CdSe quantum dots (QDs) with high quantum yield (QY) up to 76.57% are synthesized using the aqueous precipitation method. With the control of
Because of excellent size-dependent optical characteristics and chemical treatability, semiconducting CdSe quantum dots (QDs) have been widely applied in biological identification (medical diagnosis), solar cells, optoelectronic devices, display instruments, nonlinear optical equipment, and magnetic materials [
In the present work, the effective aqueous precipitation method has been explored where the control of
The Se precursor for the present work was prepared by mixing the selenium powder (0.005 mol) (A.R, Shanghai Meixing Chemical Co., Ltd.), sodium sulfite (0.01 mol) (A.R, Sinopharm Chemical Reagent Co., Ltd.), sodium hydroxide (0.01 mol) (A.R, Shanghai Aijian ready-made Reagent Co., Ltd.), and deionized water (50 mL) with N2 protection and magnetic stirring. The mixture was heated to boiling while the solution turned from red brown to the black precipitate. Then sodium hydroxide (0.01 mol) was added, and after returning for 4 hours, the reactants were cooled down to the room temperature followed by nitrogen protection for 30 min.
The Cd precursor was prepared by mixing water, ethanol, and oleic acid (A.R, Jiangsu Yonghua Fine Chemicals Co., Ltd.) together in volume ratio of 10 : 30 : 8. Cadmium acetate 0.001 mol (A.R, Sinopharm Chemical Reagent Co., Ltd.) and sodium hydroxide (0.7 g) were added as precipitation agents to the mixed solvent (48 mL). The mixture was under the N2 protection until it became clear and transparent. The reaction temperature was 50°C, 70°C, and 90°C.
The CdSe QDs were synthesized by adding 10 ml Se precursor solution to the 48 mL Cd precursor solution, under N2 protection and magnetic stirring. The reactions were carried out at 50°C, 70°C, and 90°C for 5 min, 10 min, 15 min, 20 min, and 25 min, respectively, as marked in Table
Sample numbers and reaction temperature and time.
Temperature | Time | ||||
---|---|---|---|---|---|
5 min | 10 min | 15 min | 20 min | 25 min | |
50°C | I1 | I2 | I3 | I4 | I5 |
70°C | II1 | II2 | II3 | II4 | II5 |
90°C | III1 | III2 | III3 | III4 | III5 |
X-ray diffraction spectra (XRD, D/MAX 2550 VB/PC, Japan, RIGAKU) (
The flow diagram of the whole reaction.
Figure
XRD patterns of sample I1, II1, and III1 (50°C, 70°C, and 90°C).
The average diameter of CdSe crystals in sample I1, II1, and III1 is calculated according to the broadening of XRD by the Scherrer formula
Calculated diameters of CdSe crystals in sample I1 (50°C).
2 |
25.30° | 42.24° | 49.23° |
|
12.65° | 21.12 | 24.62 |
FWHM (°) | 10.3 | 4.73 | 2.98 |
FWHM (rad) | 0.18 | 0.08 | 0.05 |
Radius (nm) | 0.78 | 1.80 | 2.95 |
Average radius (nm) | 1.84 nm |
Calculated diameters of CdSe crystals in sample II1 (70°C).
2 |
25.37° | 42.04° | 49.74° |
|
12.69° | 21.02 | 24.87 |
FWHM (°) | 3.51 | 4.11 | 5.10 |
FWHM (rad) | 0.08 | 0.07 | 0.09 |
Radius (nm) | 2.34 | 2.10 | 1.68 |
Average radius (nm) | 2.04 nm |
Calculated diameters of CdSe crystals in sample III1 (90°C).
2 |
25.51° | 42.56° | 49.25° |
|
12.76° | 21.28 | 24.63 |
FWHM (°) | 11.53 | 3.72 | 2.56 |
FWHM (rad) | 0.20 | 0.06 | 0.04 |
Radius (nm) | 0.70 | 2.43 | 3.69 |
Average radius (nm) | 2.27 nm |
HRTEM photos of samples I1, I3, I5; II1, II3, II5 and III1, III3, III5 are presented in Figures
HRTEM photos of samples I1, I3, I5, II1, II3, II5, III1, III3, and III5.
I1 (50°C)
I3 (50°C)
I5 (50°C)
II1 (70°C)
II3 (70°C)
II5 (70°C)
III1 (90°C)
III3 (90°C)
III5 (90°C)
Furthermore, HRTEM photos of samples I1, II1, and III1 for lattice parameter analysis are provided in Figure
HRTEM photos of samples I1, II1, and III1 for lattice parameter analysis.
I1 (50°C)
II1 (70°C)
III1 (90°C)
Figure
Absorption spectra of samples under different reaction temperature and time (a) 50°C, (b) 70°C, and (c) 90°C.
50°C
70°C
90°C
Figures
The emission spectra of samples under different reaction temperature and time ((a), (b), and (c); 50°C, 70°C, and 90°C) (
50°C
70°C
90°C
real samples
It is known via previously reported work that QY of CdSe QDs can be calculated according to formula (
The absorption coefficients of samples I1, II1, and III1 and rhodamine B at 460 nm are 0.0287, 0.0565, 0.0530, and 0.0139, corresponding to their integral emission areas of
It is obvious that along with the increased crystal size of CdSe QDs in the samples, QY decreases from 76.57% (I1) to 37.61% (III1) most likely due to the reduced quantum confinement effect. However, in the present work, QY of sample I1 reaches the level of 76.57%, much higher than those (~30%) reported with the aqueous precipitation method [
In our work, black powders were precipitated in preparation of the Se precursor. According to the principle of chemical reaction [
It is known that thiosulfate
The masses of
Determined Cd2+ and Se2+ % from ICP-AES analysis for sample I1 (50°C, 5 min).
Cd source | Se source | Cd2+ (tested by ICP) | Se2+ (tested by ICP) | Cd2+ (in CdSe) | Se2+ (in CdSe and unreacted to CdSe) |
---|---|---|---|---|---|
112 mg | 79 mg | 4.68 mg | 0.56 mg | 95.82% | 99.29% |
On the other hand, by taking
In this paper, CdSe quantum dots (QDs) are produced using an aqueous precipitation method. XRD patterns demonstrate the structure of products with the purely precipitated CdSe crystals. TEM manifests that the size of the CdSe QDs is between 2 nm and 2.3 nm with good monodispersity. The addition of NaOH to Se precursor speeds up the nucleation and increases the concentration of CdSe QDs. The ICP-AES shows that the calculated proportion of Cd2+ in CdSe nanocrystal is 95.82%, and the total proportion of
In this paper, CdSe quantum dots (QDs) with high quantum yield (QY) up to 76.57% are synthesized using the aqueous precipitation method. With the control of
This study was supported by Shanghai Leading Academic Discipline Project (No. B502), Shanghai Key Laboratory Project (08DZ2230500), Doctoral Fund of Ministry of Education of China (20120074110018) and the National Natural Science Foundation of China (NSFC 51072052).