The in situ growth of silver nanowires on multi-walled carbon nanotubes and their application in transparent conductive thin films

Abstract

A novel composite was constructed by the in situ growth of silver nanowires (AgNW) on multi-walled carbon nanotubes (MWCNT). Firstly, thiol groups were covalently bonded onto the surface of MWCNT by the esterification of thioglycolic acid with hydroxylated MWCNT. Reduced platinum (Pt) nanoparticles were then attached to the MWCNT via the thiol groups. In the presence of poly(vinyl pyrrolidone), Pt nanoparticles served as seeds for generating AgNW of uniform diameter. Finally, a MWCNT/AgNW composite with a conductive network structure was fabricated. The structure and morphology of the composite were measured by a variety of methods, including UV-visible extinction spectrometry, energy-dispersive spectrometry, X-ray diffractometry, Raman spectrometry and electron microscopy. An electrically conductive thin film of 80% transparency with a sheet resistance of 47.8 Ω sq⁻¹ was prepared by coating the composite onto a polyethylene terephthalate film substrate.

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1. Introduction

Since their discovery in 1991,¹ carbon nanotubes (CNT) have attracted increasing interest because of their unique one-dimensional nanostructure and their excellent electrical, mechanical and thermal properties. A variety of CNT-based composites have been developed and applied to a range of biosensor, optical, magnetic and catalytic applications.^2–6 Among these composites CNT–metal composites are of particular interest, because of their promising electrical, magnetic and optical properties.^7–9 For example, the electrical conductivity of CNT is significantly enhanced by attaching a layer of silver nanoparticles to their surface by coordination bonding.¹⁰

Because of the outstanding optical, plasmonic and electrical properties of metal nanowires, a variety of composites formed from CNT and metal nanowires have been intensively investigated.^11–15 These one-dimensional hetero-structures are of significance both in fundamental studies in nanoscience and also because of their potential industrial applications.^16–18 Luo et al.¹⁹ have fabricated a one-dimensional hetero-junction comprising a Ni nanowire, a multi-walled carbon nanotube (MWCNT) and an amorphous CNT. The interfacial structure of the hetero-junction results in it being simultaneously contacted and being able to participate in electrical transport. Arrays of one-dimensional heterojunctions of silver nanowires (AgNW) and amorphous CNT have also been synthesized and the amorphous CNT placed in chemical contact with AgNW, based on the characteristics of the electronic structure of amorphous semiconductors.²⁰ It has been concluded that the formation of a CNT–metal nanowire composite with a conductive network could provide a route to high conductivity.^21–23

Tokuno et al.²⁴ have added single-walled carbon nanotubes (SWCNT) to AgNW. The SWCNT form bridges between the AgNW and fill the surrounding spaces, resulting in high conductivity in the hybrid transparent electrodes produced. In addition to metal nanowire–CNT composites, Cui et al.²⁵ have also reported a process for growing AgNW on conductive indium-tin oxide (ITO) glass electrodes to give high conductivity.

In the present study, a novel composite of AgNW grown on the surface of MWCNT has been prepared and its properties investigated. The process for preparing the composite is shown in Scheme 1.

Scheme 1 Preparation of MWCNT/AgNW composites.

To enhance the interfacial interaction between MWCNT and AgNW, thiol groups were introduced onto the surface of MWCNT by a reaction between thioglycolic acid and the hydroxyl groups of MWCNT in the presence of acids. The immobilized thiol groups are able to serve as ligands for platinum (Pt) nanoparticles, obtained by reducing chloroplatinic acid (H₂PtCl₆) with ethylene glycol (EG). Subsequently, using the Pt nanoparticles as seeds, the AgNW are able to grow on the surface of the MWCNT. The advantage of this approach is that two conductive networks can be connected by chemical bonding. It may be anticipated that electrodes fabricated from such a composite should offer low sheet resistance, high transparency and excellent flexibility. Studies have now demonstrated the potential for the fabrication of electrical devices on plastic films by continuous drop-coating, a simple, inexpensive and easily controllable process.

2. Experimental

2.1 Materials

Multiwalled carbon nanotubes (diameter 10–90 nm, length 0.5–500 μm and purity >95%) were provided by Shenzhen Nanotech Port Co. Ltd, China. Concentrated sulfuric acid (98%), concentrated nitric acid (70%), N,N-dimethylformamide (DMF; purity 99%), tetrahydrofuran (THF; 99%), acetone (99%), EG (99%) and ethanol (99%) were purchased from Vas Chemical, China. Thioglycolic acid (90%) and H₂PtCl₆ were supplied by the Tianjin Guangfu Fine Chemical Research Institute. N,N-Dicyclohexyl carbodiimide (DCC) was purchased from Tokyo Chemical Industry Co. Ltd. 4-Dimethylaminopyridine (DMAP), silver nitrate (AgNO₃, 99%) and poly(vinylpyrrolidone) (PVP, MW 58 000) were provided by Shanghai Prolong Biochemical Co. Ltd. The THF was distilled before use.

2.2 Preparation of MWCNT attached to Pt nanoparticles (MWCNT–Pt)

Pristine MWCNT (1 g) was dispersed in 40 mL of mixed concentrated acids (H₂SO₄ : HNO₃, 1 : 1 v/v) for 1 h, and then refluxed for a further 1 h at 140 °C. The acid-treated MWCNT were filtered, washed five times with deionized water and then dried in a vacuum oven.

Acid-treated MWCNT (50 mg) were dispersed in 80 mL of DMF and 500 mg of thioglycolic acid, 100 mg of DCC and 10 mg of DMAP were added. The mixture was heated under nitrogen for 24 h at 80 °C. The thioglycolic acid-modified MWCNT were washed three times with anhydrous THF and dried in a vacuum oven.

Thioglycolic acid-modified MWCNT (4 mg) were dispersed in 10 mL of EG and the mixture heated at 160 °C with stirring for 1 h. H₂PtCl₆ (0.5 mL, 3.0 × 10⁻⁴ M) in EG was then added to the mixture and allowed to react for 4 min at 160 °C. The product was then separated by centrifugation and is denoted as MWCNT–Pt.

2.3 Preparation of silver nanowires

AgNW were synthesized by reducing silver nitrate in the presence of PVP in EG.^26,27 EG (10 mL) was heated with stirring in a flask to 160 °C for 1 h. Then 0.5 mL of H₂PtCl₆ (3.0 × 10⁻⁴ M) in EG was added to the mixture at this temperature and allowed to react for 4 min. AgNO₃ solution (2.5 mL, 0.40 M in EG) and 5 mL of PVP solution (0.72 M, also in EG) were simultaneously added dropwise over 6 min and then stirred for a further 1 h. The product was diluted with either acetone or ethanol and centrifuged for 20 min at 2000 rpm. This dispersion/centrifugation procedure was repeated eight times, and the product was dispersed in ethanol for further use.

2.4 Preparation of MWCNT/AgNW composite

MWCNT–Pt (4 mg) were dispersed in 10 mL of EG and heated in a flask with stirring to 160 °C. AgNO₃ solution (2.5 mL, 0.40 M in EG) and 5 mL of PVP solution (0.72 M in EG) were simultaneously added dropwise to the mixture over 6 min. The reaction was discontinued after 1 h, and the product was diluted with acetone or ethanol and centrifuged at 2000 rpm for 20 min. This dispersion/centrifugation procedure was repeated up to eight times and the final product (MWCNT/AgNW composite) was dispersed in ethanol for further use.

2.5 Preparation of transparent thin film

The MWCNT/AgNW composite was dispersed in ethanol at a concentration of 1.8 mg mL⁻¹. The suspension was drop-coated onto a poly(ethylene terephthalate) (PET) substrate and air-dried, giving a thin transparent film. The transparency of the film was found to be dependent on the thickness of the composite layer – for example, a film thickness of about 300 nm gave a transmittance of 80%.

2.6 Measurement

A Fourier transform infrared spectrometer (FT-IR, Thermo-Nicolet Nexus 670) was used to characterize the functionalized MWCNT in KBr pellets. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Thermo Scientific ESCALAB 250 spectrometer using a monochromatised Al K_α X-ray source at a constant rate. Thermo-gravimetric analysis (TGA, Netzsch TG 209c) was conducted at a heating rate of 10 °C min⁻¹ from 40 to 700 °C under nitrogen. The morphology of the samples was determined by transmission electron microscopy (TEM, JEOL JEM-100CX), scanning electron microscopy (SEM, Hitachi S-4700) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM 3010). X-ray diffraction (XRD, Rigaku D/Max 2400) was employed to characterize the crystal structures of AgNW and the MWCNT/AgNW composite. UV-visible extinction spectra were determined at room temperature using a Hitachi U-3010 spectrophotometer. Energy dispersive spectrometry (EDS) was undertaken using a Hitachi S-4700 SEM. Raman spectra (Horiba Jobin Yvon, HR800, wavelength 532 nm) were used to characterize the structure breakdown of MWNT after acid treatment and the formation of the MWCNT/AgNW composite. A digital multimeter (Victor Electronics VC9801A⁺) was used to measure the powder resistivity of the pulverised product and the sheet resistance of the thin film.

3. Results and discussion

3.1 Characterization of MWCNT

The FT-IR spectra of pristine MWCNT and acid-treated MWCNT are presented in Fig. 1. A broad –OH stretching peak is seen at 3400 cm⁻¹ and a peak indicating the C=O stretching of carboxylic and ester groups is found at 1716 cm⁻¹ in the spectrum of acid-treated MWCNT,^28,29 indicating that –OH groups had been successfully introduced onto the surface of the MWNT.

Fig. 1 FT-IR spectra of (a) pristine MWCNT and (b) acid-treated MWCNT.

The structures of thioglycolic acid-modified MWCNT were examined by XPS and TGA (Fig. 2(A) and (B), respectively). In the XPS spectrum, the bonding energy of S_2p at 165.8 eV, C_1s at 285.0 eV, and O_1s at 534.6 eV may be assigned to sulfydryl,³⁰ graphitic carbon, and hydroxyl or ester groups,²⁸ respectively. The emergence of S_2p confirmed that thioglycolic acid had, as expected, been grafted onto the surface of the MWCNT. The TGA curves were used to estimate the weight ratio of grafted organic material on the MWNT (Fig. 2(B)). Pristine MWNT (Fig. 2(B)(a)) maintained a stable weight up to about 700 °C. The 7% weight loss of acid-treated MWCNT (b) at 700 °C arose from the removal of –OH and –COOH groups from the surface of MWCNT during acid treatment.²⁸ After reaction with thioglycolic acid, the weight loss of acid-treated MWCNT (Fig. 2(B)(c)) increased by a further 2%, probably because of the decomposition of the grafted thioglycolic acid.

Fig. 2 (A) XPS spectrum of thioglycolic acid-modified MWCNT, and (B) TGA curves of (a) pristine MWCNT, (b) acid-treated MWCNT and (c) thioglycolic acid-modified MWCNTs.

Both the XPS and TGA results indicated that only a small amount of thioglycolic acid had been introduced onto the surface of the MWNT, but even this small amount would be of value in the formation of a conductive network, rather than excessive amounts of AgNW encapsulated on the MWCNT surface.

The morphologies of pristine MWCNT (a) and MWNT–Pt (b)–(d) were examined using TEM and HRTEM (Fig. 3). Comparing the morphology of pristine MWNT and that of MWNT–Pt, it is observed that improved dispersion has caused a number of Pt nanoparticles to become attached to the MWCNT surface. Fig. 3(d) shows that the interface between MWCNT and Pt nanoparticles is not clearly identified, which implies that the Pt nanoparticles were attached tightly to the MWCNT surface by chemical bonding. The bond concerned was a coordinate bond between Pt nanoparticles and thiol groups on the surface of MWCNT, indeed thiol groups are recognized as possessing a strong coordination facility. The lattice fringes of the Pt nanoparticles were spaced 0.23 nm apart, which is in good agreement with the d value for (111) planes of face-centered cubic (fcc) platinum (0.226 nm).²⁶ The Pt nanoparticles were available as seeds for growing AgNW on the surface of the MWCNT.

Fig. 3 TEM images of (a) pristine MWCNT, (b) MWCNT–Pt, and (c) and (d) HRTEM images of MWCNT–Pt.

3.2 Characterization of the silver nanowires

Pt nanoparticles were formed by reducing H₂PtCl₆ with EG. With the help of PVP, Ag nanoparticles reduced by EG could grow to form AgNW. The XRD pattern (Fig. 4(a)) indicated that AgNW synthesized by the solution-phase method existed purely in the fcc phase.²⁶ The peaks at 38°, 44°, 64° and 77° corresponded to Ag (111), Ag (200), Ag (220), and Ag (311), respectively. It is also important to note that the ratio of the intensity of the (111) and (200) peaks had a relatively high value of 2.7 (the theoretical ratio is 2.5), indicating enrichment of the (111) crystalline planes in the AgNW. It was deduced that AgNW with a one-dimensional nanostructure had been obtained.

Fig. 4 (a) XRD pattern, (b) UV-visible extinction spectrum, (c) and (d) SEM images of AgNW.

Fig. 4(b) shows the UV-visible extinction spectrum of AgNW. Silver nanostructures with different shapes exhibit surface plasmon resonance (SPR) bands at different wavelengths. The two SPR peaks (at ∼380 and ∼350 nm) belong to the optical signatures of AgNW.²⁶ Moreover, the one-dimensional structure of AgNW can clearly be seen in the SEM images (Fig. 4(c) and (d)), and a mean diameter of 85–105 nm for the AgNW was derived.

3.3 Characterization of the MWCNT/AgNW composite

In the presence of PVP, Pt nanoparticles on the surface of MWCNT were able to act as seeds for generating AgNW of a uniform diameter. The four similar main crystallographic planes, Ag (111), Ag (200), Ag (220) and Ag (311) were observed in the XRD pattern of the MWCNT/AgNW composite (Fig. 5(a)). The weak peak around 2θ = 26.40 corresponds to the C (100) planes of crystalline graphite-like structures,^10,31,32 and confirmed that the materials were composed of crystalline Ag and MWCNT. The strong signal of the EDS spectrum at about 3 keV (Fig. 5(b)) was proof of the connection of Ag onto the surface of MWCNT.¹⁰ In addition, the UV-visible extinction spectrum of the MWCNT/AgNW composite can be seen in Fig. 5(c). The existence of two SPR peaks at 380 and 350 nm shows that AgNW were still formed in the presence of MWCNT, and it could therefore be concluded that a MWCNT/AgNW composite had been successfully obtained.

Fig. 5 (a) XRD pattern, (b) EDS, and (c) UV-visible extinction spectra for a MWCNT/AgNW composite obtained by in situ growth.

The morphology of the MWCNT/AgNW composite was also observed by SEM and TEM (Fig. 6(a) and (b), respectively), and the short crooked MWCNT and the long straight AgNW are seen to have formed a network. The MWCNT networks were random, and the AgNW bridged the MWCNT and filled the intervening spaces. Because of the high conductivity of AgNW and their role in forming a conductive bridge connecting the MWCNT, the composite offered much higher conductivity than the MNCNT themselves, even though the volume fraction of the MWCNT was significantly greater than that of the AgNW, as illustrated in Fig. 6. This structure is highly effective in improving the conductivity of the composite.

Fig. 6 (a) SEM and (b) TEM images of a MWCNT/AgNW composite obtained by in situ growth.

The in situ growth of AgNW on the surface of MWCNT was confirmed by Raman spectrometry (Fig. 7). The Raman spectra of the MWNT contain two domains, a tangential G-band in the region 1550–1605 cm⁻¹ and a disorder-induced D-band at about 1350 cm⁻¹. The intensity ratio of the D-band and the G-band (I_D/I_G) has been widely used as a measure of the introduction of defects.³³ Compared with pristine MWNT, the I_D/I_G ratio of acid-treated MWNT and the MWCNT/AgNW composite increased from 0.81 to 1.21 and 1.44, respectively, and this can be regarded as evidence of the destruction of acid-treated MWNT and the MWCNT/AgNW composite. Furthermore, the pristine MWNT showed a G-band at 1579 cm⁻¹, whereas the acid-treated MWNT had a G-band at 1582 cm⁻¹. The shifting of the G-band to a higher wave number confirmed that –OH had been successfully introduced. When AgNW were introduced, the G-band of the MWCNT/AgNW composite shifted to 1589 cm⁻¹, which demonstrated that AgNW could affect the MWCNT by chemical interaction.³⁴ These results support the view that a branch-like MWCNT/AgNW composite had in fact been formed by in situ growth, as expected.

Fig. 7 Raman spectra of (a) pristine MWCNT, (b) acid-treated MWCNT, and (c) the MWCNT/AgNW composite.

3.4 Performance of the transparent thin film

Fig. 8(a) shows a suspension of the MWCNT/AgNW composite in ethanol at a concentration of 1.8 mg mL⁻¹. This suspension was drop-coated onto a PET substrate and air-dried. The MWCNT/AgNW composite-coated PET film remained flexible and had high transparency, as seen in Fig. 8(b). Compared to the reported transparent conductive film of AgNW grown on ITO, our process is simpler and much more cost-effective.²⁵ The visible light transmittance in the range 400 nm to 800 nm (measured on a UV-visible spectrophotometer) of the MWCNT/AgNW composite layer on a PET substrate is presented in Fig. 8(c), and shows a transmittance of approximately 80% of that of the MWCNT/AgNW composite coating layer itself.³⁵

Fig. 8 (a) Photograph of MWCNT/AgNW composite solution; (b) flexed MWCNT/AgNW film on a PET substrate; (c) optical transmittance of MWCNT/AgNW film on a PET substrate.

The electrical conductivities of the MWCNT/AgNW composite were investigated using a digital multimeter, and the thin film was used to form a transparent electrode. For comparison, a sample of mixed thioglycolic acid-modified MWCNT and AgNW was prepared, and the powder resistivities of pristine MWCNT, a mixture of thioglycolic acid-modified MWCNT and AgNW, and a MWCNT/AgNW composite, were measured and compared. The powder resistivity of pristine MWCNT was 288 ± 5 Ωm, that of the mixture was 176 ± 4 Ωm, and that of the composite 35 ± 3 Ωm. This demonstrated that in situ growth of AgNW on MWCNT had significantly reduced the electrical resistivity. In addition, when the MWCNT/AgNW composite was applied as a transparent electrode, a sheet resistance of 47.8 Ω sq⁻¹ was obtained, and the sheet resistance of AgNW applied as transparent electrode with a transmittance of 80% was 85.8 Ω sq⁻¹. The reason for the low sheet resistance was coordinate bonding within the MWCNT/AgNW composite conductive network.

The composite transparent electrode offers the possibility of being useful in applications involving electronic devices, including touch screens and organic solar cells.

4. Conclusions

An MWCNT/AgNW composite was obtained by in situ growth of AgNW, with Pt nanoparticles on the surface of the MWCNT acting as seeds. A composite transparent electrode with PET as the supporting substrate was fabricated at room temperature using a drop-coating method. Because of the method of in situ growth and the formation of a conductive network, low sheet resistance, high transparency and excellent flexibility of the composite transparent electrode were obtained. The powder resistivity of the MWCNT/AgNW composite was up to 35 Ωm, and the sheet resistance of the composite transparent electrode, with transmittance up to 80%, was as high as 47.8 Ω sq⁻¹. This technology is applicable to the development of flexible devices such as displays and organic light-emitting diodes.

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