CN103011095A - Rapid preparation method of cadmium telluride quantum dot - Google Patents
Rapid preparation method of cadmium telluride quantum dot Download PDFInfo
- Publication number
- CN103011095A CN103011095A CN2012105904022A CN201210590402A CN103011095A CN 103011095 A CN103011095 A CN 103011095A CN 2012105904022 A CN2012105904022 A CN 2012105904022A CN 201210590402 A CN201210590402 A CN 201210590402A CN 103011095 A CN103011095 A CN 103011095A
- Authority
- CN
- China
- Prior art keywords
- solution
- cadmium
- preparation
- quantum dot
- telluride quantum
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
Images
Landscapes
- Luminescent Compositions (AREA)
- Investigating, Analyzing Materials By Fluorescence Or Luminescence (AREA)
Abstract
The invention belongs to the field of nano material preparation, and in particular relates to a rapid preparation method of a cadmium telluride quantum dot. According to the invention, by using hydroxylamine hydrochloride as a reducing agent, a sulfhydryl compound as a stabilizing agent, a chlorine salt as a cadmium source, and tellurium dioxide as a tellurium source, in an air environment, a water phase system is rapidly prepared so that the cadmium telluride quantum dot with high fluorescence and good biocompatibility is directly synthesized. Compared with the prior art, the rapid preparation method has the advantages of mild synthesis conditions, simple steps, low price of used raw materials, environment friendliness, suitability of large-scale production, and higher actual application value.
Description
Technical field
The invention belongs to the preparation of inorganic semiconductor nanometer material, be specifically related to a kind of quick, green preparation method of water miscible cadmium telluride quantum dot.
Background technology
Quantum dot is a kind of emerging nano-luminescent material, compare with traditional organic fluorescent dye, quantum dot has the excitation spectrum wide ranges, emmission spectrum is narrow and peak shape good, emmission spectrum is adjustable, the optical property of the high excellence of quantum yield, demonstrates huge potentiality at aspects such as biomarker and bio-sensings.Cadmium telluride quantum dot be at present research at most and the most widely II of using-IV family quantum dot, the preparation method of high-quality water soluble cadmium telluride quantum point is researchist's focus always.The quantum dot of biologically using at present is still mainly from organic synthesis, and the synthetic quantum dot of this method has single dispersion, high quantum production rate and the good advantages such as degree of crystallinity; The shortcomings such as but also there is severe reaction conditions in the method, and synthetic cost is high, and toxicity is large, and product is water insoluble.2002, Rogach, A.L. used a series of sulfhydryl reagent used as stabilizers such as Thiovanic acid, mercapto glycerol, mercaptoethanol, halfcystine to prepare water miscible cadmium telluride quantum dot.The synthetic cadmium telluride quantum dot of water method has water-soluble and biocompatibility, but usually will prepare first telluretted hydrogen (H in the method for the synthetic cadmium telluride quantum dot of present water method
2Te) or sodium hydrogen telluride (NaHTe) as tellurium source (telluretted hydrogen and sodium hydrogen telluride have severe toxicity), and preparation cost is high, building-up process needs substep, operates comparatively loaded down with trivial details.Therefore, take tellurium powder, tellurite and tellurium dioxide as the tellurium source, be reduced to H by reductive agent
2Te or NaHTe prepare quantum dot with the Cd ionic reaction again.
Literature survey and practice show that when preparation CdTe quantum dot, used reductive agent mostly is sodium borohydride, and sodium borohydride is made reductive agent, is easy to generate in solution salt and the precipitation of solubility, so that the purity of quantum dot is not high.In the document, the hydrazine hydrate that the preparation method take hydrazine hydrate as reductive agent consumes is more, generally is hundreds of times of tellurium source.2011, D.Zhou adopted sodium borohydride-hydrazine hydrate to unite the method for reduction.This method is first with the sodium borohydride reduction tellurite, and then add large excess hydrazine hydrate (the hydrazine hydrate add-on is 120000 times of cadmium source), although this method can solve salt and the sedimentation problem that generates solubility preferably, but need a large amount of hydrazine hydrate reagent of consumption and hydrazine hydrate to have than high toxicity, can cause environmental pollution and improve quantum dot industrial production cost.
Summary of the invention
The object of the invention is to for the defects that exists in the prior art, a kind of environmental protection is provided, produces without new soluble salt, realize that in aqueous phase system a step prepares the directly method of the cadmium telluride quantum dot of synthetic high fluorescence, good water solubility fast.
The present invention seeks to realize that by following method this comprises the step of following order:
(1) oxammonium hydrochloride is dissolved in the deionized water, regulator solution acidity is pH=9～13;
(2) with tellurium dioxide concentration be the dissolving of 1mol/LNaOH solution;
(3) under the room temperature, cadmium salt is dissolved in the deionized water, then adds sulfhydryl compound, stir; Wherein, the mol ratio of cadmium salt and sulfhydryl compound is 1:1.2～5.0;
(4) dripping concentration in the solution of step (3) preparation is the sodium hydroxide solution of 1mol/L, and regulator solution acidity is pH=9～13, obtains the precursor solution of cadmium;
(5) under whipped state, the solution that step (1), step (2) and step (4) are prepared adds in the three-necked bottle bottle successively, sealing, stirring; Wherein, the mol ratio of tellurium dioxide and cadmium salt is 1:0.02～0.40, and the mol ratio of tellurium dioxide and oxammonium hydrochloride is 1:7～80;
(6) with the solution of step (5) preparation reflux 10 minutes to 210 minutes in 85-100 ℃ of temperature range, whole process is carried out under magnetic agitation, namely obtains water soluble cadmium telluride quantum point solution.
More particularly, described cadmium salt is a kind of in Cadmium chloride fine powder, cadmium perchlorate and the cadmium acetate.
Described sulfhydryl compound is a kind of in halfcystine, Thiovanic acid, thiohydracrylic acid, dimercaptosuccinic acid, gsh and N-(the 2-sulfydryl propionyl) glycine.
Preferably, step (1) is identical with the acidity value of regulating described in the step (4).
The present invention is N2 by take oxammonium hydrochloride as reductive agent based on the oxidation products of oxammonium hydrochloride in alkaline environment, can not introduce new soluble salt, meets the requirement of Green Chemistry; The present invention is take cadmium salt as the cadmium source, and tellurium dioxide is the tellurium source, and take sulfhydryl compound as stablizer, preparation process is simple, quick, and used reductive agent consumption is few, easy to implement; Whole preparation process satisfies the requirement of Green Chemistry, safety, environmental protection; The cadmium telluride quantum dot that makes has higher quantum yield, good stability.
Description of drawings
Fig. 1 is CdTe quantum dot homogenization fluorescence spectrum and uv-visible absorption spectroscopy figure prepared among the embodiment 1.
Fig. 2 is CdTe quantum dot homogenization fluorescence spectrum and uv-visible absorption spectroscopy figure prepared among the embodiment 2.
Fig. 3 is CdTe quantum dot homogenization fluorescence spectrum and uv-visible absorption spectroscopy figure prepared among the embodiment 3.
Fig. 4 is CdTe quantum dot homogenization fluorescence spectrum and uv-visible absorption spectroscopy figure prepared among the embodiment 4.
Fig. 5 is CdTe quantum dot homogenization fluorescence spectrum and uv-visible absorption spectroscopy figure prepared among the embodiment 5.
Fig. 6 is CdTe quantum dot homogenization fluorescence spectrum and uv-visible absorption spectroscopy figure prepared among the embodiment 6.
Fig. 7 is CdTe quantum dot homogenization fluorescence spectrum and uv-visible absorption spectroscopy figure prepared among the embodiment 7.
Embodiment
The present invention is described in further detail with concrete EXPERIMENTAL EXAMPLE below in conjunction with accompanying drawing.
Embodiment 1:
(1) 0.7m mol oxammonium hydrochloride is dissolved in the 50ml deionized water, regulator solution acidity is pH=9;
(2) with 0.05m mol tellurium dioxide concentration be the NaOH solution dissolving of 1mol/L;
(3) under the room temperature, 1.0m mol Cadmium chloride fine powder is dissolved in the 50ml deionized water, then adds the 1.6mmol Thiovanic acid, stir;
(4) dripping concentration in the solution of step (3) preparation is the sodium hydroxide solution of 1mol/L, and regulator solution acidity is pH=9, obtains the precursor solution of cadmium;
(5) under agitation, the solution with step (1), (2) and (4) preparation adds in the 250ml three-necked bottle bottle sealing, stirring successively;
(6) with the solution of step (5) preparation reflux 30 minutes under 100 ℃ of temperature condition, whole process is carried out under magnetic agitation, obtains water soluble cadmium telluride quantum point solution.
Figure 1 shows that water soluble cadmium telluride quantum point solution homogenization fluorescence spectrum and the uv-visible absorption spectroscopy figure of present embodiment preparation, among the figure, curve 1 is uv-visible absorption spectroscopy, and curve 2 is fluorescence spectrum.
Embodiment 2:
(1) 1m mol oxammonium hydrochloride is dissolved in the 50ml deionized water, regulator solution acidity is pH=10;
(2) with 0.05m mol tellurium dioxide concentration be the NaOH solution dissolving of 1mol/L;
(3) under the room temperature, 1.0m mol cadmium perchlorate is dissolved in the 50ml deionized water, then adds the 2.4mmol Thiovanic acid, stir;
(4) dripping concentration in the solution of step (3) preparation is the sodium hydroxide solution of 1mol/L, and regulator solution acidity is pH=10, obtains the precursor solution of cadmium;
(5) under agitation, the solution with step (1), (2) and (4) preparation adds in the 250ml three-necked bottle bottle sealing, stirring successively;
(6) with the solution of step (5) preparation reflux 30 minutes under 100 ℃ of temperature condition, whole process is carried out under magnetic agitation, obtains water soluble cadmium telluride quantum point solution.
Figure 1 shows that water soluble cadmium telluride quantum point solution homogenization fluorescence spectrum and the uv-visible absorption spectroscopy figure of present embodiment preparation, among the figure, curve 1 is uv-visible absorption spectroscopy, and curve 2 is fluorescence spectrum.
Embodiment 3:
(1) 1.5m mol oxammonium hydrochloride is dissolved in the 50ml deionized water, regulator solution acidity is pH=10;
(2) with 0.05m mol tellurium dioxide concentration be the NaOH solution dissolving of 1mol/L;
(3) under the room temperature, 1.0m mol cadmium acetate is dissolved in the 50ml deionized water, then adds the 3.2mmol Thiovanic acid, stir;
(4) dripping concentration in the solution of step (3) preparation is the sodium hydroxide solution of 1mol/L, and regulator solution acidity is pH=10, obtains the precursor solution of cadmium;
(5) under agitation, the solution with step (1), (2) and (4) preparation adds in the 250ml three-necked bottle bottle sealing, stirring successively;
(6) with the solution of step (5) preparation reflux 60 minutes under 100 ℃ of temperature condition, whole process is carried out under magnetic agitation, obtains water soluble cadmium telluride quantum point solution.
Figure 1 shows that water soluble cadmium telluride quantum point solution homogenization fluorescence spectrum and the uv-visible absorption spectroscopy figure of present embodiment preparation, among the figure, curve 1 is uv-visible absorption spectroscopy, and curve 2 is fluorescence spectrum.
Embodiment 4:
(1) 0.4m mol oxammonium hydrochloride is dissolved in the 50ml deionized water, regulator solution acidity is pH=10;
(2) with 0.025m mol tellurium dioxide concentration be the NaOH solution dissolving of 1mol/L;
(3) under the room temperature, 1.0m mol Cadmium chloride fine powder is dissolved in the 50ml deionized water, then adds 2.0mmol N-(2-sulfydryl propionyl) glycine, stir;
(4) dripping concentration in the solution of step (3) preparation is the sodium hydroxide solution of 1mol/L, and regulator solution acidity is pH=10, obtains the precursor solution of cadmium;
(5) under agitation, the solution with step (1), (2) and (4) preparation adds in the 250ml three-necked bottle bottle sealing, stirring successively;
(6) with the solution of step (5) preparation reflux 120 minutes under 100 ℃ of temperature condition, whole process is carried out under magnetic agitation, obtains water soluble cadmium telluride quantum point solution.
Figure 1 shows that water soluble cadmium telluride quantum point solution homogenization fluorescence spectrum and the uv-visible absorption spectroscopy figure of present embodiment preparation, among the figure, curve 1 is uv-visible absorption spectroscopy, and curve 2 is fluorescence spectrum.
Embodiment 5:
(1) 1.4m mol oxammonium hydrochloride is dissolved in the 50ml deionized water, regulator solution acidity is pH=10;
(2) with 0.15m mol tellurium dioxide concentration be the NaOH solution dissolving of 1mol/L;
(3) under the room temperature, 1.0m mol Cadmium chloride fine powder is dissolved in the 50ml deionized water, then adds 2.0mmol, Thiovanic acid stirs;
(4) dripping concentration in the solution of step (3) preparation is the sodium hydroxide solution of 1mol/L, and regulator solution acidity is pH=10, obtains the precursor solution of cadmium;
(5) under agitation, the solution with step (1), (2) and (4) preparation adds in the 250ml three-necked bottle bottle sealing, stirring successively;
(6) with the solution of step (5) preparation reflux 150 minutes under 100 ℃ of temperature condition, whole process is carried out under magnetic agitation, obtains water soluble cadmium telluride quantum point solution.
Figure 1 shows that water soluble cadmium telluride quantum point solution homogenization fluorescence spectrum and the uv-visible absorption spectroscopy figure of present embodiment preparation, among the figure, curve 1 is uv-visible absorption spectroscopy, and curve 2 is fluorescence spectrum.
Embodiment 6
(1) 1m mol oxammonium hydrochloride is dissolved in the 50ml deionized water, regulator solution acidity is pH=11;
(2) with 0.075m mol tellurium dioxide concentration be the NaOH solution dissolving of 1mol/L;
(3) under the room temperature, 1.0m mol Cadmium chloride fine powder is dissolved in the 50ml deionized water, then adds the 2.4mmol thiohydracrylic acid, stir;
(4) dripping concentration in the solution of step (3) preparation is the sodium hydroxide solution of 1mol/L, and regulator solution acidity is pH=11, obtains the precursor solution of cadmium;
(5) under agitation, the solution with step (1), (2) and (4) preparation adds in the 250ml three-necked bottle bottle sealing, stirring successively;
(6) with the solution of step (5) preparation reflux 60 minutes under 100 ℃ of temperature condition, whole process is carried out under magnetic agitation, obtains water soluble cadmium telluride quantum point solution.
Figure 1 shows that water soluble cadmium telluride quantum point solution homogenization fluorescence spectrum and the uv-visible absorption spectroscopy figure of present embodiment preparation, among the figure, curve 1 is uv-visible absorption spectroscopy, and curve 2 is fluorescence spectrum.
Embodiment 7:
(1) 15m mol oxammonium hydrochloride is dissolved in the 50ml deionized water, regulator solution acidity is pH=12;
(2) with 0.35m mol tellurium dioxide concentration be the NaOH solution dissolving of 1mol/L;
(3) under the room temperature, 1.0m mol Cadmium chloride fine powder is dissolved in the 50ml deionized water, then adds the 2.4mmol dimercaptosuccinic acid, stir,
(4) dripping concentration in the solution of step (3) preparation is the sodium hydroxide solution of 1mol/L, and regulator solution acidity is pH=12, obtains the precursor solution of cadmium;
(5) under agitation, the solution with step (1), (2) and (4) preparation adds in the 250ml three-necked bottle bottle sealing, stirring successively;
(6) with the solution of step (5) preparation reflux 90 minutes under 100 ℃ of temperature condition, whole process is carried out under magnetic agitation, obtains water soluble cadmium telluride quantum point solution.
Figure 1 shows that water soluble cadmium telluride quantum point solution homogenization fluorescence spectrum and the uv-visible absorption spectroscopy figure of present embodiment preparation, among the figure, curve 1 is uv-visible absorption spectroscopy, and curve 2 is fluorescence spectrum.
Claims (4)
1. the fast preparation method of a cadmium telluride quantum dot is characterized in that comprising the step of following order:
(1) oxammonium hydrochloride is dissolved in the deionized water, regulator solution acidity is pH=9～13;
(2) with tellurium dioxide concentration be the dissolving of 1mol/LNaOH solution;
(3) under the room temperature, cadmium salt is dissolved in the deionized water, then adds sulfhydryl compound, stir; Wherein, the mol ratio of cadmium salt and sulfhydryl compound is 1:1.2～5.0;
(4) dripping concentration in the solution of step (3) preparation is the sodium hydroxide solution of 1mol/L, and regulator solution acidity is pH=9～13, obtains the precursor solution of cadmium;
(5) under whipped state, the solution that step (1), step (2) and step (4) are prepared adds in the three-necked bottle bottle successively, sealing, stirring; Wherein, the mol ratio of tellurium dioxide and cadmium salt is 1:0.02～0.40, and the mol ratio of tellurium dioxide and oxammonium hydrochloride is 1:7～80;
(6) with the solution of step (5) preparation reflux 10 minutes to 210 minutes in 85-100 ℃ of temperature range, whole process is carried out under magnetic agitation, namely obtains water soluble cadmium telluride quantum point solution.
2. the fast preparation method of cadmium telluride quantum dot according to claim 1 is characterized in that: described cadmium salt is a kind of in Cadmium chloride fine powder, cadmium perchlorate and the cadmium acetate.
3. the fast preparation method of cadmium telluride quantum dot according to claim 1 is characterized in that: described sulfhydryl compound is a kind of in halfcystine, Thiovanic acid, thiohydracrylic acid, dimercaptosuccinic acid, gsh and N-(the 2-sulfydryl propionyl) glycine.
4. the fast preparation method of cadmium telluride quantum dot according to claim 1 is characterized in that: step (1) is identical with the acidity value of regulating described in the step (4).
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN201210590402.2A CN103011095B (en) | 2012-12-29 | 2012-12-29 | Rapid preparation method of cadmium telluride quantum dot |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN201210590402.2A CN103011095B (en) | 2012-12-29 | 2012-12-29 | Rapid preparation method of cadmium telluride quantum dot |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CN103011095A true CN103011095A (en) | 2013-04-03 |
| CN103011095B CN103011095B (en) | 2014-10-29 |
Family
ID=47960280
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CN201210590402.2A Expired - Fee Related CN103011095B (en) | 2012-12-29 | 2012-12-29 | Rapid preparation method of cadmium telluride quantum dot |
Country Status (1)
| Country | Link |
|---|---|
| CN (1) | CN103011095B (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN103539082A (en) * | 2013-10-25 | 2014-01-29 | 湖南科技大学 | Quick green preparation method of cadmium telluride quantum dot |
| CN103539945A (en) * | 2013-09-29 | 2014-01-29 | 南京医科大学 | P-aminophenol imprinted polymer on surface of cadmium telluride quantum dot as well as preparation method and application thereof |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102634342A (en) * | 2012-03-31 | 2012-08-15 | 华东师范大学 | Preparation method of water-soluble CdTe quantum dot |
-
2012
- 2012-12-29 CN CN201210590402.2A patent/CN103011095B/en not_active Expired - Fee Related
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102634342A (en) * | 2012-03-31 | 2012-08-15 | 华东师范大学 | Preparation method of water-soluble CdTe quantum dot |
Non-Patent Citations (2)
| Title |
|---|
| JIYING PEI ETAL: "Synthesis of cysteamine-coated CdTe quantum dots and its application in mercury (II)detection", 《ANALYTICA CHIMICA ACTA》, 1 November 2012 (2012-11-01) * |
| YI LIN WANG ETAL: "An alternative aqueous synthetic route to preparing CdTe quantum dots with tunable photoluminescence", 《CHINESE CHEMICAL LETTERS》, 26 January 2012 (2012-01-26) * |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN103539945A (en) * | 2013-09-29 | 2014-01-29 | 南京医科大学 | P-aminophenol imprinted polymer on surface of cadmium telluride quantum dot as well as preparation method and application thereof |
| CN103539945B (en) * | 2013-09-29 | 2015-12-23 | 南京医科大学 | Cadmium telluride quantum dot surface p-aminophenol imprinted polymer and its preparation method and application |
| CN103539082A (en) * | 2013-10-25 | 2014-01-29 | 湖南科技大学 | Quick green preparation method of cadmium telluride quantum dot |
| CN103539082B (en) * | 2013-10-25 | 2015-06-17 | 湖南科技大学 | Quick green preparation method of cadmium telluride quantum dot |
Also Published As
| Publication number | Publication date |
|---|---|
| CN103011095B (en) | 2014-10-29 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CN108217718A (en) | A kind of ABX3Nanocrystalline synthetic method of perovskite and products thereof and purposes | |
| CN102649583B (en) | Method for synthesizing monoclinic phase nano vanadium dioxide powder | |
| CN101220275A (en) | Hydrothermal production method for water-soluble ZnCdSe quantum dot | |
| CN110551304A (en) | Cesium-lead halogen inorganic perovskite quantum dot/transparent polymer composite film | |
| CN105384189A (en) | Method for preparing caesium-lead halide nanorod and product obtained through method | |
| CN102786037A (en) | CdX quantum dot and preparation method thereof | |
| CN103011095B (en) | Rapid preparation method of cadmium telluride quantum dot | |
| Chen et al. | Room-temperature ionic-liquid-assisted hydrothermal synthesis of Ag-In-Zn-S quantum dots for WLEDs | |
| CN103436256A (en) | Aqueous-phase synthesis method of Cu-doped Zn1-xCdxS quantum dot and core/shell structure thereof | |
| CN103539082B (en) | Quick green preparation method of cadmium telluride quantum dot | |
| CN101948686A (en) | Method for aqueous phase synthesis of manganese-doped zinc selenide adjustable-colour fluorescent quantum dot | |
| Umemoto et al. | Separation of mono-dispersed CH 3 NH 3 PbBr 3 perovskite quantum dots via dissolution of nanocrystals | |
| CN111057542A (en) | Method for preparing CsPbX by room-temperature water emulsion method3Method for perovskite quantum dots | |
| CN100494309C (en) | Synthesis method of core/shell type cadmium telluride/cadmium sulfide water-soluble quantum dot | |
| CN115433575B (en) | CsPbX 3 Preparation method of nanocrystalline material | |
| CN105399128A (en) | Micron spindle-fiber-like structure vaterite calcium carbonate preparation method | |
| CN102191038A (en) | Method for preparing CdTe quantum dots in aqueous phase and at low temperature | |
| CN103979506B (en) | A kind of preparation method of hyperfluorescenceZeng Yongminggaoyingguang cadmium telluride quantum dot | |
| CN103484122A (en) | Method for rapidly preparing thick-wall CdTe/CdS quantum dot with controllable shell thickness | |
| CN102644089B (en) | Preparation method for cadmium sulfide quantum dots | |
| CN113292096B (en) | Preparation method for synthesizing inorganic perovskite nanocrystal by non-injection one-step method | |
| Chandran et al. | Facile, seedless and surfactant-free synthesis of ZnO nanostructures by wet chemical bath method and their characterization | |
| Wang et al. | One-pot synthesis of CdTe quantum dots using tellurium dioxide as a tellurium source in aqueous solution | |
| CN103552999B (en) | Method for preparing quantum dots at low temperature | |
| CN109456763B (en) | Synthesis method of lead-iodine perovskite quantum dots |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| C06 | Publication | ||
| PB01 | Publication | ||
| C10 | Entry into substantive examination | ||
| SE01 | Entry into force of request for substantive examination | ||
| C14 | Grant of patent or utility model | ||
| GR01 | Patent grant | ||
| CF01 | Termination of patent right due to non-payment of annual fee | ||
| CF01 | Termination of patent right due to non-payment of annual fee |
Granted publication date: 20141029 Termination date: 20201229 |