Preparation of thermally stable well-dispersed water-soluble CdTe quantum dots in montmorillonite clay host media

Preparation of thermally stable well-dispersed water-soluble CdTe quantum dots in montmorillonite clay host media

Journal of Colloid and Interface Science 368 (2012) 139–143 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Scie...

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Journal of Colloid and Interface Science 368 (2012) 139–143

Contents lists available at SciVerse ScienceDirect

Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Preparation of thermally stable well-dispersed water-soluble CdTe quantum dots in montmorillonite clay host media Yuan-Cheng Cao ⇑ School of Chemical Engineering and Advanced Materials, University of Newcastle, Newcastle Upon Tyne NE1 7RU, UK

a r t i c l e

i n f o

Article history: Received 18 August 2011 Accepted 18 November 2011 Available online 1 December 2011 Keywords: Quantum dots Layered materials Clay Luminescent film Polymer nanocomposites

a b s t r a c t In this work, a method to prepare a thermally stable QDs/clay powder is reported. First, several water soluble CdTe QDs characterised by different size-dependent emission wavelengths were synthesised through wet chemistry. Montmorillonite-Na+ clay in water was dispersed into a muddy suspension by sonication. Then, the clay-water suspension was used as the host media for CdTe QDs to prepare the QDs/clay powder by freeze drying. The experiments showed that QDs/clay powder could be re-dispersed in water without changing the luminescent property of the QDs; this process was reversible. EDX showed that Cd and Te elements existed in the QDs/clay powder and the XRD tests showed that the clay [0 0 1] reflection peaks for raw clay, QDs (kem = 514 nm)/clay and QDs (kem = 560 nm)/clay were the same, namely 2h = 7.4°. Finally, QDs/ clay powder was applied to the HDPE polymer extrusion process at 200 °C to produce thin films; the resultant QDs-polymer nanocomposite film exhibited strong fluorescence. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction In recent decades, nanotechnology and nanoscience have made great progress in the materials field and a number of novel materials have been developed for different applications [1–3]. One of the most attractive materials is quantum dots (QDs), highly photoluminescent nanocrystal semiconductors, which have attracted considerable attention due to their unique physical characteristics such as their size-tunable optical properties, high photostability and wide absorption spectrum [3–7]. QDs are only several nanometres in size, which is less than the distance between the electron and electron–hole (the so-called Bohr excitation radius). The electronic structure of QDs consists of continuous bands on this scale, which leads to a broad absorption range and a narrow emission range which correlates to the band gap energy [5–7]. QDs can thus be tuned to give the desired wavelength of fluorescence emission by controlling particle size [4]. And also, compared with conventional organic dyes, the fluorescence of QDs is 20 times brighter than currently available fluorescent dyes [1,3]. Due to these unique properties, the study of novel materials based on semiconductor QDs is becoming an attractive area of research and various QDs based applications such as fluorescent materials [1,3], biological imaging [6–8], photovoltaic devices [9,10] and light-emitting diodes (LED) [11,12] have been widely studied in many fields, including science and engineering.

⇑ Fax: +44 191 222 5292. E-mail addresses: [email protected], [email protected] 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.11.044

In most applications, QDs are used in a suspension colloidal format, either in an organic solvent or water system [7–12]. However, in some situations, QDs are required in a solid powder format in order to undergo a special preparation process without changing the optical properties. For example, in the extrusion process to produce polymer nanocomposites, the materials have to be in powder form to undergo melting. As nanoparticles, like many other materials, QDs show a strong tendency towards agglomeration upon their isolation from colloidal suspensions; this will lead to a red shift for both the optical absorption and emission [9–15]. Agglomerated QDs lose the advantages resulting from their nanodimensions, and this agglomeration is not reversible for most nanoparticles. For example, QDs aggregation in polymers may affect the efficiency of photovoltaic devices in solar cells, leading to lower power-conversion efficiency [9–11]. Therefore, it is a challenge to prepare a powder of QDs, which has significantly impeded the development of practical applications in materials science. One strategy to solve this problem is by introducing QDs into the dispersant media which can facilitate uniform incorporation to enhance their stability and luminescence without aggregation [12–17]. For example, Sundar et al. have demonstrated an inorganic sol–gel Titania matrix to stabilise high volume fractions of nanoparticles [16]; the resultant Titania composites possessed a narrow size distribution and high PL efficiency. Dilag et al. have reported a chitosan-supported CdS QDs powder for unfumed cyanoacrylate latent fingerprint detection [17]. Clay minerals are well-studied layered materials, and are attractive as inorganic materials for the production of polymer nanocomposites because they show unique intercalation and

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swelling properties; thus, they have been widely investigated [2,18,19]. As a cheap, easily available, and most importantly, environmentally friendly material, clay is preferred for polymer nanocomposite materials. Recently, intercalation of photoluminescent organic molecules into layered clay minerals has attracted attention as one of the prominent synthetic methods for photoluminescent inorganic–organic complexes [20]. Based on these previous studies, layered clay may be applied for use as a QDs host media to prepare a QDs/clay powder. The QDs aggregation problem can be solved by QDs dispersion in a chemically inert clay layered matrix. This dispersion can also prevent the release of QDs. Thus, clay as a QDs host media can greatly improve the stability of QDs and prevent their aggregation.

2.4. QDs/clay polymer nanocomposite film Two grams of the QDs/clay powder with 98 g of LDPE pellets was mechanically blended using a Philips HR2160/50 blender (600 Watt) for 2 min prior to extrusion. Next, the QDs/clay LDPE pellet mixture was fed into a 16 mm twin extruder (length/diameter = 24:1); the process time for the materials in the extruder was estimated to be 2 min at 200 °C. The resultant QDs/clay polymer materials (about 90 g) were collected after extrusion and the extruder was cleaned using the raw HDPE polymer to avoid cross-contamination of the different samples. Then, injection moulding was applied to the collected QDs/clay polymer materials to produce a thin film (length  width  thickness = 5 cm  2 cm  300 lm). The extrusion and injection moulding temperatures were set to 200 °C and the extrusion speed was 500 rpm.

2. Experimental 2.1. Materials

3. Results and discussion

The MMT-Na+ clay (d-space = 11.7 Å) was purchased from Southern Clay Products (USA). Tellurium power (>99.999%), CdCl22.5H2O (>98%), NaBH4 (>96%), 3-mercaptopropionic acid (MPA, 99%) and LDPE polymer (pellets) with a melt index of 40 g/10 min (190 °C/2.16 kg) were purchased from Sigma. An ultrasonic liquid processor (Misonix Sonicator 4000) was used in this work. All other chemicals used in this work were used as received and the water used in this work was distilled water.

Semiconductor QDs have been extensively studied due to their novel properties, which are determined by their size, shape and surface modifications. CdTe nanomaterials are easy to fabricate using colloidal chemistry approaches by aqueous synthesis [6–8]. Due to their easy synthesis and low toxicity, water-soluble CdTe QDs are widely used in various fields [6–12]. Fig. 1 shows assynthesised mercaptopropionic acid (MPA) stabilised CdTe QDs

2.2. TeCd QD preparation Briefly, 57.25 mg of CdCl2 were dissolved in 250 mL of double distilled water in a round-bottomed flask and 68 lL of MPA was added while stirring; the pH was adjusted to 11–12 which was followed by N2 bubbling for 40 min. Next, 80 mg of NaBH4 powder were added to 5 mL of O2-free double distilled water in a flask on ice with an N2 supply. Then, 127.6 mg of Te powder were added to the NaBH4 solution on ice (0 °C) for 2 h to prepare the NaHTe solution. Then, 50 mL of the CdCl2 solution were taken out to mix with 150 lL of the NaHTe solution in a round-bottomed flask with a condenser; the mixture was stirred at 100 °C to reflux. Different emission wavelength QDs were obtained by varying the reflux time from 30 min to 4 h. Acetone (about 20 mL) was used to precipitate the raw QD solutions several times and centrifugation was applied to separate the precipitate. The resultant QDs of different wavelengths were then redissolved in deH2O.

2.3. Preparation of the QDs/clay powder Two grams of MMT-Na+ clay were first suspended in 100 mL of distilled water in a 250 mL flask, then the ultrasonic liquid processor (Sonicator 4000) was used to disperse the particles of the clay until the clay became a muddy colloidal suspension. Water soluble QDs (kem = 514 nm, 5 mL) were added into the flask as followed by a further 30 min of ultrasonication. The resultant mixture was first treated with liquid N2 until it became an icy solid. Then, it was loaded into the freeze dryer where it was kept overnight to dry before the harvest of QDs (kem = 514 nm)/clay. The same method as described above was applied to obtain other QDs/clay samples, namely QDs (kem = 560 nm)/clay, QDs (kem = 540 nm)/clay, QDs (kem = 524 nm)/clay. A fluorescent microscope (Olympus IX-71, Japan), which was connected to a camera (PCO1600, PCO, GMbH, Germany) and an imaging spectroscope (PARISS, Lightform Inc., USA), was used to collect the sample fluorescent spectra.

Fig. 1. Water soluble QDs colloids in room light and under UV light (from left: kem = 560 nm, kem = 540 nm, kem = 524 nm, kem = 514 nm) with a colour from orange to green (A) and the individual normalised fluorescent spectra (B) (kex = 350 nm). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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crystal can grow from 1.5 nm to 6 nm in diameter with an increase in reaction time from 30 min to 4 h. As the individual quantum dots increase in size, their optical absorption and emission undergo a red shift, and their unique size-dependent optical properties thus can be tuned simply by controlling the size of the nanocrystals during synthesis [7,8]. Energy transfer occurs between QDs at interparticle separations of up to 10 nm when they come into direct contact and reach a critical aggregate size, so the preparation of powder format QDs is a challenge which hinders wider applications of this novel material in polymer matrices. In order to obtain powder format CdTe QDs, the well-studied layered material MMT-Na+ clay was used as the host media in this work. As MMT-Na+ clay can be easily dispersed in water, the QDs and MMT-Na+ clay mixture in an aqueous environment can be well-dispersed after the ultrasonic process before freeze drying. For comparison, raw MMT-Na+ clay and pure CdTe QDs samples were also applied to the freeze dryer. Typical images of the resulting QDs powders after freeze drying (QDs with kem = 540 nm, for example) are shown in Fig. 2 for the MMT-Na+ clay (Fig. 2A), the QDs/clay (Fig. 2B) and QDs (Fig. 2C). From these results, we can see that the MMT-Na+ clay was a white powder while the QDs/clay was a yellowish powder which was close to the solution colour of the corresponding wavelength QDs in Fig. 1; the wavelength of the QDs/clay was 540 nm as shown in Fig. 2D, which was the same as

Fig. 2. The typical images of MMT-Na+ clay powder (A), QDs/clay powder (B) and QDs powder (C) after freeze drying, and the fluorescent spectra of these powders (D). These samples were prepared under the same conditions; QDs kem = 540 nm.

(Fig. 1A) and their fluorescent spectra (Fig. 1B) under the excitation wavelength of kex = 350 nm. The MPA makes these high photoluminescence QDs water soluble. During synthesis, the QDs nanocrystals grow gradually as the reflux time increases. The CdTe

Fig. 3. Typical SEM image of the QDs/clay powder material (A) and EDX spectra (B). kem = 540 nm QDs were used in the clay.

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(Fig. 4A(a)). This resultant colloidal solution was still highly luminescent under UV light (Fig. 4B(a)), while for the freeze dried neat QDs powder, it could not be re-dispersed in water. It can be seen that the colour became dark, which means that the QDs particle size increased as aggregation occurred (Fig. 4A(b)); and the resultant material was not fluorescent (Fig. 4B(b)) under UV light. Good reproducibility was observed even after six rounds of continuous dispersion/freeze drying, demonstrating the high stability of the QDs/clay material. These results indicate that QDs can be well-dispersed in MMT-Na+ clay without aggregation and the frozen dried QDs/clay powders can be re-dispersed in water without loss of their highly luminescent properties. Further powder diffraction XRD tests were applied to study the d-space of the raw MMT-Na+ clay and QDs/clay samples with different emission wavelength QDs (kem = 514 nm and 560 nm); the results are shown in Fig. 5. From the results, it can be seen that the raw clay’s [0 0 1] reflection peak 2h was 7.4 (Fig. 5A(a)), while for the QDs (kem = 516 nm)/clay and QDs (kem = 560 nm)/clay samples, the [0 0 1] reflection peaks were 2h = 7.4, the same as the raw clay (Fig. 5A(b and c)), which means that the resultant sample’s gallery distance was the same as that of the raw MMT-Na+ clay. These results indicate that the QDs did not intercalate into the layers of the clay since the d-space did not change. In other words, the QDs were dispersed into the clay particles, not in the clay galleries.

Fig. 4. Dispersed freeze dried QDs powder and QDs/clay powder (QDs kem = 540 nm) in water (A) and their images under UV (B) (kex = 350 nm). (a) QDs/clay; (b) QDs. The samples were dispersed by a supersonic process for 10 min before the images were taken.

the pure QDs colloid. However, for the QDs after freeze drying, the solid was much darker in colour (nearly red) for the sample of QDs with kem = 540 nm in this case. The solid fluorescent wavelength of this preparation was 579 nm (Fig. 2D), which showed a 39 nm redshift after the freeze drying. Moreover, the full width at half maximum (FWHM) was much broader than that of the QDs colloid spectrum. In the systems where colloidal aggregates were formed, QDs exhibited red-shifting as the aggregate size increased. These results indicate that the QDs in the QDs/clay preparation were well-dispersed without aggregation, so the fluorescent spectrum of the powder did not change compared to the pure QDs colloid. However, for the QDs which were freeze dried directly, these samples clearly underwent a red-shift, which means that aggregation occurred during freeze drying and QDs began to display bulk material properties. Fig. 3A shows the microstructure of the QDs (kem = 540)/clay powder sample under SEM observation. From the image, we can clearly see the porous structure of the freeze dried QDs/clay sample was a flake-like material. Fig. 3B shows the results of EDX elemental analysis. From the EDX spectra, we can see the element characteristic peaks of Na, Mg, Al, Si, Cd and Te, which indicated the components of the sample material. Among these elements, Na, Mg, Al and Si were from the clay host media [18,19], while Cd and Te were from the QDs. These results indicate that the QDs existed in the clay flakes and were dispersed in the clay matrix. The re-dispersion ability of the powder samples (QDs/clay and QDs) were studied by dispersing the frozen dried samples in water (Fig. 4A) and the resultants were observed under the room light and UV light (kex = 350 nm) (Fig. 4B). From the results we can see that the QDs/clay powder could be re-dispersed in the water and the resultant solution became a well-dispersed colloidal QDs solution; again, and the optical properties were the same

Fig. 5. XRD spectra of the MMT-Na+ clay, QDs (kem = 514 nm)/clay, QDs (kem = 560 nm)/clay (A) and the structural illustration of the QDs/clay materials (B). The curves were smoothed from the raw XRD data.

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This proposed structure can be seen in Fig. 5B. These results also indicate that the QDs nanocrystal size did not affect their dispersion because both the small QDs (kem = 516 nm) and large QDs (kem = 560 nm) clay samples had the same 2h [0 0 1] reflection peak. As the surface of montmorillonite-Na+ clay is negatively charged, many of the aggregation and adsorption properties of clays can be related to the layer charge [20–22] and the MPA-stabilised QDs also have a negative surface due to the –COO functional group. The dispersion of QDs in clay (orientation and aggregation) was assumed to be affected by several factors such as the clay layer charge as well as the QDs size and surface charge. In order to study the thermal stability and the luminescence reproducibility of the QDs/clay powder sample and its resulting QDs-polymer nanocomposites, the QDs/clay sample and the LDPE polymer were extruded in the twin extruder first and then loaded into the injection moulding apparatus to produce a thin film (300 lm in thickness), which followed typical polymer processing conditions. Fig. 6 shows the optical and fluorescent images of the thin films. From these results, we can see that the QDs/clay LDPE thin film was slightly darker in colour in room light (Fig. 6A(b))

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with loading of 2% (wt, QDs/clay to polymer) while it was highly fluorescent under UV light (Fig. 6B(b)) after extrusion. The resultant thin film had the same wavelength as the pure QDs namely, kem = 540 nm (Fig. 6C). These results show that the QDs/clay sample exhibited good thermal stability and luminescence reproducibility even after experiencing temperatures up to 200 °C. Although some literature reports show that the thermal stability and optical properties of QDs can be improved by coating the QDs with inorganic or organic materials, they are only reversible in the near ambient temperature range (5–80 °C) [23–26]. Overall, these results indicate that clay is a good solid host media for QDs and that QDs/clay can be applied to the polymer process without changing the luminescent properties of the QDs. 4. Conclusions Montmorillonite-Na+ clay can be used as the host media for water-soluble CdTe QDs by preparing a QDs/clay powder by freeze drying. CdTe QDs can be well-dispersed in montmorillonite-Na+ clay under aqueous conditions. The QDs/clay powder can be re-dispersed in water after freeze drying without changing the luminescent properties of the QDs, which showed good fluorescence reproducibility. No obvious aggregation was observed for the QDs/clay materials in the freeze drying/dissolution process, while serious aggregation occurred with neat QDs samples. QDs/clay showed good thermal stability in the polymer extrusion and injection moulding process and the resultant QDs-polymer nanocomposite exhibited strong luminescence. Acknowledgment The author gratefully acknowledges support from the Electron Microscopy and Chemical Analysis Services (ACMA) at Newcastle University, UK. References

Fig. 6. Optical image (A) and fluorescent image (B) of the QDs/clay LDPE polymer nanocomposite matrix and its fluorescent spectrum (C). (a) LDPE polymer thin film; (b) QDs/clay LDPE fluorescent nanocomposite thin film. The QDs/clay comprised 2% of the nanocomposite material, QDs kem = 540 nm, UV light kex = 350 nm.

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