Quantum Dot Surface Mediated Unprecedented Reaction of Zn2+ and

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Quantum Dot Surface Mediated Unprecedented Reaction of Zn and Copper Quinolate Complex 2+

Shilaj Roy, Satyapriya Bhandari, and Arun Chattopadhyay J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05952 • Publication Date (Web): 20 Aug 2015 Downloaded from http://pubs.acs.org on August 21, 2015

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The Journal of Physical Chemistry

Quantum Dot Surface Mediated Unprecedented Reaction of Zn2+ and Copper Quinolate Complex Shilaj Roy,a Satyapriya Bhandari,a and Arun Chattopadhyay a,b* a

Department of Chemistry, bCentre for Nanotechnology, Indian Institute of Technology Guwahati, Assam-781039, India *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT We report the reaction between Zn2+ ions, being present on the surface of ZnS quantum dot (Qdot), and copper(II)-bis(8-hydroxyquinoline) (copper quinolate; CuQ2) complex, leading to the formation of luminescent ZnQ2 on the surface. This is contrary to the reactivity of copper complex based on Irving-William series in the liquid medium and thus indicating catalytic role of the Qdot surface. The rate of the reaction was observed to be first order with respect to the concentrations of both the reactants, with activation energy measured to be 74.3 kJ mol-1. Further, the reaction of the Zn2+ ions on the Qdot was observed to be fastest with HQ, slower with MnQ2 and slowest with CuQ2, all leading to the formation of ZnQ2 on the surface of the Qdot. The enhanced stability of the product complex on the surface, in the presence of dangling sulphide ions, indicated a new type of chemical reaction deserving special attention.

KEYWORDS Nanocrystal; Surface chemistry; Complexation reaction; Reaction Kinetics; Photoluminescence.


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1. INTRODUCTION The canonical surface of a nanoparticle defines a majority of its physical and chemical properties.1-3 For a conventional quantum dot (Qdot), the cations and anions present on the surface also control its reactivity and thus functionalization with other molecules and ions, which are required for their practical utility. The recent surge of research activities portrays the importance of the surface chemistry of Qdots in fields as diverse as chemical catalysis, photovoltaics and biodiagnostics.4,5 For example, surface ligand exchange reaction has been used extensively for solubilisation of Qdots in aqueous medium for applications in bio-imaging and diagnostics.6,7 On the other hand, for Qdot based efficient thin film solar cell, assembly formation and ordering of Qdot require appropriate surface functionalization.8,9 Interestingly, the chemical activity of the surface cations have led to redox tuning of the emission properties of the Qdots.10,11 A key achievement is the realization of the importance of coordination chemistry involving the nanocrystal surface cations vis-a-vis the electronic and optical properties of the crystals.1,2,12,13 While chelation of the surface with organic moieties 14,15 and even inorganic complexes16 has been the main target of researchers, we have argued and demonstrated that inorganic complexes on the surface of the crystal can be formed by reaction involving the surface cations with ligand(s).17-20 We have defined the new composite as quantum dot complex (QDC), which consists of inorganic complex(s) on the surface of the Qdot. This has led to the enhancement of optical properties – especially due to dual emission from the complex and the Qdot, thermal stability of the complex and the ability to disperse the Qdot in water medium.17-20


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Now, a question can be asked about the reactivity of the surface cation of Qdot vis-à-vis the free ion when present in the liquid medium. Simply put, for example, would a reaction between the Zn2+ ions present on the surface of ZnS Qdot with HQ be more facile than that of free Zn2+ ion in the liquid medium? This leads to an aspect associated with ligand exchange of Qdot - which can be considered important and which has received scant attention 21,22 – that is the reaction kinetics. In addition, understanding the structure of the complex formed on the surface 23 is key to achieving further systematic progress in the field. Furthermore, the ability to monitor reaction kinetics, through the time-dependent changes in the rather easy to observe properties (such as photoluminescence, PL) of either the reactant or product or both, would provide impetus to pursue such a goal.21 Herein we report that a chemical reaction, between the metal ion present on the surface of a Qdot and an inorganic complex - both being present in a liquid medium - led to the formation of QDC, which was more facile than the reaction between the free metal ion and the inorganic complex in the same medium. Thus, ZnS Qdot (capped by ethylenediamine, en) and copper quinolate complex (CuQ2), in ethanolic medium, led to the formation of ZnQ2 on the surface of the Qdot, whereas there was no product formation for the reaction of Zn (acetate)2 with CuQ2 in the same medium. The higher reactivity of the Zn2+ - present on the surface of the Qdot - was further supported by faster reaction with MnQ2 and even faster one with HQ. Furthermore, the reaction followed first-order kinetics with respect to the concentrations of both the Qdot and CuQ2. The rate constant was measured to be (2.93 ± 0.45) x 104 mol-1 s-1 L and the activation energy of the reaction was calculated to be 74.3 kJ mol-1.


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2. EXPERIMENTAL SECTION 2.1. Materials Zinc acetate dihydrate (Merck), ethylene diamine (en, Merck), 8-hydroxyquinoline (HQ, Merck), sodium sulphide (58%, Merck), copper (II) acetate monohydrate (Merck), manganese acetate tetra hydrate (99%, Merck), quinine sulphate (Sigma Aldrich) and ethanol (Merck) were purchased and used without further purification. Mili-Q grade water was used for the synthesis and in other experiments. 2.2. Synthesis and characterization of ethylene diamine capped ZnS Qdots The ZnS Qdots were synthesized following precipitation method using ethylene diamine (en) as the capping ligand. Briefly, 500.0 L of ethylene diamine (en) was poured into a round bottom flask containing 50.0 mL of Mili-Q water. To that solution, 0.25 millimoles of zinc acetate dihydrate (Zn(OAc)2.2H2O) and 0.25 millimoles of sodium sulphide (Na2S) were sequentially added and the resulting mixture was allowed to stir for 1 h at 100-120 ºC temperature under reflux condition. The resulting colloidal dispersion was centrifuged at 25,000 rpm speed for 10 min. The obtained pellet was washed repeatedly with water and then was redispersed into same amount of solvent and the cycle was repeated for another two times. Finally, the pellet was dispersed into 100.0 mL ethanol and was used for further experiments. 2.3. Preparation of CuQ2 complex The complex of CuQ2.2H2O was synthesized based on an earlier reported protocol.24 Briefly, 10.0 mL ethanolic solution of 20.0 mM of HQ was added drop-wise to 10.0 mL aqueous solution of 10.0 mM copper acetate and was kept under sonication for 30 min. After sonication, the reaction mixture was filtered using Whatman filter paper. The filtrate was then thoroughly washed with water and hexane, respectively, in order to remove the unreacted salts. The obtained


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precipitate was then dried at room temperature. Further, the precipitate was crystallized by dissolving into a minimum amount of ethanol. Finally, the crystals were collected and purified followed by washing with water and hexane. The purified crystals were further used for experiments. The x-ray diffraction pattern, thermo-gravimetric analysis (TGA), Fourier transform infrared (FTIR) and UV-Vis spectroscopy were used to characterize the CuQ2.2H2O complex (Figure S1). Similarly, MnQ2.2H2O complex was synthesized using manganese acetate tetrahydrate (99%, Merck) instead of copper acetate and following the same procedure of the synthesis of CuQ2.2H2O complex. 2.4. Synthesis of Qdot complex (QDC) To a 2.0 mL ethanolic dispersion of as prepared Qdots, 40.0 L of 0.1 mM CuQ2 (in ethanol) was added and kept at room temperature for 20 min. Then the resulting mixture was centrifuged at a speed of 25000 rpm for 10 min. The so-obtained pellet was redispersed into the same amount of Mili-Q water. The resulting dispersion was characterized by using UV-Vis analysis, photoluminescence (PL), zeta potential measurement, atomic absorption spectroscopy (AAS), transmission electron microscopy (TEM), while the solid form of the dispersion (following centrifugation) was used for Fourier transform infrared (FTIR) spectroscopy and Xray diffraction (XRD) analysis. 2.5. Effect of different concentration of Zn2+ ions (added as salt) on the luminescence of CuQ2 in absence and presence of S2- ions (i) To a solution containing 10.0 L of 0.1 mM CuQ2 and 3.0 mL ethanol, 2.4 L of 2.5 mM zinc acetate (in ethanol) was added and the PL was measured. Next, the PL of the same resulting mixture was recorded after 12 h. To that resulting mixture, 2.4 L of 2.5 mM sodium


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sulphide (in ethanol) was added and the PL was recorded just after the addition and after 12 h of sodium sulphide addition. (ii) To a solution of 10.0 L 0.1 mM CuQ2 and 2.59 mL ethanol, 200.0 L 2.5 mM of zinc acetate (in ethanol) was added and the PL was measured. To that resulting mixture, 200.0 L of 2.5 mM sodium sulphide (in ethanol) was added and the PL was recorded. (iii) On the other hand, to a solution of 10.0 L 0.1 mM CuQ2 (in ethanol) and 2.59 mL ethanol, 200.0 L of 2.5 mM sodium sulphide (in ethanol) were added and the PL was monitored. To that, 200.0 L 2.5 mM of zinc acetate (in ethanol) and the PL was measured. The excitation wavelength used to monitor the PL was 361 nm. 2.6. Effect of Zn2+ (added as salt) on the luminescence of ZnQ2 in absence and presence of S2- ions To a solution containing 10.0 L of 0.1 mM ZnQ2 and 2.59 mL ethanol, 200.0 L of 2.5 mM sodium sulphide (in ethanol) was added and the PL was measured. To that resulting mixture, 200.0 L of 2.5 mM zinc acetate (in ethanol) was added and the PL was recorded. 2.7. Study of kinetics of the formation of QDC following complexation between en-capped ZnS quantum dots and CuQ2.2H2O In order to calculate the order of the reaction and the rate of the formation of QDC, with respect to both the reactants (CuQ2 and Qdot), the time-dependent change in area under the emission curve was measured, by varying the concentration of one reactant (at five different concentrations), while the concentration of other was kept fixed (refer to supporting information). This was done at five different temperatures.


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(i) Calculation of order of reaction for the formation of QDC with respect to CuQ2 and Qdot 2.0 L, 4.0 L, 6.0 L, 8.0 L and 10.0 L of 0.1 mM CuQ2 (in ethanol) were separately added to solutions containing each of 2.8 mL ethanol and 200.0 L of as-prepared Qdots (with absorbance of 0.106 at 317 nm) in a cuvette set at a particular temperature (T = 20 ºC). Further details of the solutions are tabulated in the Supporting Information (SI), Table S5. Then the timedependent changes in emission intensity (at 361 nm excitation) were monitored for each set of the resulting solution until the maximum emission intensity was reached. On the other hand, for five different amounts of as-prepared Qdot dispersion (50.0 L, 100.0 L, 150.0 L, 200.0 L and 250.0 L of Qdot dispersion), 10.0 L of 0.1 mM CuQ2 (in ethanol) was added to each of them and finally the total volume of the resulting mixture was adjusted to 3.0 mL by adding ethanol. The resulting mixtures were used to monitor the time-dependent changes in emission intensity at 20 ºC. (ii) Calculation of activation energy of the formation of QDC (with respect to CuQ2) Experiments similar to (i) above (for finding the order with respect to [CuQ2]) were carried out at different temperatures, namely, T = 15 ºC, 20 ºC, 25 ºC and 30 ºC, in order to calculate activation energy of the formation of QDC. The rate constant was calculated from the intercept value obtained from the plot of ln(Rate) vs ln([CuQ2]) of the reaction at different temperatures. The results were further used to make Arrhenius plot (lnk vs 1/T), in order to calculate the activation energy of the reaction with regard to CuQ2. 2.8. Kinetic study of the formation of QDC following complexation of en-capped ZnS Qdots with HQ and MnQ2.2H2O


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To a solution containing 2.8 mL ethanol and 200.0

L of as prepared Qdots (with

absorbance 0.106 at 317 nm), 10.0 L 0.2 mM HQ and 10.0 L 0.1 mM MnQ2 (in ethanol) were separately added in the cuvette and the time-dependent photoluminescence spectra were recorded until the emission intensity of the resulting mixture stopped increasing. 2.9. Calculation of equilibrium constant of the formation of QDC To a solution containing 2.8 mL ethanol and 200.0 L ethanolic dispersion of as-prepared Qdots (with absorbance of 0.106 at 317 nm) in a cuvette at a particular temperature (T = 20 ºC), 2.0 L of 0.1 mM CuQ2 (in ethanol) was added sequentially in order to obtained saturation in emission intensity (at 497 nm) of the resulting mixture. Additionally, the resulting mixture after each sequential addition was kept for 20 min, under same experimental condition, before the measurement was made. 2.10. Instruments The visual images of the samples were recorded under UV light (365 nm) using a UVlamp. Perkin Elmer Lambda-750 and HORIBA-Fluoromax4 spectrofluorimeter were used to record the UV-Vis and PL spectra, respectively. The pH of the samples were measured using a JENWAY 3510- pH meter. HORIBA-JOBIN Yvon Flurolog spectrofluorimeter (using 340 nm LED excitation source with pulse width

Quantum Dot Surface Mediated Unprecedented Reaction of Zn2+ and

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