Controlled transformation of aqueous CdTe quantum dots → Te-rich CdTe nanorods → second CdTe QDs

Abstract

The transformation of existing inorganic nanomaterials from one structure into another represents a straightforward, versatile and effective approach for the synthesis of nanomaterials. Hence, in this paper, we focus on the controlled transformation of water-soluble CdTe quantum dots (QDs) → Te-rich CdTe nanaorods (NRs) → second CdTe QDs. It was found that in the presence of L-cysteine (L-cys), aqueous thiolglycolic acid (TGA)-stabilized CdTe QDs might spontaneously assemble and recrystallize into luminescent Te-rich CdTe nanorods under ambient conditions. However, increasing the temperature of the nanorod dispersion to 90 °C or adding a specific amount of TGA into the dispersion of NRs might effectively induce the transformation of Te-rich CdTe NRs → second CdTe QDs. These unique transformation processes that involve simultaneous complicated changes in the chemical compositions, structures and morphologies of nanocrystals were systematically characterized by optical techniques, transmission electron microscopy (TEM) and powder X-ray diffractometry (XRD).

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1. Introduction

Due to the quantum confinement effect, semiconductor nanocrystals exhibit many unique size- and shape-dependent optical and electrical properties.^1–5 Hence, up to now, significant efforts have been invested toward developing these high-quality nanomaterials.^4,5 Besides quasi-spherical quantum dots (QDs), recently, one-dimensional (1D) nanostructures, such as nanorods and nanowires, as a particularly important family, have attracted considerable attention.^4–6 They are not only fundamentally interesting, but also potentially useful for fabricating new types of nanoscale photonic, electronic and optoelectronic devices, and for biomedical applications.^7–11 At present, several different synthetic strategies have been exploited for fabricating 1D nanocrystals, such as the template-directed method, seed-mediated approach and solution-phase growth based on selective adhesion of ligands.^4–6

The transformation of existing inorganic nanomaterials from one structure into another (e.g., from dots to rods) represents a straightforward, versatile and effective strategy for nanostructure synthesis.¹² As a unique feature, this strategy allows one to easily and independently control the chemical compositions, structures and morphologies of nanostructured materials. Hence, currently because of the rapid development of the QD synthetic techniques, using existing inorganic QDs as the starting point for making other nanomaterials is receiving increasing interest.^12,13 Among various kinds of semiconductor nanomaterials, colloidal CdTe nanocrystals are undoubtedly the most widely studied due to the strong tunable emission in the visible-near-infrared range, and the wide applications in solar cell and biological labeling.^14–17 To our knowledge, the shape transformation of aqueous CdTe nanocrystals from QDs to nanorods, initiated by Kotov et al. in 2002,¹⁸ is probably the most successful example.^19–22 Subsequently, they further prepared other inorganic nanomaterials, such as nanosheets,²³ Se nanowires, Te nanowires^24,25 and angled Te nanocrystals²⁶ using CdSe or CdTe QDs as the starting material of the synthesis. Here, it should be mentioned that spontaneous CdTe → alloy → CdS transition of stabilizer-depleted CdTe nanoparticles induced by EDTA was found for the first time in 2006.²⁷ These chemical transformations in bulk solids should be very slow due to the high activation energy for the diffusion of reactant atoms and ions.¹² In contrast, the high surface-to-volume ratio of a nanocrystal can effectively reduce the kinetic barrier for diffusion,^28,29 which allows the fast and even complete transformation of nanostructures from one material into another.

Water-soluble CdTe nanocrystal is a promising group II–VI nanomaterial owing to the strong photoluminescence emission.^14–17 Therefore, exploring its stability under different experimental conditions is also very interesting for the future applications.^14,15 Thus, in this report, we investigated systematically the controlled transformation between CdTe QDs and 1D nanocrystals in water. The addition of L-cys was found to efficiently initiate the self-assembly of TGA-stabilized CdTe QDs into Te-rich CdTe NRs under ambient conditions. Subsequently, by increasing the temperature or adding thiolglycolic acid to the nanorod dispersion, Te-rich CdTe NRs inversely transformed into second CdTe QDs. The reasons for these unique transformations have been revealed using optical techniques, TEM and powder XRD. To our knowledge, these experimental observations have not been reported previously. Hence, they should be interesting for enhancing our current understanding of the transformation mechanism of inorganic nanostructured materials.

2. Experimental

2.1 Materials

CdCl₂·2.5H₂O (99+%), tellurium powder (Te, 99.8%), thiolglycolic acid (TGA, 90+%), L-cysteine hydrochloride monohydrate (L-cys, 99%), sodium borohydride (99%), absolute ethanol, sodium hydroxide (96+%) and sulfuric acid are commercially available products and were used as received. Re-distilled water was used in all preparations.

2.2 Synthesis of aqueous TGA-stabilized CdTe QDs

TGA-stabilized CdTe QDs were prepared according to the methods described in the previous reports.^30–34 Briefly, Te powder is used as a tellurium source to prepare NaHTe solution. Then, the freshly prepared NaHTe solution was injected into an oxygen-free CdCl₂ solution containing TGA at pH 10 by the pressure of N₂. CdTe QD precursors are formed at this stage (the total volume is 50 mL; the precursor ratio of Cd²⁺/Te²⁻/TGA was 1 : 0.5 : 1.5, [Cd²⁺] = 6 mM) and subjected subsequently to a refluxing at 100 °C under open-air conditions with a condenser. The particle sizes of the QDs increase with the reflux period. Accompanied by the growth of CdTe QDs, the absorption and photoluminescence from the QD solution go to the red gradually.

2.3 Synthesis of Te-rich CdTe nanorods

Te-rich CdTe NRs were prepared as follows. In brief, L-cysteine solution ([L-cys] = 27 mM, pH = 10.5) was added dropwise directly into TGA-stabilized CdTe QD solutions, where the volume ratio of the L-cys solution and the initial CdTe QD solution was fixed at 1 : 2 (in this case, [L-cys] : [TGA] = 9 : 6 or [L-cys] : [QDs] = 9 : 2 (the concentration of the QDs refers to Te)). Next, the obtained dispersions were left in an ambient atmosphere and allowed to age in the dark at room temperature for two days. During the storage, the dispersions turned from yellow to dark green. Finally, the resulting solutions of Te-rich CdTe NRs were stored in an icebox (4 °C). In addition, TGA-stabilized CdTe QDs with other PL colors could be used as the starting reagents for the nanorod preparation.

2.4 Characterization

Absorption and PL emission spectra were measured using a Shimadzu 3100 UV-Vis-near-IR spectrophotometer and a Shimadzu RF-5301 fluorescence spectrometer, respectively. All optical measurements were performed at room temperature. The room-temperature PL quantum yields (QYs) of CdTe QDs in water were estimated using Rhodamine 6G as the PL reference.^30–34 Transmission electron microscopy (TEM) analysis and energy dispersion X-ray (EDX) spectroscopy were performed on a Philips FEI Tecnai G² 20 S-TWIN or a JEOL JEM-200CX TEM. Powder X-ray diffraction (XRD) measurement was carried out using a Philips X'Pert PRO X-ray diffractometer.

3. Results and discussion

3.1 Synthesis of initial aqueous TGA-capped CdTe QDs

The synthesis of TGA-capped CdTe quantum dots followed a procedure similar to those reported by Gaponik et al.^30–34 Here, Te powder is firstly used as a tellurium source to prepare NaHTe solution. Then, the as-prepared NaHTe solution was injected into a CdCl₂ solution containing TGA by a N₂ flow. Finally, the CdTe precursors formed are converted to highly luminescent QDs by refluxing at 100 °C under open-air conditions. Fig. 1 shows the absorption and PL spectra of TGA-stabilized CdTe QDs prepared at different reflux times. With prolonging of the reflux time from 0.2 to 40 h, both absorption and photoluminescence spectra of QDs red-shift gradually, indicating that the heating results in the increase in the particle size of QDs (the average diameters of the QDs are 2.5 (0.2 h), 2.8 (1.5 h). 3.1(5 h), 3.3 (15 h) and 3.5 (40 h) nm, which were estimated from their corresponding excitonic absorption peaks or TEM images). In this way, highly luminescent water-soluble TGA-stabilized CdTe QDs with green–red emission can be obtained, as shown in the inset of Fig. 1. The maximum PL quantum yield (QY) is about 50% and the typical PL fwhm (full width at half maximum) is ∼50 nm. In addition, TEM and energy-dispersive X-ray (EDX) measurements have confirmed that the as-prepared CdTe nanocrystals are spherical TGA-stabilized CdTe QDs with low polydispersity (∼15%).

Fig. 1 Temporal evolution of absorption and PL spectra of water-soluble TGA-stabilized CdTe QDs obtained during the reflux at 100 °C. The inset shows the fluorescent images of the as-prepared QD solutions taken under UV lamp excitation.

3.2 Transformation of aqueous CdTe QDs → Te-rich CdTe NRs

Using existing QDs as the starting point for making other nanomaterials is receiving increasing attention.^13,18–28 This method allows one to easily and independently control the chemical compositions, structures and morphologies of nanostructured materials. Hence, in this study, using aqueous CdTe QDs as the starting material, Te-rich CdTe NRs were synthesized following a method modified from those of previous works for water-soluble CdTe nanorods (see the Experimental section).^18–22 Specifically, a certain amount of L-cysteine was added dropwise into aqueous solutions of TGA-stabilized CdTe QDs (the precursor ratio of Cd²⁺/Te²⁻/TGA was 1 : 0.5 : 1.5, [Cd²⁺] = 6 mM). Next, the obtained CdTe dispersions were stored in the dark under ambient conditions for two days. During the storage, the color of the solution changed from initial yellow to dark green. Fig. 2 shows the TEM images of the initial TGA-capped CdTe QDs with green emission and the resulting nanocrystals obtained in the presence of different concentrations of L-cys, which represent the evolution of the self-assembly of QDs into nanorods induced by L-cys. The initial TGA-capped CdTe nanocrystals with green PL were proven to be spherical quantum dots (∼3 nm in diameter) (Fig. 2A). The TEM image in Fig. 2B reveals that the addition of L-cys ([L-cys] = 5 mM) does cause the transformation of QDs to NRs. At this time, we noted that the structure of the as-prepared nanorods is relatively incompact, which provides the direct evidence that the nanorods might be produced via linearly oriented attachment (or the self-assembly) of nanoparticles. When the concentration of L-cys is increased to 9 mM, high-quality nanorods (∼200 nm in length, ∼20 nm in diameter; the average aspect ratio of NRs is 10) were obtained, as shown in Fig. 2C. In this case, almost no QDs were observed on the TEM grid. These results confirm that the addition of L-cys might destabilize the initial TGA-stabilized CdTe QDs and highly efficiently induce the transformation of QDs to NRs.

Fig. 2 TEM images of (A) the initial CdTe QDs and the resulting nanocrystals obtained in the presence of different concentrations of L-cysteine: (B) 5, (C) 9 mM. The storage time is two days.

To reveal further the actual reason for the formation of nanorods, the transformation of QDs to nanorods was characterized systematically by optical techniques, TEM and powder XRD. Fig. 3 shows the absorption and PL spectra of the initial TGA-capped CdTe QDs and the resulting NRs. As compared to the initial CdTe QDs, the formation of nanorods results in 1) a new broad absorption peak appearing at ∼600 nm; 2) the first excitonic absorption peak and PL emission peak blue-shifting about 10 nm. These results are very different from those in previous reports, where the red-shifts in the absorption and PL emission spectra were observed during the growth of one dimensional CdTe nanocrystals.^13,18–21 This might suggest a different formation mechanism for our nanorods.

Fig. 3 The (A) absorption and (B) PL spectra of the initial TGA-capped CdTe QDs and the resulting NRs.

Next, TEM with energy-dispersive X-ray (EDX) spectrometry and powder XRD were employed to further characterize the elemental compositions and the crystal structures of the initial QDs and the resulting nanorods. The results obtained are shown in Fig. 4 and 5 (and Fig. S1, ESI†). As shown in Fig. 4, the transformation of QDs to NRs is accompanied by the evolution in the molar ratio of Cd/Te/S from 1 : 1 : 0.8 (QDs) to 1 : 4 : 1.2 (NRs). High-resolution TEM (HRTEM) images and XRD patterns in Fig. 5 clearly indicate that 1) the initial CdTe QDs belong to the cubic (zinc blende) structure, which is in good agreement with data published in previous reports;^30–34 2) the crystal structure of the resulting nanorods should be assigned as a mixture of the cubic structure of CdTe and the hexagonal structure of tellurium, rather than CdTe (Fig. S2 and S3, ESI†). Hence, these data fully confirm that under ambient conditions, in the presence of L-cys, TGA-stabilized CdTe QDs spontaneously assemble and re-crystallize into luminescent Te-rich CdTe NRs.^35,36

Fig. 4 EDX spectra of (A) the initial TGA-stabilized CdTe QDs and (B) the resulting Te-rich CdTe NRs.

Fig. 5 HRTEM images of (A) the initial TGA-stabilized CdTe QDs and (B) the resulting Te-rich CdTe NRs (the insets are higher magnification images). Clear crystalline lattices of the QD in the inset of panel A show the 3.8 Å separated (111) planes extending over ∼30 Å. The lattice spacings, as marked in panel B, are 5.9 Å, 3.9 Å and 3.2 Å, which are close to those for (001), (100) and (101) planes of hexagonal tellurium, respectively. These data all suggest that the growth direction of nanorods is along the (001) axis of hexagonal crystal lattice, along with the experimental result from fast Fourier transform diffraction (FFT). (C) The typical XRD patterns of the initial TGA-capped CdTe QDs and the resulting Te-rich CdTe NRs. The positions of the new narrow peaks in the XRD pattern of the nanorods match with those of hexagonal tellurium well.

3.3 Transformation of Te-rich CdTe NRs → second CdTe QDs

The dispersion of Te-rich CdTe NRs prepared above can remain stable for about two weeks, when stored in an icebox at 4 °C. Here, the temperature of the nanorod dispersion was increased to 90 °C and aliquots were taken at different time intervals. Fig. 6 shows the absorption and PL spectra of the resulting dispersions obtained at different heating times. As shown, with the heating time the new broad absorption peak at ∼600 nm disappears gradually, and both absorption and PL spectra of the dispersions red-shifted rapidly. For instance, within about 30 min, the PL peak red-shifted quickly from 530 to 590 nm. As compared to TGA-stabilized CdTe QDs,^14,15 the time needed for achieving the same red-shift (from 530 to 590 nm) is more than 10 h, even at 100 °C. To reveal further the reason for the rapid spectral shifts, TEM was used to characterize the shapes of the dispersions. TEM images in Fig. 7 indicate clearly that increasing the reaction temperature causes the decomposition of Te-rich CdTe NRs; prolonging the heating time results in the gradual transformation of NRs → second CdTe QDs (Fig. S4, ESI†). In the meantime, in this study, we noted that by adding a specific amount of thiolglycolic acid (pH 10) into the dispersion of Te-rich CdTe NRs (e.g. TGA : L-cys = 2 : 1, [L-cys] = 6 mM), a similar transformation from Te-rich CdTe NRs to CdTe QDs was also observed at room temperature (Fig. S5, ESI†). This shape transformation leads to the similar evolution of the optical properties, i.e., from the typical spectra for Te-rich CdTe NRs to these for CdTe QDs (Fig. 8). As will be discussed later, the transformation of Te-rich CdTe NRs → second CdTe QDs could be attributed to the disproportionation reaction of nanoscale Te in the nanorods.

Fig. 6 The absorption (A) and PL (B) spectra of the dispersions prepared at different heating times (90 °C).

Fig. 7 TEM images of the dispersions prepared at different heating times (90 °C): (A) 0 min, (B) 10 min, (C) 30 min.

Fig. 8 The absorption (A) and PL (B) spectra of the dispersions prepared at different times after the addition of TGA (room temperature).

3.4 Transformation mechanism of aqueous CdTe QDs → Te-rich CdTe NRs → second CdTe QDs

By using a combination of optical techniques, TEM and powder XRD, the unique transformation from initial water-soluble CdTe QDs to Te-rich CdTe NRs, and finally to second CdTe QDs has been confirmed. To our knowledge, these observations have not been reported previously. The reasons for this unique transformation are shown in the scheme in Fig. 9A. The chemical transformation of CdTe to Te-rich CdTe in water should be attributed to the introduction of L-cys. In previous studies,^24–26 the addition of EDTA (a strong complexing agent for Cd²⁺ ions) has been proven to induce the fast decomposition of CdTe QDs and the resulting complete transformation of CdTe to Te (Te²⁻ anions released from the decomposition of CdTe QDs could be easily oxidized to Te in water (i.e., 2Te²⁻ + O₂ + 2H₂O → 2Te + 4OH⁻; the redox potential is −1.14 V for Te²⁻/Te; 0.401 for O₂/OH⁻)). Here, similar to EDTA, L-cys can also destabilize CdTe QDs to release Te²⁻ anions due to the strong complexing ability, whereas different from EDTA, L-cys, a thiol molecule, can act as the stabilizer for the synthesis of water-soluble CdTe nanocrystals simultaneously (the complexing ability of L-cys with Cd²⁺ ions might be weaker than that of EDTA).^{14,15,19–22} Hence, the addition of L-cys only causes a part chemical transformation from initial CdTe to final Te-rich CdTe, as presented in Fig. 4 and Fig. S1, ESI†.

Fig. 9 (A) A general scheme for the transformation from initial aqueous CdTe QDs to Te-rich CdTe NRs, and finally to second CdTe QDs. (B) The optical photographs of the dispersions during the transformation of initial aqueous CdTe QDs → Te-rich CdTe NRs → second CdTe QDs.

Concomitantly with the chemical change in the elemental composition, the shape transformation from QDs to NRs in water occurs (Fig. 2), which could be ascribed to the three key factors. 1) The unique molecular structure of L-cys-L-cys can serve as a particular stabilizing ligand for the fabrication of water-soluble 1D CdTe nanocrystals,^19–22 besides QDs.^14,15 2) The intrinsic crystal structure of tellurium is known to have a highly anisotropic crystal structure consisting of 1D helical chains of covalently bound atoms, thus having a strong tendency toward 1D growth.^37–39 3) The probable dipole–dipole attraction between nanoparticles favors the self-assembly of nanoparticles via oriented attachment, as shown in Fig. 2B.^16–18 Thus, the addition of L-cys results in the chemical and shape transformation of aqueous CdTe QDs to Te-rich CdTe NRs, rather than the transformation of CdTe QDs to CdTe NRs or to Te NRs. In addition, during this transformation process, the small blue-shift in PL emission peak observed in Fig. 3 could be ascribed to the combination of two reasons: the red-shift in the spectrum caused by the 1D growth process of nanocrystals from initial CdTe QDs and the blue-shift induced by the part transformation of CdTe to Te.

According to previous reports,^38,39 in hot and condensed alkaline solution (e.g., 100 °C and [OH⁻] = 2.5 M), Te powder could disproportionate into Te²⁻ and TeO₃²⁻ (i.e., 3Te + 6OH⁻ → TeO₃²⁻ + 2Te²⁻ + 3H₂O). Hence, under our experimental conditions (room temperature and pH 10), the disproportionation process in a bulk solid of elemental Te is very slow and even negligible due to the high activation energy for the diffusion of reactant atoms and ions.¹² However, in contrast, for Te-rich CdTe nanorods prepared in the present study, the gradual disproportionation of Te in the nanorods into Te²⁻ and TeO₃²⁻ could be achieved by increasing the temperature or introducing TGA (pH 10; the molecular structure of TGA probably favors the stabilization of aqueous CdTe QDs), under the same conditions (Fig. 7 and S5†). This might be because the large surface-to-volume ratio of nanocrystals can effectively reduce the kinetic barrier for diffusion.¹² At the same time, the released Te²⁻ anions may react with Cd²⁺ ions existing in the dispersion to make the QDs formed from the decomposition of NRs bigger (CdTe, K_sp = 1 × 10⁻⁴²), resulting in the red-shift in the spectrum, as observed in Fig. 6 and 8 (in addition, heating at 90 °C is another reason for the rapid spectral shifts in Fig. 6). It is worth noting that the excessive amount of L-cysteine in the solution can inhibit the oxidation speed of Te²⁻ by dissolved oxygen in water, since the reductive thiol molecules can be also oxidized by air.²⁷ Thus, under ambient atmospheres, Te-rich CdTe NRs gradually transform into second CdTe QDs. This represents one of the unique features of the nanoscale materials that are different from those of their corresponding bulk form.

Fig. 9B further shows the optical photographs of the dispersions during the transformation of initial aqueous CdTe QDs → Te-rich CdTe NRs → second CdTe QDs. As shown, under ambient conditions, the self-assembly (or transformation) of TGA-stabilized CdTe QDs into Te-rich CdTe NRs in water due to the introduction of L-cys results in a color change of the dispersions from the initial yellow to dark green within two days. Here, the resulting Te-rich CdTe NRs were metastable. By increasing the temperature of the nanorod dispersion to 90 °C or adding a specific amount of TGA into the dispersion of NRs, Te-rich CdTe NRs might inversely transform into second CdTe QDs. In this process, the color of the dispersion changes from dark green to yellow (+ TGA) or orange (heat), accompanied by a red-shift in the emission color. Here, it should be mentioned that the initial CdTe QDs, Te-rich CdTe NRs and second CdTe QDs all exhibit favorable PL emission. Their PL colors can be seen easily under room light. These results are coincident with those from spectral analysis.

4. Conclusions

In summary, we systematically explored the controlled transformation of CdTe QDs → Te-rich CdTe NRs → second CdTe QDs in water using a combination of optical techniques, TEM and powder XRD. On the one hand, the addition of L-cys was found to efficiently promote the self-assembly of TGA-stabilized CdTe QDs into Te-rich CdTe NRs under ambient conditions. The duplicate role of L-cys (both destabilizer and stabilizer), the intrinsic crystal structure of Te and the dipole–dipole interaction were considered to play key roles in the transformation of initial CdTe QDs into Te-rich CdTe NRs. On the other hand, increasing the dispersion temperature or introducing TGA to the nanorod dispersion was found to induce the reverse transformation of Te-rich CdTe NRs → second CdTe QDs. This transformation was attributed to the disproportionation reaction of tellurium in the nanorods. These experimental observations are interesting for enhancing our current understanding of the transformation mechanism of inorganic nanostructured materials.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (30800257, 30970776, 31050110123, and 81071194), the Natural Science Foundation of Jiangsu Province (BK2011634) and the Fundamental Research Funds for the Central Universities (Program No. JKP2011017).

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Footnote

† Electronic supplementary information (ESI) available: EDX spectrum of the intermediate in the NR growth process; XRD pattern, TEM image and EDX spectrum of Te NRs; XRD pattern of second CdTe QDs; TEM images of transformation of Te-rich CdTe NRs to CdTe QDs upon addition of TGA. See DOI: 10.1039/c2ra21980h

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