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Influence of pH on the thermo-optic properties of CdSe QDs prepared by a microwave irradiation method

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Laser Phys. Lett. 11 115901 (http://iopscience.iop.org/1612-202X/11/11/115901) View the table of contents for this issue, or go to the journal homepage for more

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Laser Physics Letters

Astro Ltd Laser Phys. Lett. 11 (2014) 115901 (6pp)

doi:10.1088/1612-2011/11/11/115901

Influence of pH on the thermo-optic properties of CdSe QDs prepared by a microwave irradiation method Anju K Augustine, C P Girijavallabhan, V P N Nampoori and M Kailasnath International School of Photonics, Cochin University of Science and Technology, Kochi-682022, Kerala, India E-mail: [email protected] Received 8 May 2014, revised 30 July 2014 Accepted for publication 8 September 2014 Published 29 September 2014 Abstract

In this letter the optical behavior as well as the thermal properties of CdSe quantum dots (QDs) capped with mercapto succinic acid (MSA) are studied and analyzed. CdSe QDs with an average particle size of 7.0 nm are prepared by a microwave irradiation method. The unique structure of MSA plays an important role in determining the PL intensity and better stability by controlling the pH of the medium. A significant increase in thermal diffusivity with pH values is observed with a mode matched thermal lens method. At the optimum value of pH, the surface charge of nanoparticles increases, which increases the repulsive forces. The resulting reduced agglomeration of QDs enhances mobility and improves heat transport. There is a clear correlation between luminous intensity and thermal diffusivity in these nano fluids containing CdSe QDs. Keywords: MSA, CdSe QDs, luminous intensity, thermal diffusivity (Some figures may appear in colour only in the online journal)

1. Introduction

coagulation of QDs in the nanofluid becomes the primary issue in order to exploit their potential benefits and applications. In the case of nanofluids, it is an important evaluation for understanding the dispersion stability behavior [13, 14]. However, to our knowledge, recent efforts have mainly focused on optical behavior and there are few reports regarding the nanofluid thermal diffusivity changes with pH. In this letter, by measuring the optical changes as well as the thermal diffusivity changes of nanofluids with pH, we investigate the effect of pH value on the dispersive stability and the heat transfer enhancement of the nanosuspension. The high sensitivity character of the thermal lens (TL) technique makes it very appropriate for measuring the thermal diffusivity in the samples that are dispersed in a fluid medium. It is expected to provide a means for designing nanofluids with desirable thermal and optical properties.

During the last two decades, there has been an enormous interest in nanomaterials due to their novel physical and chemical properties, which arise from quantum confinement of charge carriers that differ clearly from those of bulk materials [1–5]. Among the various kinds of quantum dot (QD) semiconductors, colloidal CdSe is the most widely investigated kind, because their emission can be easily tuned to cover from red (centred at 650 nm) to blue (centred at 450 nm) as the size of QDs decrease because of their strong size-dependent optical properties [6–8]. CdSe QDs, prepared in a non aqueous method are only soluble in some nonpolar organic solvents, which creates problems if they have to be used for thermal or biological applications [9, 10]. In the present work, CdSe QDs are prepared by a microwave irradiation (MWI) method in an aqueous medium [11, 12]. However, due to the high surface energy of QDs, they can easily coagulate and are difficult to disperse in the base fluid. This leads to changes of the morphology and of the volume fraction resulting in low fluidity. Therefore, controlling the 1612-2011/14/115901+6$33.00

2.  Experimental details CdSe QDs were prepared by a rapid MWI method. The precursors used were CdCl2.H2O and Na2SeO3. The capping 1

© 2014 Astro Ltd  Printed in the UK

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Laser Phys. Lett. 11 (2014) 115901

Figure 1.  TL experimental setup with a DSO.

agent used was mercaptosuccinic acid (MSA). The pH of the corresponding colourless solution was adjusted by adding 1 M NaOH. We have selected samples with different pH values. MWI provided the required thermal energy to the reaction. The power level of the reaction system was maintained at 60 W. During the reaction, the colour of the solution gradually changed and QDs began to form. Aliquots were taken out of the flask for the optical measurements to monitor the growth of the QDs. The samples taken for optical characterization are named as pH 3.7, pH 4.7, pH 7.6, and pH 8.9. Absorption spectra of the prepared CdSe QDs were recorded using a UV–Visible Spectrophotometer (Jasco V-570 UV/VIS/IR). Fluorescence spectra of the nanoparticles were obtained on a Cary Eclipse fluorescence spectrophotometer (Varian). The excitation wavelength was kept at 390 nm, and the emission spectra were recorded. All absorption and fluorescence spectra were measured without any post preparative size separation. The Electron micrograph images of CdSe QDs in colloidal form were taken using a Transmission electron microscope (Philips Technai G2 at 120 kV). TL spectrometry (TLS) is one of the important photothermal techniques that depends on a temperature gradient generated by the absorption of electromagnetic radiation and subsequent non-radiative relaxation of the excited molecules. For TL studies, the schematic experimental setup is shown in figure 1. The excitation source used was a continuous wave (cw), 532 nm diode pumped solid state laser (DPSS) with a maximum power of 150 mW. The power at the sample was suitably adjusted using attenuators so that the probe beam spot was free from aberrations. A 2 mW He–Ne gas laser emitting at 632.8 nm used as the probe was arranged to be collinear with the pump using a dichroic mirror. The two beams were focused into the sample cell such that the beam area at the sample plane was the same for both pump and probe resulting in a mode matched TL arrangement. A sample was taken in a cuvette of 1 cm path length for making the measurements. A low frequency mechanical chopper with 3 Hz was used to modulate the intensity of the pump, until the TL peak-to-peak signal reached its maximum. This also enabled us to determine the thermal recovery of the sample. In the TL experiment the excitation laser must have a Gaussian profile, so that when the sample absorbs the beam with a Gaussian intensity profile, the temperature distribution has a radial dependence. The temperature gradient causes a

refractive index gradient which behaves like a converging or diverging lens depending on whether the rate of change of the refractive index with respect to temperature is positive or negative [15, 16]. The TL signal was collected using an optical fiber, which serves as the finite aperture and the same was mounted on the xyz translator. It was positioned at the centre of the probe beam spot and connected to a photo detector– digital storage oscilloscope (DSO) system. A filter to cut off 532 nm was used before the detector to remove the residual pump. The probe beam from the He–Ne laser which passes collinearly with the pump beam experienced divergence and the beam shape expanded in the presence of the TL. The change in intensity of the probe beam was measured using a fast photodetector from which the relative intensity and initial slope was measured. The data was analyzed by using the procedure reported earlier [17].

3.  Results and discussion Figures 2 and 3 show the absorption and florescence spectra of the prepared CdSe QDs with a different pH at room temperature. It can be clearly seen that the absorption and luminescence peaks of the CdSe QDs were red-shifted with increasing value of pH. It is observed that, for the sample with pH of less than 4.7 there are signs of agglomeration and for a higher pH the intensity of emission is decreased, and it shows much less intensity with a large FWHM. Figure 3 shows the pH effects of the reaction solution on the fluorescent properties of CdSe QDs. It can be seen that the pH in our synthesis is much lower than with glutathione (GSH) (~11.0–12.0) or MPA (>8.0) as thiol stabilizers used in common aqueous synthesis. For example, when GSH was used as a stabilizer for CdSe QDs, the prepared colloid solution formed some white precipitates if the pH of the Cd precursor was reduced to acid or was weakly basic, which restricted the preparation of the CdSe QDs in a relatively lower pH aqueous solution. However, MSA–CdSe can be synthesized even in a weak acidic solution, mostly because of the special structure of MSA with two carbonyl groups. In this study, we also obtained high-quality CdSe QDs in a weak acid solution, the possible reasons for which are as follows. First, previous studies indicated that thiols had a strong complexation to CdS particles rather than to free cadmium ions under acidic conditions, which resulted in a smaller trap site on the 2

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Laser Phys. Lett. 11 (2014) 115901

Figure 2. (a) Absorption spectra of the prepared CdSe QDs with different pH. (b) Red shift in wavelength of CdSe QDs with pH.

Figure 3. (a) Fluorescence spectra of the prepared CdSe QDs with different pH. (b) Change in intensity of CdSe QDs with pH.

CdSe surface. Secondly, a thick layer of cadmium thiol complexes under acidic conditions was also an important factor for determining high-quality QDs, because more trap sites on the CdSe surface are removed after being covered with cadmium thiol complexes. Last, but not least, the unique structure of MSA plays an important role in reducing the surface trap of CdSe QDs [18–20]. The two carbonyl groups of MSA can provide better stability than other thiol compounds (MPA, GSH, etc), as seen from figure 2. Nevertheless, when the pH of the precursor solution was decreased further, it led to weakening of the protection abilities of MSA due to the protonation of MSA. Because the strong acidic solution was unfavorable for the formation of a defect-free surface, we choose pH ~ 4.7 for the synthesis of CdSe QDs. Figure 4 represents the TEM image and SAED of the CdSe sample. It shows that the nanoparticles formed are spherical in shape. Figure 5 represents the thermal decay at an input power of 136 mW. Under the same experimental conditions, the experiment was repeated for various pH conditions of CdSe QDs dissolved in water. Two typical results are shown in figure 6 The time dependent probe beam intensity follows the expression [20, 21].

Figure 4. (a) TEM image and (b) SAED of the CdSe sample.



I (t )= I0 ⎡1 − θ (1 + tc 2t )−1 + ⎣

(1 2 ) θ 2(1 + t 2t )−2 ⎤⎦ . c

(1)

Here, the parameter θ is related to the thermal power radiated as heat, Path, and can be obtained with I =(I0 − I∞)/ I∞ 3

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Laser Phys. Lett. 11 (2014) 115901

Figure 5.  Fitting of the data (pH −9.6) using equation (1) with the probe beam intensity as a function of time with fit parameters I0 = 0.25,

θ = 36.22 and tc =1.94 s The beam spot size at the sample position is 0.225 mm.

Figure 6.  TL fitting of the CdSe QD data with different pH using equation (1) with the probe beam intensity as a function of time with error ±0.03. The beam spot size at the sample position is 0.22 mm.

and θ = 1 − (1 + 2I ) , where I0 is the initial intensity and I∞ is the intensity after the steady state. A detailed curve fitting of this experimental data to equation  (1) gives the time constant tc of the thermal decay process. Finally the thermal diffusivity D of the sample can be calculated from the equation  tc = ω2 / 4D where ω is the beam radius at the sample position and tc, the time response to attain the steady state focal length. Similarly TL signal evolution was obtained for CdSe QDs of different pH values and their corresponding cluster size (d) or hydro dynamic particle size, tc’s, θ’s, and diffusivity(D)s were obtained and are tabulated in table 1.

Table 1.  Data showing pH, θ, time constant and diffusivity of CdSe

QDs with error ±0.03.

4

pH

Hydro dynamic particle size (d) (nm)

θ

tc (ms)

Diffusivity 10−5 cm2 s−1

3.7 4.7 7.6 8.9 9.6 10 11

8.4 10 10.1 10.3 8.5 9.2 7.9

467.9730 0.9801 13.2410 103.1097 36.2265 463.5255 465.0139

12.9078 0.0007 0.7934 4.9154 1.9444 48.2313 30.0740

0.2451 4520 3.987 0.6436 1.6272 0.0656 0.1052

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Laser Phys. Lett. 11 (2014) 115901

Figure 7. (a) tc of CdSe QDs plotted against pH and (b) thermal diffusivity of CdSe QDs plotted against pH.

The observed reduction (from figure 7(a)) in tc at optimum pH could be the effect of shortening the mean free path of the phonons. It has to be noted from table 1. As the pH increased continuously the mean free path of the phonon in the solid increases the value of tc and thermal diffusivity was reduced. From the figure it is observed that at optimum pH, the tc value is very small compared to pH other than this. A large enhancement of thermal diffusivity is observed (from figure 7(b)) with the sample of pH in the region of ~4.7 Note that thermal diffusivity of the base fluid is nearly constant at different doses of electrolyte salt and acid or base. The enhancement seems to be related only to the particles. When the QDs are dispersed into water, the overall behavior of the particle-water interaction depends on the properties of the particle surface. Experimental results also indicate that the stabilities of these nanofluids are influenced by pH values. The Derjaguin theory explains the aggregation of aqueous dispersions quantitatively and describes the force between charged surfaces interacting through a liquid medium. It combines the effects of the van der Waals attraction and the electrostatic repulsion due to the so-called double layer of counter ions. According to this, when the pH is increased to the optimized value, (pH less than 4.7) the surface charge increases because of the more frequent attacking of the surface hydroxyl groups and the phenyl sulfonic group by potential-determining ions (H+, OH− and the phenyl sulfonic group). This leads to an increase of the electrostatic repulsion force between the particles, and the suspensions show significantly reduced agglomeration and enhanced mobility, ultimately improving the heat transport [21]. When the pH is equal to the isoelectric point (IEP), which is the optimized pH(~4.7) in our case the QDs tend to be unstable, form clusters, and precipitate where IEP is the pH at which a particular molecule or surface carries no net charge. The repulsive forces among metal oxides are zero and QDs coagulate together at this pH value. The resulting big clusters formed at the IEP will trap water and the structures of trapped water vary due to the strong atomic force among QDs. Water is packed well inside and the volume fraction of the QDs will be larger. In addition, the shapes of clusters containing trapped water will not be spherical but rather will have an

Figure 8.  Variation of ξ-potential with pH.

irregular structure like chains. Such a structure favors thermal transport because they provide a long link [22]. Therefore, the overall thermal conductivity and hence the thermal diffusivity of nanofluids are enhanced. When the pH is too large, the concentration of the pH adjustment reagent (NaOH) in the system increases, which causes compression of the electrical doublelayer. At pH > 10, the surface charge of QDs increases, which creates repulsive forces between the QDs. As a result of this effect, substantial clustering of Quantum dots is prevented. We calculated the variation of ξ-potential and the cluster radius of the QDs (table 1) as a function of pH by using dynamic light scattering (DLS) measurement. The effect of charge on the clustering process with varying pH has also been plotted in figure 8 which shows an almost neutral charge at optimum pH. Therefore, it is reasonable to infer that optimizing the pH or a high surface charge facilitates heat transport through increases in transport efficiency. We also attempt to link the concept of this interesting phenomenon of change in thermal diffusivity with pH from the emission intensity in figure  4. The total relaxation cross section (σT) = σR + σNR; where σR is radiative relaxation and σNR is the nonradiative relaxation cross section. So that σNR = σT − σR. From figure 4, at optimum pH value, the fluorescence intensity maximum indicating 5

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Laser Phys. Lett. 11 (2014) 115901

References

maximum fluorescence quantum yield and minimum thermal energy is generated in the medium. Our calculation showed that maximum fluorescence quantum yield is obtained at pH ~ 4.7 where the probe beam (TL signal) intensity is at a minimum. It is also observed in figure 6. This enhanced radiative process will reduce the heat evolved through the nonradiative process. This may cause reduction of tc which is the time taken to transport heat and hence enhance the magnitude of the diffusion coefficient. The major advantages of such materials are that they can be used as both a coolant and an insulator by adjusting the corresponding pH values. At a low pH (~4.7), they act as a good coolant which can be used to diffuse heat energy to the surroundings, and at a greater pH they can be an efficient material to trap thermal energy and act as good insulators.

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4. Conclusion A thermo-optic study of MSA capped CdSe QDs synthesized by a MWI method shows a significant increase in thermal diffusivity at optimum pH value. It was observed that QDs become unstable and aggregated at pH equal to or close to the isoelectric point and the resulting big clusters that are formed trap water molecules and form long chain like structures. This may be the reason for large enhancement in the thermal diffusivity of nanofluids at optimum pH. When the pH is too large, the repulsive force of the QDs prevents agglomeration and causes a decrease in thermal diffusivity. The reduced nonradiative process at pH ~ 4.7 may also be the reason for the highest diffusion coefficient. Acknowledgment The authors acknowledge DST for financial assistance.

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