample was dispersed in N,N-dimethyl formamide (DMF) with a concentration of 0.1 mg mL1 before measurement. The suspension of acid-purified MWCNTs gives the typical featureless absorption of MWCNTs (Fig. 6a). For MWCNT–PSS–NP-1 and MWCNT–PSS–NP-2, a broad absorption band appears at the wavelength range of 400–800 nm. It has been reported that the absorption spectra of Cu2S nanocrystals present a broad absorption peak within this wavelength range [34]. Meanwhile, it has been found that the pure Cu3SnS4 NPs also display an absorption peak at 300–600 nm (Fig. S2). Therefore, the broad absorption bands observed in the absorption spectra

(b)

Cu2p3/2

Sn3d5/2

932.6 Counts (s)

Counts (s)

Sn3d3/2 Cu2p1/2 934.7

930

940

950

485

960

490

495

500

Binding energy (eV)

Binding energy (eV)

(c)

(d) Cu2p3/2

Cu2p1/2

930

940

950

Binding energy (eV)

Sn3d3/2

Counts (s)

Counts (s)

Sn3d5/2

960

485

490

495

500

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Fig. 5 – Cu2p and Sn3d core level spectra of MWCNT–PSS–NP-2 (a and b) and MWCNT–PSS–NP-5 (c and d).

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Purified MWCNTs MWCNT-PSS-NP-1 MWCNT-PSS-NP-2 MWCNT-PSS-NP-3 MWCNT-PSS-NP-4 MWCNT-PSS-NP-5

0.8

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0.6 0.4 0.2 0.0 400

600

800

1000

1200

Wavelength (nm) Fig. 6 – UV–vis-NIR spectra of purified MWCNTs, MWCNT– PSS–NP-1, MWCNT–PSS–NP-2, MWCNT–PSS–NP-3, MWCNT–PSS–NP-4, and MWCNT–PSS–NP-5.

of MWCNT–PSS–NP-1 and MWCNT–PSS–NP-2 are mainly due to Cu3SnS4 and Cu2S NPs. The absorption peak at about 275 nm may be partly due to the absorption of MWCNTs. The energy transfer between the CNTs and the semiconductors may also result in this absorption peak [35]. Besides an absorption peak at 275 nm, MWCNT–PSS–NP-3 also shows a weak broad absorption band at 400–800 nm. In the case of MWCNT–PSS–NP-4 and MWCNT–PSS–NP-5, only a strong absorption peak mainly due to SnO2 NPs appears at about 275 nm [36], while no visible absorption peak was detected at wavelengths longer than 400 nm. For MWCNT–PSS–NP-4 and MWCNT–PSS–NP-5, there are no distinguishable absorption peaks relative to Cu2SnS3 NPs [37]. In addition to the above mentioned absorption bands, the absorbance of all these samples decreases gradually from UV to near-IR region. The FT-IR spectrum of MWCNT–PSS–NP-2 is presented in Fig. S3. The characteristic absorbance peaks, which appear at 2923, 2847 cm1 (CH2 stretching), 1402 cm1 (CH2 in-plane bending), and 1114 cm1 (O@S@O stretching), confirm the still presence of PSS in the resulting MWCNT–PSS–NP-2 hybrids [27]. To investigate the formation mechanism of the Cu–Sn–S ternary chalcogenides, a series of comparison experiments were studied. First, three groups of comparison experiments were carried out in ethanol at 70 C as follows [16]: (A) thiourea; (B) 1.2 mmol 1.2 mmol CuCl2 + 1.2 mmol SnCl2 + 1.2 mmol thiourea; (C) CuCl2 + SnCl2 + 1.2 mmol thiourea (total metal ions: 1.2 mmol; molar ratio of Cu2+ to Sn2+: 3:1, 2:1, 1:1, 1:2, 1:3). White precipitates were formed in experiment (A) and (C), while no precipitate could be obtained in experiment (B). Furthermore, the quantities of the white precipitates in the experiment group (C) decreased with the decrease of the feed amount of Cu2+. The white products were collected by centrifugation, washed repeatedly with ethanol and water, and dried in vacuum for characterization. The FT-IR spectrum and the Cu2p core level spectrum of the white precipitate obtained in experiment (A) are shown in Fig. S4, which indicate that the thiourea uses sulfur atom to coordinate with Cu+ cation to form Cu+–thiourea complex precursor at the initial

stage of the reaction. The white precipitates obtained from experiment (C) show similar FT-IR and XPS spectra. Next, another three groups of comparison experiments with preparation conditions similar to that of MWCNT–PSS– NP-n (n = 15) were carried out as follows: (D) MWCNT–PSS + CuCl + SnCl2 + thiourea (Cu/Sn ratio = 3:1); (E) MWCNT– PSS + CuCl + SnCl2 + thiourea (Cu/Sn ratio = 1:3); (F) MWCNT– PSS + SnCl2 + thiourea. The resulting products of experiment (D), (E) and (F) were named as MWCNT–PSS–NP-6, MWCNT– PSS–NP-7 and MWCNT–PSS-NP-8. According to the XRD analysis (Fig. S5(A)), the detected main phase of MWCNT–PSS–NP-6 can be assigned to the rhombohedral Cu8S5 (JCPDS card No. 33-0491), while both rhombohedral Cu8S5 (JCPDS card No. 33-0491) and tetragonal SnO2 (JCPDS card No. 41-1445) were existing in MWCNT– PSS–NP-7. The Cu3SnS4 failed to be obtained under a 3:1 feed ratio of CuCl to SnCl2, because Cu+ ions could not be oxidized to Cu2+ in this reaction system. Meanwhile, Cu2SnS3 cannot be formed under a 1:3 feed ratio of CuCl and SnCl2, because the Sn2+ was directly oxidized to SnO2 by O2 without the presence of Cu2+. To further confirm the formation of Cu8S5 instead of Cu3SnS4, the Cu2p and Sn3d core level XPS spectra of MWCNT-PSS-NP-6 were measured (Fig. S5 (C and D)). The molar ratio of Cu/Sn in this sample is 6.1:0.1. The existence of Sn4+ can be attributed to the oxidation of a little Sn2+ by O2. The binding energies of Cu2p3/2 (932.4 eV) and Cu2p1/2 (952.4 eV) and the absence of shake-up satellite at about 942 eV indicate that only Cu+ existed in this sample. Therefore, no Cu3SnS4 was formed in MWCNT–PSS–NP-6. In all the obtained MWCNT–PSS–NP products, no tin sulfides were found. Under the conditions of excessive tin sources and thiourea, SnO2 was formed instead of tin sulfides. Even when only MWCNT–PSS, tin sources and thiourea were added into the reaction system (Fig. S5(B)), The XRD pattern of the resulting MWCNT–PSS–NP-8 reveals only the presence of tetragonal SnO2 phase (JCPDS card No. 41-1445), and no impurity phases can be detected. These results indicated that the Cu3SnS4 and Cu2SnS3 were not formed through the solid solution intermediate of tin sulfides [13]. According to the above results, the possible formation mechanism of Cu3SnS4 and Cu2SnS3 nanocrystals can be briefly described as follows [13,16,37]. In the reaction system, Cu2+ can be reduced to Cu+ by thiourea, and thiourea easily chelates Cu+ to form stable Cu+-thiourea complexes, while Sn2+ can be slowly oxidized to Sn4+ by Cu2+ ion. As the temperature and pressure increases, the Cu+–thiourea complexes will decompose and release the Cu+ ions. Meanwhile, the hydrolyzation of thiourea will produce H2S species. Under Cu/Sn ratios of 3:1, 2:1 and 1:1, the newly produced Sn4+ rapidly reacts with H2S to produce monomeric [SnS4]4 anion [13,38], which then combines with copper ions (Cu+ and Cu2+) and results in the formation of Cu3SnS4 crystal nuclei. Simultaneously, some released Cu+ ions react with H2S to produce Cu2S. In the case of 1:2 and 1:3, all Cu2+ ions can be transformed to Cu+ by thiourea or Sn2+. The reaction of Sn4+ with H2S can induce the formation of [Sn2S6]4 anion [37], and then the Cu+ combines with [Sn2S6]4 to form Cu2SnS3 product. The SnO2 phase may be formed by the reaction between Sn2+ cations and dissolved oxygen in the solvent [39],

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since the amount of Cu2+ ions in the reaction systems of 1:1, 1:2 and 1:3 is not enough to turn all Sn2+ cations into Sn4+.

3.2.

[40]. Moreover, the NP-modified MWCNT samples show significantly lower normalized transmission than purified MWCNTs at the same pulse intensity at the focus. Therefore, the surface modification of CNTs with the NPs improves the OL performance of CNTs. To directly show the OL performance of the samples, the relation between the transmitted signal and the sample position is transformed into the normalized transmission versus the incident fluence. Herein, the incident fluence is defined as Fpulse = Epulse/px(z)2 [41,42], where Epulse is the energy of a single pulse and x(z) is the radius of the propagating Gaussian pulse as a function of position z. The single incident pulse energy was 30.7 lJ, and the beam waist radius at the focus was 17.5 lm. The resulting curves of normalized transmission versus incident fluence calculated from open-aperture z-scan data for these samples are shown in Fig. 7b. As shown in Fig. 7b, the transmittance of these four samples decreases as the fluence increases. This fluencedependent transmittance feature is a typical OL response. The normalized transmission of MWCNT–PSS–NP-2, MWCNT–PSS–NP-3 and MWCNT–PSS–NP-4 starting to deviate from unity is determined to be 0.2, 0.6 and 0.4 J cm2, respectively, lower than that of purified MWCNTs (0.9 J cm2). The OL properties can be quantitatively compared by the limiting threshold, defined as the input fluence at which the transmittance falls to 50% of the linear transmittance [43]. The limiting thresholds of MWCNT–PSS–NP-2, MWCNT–PSS– NP-3 and MWCNT–PSS–NP-4 are about 1.2, 3.2 and 2.3 J cm2 at 532 nm, respectively. In contrast, the normalized transmission of purified MWCNTs only falls to 67% at an input fluence of 3.20 J cm2. Therefore, the MWCNT–PSS–NP samples show better OL behavior than do purified MWCNTs, and the semiconductor NPs loaded on CNTs have made an important contribution to the improved OL performance. Among these three samples, the OL performance is MWCNT–PSS–NP2 > MWCNT–PSS–NP-4 > MWCNT–PSS–NP-3, indicating that the decoration of Cu2S and Cu3SnS4 NPs on MWCNTs can significantly improve the OL properties of CNTs. The

NLO properties of MWCNT–PSS–NP hybrids

The z-scan technique was used to determine the NLO characteristics of MWCNT–PSS–NP suspensions. The z-scan measurements were performed on three typical hybrid materials including MWCNT–PSS–NP-2, MWCNT–PSS–NP-3 and MWCNT–PSS–NP-4. The NLO properties of purified MWCNTs and MWCNT–PSS were also measured for comparison. The samples were suspended in DMF with a concentration of 1 mg mL1 for the experiments. Since MWCNT–PSS–NP hybrids were prepared by direct growth of NPs on CNTs and no pristine NPs were synthesized, the NPs were not measured in our experiments. Both the open- and closed-aperture z-scan data of MWCNT–PSS–NP samples were measured under exposure to a Nd:YAG laser at wavelength of 532 nm. The closed-aperture z-scan curves of the hybrids show only a valley similar to their open-aperture z-scan data, and there are no alternant increase and decrease of normalized transmittance, indicating that the MWCNT–PSS–NP suspensions have no nonlinear refraction effect under the experiment conditions. Therefore, only the open-aperture z-scan data of the samples were presented and discussed in the following text. Fig. 7a shows the typical open-aperture z-scan traces of purified MWCNTs, MWCNT–PSS–NP-2, MWCNT–PSS–NP-3 and MWCNT–PSS-NP-4 suspensions at the wavelength of 532 nm. Only the data of purified MWCNTs are shown for comparison, because both purified MWCNTs and MWCNT– PSS suspensions show similar NLO behavior. In the vicinity of the lens focus, all samples suspended in DMF exhibit a decrease in the transmittance, as indicated in Fig. 7a. Meanwhile, there is no similar phenomenon observed from DMF. Consequently, the open z-scan data reveal that the normalized transmittance of the samples decreases as the light intensity increases, indirectly showing their OL responses

1.0

1.0

Normalized transmission

(b) 1.2

Normalized transmission

(a) 1.2

0.8 0.6 0.4 Purified MWCNTs MWCNT-PSS-NP-2 MWCNT-PSS-NP-3 MWCNT-PSS-NP-4

0.2 0.0 -2

-1

0

Z (cm)

1

4853

5 0 ( 20 1 2 ) 4 8 4 7–48 5 5

2

0.8 0.6 0.4

Purified MWCNTs MWCNT-PSS-NP-2 MWCNT-PSS-NP-3 MWCNT-PSS-NP-4

0.2 0.0 0.01

0.1

1 -2

Fpulse (J cm )

Fig. 7 – Typical open-aperture traces (a) and plots of normalized transmission versus incident fluence calculated from openaperture z-scan data (b) for purified MWCNTs, MWCNT–PSS–NP-2, MWCNT–PSS–NP-3 and MWCNT–PSS–NP-4 dispersed in DMF at a wavelength of 532 nm with a pulse intensity of I0 = 7.98 · 1010 W cm2 at the focus.

4854

CARBON

5 0 ( 2 0 1 2 ) 4 8 4 7 –4 8 5 5

open-aperture z-scan data also reveal that the composition of NPs has a significant effect on the OL behavior of the resultant MWCNT–PSS–NP hybrids. In recent years, the OL properties of CNT-based hybrid materials have been concerned. Several kinds of inorganic nanomaterials such as Au [44,45], Ag [44], TiO2 [45], SiO2 [45], FeOx [46], ZrOx [46], and metal sulfide (MS) [47,48] have been loaded on the surfaces of CNTs, and the OL performance of the resulting hybrid materials has been evaluated. Only coating carbon nanotubes with Au or Ag or MS particles can clearly enhances the overall OL effect of the MWNTs. In this paper, the three typical MWCNT–PSS–NP hybrids all show enhanced OL performance after the decoration of different kinds of NPs. Furthermore, the OL responses of the hybrids can be regulated by tuning the composition of the NPs. Therefore, such MWCNT–PSS–NP hybrids have favorable OL properties. The nonlinear absorption coefficient b can be calculated from open-aperture z-scan data [25,49]. By theoretical fits, the nonlinear absorption coefficient b at 532 nm of purified MWCNTs, MWCNT–PSS–NP-2, MWCNT–PSS–NP-3 and MWCNT–PSS–NP-4 was determined to be 1.3 · 1010, 3.3 · 1010, 2.1 · 1010, and 2.7 · 1010 cm W1, respectively, with the same order of magnitude as those reported in the manuscript [45]. The MWCNT–PSS–NP samples possess nonlinear absorption coefficients larger than purified MWCNTs, indicating that the loading of NPs on MWCNTs can improve the nonlinear absorption coefficients of the resultant materials. For CNTs and semiconductor nanostructures, nonlinear scattering is also involved in their OL performance [45]. Therefore, it should be noted that the coefficient b derived from open-aperture z-scan data is actually an integrated contribution from nonlinear absorption and nonlinear scattering. The above results of OL experiments indicate that MWCNT–PSS–NP-2, MWCNT–PSS–NP-3, and MWCNT–PSS– NP-4 all show better OL properties at 532 nm compared with that of purified MWCNTs. There may be two factors that contribute to the enhancement of the NLO performance of MWCNT–PSS–NP samples. First, the semiconductor NPs in MWCNT–PSS–NP, especially Cu3SnS4 and Cu2SnS3 NPs, should have good NLO properties. Second, the deposition of semiconductor NPs on MWCNTs may result in an enhanced nonlinear scattering due to the increase of the size of the scattering center over that of the pristine MWCNTs [47]. Among these three samples, MWCNT–PSS–NP-2 showed the best OL performance at 532 nm. MWCNT–PSS–NP-2 has a broad absorption band in the wavelength range of 400–800 nm. Its superior OL performance may be partly due to the band-gap absorption processes in Cu3SnS4 and Cu2S NPs [50]. In addition, the possible contribution of free-carrier absorption of Cu3SnS4 and Cu2S NPs to OL improvement at 532 nm should also be considered [23].

4.

Conclusions

The sidewalls of MWCNTs has been loaded with two (Cu3SnS4 and Cu2S, or Cu2SnS3 and SnO2) or three (Cu3SnS4, Cu2S and SnO2) types of semiconductor NPs by a solvothermal treatment of a mixture of PSS wrapped MWCNTs, metal chloride,

and thiourea. The composition and species of NPs on MWCNTs can be altered by changing the Cu/Sn ratio. The OL properties of three typical MWCNT–PSS–NP samples have been studied by open-aperture z-scan measurements. The MWCNT–PSS–NP samples all show better OL performance than MWCNTs due to the presence of semiconductor NPs. And importantly, the OL responses of the hybrids can be regulated by tuning the composition of the NPs. The MWCNT– PSS–NP hybrids with different types of NPs simultaneously loaded on CNTs can be considered as potential candidates for limiting applications to protect human eyes and sensors.

Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 50972092), the Shanghai Key Laboratory of the Rare Earth Functional Materials (07dz22303), the Key Subject of Education Ministry of China (210075), and the Shanghai Municipal Education Commission (10ZZ84).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2012.06.011.

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