CN111003725B - Low-temperature preparation method of monodisperse lead sulfide nanocluster - Google Patents

Low-temperature preparation method of monodisperse lead sulfide nanocluster Download PDF

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CN111003725B
CN111003725B CN201911325895.5A CN201911325895A CN111003725B CN 111003725 B CN111003725 B CN 111003725B CN 201911325895 A CN201911325895 A CN 201911325895A CN 111003725 B CN111003725 B CN 111003725B
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王向华
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Shanghai Biying Semiconductor Technology Co ltd
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Abstract

The invention discloses a low-temperature preparation method of monodisperse lead sulfide nanoclusters, which comprises three stages of cluster growth, surface lead atom mercaptosilane passivation and sulfur atom halogen substitution passivation. The preparation method can obtain superfine (not more than 1.5nm) monodisperse PbS nanoclusters under the condition of low temperature, is simple, does not need to isolate water and oxygen, and has the advantages of high synthesis yield and suitability for batch production.

Description

Low-temperature preparation method of monodisperse lead sulfide nanocluster
Technical Field
The invention belongs to the field of semiconductor materials, and particularly relates to a preparation method of a nanocluster material with uniform size distribution.
Background
The electronic energy band of the lead sulfide material has an energy gap of 0.41eV, the exciton Bohr radius is 18nm, different electronic energy gaps can be obtained by preparing nano particles with different sizes, and the lead sulfide material has a good photoelectron application prospect. The lattice structure is face-centered cubic, the lattice constant is 0.593nm, atoms are combined by covalent bonds, and the chemical stability is good. The hole effective mass of most II-VI and III-V semiconductors is large and highly confined in momentum space is difficult to achieve. The lead sulfide has electrons and holes with large Bohr radius and high exciton binding energy, and is an important nanometer material for realizing efficient exciton luminescence and nonlinear optical characteristics. The size of the lead sulfide nano-particles is further reduced by utilizing the excellent electronic energy state structure of the lead sulfide nano-particles, and better optical characteristics are expected to be obtained.
The prior art can obtain the lead sulfide quantum dot material with the particle size of 3nm and suitable for infrared band application. If the particle size of the nano material is further reduced to be below 2.5nm, the energy gap can be further increased, so that the relevant application is extended to a visible light wave band. Since the properties of nanomaterials are closely related to size, controlling size distribution is a common requirement for the preparation of nanomaterials. In particular, size control becomes more difficult to produce such nanocluster materials below 2.5nm in size.
There are two main technical routes for the laboratory preparation of lead sulfide quantum dots, one is based on the reaction of lead oleate and bis (methoxysilicon sulfide) in octadecene, and the other is based on the reaction of lead chloride with elemental sulfur in oleylamine. Both of these routes require the use of the Schicker technique (Schlenk line) to complete the reaction under the conditions of dehumidification and oxygen removal, and the reaction temperature is typically greater than 100 ℃. The lead sulfide nano material with smaller particles has better stability and higher fluorescence quantum efficiency in air; the lead-sulfur molar ratio Pb to S in the nano particles is higher and is close to 2:1 [1] . Lead sulfide quantum dots with the size as low as 3nm can be obtained by adopting TOP ligandHowever, the synthesis yield is low and the size distribution is remarkably broadened [2] . The preparation of small-sized lead sulfide quantum dots or nanoclusters with uniform size distribution usually requires the adoption of a lead-sulfur molar ratio greater than 8, which is much greater than the lead-sulfur molar ratio in the lead sulfide nanoparticle product, thereby limiting the improvement of the synthesis yield of the nano material. The mass preparation technology of the superfine nanocluster with the particle size smaller than 2.5nm is a key technology for opening the application market of the lead sulfide nanometer material.
Reference documents:
[1]Tang J,Brzozowski L,Barkhouse DAR,Wang XH,Debnath R,Wolowiec R,et al.Quantum Dot Photovoltaics in the Extreme Quantum Confinement Regime:The Surface-Chemical Origins of Exceptional Air-and Light-Stability.Acs Nano.2010；4(2):869-78.
[2]Moreels I,Justo Y,De Geyter B,Haustraete K,Martins JC,Hens Z.Size-Tunable,Bright,and Stable PbS Quantum Dots:A Surface Chemistry Study.Acs Nano.2011；5(3):2004-12.
disclosure of Invention
Based on the defects of the prior art, the invention discloses a low-temperature preparation method of monodisperse lead sulfide nanoclusters, so that the monodisperse lead sulfide nanoclusters with the particle size smaller than 2.5nm can be obtained at low temperature, the application of lead sulfide nanomaterials in visible light and shorter optical bands is extended, and the optical performance of the materials is improved.
In order to realize the purpose of the invention, the following technical scheme is adopted:
the low-temperature preparation method of the monodisperse lead sulfide nanocluster is characterized by comprising three stages of cluster growth, surface lead atom mercaptosilane passivation and sulfur atom halogen substitution passivation;
the cluster growth is characterized in that thiourea and lead acetate are used as raw materials and react in a polar solvent to obtain a dispersion liquid of the PbS nanocluster without a passivation layer, and the dispersion liquid is marked as a dispersion liquid of the UP-PbS nanocluster;
the step of passivating the mercaptosilane of the lead atoms on the surface is to add 3-mercaptopropyltrimethoxysilane (MPTMS) into the dispersion liquid of the UP-PbS nanocluster so that the lead atoms on the surface of the nanocluster are passivated, and the further growth of the nanocluster is stopped to obtain the dispersion liquid of the MP-PbS nanocluster;
and the halogen substitution passivation of the sulfur atoms is to quickly inject a dilute hydrochloric acid solution into the dispersion liquid of the MP-PbS nanocluster to substitute and passivate the sulfur atoms on the surface of the nanocluster by Cl atoms, so that the monodisperse lead sulfide nanocluster with uniform size is obtained and is marked as the HP-PbS nanocluster.
In the above preparation process: the size of the UP-PbS nano cluster obtained by cluster growth is not more than 2nm, and the UP-PbS nano cluster has the characteristics of absorption and fluorescence in a visible light wave band; stopping the growth of the UP-PbS nanocluster by MPTMS passivation to form a stable MP-PbS nanocluster; after halogen substitution passivation, the absorption spectrum of the product has a strong exciton absorption peak, the cluster size is obviously reduced and more uniform, and the superfine monodisperse HP-PbS nanocluster is formed, has sharp exciton absorption characteristics in near ultraviolet and visible light bands, and has fluorescence characteristics in visible light bands.
Further: the polar solvent is a mixed solvent formed by mixing ethylene glycol methyl ether serving as a main solvent and acetylacetone and acetic acid serving as auxiliary solvents; in the polar solvent, the volume percentage of acetylacetone is 20-35%, and the volume percentage of acetic acid is 1.5-8%. The addition of acetylacetone and acetic acid can control the size distribution and average particle size of the lead sulfide clusters. Acetic acid also regulates the reaction rate so that the reaction can be completed in a lower temperature range and in a shorter time.
Further, the specific steps of the cluster growth stage are as follows: dissolving thiourea in a polar solvent to obtain a thiourea solution with the concentration of 0.4 mol/L; dissolving lead acetate trihydrate in a polar solvent to obtain a lead acetate solution with the concentration of 0.125 mol/L; respectively heating the thiourea solution and the lead acetate solution to a temperature T 1 Then, uniformly mixing to obtain a mixed solution; subjecting the mixture to a temperature T 1 Stirring and reacting for t 1 (ii) a After the reaction is finished, the dispersion liquid of the UP-PbS nanoclusters is obtained.
Furthermore, the molar ratio of the thiourea to the lead acetate raw material in the mixed solution is 0.2-2: 1, and preferably 0.5: 1.
Further, T 1 ＝25℃～80℃、t 1 10min to 3 d. At different T 1 Reaction time t required for lower cluster growth 1 In contrast, the reaction time t can be determined by judging whether the reaction is completed or not as follows 1 : and sampling and detecting the ultraviolet visible absorption spectrum or the PL spectrum, and completing the reaction when the absorption edge of the absorption spectrum or the emission peak of the PL spectrum does not generate red shift any more. The reaction is completed to increase the yield of the product.
Further, the step of passivating the surface lead atoms with mercaptosilane comprises the following steps: diluting MPTMS with ethylene glycol monomethyl ether to obtain a dilute solution of MPTMS with the concentration of 0.4-0.54 mol/L, and then preheating to the temperature T 2 (ii) a Cooling the dispersion of UP-PbS nanoclusters obtained in the cluster growth stage to T 2 Then injecting the MPTMS diluent, stirring and reacting for t 2 (ii) a After the reaction is finished, the MP-PbS nanocluster dispersion liquid is obtained.
Furthermore, the molar ratio of MPTMS to lead acetate is 0.2-1: 1.
Further, T 2 ＝25℃～50℃、t 2 ＝1h，T 2 Should not be greater than T 1 。
Further, the halogen substitution passivation stage of the sulfur atom comprises the following specific steps: diluting hydrochloric acid (commercially available hydrochloric acid with the mass concentration of 37%) with ethanol or ethylene glycol monomethyl ether to the mass concentration of 3-4%; and (3) cooling the dispersion liquid of the MP-PbS nanoclusters obtained in the silane passivation stage of the lead atoms on the surface to room temperature, then quickly injecting diluted hydrochloric acid, uniformly stirring, and then standing and curing for 10 hours to obtain the dispersion liquid of the HP-PbS nanoclusters.
Furthermore, the molar ratio of the injected hydrochloric acid to the thiourea raw material is 0.3-3: 1.
Compared with the prior art, the invention has the beneficial effects that:
1. the preparation method can obtain the ultrafine (not more than 1.5nm) and monodisperse PbS nanoclusters under the low-temperature condition, is simple, does not need to isolate water and oxygen, and has the advantages of high synthesis yield and suitability for batch production.
2. The method of the invention comprises the following steps: the raw materials of lead acetate and thiourea are easy to dissolve in polar solvents, have large process window and are suitable for mass production. The ethylene glycol methyl ether is used as a main solvent, and the viscosity is low, so that the precursor material can complete the reaction in a low-temperature high-concentration solution. The reaction between lead acetate and thiourea is very slow at room temperature, and the appropriate reaction speed can be obtained by controlling the solvent components or the reaction temperature, and the nucleation growth process can be completed within minutes to hours. Lower reaction temperatures and relatively slow reaction rates are suitable for scale-up into a mass production process.
3. The mol ratio of sulfur atoms to lead atoms in the small-size lead sulfide cluster product is close to 0.5, the small-size lead sulfide cluster product is consistent with the preferable precursor ratio in the invention, and the cluster with the size less than 2.5nm is formed by combining with diluted hydrochloric acid treatment, so that the synthesis yield is high. Such small-sized nanoparticles have uniform size distribution, excellent chemical stability, enhanced exciton absorption and light emission characteristics.
4. The target product lead sulfide cluster formed by the invention is dispersed in ethylene glycol monomethyl ether, can be mutually soluble with various solvents with different polarities, is suitable for forming films on different substrates, and the preparation of related devices is easier to realize by a solution method process. The lead sulfide clusters prepared by the method have monodispersity or discontinuous particle size distribution, can be separated by one step through a centrifugal separation technology, and are simple in purification step.
Drawings
FIG. 1 shows the results of example 1, at room temperature (T) 1 25 ℃) lead acetate reacts with thiourea for different times t 1 After (t) 1 ＝1h，t 1 3d) uv-vis absorption spectrum of the obtained UP-PbS nanoclusters.
FIG. 2 shows the results of example 1, at room temperature (T) 1 25 ℃) lead acetate reacts with thiourea for different times t 1 After (t) 1 ＝1h，t 1 3d) fluorescence spectrum of the obtained UP-PbS nanoclusters.
FIG. 3 shows the results of example 2 at T 1 ＝T 2 Condition of 50 ℃With different reaction times t 1 Absorption spectrum of the obtained MP-PbS nanocluster dispersion.
FIG. 4 shows the results of example 2 at T 1 ＝T 2 With different reaction times t at 50 ℃ 1 Fluorescence spectrum of the obtained MP-PbS nanocluster dispersion.
FIG. 5 shows the results of example 2 at T 1 ＝50℃、T 2 At 25 ℃ with different reaction times t 1 Absorption spectrum of the obtained MP-PbS nanocluster dispersion.
FIG. 6 shows the results of example 2 at T 1 ＝50℃、T 2 At 25 ℃ with different reaction times t 1 Fluorescence spectrum of the obtained dispersion of MP-PbS nanoclusters.
FIG. 7 shows the results of example 2 at T 1 ＝50℃、T 2 Reaction at 25 ℃ t 1 TEM photographs of the resulting MP-PbS nanoclusters were taken for 10 minutes.
FIG. 8 is a graph of the UV-VIS absorption spectra of the MP-PbS nanoclusters and the HP-PbS nanoclusters obtained in example 3 at different molar ratios of thiourea to lead acetate.
FIG. 9 is a graph showing fluorescence spectra of MP-PbS nanoclusters and HP-PbS nanoclusters obtained in example 3 at different molar ratios of thiourea to lead acetate.
Fig. 10 is an absorption spectrum of the HP-PbS nanocluster dispersion obtained in example 3 diluted 30 times, 20 times, and 10 times in the molar ratio of thiourea to the lead acetate raw material of 0.67:1, 0.50:1, and 0.2:1, respectively.
FIG. 11 is a fluorescence spectrum of HP-PbS nanocluster dispersions obtained in example 3 diluted 30 times, 20 times, and 10 times in the molar ratios of thiourea to lead acetate raw material of 0.67:1, 0.50:1, and 0.2:1, respectively.
Fig. 12 is a graph showing fluorescence spectra of MP-PbS nanoclusters prepared in example 4 and HP-PbS nanoclusters prepared by injecting different amounts of diluted hydrochloric acid before dilution.
Fig. 13 is a graph showing absorption spectra of HP-PbS nanoclusters prepared in example 4 by injecting dilute hydrochloric acid at a molar ratio of 6.65:1 before and after dilution.
FIG. 14 is a graph showing fluorescence spectra of HP-PbS nanoclusters prepared in example 4 by injecting diluted hydrochloric acid at a molar ratio of 6.65:1 before and after dilution.
Detailed Description
The following examples are given for the detailed implementation and specific operation of the present invention, but the scope of the present invention is not limited to the following examples.
Examples 1,
A certain reaction time t is needed in the cluster growth stage 1 To complete the process of nucleation and growth, and t 1 And T 1 And (4) correlating. The synthesis yield of the UP-PbS nanocluster is related to whether the reaction is completed or not, and whether the reaction is completed or not can be judged by an ultraviolet visible absorption spectrum or a PL spectrum: and sampling and detecting the ultraviolet visible absorption spectrum or the PL spectrum, and completing the reaction when the absorption edge of the absorption spectrum or the emission peak of the PL spectrum does not generate red shift any more.
Mixing ethylene glycol methyl ether, acetylacetone and acetic acid according to the volume percentage of 64 percent to 29 percent to 7 percent to obtain a standby polar solvent; dissolving thiourea in a polar solvent to obtain a thiourea solution with the concentration of 0.4 mol/L; dissolving lead acetate trihydrate in a polar solvent to obtain a lead acetate solution with the concentration of 0.125 mol/L; at room temperature (T) 1 At 25 ℃, uniformly mixing 1.5mL of thiourea solution and 2.4mL of lead acetate solution to obtain a mixed solution (the molar ratio of thiourea to the lead acetate raw material is 2: 1); mixing the mixed solution at T 1 Reaction under continuous stirring at 25 ℃ for t 1 (ii) a After the reaction is finished, the dispersion liquid of the UP-PbS nanoclusters is obtained.
At room temperature (T) 1 25 ℃), lead acetate and thiourea are subjected to slow chemical reaction in the solution environment with ethylene glycol monomethyl ether as a main solvent. Detecting the UV-visible absorption spectrum of the reaction solution at different time points to determine the reaction process, wherein t is shown in FIG. 1 1 1h and t 1 Uv-vis absorption spectrum of the obtained UP-PbS nanoclusters at 3 d. Corresponds to (t) 1 1h) and (t) 1 PL spectrum of 3d) is shown in fig. 2, which is seen withWith increasing time, the emission peak wavelength of the fluorescence spectrum red-shifts from 405nm to 495nm, and a significant red shift occurs, representing the average size of the nanoclusters with t 1 Increasing; while the full width at half maximum (FWHM) also increases significantly, with a trend towards broadening of the size distribution from 61nm to 110 nm.
The results show that the reaction speed is slow under the condition of room temperature, and the reaction needs several hours or even tens of hours to be finished. The reaction speed in the first stage can be increased by increasing the temperature of the reaction liquid.
Example 2
The first-stage reaction rate may be increased by raising the temperature of the reaction solution, so that the growth of the nanoclusters is completed in several tens of minutes or less. The specific judgment method comprises the following steps: the time required for the reaction to be basically completed is judged according to the difference of absorption values of lead sulfide nanoclusters and reaction precursors of the lead acetate, thiourea and ligand MPTMS near 400nm and the corresponding fluorescence spectrum characteristic peak, and relevant experiments are provided below.
Mixing ethylene glycol methyl ether, acetylacetone and acetic acid according to the volume percentage of 68.2 percent to 30.3 percent to 1.5 percent to obtain a standby polar solvent; dissolving thiourea in a polar solvent to obtain a thiourea solution with the concentration of 0.4 mol/L; dissolving lead acetate trihydrate in a polar solvent to obtain a lead acetate solution with the concentration of 0.125 mol/L;
respectively heating the thiourea solution and the lead acetate solution to a temperature T 1 Mixing 0.15mL of thiourea solution and 2.4mL of lead acetate solution uniformly at 50 ℃ to obtain a mixed solution (the molar ratio of thiourea to the lead acetate raw material is 0.2: 1); mixing the mixed solution at T 1 Reacting under the condition of continuously stirring at 50 ℃ for t 1 (ii) a After the reaction is finished, the dispersion liquid of the UP-PbS nanoclusters is obtained.
MPTMS is diluted to 0.54mol/L by ethylene glycol monomethyl ether and then preheated to the temperature T 2 The temperature is 50 ℃; cooling the dispersion of UP-PbS nanoclusters to T 2 Adding diluted MPTMS solution at 50 deg.C, stirring, and reacting for t 2 1 h; after the reaction is finished, the MP-PbS nanocluster dispersion liquid is obtained. The molar ratio of MPTMS to lead acetate was 0.36: 1.
The absorption and fluorescence spectra of the precursor solutions (thiourea solution, lead acetate solution and MPTMS dilution) were measured to find that: thiourea has a certain absorption near 400nm, while MPTMS has a weak absorption near 400nm, which can be ignored. In the aspect of fluorescence characteristics, MPTMS has obvious fluorescence peaks near 410nm and 436nm respectively.
FIG. 3 shows the use of different reaction times t 1 Absorption spectrum of the obtained MP-PbS nanocluster dispersion, wherein t 1 -10min is a control obtained by the following method: at T 1 Injecting the diluted MPTMS solution into a lead acetate solution at 50 ℃, adding a thiourea solution after 10min, uniformly mixing (the molar ratio of thiourea to the lead acetate raw material is 0.2:1), and continuously reacting at 50 ℃ for 1 h; after the reaction is finished, the MP-PbS nanocluster dispersion liquid is obtained.
As can be seen from fig. 3: adding the reaction solution of MPTMS 10min in advance according to the light absorption of 400nm (t) 1 -10min) absorption was minimal, indicating that the addition of MPTMS ligand ahead of time inhibited the formation of lead sulfide nanoclusters. The invention thus employs the addition of the MPTMS ligand after the reaction has been completed for a period of time in the first stage. The absorption of the dispersion at 400nm increased significantly with increasing reaction time, and figure 3 visually reflects the first stage of PbS nanocluster growth, the process of nucleation within minutes, but no steep absorption edge is seen.
FIG. 4 shows the use of different reaction times t 1 The fluorescence spectrum of the resulting dispersion of MP-PbS nanoclusters can be seen as: all samples have obvious MPTMS characteristic emission peak along with t 1 From 5 seconds to 5 minutes, the intensity of the MPTMS characteristic emission peak decreases in turn, and the fluorescence intensity around 460nm has a tendency to be significantly enhanced relative to the characteristic peaks at 410nm and 436 nm.
It is clear that the addition of the MPTMS ligand in advance to the lead acetate solution is detrimental to the formation of lead sulfide clusters. Thus, in another set of experiments, the normal mixing sequence was used, using t 1 5 seconds, 2 minutes, 5 minutes and 10 minutes. While simultaneously adjusting the reaction temperature T of the second stage 2 The temperature was lowered to room temperature of 25 ℃.
FIG. 5 is a graph at T 1 ＝50℃、T 2 At 25 ℃ with different reaction times t 1 The absorption spectrum of the resulting MP-PbS nanocluster dispersion can be seen as: the four samples all show a weak absorption shoulder and the reaction t 1 The absorption edge appeared to be significantly red shifted relative to the other three samples for the 10min sample, indicating that thiourea and lead acetate were nucleating within 5 minutes after mixing and only started to grow significantly thereafter. The fluorescence spectrum shown in FIG. 6 shows the same trend as that of FIG. 4, but the MPTMS characteristic peak is plotted as t 1 Exhibit a more pronounced attenuation, in particular t 1 The MPTMS characteristic peak of the 10min sample completely disappeared, confirming that the reaction was substantially complete, but no fluorescence peak corresponding to the lead sulfide nanoclusters was observed because the concentration of the lead sulfide nanoclusters was too high. Shown in FIG. 7 is a graph corresponding to t 1 TEM pictures of the sample at 10min, PbS particles with a size of less than 2nm can be seen.
Under the condition of relatively sufficient reaction, the concentration of the nanocluster dispersion liquid prepared by adopting the concentration of the raw materials is very high, and the optical absorption in a near ultraviolet band is more than 5. Since the concentration of the nanocluster is very high, the fluorescence peak disappears due to strong light absorption, and the fluorescence characteristics can be measured after diluting the reaction solution several times, which will be described later with reference to examples. This example demonstrates that the reaction time t required for completing the reaction under a given reaction temperature condition can be determined by observing the movement of the absorption edge in the absorption spectrum or the change rule of the peak shape of the fluorescence spectrum 1 (ii) a By increasing the first stage reaction temperature, t can be greatly reduced 1 About 10 min.
Example 3
Mixing ethylene glycol methyl ether, acetylacetone and acetic acid according to the volume percentage of 75 percent to 20 percent to 5 percent to obtain a standby polar solvent; dissolving thiourea in a polar solvent to obtain a thiourea solution with the concentration of 0.4 mol/L; dissolving lead acetate trihydrate in a polar solvent to obtain a lead acetate solution with the concentration of 0.125 mol/L; heating 0.5mL, 0.375mL and 0.15mL of thiourea solution and lead acetate solution to temperature T respectively 1 The appropriate amount of thiourea solution was then mixed with 2.4mL of lead acetate solution at 50 deg.CHomogenizing to obtain mixed solution (molar ratio of thiourea to lead acetate is 0.67:1, 0.50:1, and 0.2:1, respectively); mixing the mixed solution at T 1 Reacting under the condition of continuously stirring at 50 ℃ for t 1 10 minutes; after the reaction is finished, the dispersion liquid of the UP-PbS nanoclusters is obtained.
MPTMS is diluted to the concentration of 0.4mol/L by ethylene glycol monomethyl ether and then preheated to the temperature T 2 The temperature is higher than 40 ℃; cooling the dispersion of UP-PbS nanoclusters to T 2 Adding MPTMS diluted solution, stirring and reacting at 40 deg.C for t 2 1 h; after the reaction is finished, the MP-PbS nanocluster dispersion liquid is obtained. The molar ratio of MPTMS to lead acetate was measured with the ratio of the total moles of thiourea and MPTMS to the moles of lead acetate constant at 1.07, keeping the total S/Pb molar ratio constant.
Diluting hydrochloric acid with ethylene glycol monomethyl ether to a mass concentration of 3.7%; and (3) cooling the dispersion liquid of the MP-PbS nanoclusters to room temperature, then quickly injecting diluted hydrochloric acid, simultaneously supplementing glycol methyl ether solvent until the molar concentration of lead in each system is 0.0638mol/L, uniformly stirring, and then standing and curing for 10 hours to obtain the dispersion liquid of the HP-PbS nanoclusters. The ratio of the molar amount of hydrochloric acid injected to the molar amount of thiourea feed was 3: 1.
The obtained MP-PbS nanocluster dispersion and HP-PbS nanocluster dispersion were diluted 10 times with ethylene glycol monomethyl ether, respectively, and then uv-vis absorption spectrum and PL spectrum were measured, and the results are shown in fig. 8 and 9. The results show that: the absorption at 400nm of all MP-PbS nanoclusters was less than 0.6, whereas the absorption of HP-PbS nanoclusters increased significantly and the exciton absorption shoulder was more pronounced with increasing thiourea ratio. According to PL of the MP-PbS nanocluster dispersion, when the molar ratio of thiourea to lead acetate is 0.5, the intensity of the MPTMS characteristic peak is strong, indicating that the reaction in the first stage is insufficient. The PL of the HP-PbS nanoclusters in fig. 9 is weak due to the too high concentration.
The uv-vis absorption spectrum and PL spectrum were measured after diluting the dispersion of HP-PbS nanoclusters obtained under the conditions of molar ratios of thiourea to lead acetate raw material of 0.67:1, 0.50:1, and 0.2:1 by 30 times, 20 times, and 10 times, respectively, as shown in fig. 10 and 10As shown in fig. 11. The absorption curves of the 30-fold diluted HP-PbS nanoclusters (0.67:1) in fig. 10 and the 20-fold diluted HP-PbS nanoclusters (0.5:1) almost coincide, but the PL intensity of the latter is lower in fig. 11, because the reaction in the first stage is not sufficient. Therefore, by detecting PL of the corresponding MP-PbS nanocluster, a sufficiently long reaction time t is determined according to the change of MPTMS characteristic peak 1 It is necessary to improve the fluorescence characteristics of the HP-PbS nanoclusters. The absorption spectrum of the HP-PbS nanoclusters in this example exhibited an absorption shoulder around 360nm, and the 400nm absorption was greatly enhanced relative to the MP-PbS nanoclusters, indicating that dilute hydrochloric acid treatment promoted the formation of lead sulfide cluster products, i.e., increased yield, and made the nanoclusters tend to be uniform in size.
Example 4
Mixing 68.2 percent by volume of ethylene glycol methyl ether, 30.3 percent by volume of acetylacetone and 1.5 percent by volume of acetic acid to obtain a spare polar solvent; dissolving thiourea in a polar solvent to obtain a thiourea solution with the concentration of 0.4 mol/L; dissolving lead acetate trihydrate in a polar solvent to obtain a lead acetate solution with the concentration of 0.125 mol/L; respectively heating the thiourea solution and the lead acetate solution to a temperature T 1 Uniformly mixing a proper amount of thiourea solution with 2.4mL of lead acetate solution at the temperature of 80 ℃ to obtain a mixed solution (the molar ratio of thiourea to the lead acetate raw material is 0.50:1 respectively); mixing the mixed solution at T 1 Reacting at 80 ℃ under continuous stirring for t 1 10 minutes; after the reaction, a dispersion of the UP-PbS nanoclusters was obtained.
Diluting MPTMS with ethylene glycol monomethyl ether to a concentration of 0.54M, and preheating to a temperature T 2 The temperature is higher than 40 ℃; cooling the dispersion of UP-PbS nanoclusters to T 2 Adding MPTMS diluted solution, stirring and reacting at 40 deg.C for t 2 1 h; after the reaction is finished, the MP-PbS nanocluster dispersion liquid is obtained. The molar ratio of MPTMS to lead acetate was 0.72: 1. The fluorescence spectrum of the obtained MP-PbS nanocluster is shown in FIG. 12, the PL peak of the product is around 490nm, and no MPTMS characteristic peak exists, which indicates that the reaction in the first stage is relatively sufficient.
Diluting hydrochloric acid with ethanol to a mass concentration of 3.7%; and (3) cooling the dispersion liquid of the MP-PbS nanoclusters to room temperature, then quickly injecting diluted hydrochloric acid, simultaneously supplementing ethylene glycol monomethyl ether until the molar concentration of lead in each system is 0.0638mol/L, uniformly stirring, and then standing and curing for 10 hours to obtain the dispersion liquid of the HP-PbS nanoclusters. The molar ratio of the injected hydrochloric acid to the thiourea raw material is 1.33:1 and 6.65:1 respectively.
The fluorescence spectrum of the obtained HP-PbS nanoclusters is shown in FIG. 12, and it can be seen that its PL peak is around 455 nm. Of these, HP-PbS injected with diluted hydrochloric acid at a 6.65:1 molar ratio required 10-fold dilution due to too high concentration to detect distinct exciton absorption peaks (FIG. 13) and PL peaks (FIG. 14). The blue shift of PL of HP-PbS nanoclusters relative to PL of MP-PbS nanoclusters is about 35nm, which is believed to be due to the substitution of S atoms on the surface of the lead sulfide clusters by chloride ions. Under the condition of a given polar solvent proportion, the molar fraction of the ligand is selected to be proper, so that the Tyndall phenomenon caused by the agglomeration of the MP-PbS nanoclusters can be avoided. In addition, through the hydrochloric acid treatment of the third stage, clear nanocluster dispersion liquid is obtained more easily, and the absorption of the corresponding absorption spectrum in a visible light band with the wavelength of more than 500nm is close to zero. As shown in fig. 13, it was also observed that the absorption of the reaction solution in the near ultraviolet band was so sharply increased that a band edge structure of the absorption spectrum could not be observed. The absorption spectrum of the HP-PbS nanocluster dispersion diluted 10 times can show a more pronounced exciton absorption shoulder with a peak around 360nm, from which it is assumed that the nanocluster has a particle size of about 1nm and a volume equivalent to 3 unit cells. The diluted HP-PbS nanocluster dispersion liquid has a distinct fluorescence peak in the blue light band around 455nm (fig. 14). Further dilution of the HP-PbS solution tends to increase the fluorescence intensity.
The above examples are summarized:
example 1 and FIGS. 1-2 illustrate the case where T 1 At room temperature at 25 ℃, the UP-PbS stage is a slow reaction process in which clusters increase in size resulting in a red shift of the spectrum. To increase the reaction rate, the subsequent examples increase the reaction temperature T 1 . Examples 2 and 3 use T 1 ＝50℃。
FIGS. 3-4 of example 2 illustrate that the addition of the MPTMS ligand should be doneAfter the thiourea is mixed with the lead acetate, the first stage reaction time t is passed 1 Then, the ligand reaction solution was injected. FIGS. 5 to 7 in example 2 illustrate that in the initial stage of the reaction, for example, t in this example 1 <5 minutes, is a UP-PbS nucleation stage, the cluster size does not grow obviously, and after a sufficiently long reaction time, namely t 1 The UP-PbS and corresponding MP-PbS clusters increase significantly in size for 10 minutes, primarily characterized by a red-shift in absorption edge.
In example 3, T was used 1 ＝50℃、t 1 10min, T 2 Keeping the total S/Pb molar ratio constant at 40 ℃, i.e. the ratio of the total number of moles of thiourea and MPTMS relative to the number of moles of lead acetate, 1.07, the strength of the MPTMS characteristic peak is stronger when the molar ratio of thiourea to lead acetate is 0.5, according to the PL of the MP-PbS dispersion, resulting in a weaker PL of the corresponding HP-PbS. The absorption spectrum of HP-PbS in example 3 shows an absorption shoulder around 360nm, and the 400nm absorption is greatly enhanced relative to MP-PbS, indicating that dilute hydrochloric acid treatment increases the yield of the lead sulfide cluster prepared and makes the size of the nanocluster tend to be uniform.
Example 4 further increase in temperature T 1 ＝80℃、t 1 10min, T 2 The amount of dilute hydrochloric acid was varied maintaining an overall S/Pb molar ratio of 1.22 at 40 ℃ and a molar ratio of thiourea to lead acetate of 0.5. The PL of MP-PbS indicates that the first stage reaction is relatively complete and that the PL of the corresponding HP-PbS is relatively strong. In addition, when the injection amount of the dilute hydrochloric acid is increased, the exciton absorption peaks of 400nm absorption and 360nm absorption are enhanced, and the size uniformity of the cluster is improved. However, the addition amount of hydrochloric acid is too high, so that a large amount of lead chloride precipitate is formed, and the yield of the lead sulfide nanocluster is reduced.

Claims (8)

1. The low-temperature preparation method of the monodisperse lead sulfide nanocluster is characterized by comprising three stages of cluster growth, surface lead atom mercaptosilane passivation and sulfur atom halogen substitution passivation;
the cluster growth is carried out by taking thiourea and lead acetate as raw materials, and reacting in a polar solvent to obtain a dispersion liquid of the PbS nanocluster without a passivation layer, which is marked as a dispersion liquid of the UP-PbS nanocluster;
the step of passivating the mercaptosilane of the lead atoms on the surface is to add 3-mercaptopropyltrimethoxysilane (MPTMS) into the dispersion liquid of the UP-PbS nanocluster so that the lead atoms on the surface of the nanocluster are passivated, and the further growth of the nanocluster is stopped to obtain the dispersion liquid of the MP-PbS nanocluster;
the halogen substitution passivation of the sulfur atom comprises the following specific steps: diluting hydrochloric acid with ethanol or ethylene glycol monomethyl ether to a mass concentration of 3-4%; cooling the dispersion liquid of the MP-PbS nanocluster obtained in the silane passivation stage of the surface lead atoms to room temperature, quickly injecting diluted hydrochloric acid, wherein the molar ratio of the injected hydrochloric acid to the thiourea raw material is 0.3-3: 1, uniformly stirring, standing and curing for 10 hours to enable sulfur atoms on the surface of the nanocluster to be replaced and passivated by Cl atoms, and obtaining the monodisperse lead sulfide nanocluster with uniform size, which is marked as HP-PbS nanocluster, wherein the particle size of the lead sulfide nanocluster is smaller than 2.5 nm.
2. The method of claim 1, wherein: the polar solvent is a mixed solvent formed by mixing ethylene glycol methyl ether serving as a main solvent and acetylacetone and acetic acid serving as auxiliary solvents; in the polar solvent, the volume percentage of acetylacetone is 20-35%, and the volume percentage of acetic acid is 1.5-8%.
3. The preparation method according to claim 2, wherein the specific steps of the cluster growth stage are as follows:
dissolving thiourea in a polar solvent to obtain a thiourea solution with the concentration of 0.4 mol/L; dissolving lead acetate trihydrate in a polar solvent to obtain a lead acetate solution with the concentration of 0.125 mol/L;
respectively heating the thiourea solution and the lead acetate solution to a temperature T 1 Then, uniformly mixing to obtain a mixed solution; subjecting the mixture to a temperature T 1 Stirring and reacting for t 1 (ii) a After the reaction is finished, the dispersion liquid of the UP-PbS nanoclusters is obtained.
4. The production method according to claim 1 or 3, characterized in that: the molar ratio of the thiourea to the lead acetate raw material is 0.2-2: 1.
5. The production method according to claim 3, characterized in that: t is 1 ＝25℃～80℃、t 1 ＝10min～3d。
6. The method of claim 1, wherein: the specific steps of the surface lead atom mercaptosilane passivation stage are as follows:
diluting MPTMS with ethylene glycol monomethyl ether, and preheating to temperature T 2 ；
Cooling the dispersion of UP-PbS nanoclusters obtained in the cluster growth stage to T 2 Then injecting the MPTMS diluent, stirring and reacting for t 2 (ii) a After the reaction is finished, the MP-PbS nanocluster dispersion liquid is obtained.
7. The production method according to claim 1 or 6, characterized in that: the molar ratio of MPTMS to lead acetate is 0.2-1: 1.
8. The method of claim 6, wherein: t is 2 ＝25℃～50℃、t 2 ＝1h。
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