30 Copyright © 2018 -. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International License (http://creativecommons.org/licenses/by-nc/4.0/), permitting all non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Characterization and Application of Nanomaterials（2018） Original Research Article Solvent Thermal Method to control Lead Sulfi de Nano/Micron Crystal and Its Ni/PbS Composites Yajia Ding,Bing Bai,Jianan Peng Key Laboratory of Nanometer Materials, University of Architecture and Technology, Fujian, China ABSTRACT Lead sulfide (PbS) is an important IV-VI semiconductor material with narrow bandwidth and wide wave width, which attracts people's attention. Nano-level PbS has many novel optoelectronic properties and has a wide range of applications in the fi eld of optoelectronics, such as infrared optoelectronic devices, photovoltaic devices, light-emitting devices and display devices. In this paper, Pbs is produced by solvent thermal method by using lead acetate as lead source, sulfur power as sulfur source, ethylene glycol as solvent, and acetic acid to provide acidic environment. The reaction acidity, type of lead source, amount of sulfur source and other aspects will be explored. The products obtained under diff erent conditions were characterized by X-ray diff raction (XRD), optical microscopy and scanning electron microscopy (SEM). The results showed that PbS produced at 140°C for 24 hours, using 14mL ethylene glycol and 1.2mL acetic acid has the best morphology. It has a non-planar six-arm symmetrical structure. Finally, we prepare the lead sulfi de composite Ni/PbS, and characterized it. KEYWORDS: PbS; solvent heat; nanomaterials; composites 1. Introduction 1.1. Nanomaterials 1.1.1 Introduction to Nanomaterials Nano-materials[1], also known as ultra-fi ne particles, are composed of nanoparticles. Nanoparticles, also known as ultrafi ne particles, generally refer to particles in the range of 1 to 100 nm which are in the transitional region between atomic cluster and the macroscopic object boundary. Such a system is neither typical in the general point of view of microcosmic and macroscopic system. This system is a typical mesoscopic system with surface eff ect, small size eff ect and macroscopic quantum tunneling eff ect. When subdividing macroscopic objects into ultrafi ne particles (nanoscale), it will show many properties namely optical, thermal, electrical, magnetic, mechanical and chemical properties, and has signifi cant diff erences when compared to its large solid phases. Nanoparticles are composed of a limited number of atoms or molecules, able to maintain the chemical properties of the original material and maintain in a metastable atomic group or molecular group. When the linearity of the material decreases, the relative proportion of the number of atoms on the surface increases, so that the surface energy of the single atom can increase rapidly. To the nano-scale, the changes in this form refl ect on the material structure and performance, and will show unique eff ects which can be divided into the following basic characteristics [3]. (1) Small size eff ect [4, 5] When the volume of a substance is reduced to a size equal to or smaller than the size of the physical characteristic of light wavelength, Deborah wavelength, coherence length or transmittance depth of superconductivity, the periodic boundary conditions of the crystal will be destroyed which lead to a great change in magnetism, internal pressure, light absorption, thermal resistance, chemical activity, catalytic activity, melting point, etc. when compared to ordinary crystal. This is the volume eff ect of nano-materials. When the crystal is in the nano-scale category, the average free path of free electrons in the metal will be reduced resulting in a decrease in conductivity which may cause the original material to convert from metal conductor to an insulator. The stress-strain of a material in nano-scale will change from Hall-Petch eff ect to anti-Hall-Petch eff ect. For example, under normal conditions the melting point of gold is 1337 K, Yajia Ding, et al 31 while the melting point of 2 nm gold particles is 600 K, and the melting point of nano silver can be reduced from 1173 K to 373 K. (2) Surface eff ect The surface eff ect of nano-materials is the abrupt increase on ratio of the number of atoms on the surface of the nanoparticles to the total number of atoms with the change of the particle size, which leads to the change of the material properties. It is well known that the specific surface area of the material is inversely proportional to the size of the material. As the size of the material decreases, the number of atoms in the surface area and surface atoms will increase significantly. The increase in the number of atoms on the surface causes a lack of surface atomic coordination and high surface energy, these atoms combine easily with other atoms and stabilize, which has a high chemical activity. It can be widely used in catalysis, adsorption and other fi elds. The ratio of the number of surface atoms to the total number of nanoparticles increases sharply with the decrease of particle size. When the particle size is reduced to 1 nm, the proportion of the number of atoms on the surface has reached 99% and the atoms are almost all concentrated on the surface of the nanoparticles. The increase in the number of atoms on the surface causes a lack of surface atomic coordination and high surface energy, these atoms combine easily with other atoms and stabilize, which has a high chemical activity [6, 7]. This causes the change of surface electron spin conformation and electron spectrum, and the change of atomic transport and confi guration on the surface of nanoparticles. (3) Quantum size eff ect When the particle size drops to a certain value, the electron energy level near the Fermi level is reduced from quasi- continuous to discrete energy levels, conductive nano particles have discontinuous highest occupied molecular orbit and lowest unoccupied molecular orbit, band gap widening and other phenomena, are known as quantum size eff ect [8] . The band theory suggests that the electron level near the Fermi level is generally continuous, which is only true at high temperature or macro size. For ultrafi ne particles with only a limited number of conductive electrons, in low temperature energy level is discrete; for macroscopic material which contains infi nite atoms, by the Kubo formula: δ = (4/3) EF / N (1-1) It can be seen that the energy level spacing is δ → 0. Which means that the spacing of large particles or macroscopic objects is almost zero; and for nanomaterials, the number of contained atoms is limited, N value is very small, and caused a certain value to δ. The separation of energy level causes the Kubo eff ect. When the energy level is greater than the cohesive energy of thermal energy, magnetic energy, magnetostatic energy, electrostatic energy, photon energy, or superconductivity, quantum size eff ects must be taken into account. This can lead to significant difference between nanoparticle magnetism, light, sound, thermoelectric, superconductivity and macroscopic characteristics. (4) Macro quantum tunneling eff ect The tunneling eff ect is one of the basic quantum phenomena. When the total energy of the microscopic particles is less than the barrier height, the particles can still pass through the barrier. In recent years, it has been found that some macroscopic quantities such as the magnetization of ultrafine particles and the magnetic flux in quantum coherent devices also have tunneling effects which can change through the potential well of the macroscopic system, known as the Macroseopie Quantum Tunneling [9]. Many of the magnetic electrons in the atom (referring to the electrons in the 3d and 4f shells) pass through the barrier in a tunneling manner, resulting in a change in magnetization which is a magnetic macroscopic quantum effect. It can be used to explain the phenomenon where nano-particles at low temperature remain Superparamagnetic. The study of macroscopic quantum tunneling is of great signifi cance to basic research and application, which limits the time limit of information storage of magnetic tape and disk. The quantum size eff ect and the macroscopic quantum tunneling eff ect will be the foundation of future microelectronic devices which establishes the limits of further miniaturization of existing microelectronic devices. When the microelectronic devices are further refi ned, the above-mentioned quantum eff ects must be considered. (5) Volume eff ect Due to the small size of the nanoparticles, the number of atoms contained is very small. Therefore, many phenomena such as adsorption, catalysis, diff usion, sintering and other physical and chemical properties related to the interface state will be significantly different from those of the large particle conventional materials, and can not be explained by the nature of the bulk material which is usually infi nitely atomized. This particular phenomenon is often called the volumetric eff ect [10]. (6) Dielectric limit eff ect The dielectric confi nement is the phenomenon that the nanoparticles are dispersed in the heterogeneous medium due to the dielectric enhancement of the interface caused by the interface. This dielectric enhancement is usually called the dielectric limit, mainly derived from the enhancement of the surface and the internal of the particles. When the refractive Solvent Thermal Method to control Lead Sulfi de Nano/Micron Crystal and Its Ni/PbS Composites 32 index of the medium is larger than the refractive index of the microparticles, the refractive index boundary is generated which leads to the enhancement of the fi eld strength of the particles and the internal fi eld. The enhancement of the local field strength is called dielectric limited field. Generally, transition metal oxides and semiconductor microparticles may produce dielectric limiting effects. The dielectric confinement of nanoparticles has an important effect on light absorption, photochemistry and optical nonlinearity. Therefore, when we analyze the optical phenomenon of a material, it is necessary to consider the quantum size eff ect and dielectric limit eff ect. 1.2. Hydrothermal / Solvent thermal method 1.2.1 Defi nition of hydrothermal method / solvent thermal synthesis In hydrothermal method, a water solution is used as a reaction medium in a specially made closed reactor to create a high temperature and high pressure reaction environment by heating the reaction vessel so that the normally insoluble material dissolves and recrystallizes. The hydrothermal reaction can treat some organic reaction or treat organic wastes that endanger our environment, as well as sintering ceramic materials at relatively low temperatures. However, the process of hydrothermal preparation of nanocrystals using water as solvent reaction is subjected to certain restrictions such as reactants decomposition or some reaction are unable to occur. For example: carbides, nitrides, phosphides and silicides. Therefore, by replacing water with non-aqueous solvent such as ethanol, methanol, benzene etc. as a solvent, and by replacing hydrothermal reaction with solvothermal reaction, a large amount of precursors to water-sensitive nanocyrstaline compound. Solvent thermal reaction is in the lime light in recent years. Solvent thermal reaction is the development of hydrothermal reaction, which is diff erent from hydrothermal reaction where an organic solvent is used rather than water. Compared with other preparation method, the main characteristic of solvent thermal reaction is very mild reaction conditions (such as the preparation of diamond), stabilizes phase, prepares new substances, develop new preparation routes and more. In the solvent thermal reaction process, the chemical component involved in the reaction acts as a solvent, mineralization accelerator, and also transmission medium for pressure. Solvent thermal reaction is mainly researched and developed by research group lead by Mr Qian Yitai. The most widely used solvent is ethylenediamine. In ethylenediamine system, in addition to being a solvent, ethylenediamine is also a complexing agent or chelating agent. Ethylenediamine as a bidentate ligand, due to the strong chelating eff ect of N, together with ion precursor become stable complex ion, where the complex ion then react with reactants to produce products. The reduction properties of methanol, ethanol, etc. in addition to solvent can also be used as a reducing agent. It can be seen, that solvent thermal reaction greatly expands the fi eld of nano-functional materials synthesis. The method is simple and convenient. As long as the suitable solvent is found, the development and application of solvent thermal reaction will have broad prospects. 1.2.2 Characteristics of hydrothermal / solvent thermal method (1) Advantages and disadvantages of hydrothermal method: (a) Hydrothermal method uses medium temperature in liquid phase control. Energy consumption is relatively low and has wide applicability. (b) Raw material is relatively cheap and easy to obtain, quickly convection in the liquid phase reaction, high yield, uniform phase, high purity, good crystallization, and controllable shape and size. (c) In hydrothermal process, the purpose of effective control of reaction and crystal growth can be achieved by adjusting the reaction temperature, pressure, treatment time, solution composition, pH value, the type of precursor and mineralizer. (d) Reaction is carried out in a closed vessel and the reaction atmosphere can be controlled to form a suitable redox reaction condition, and obtain some special phases, which is particularly advantageous for the synthesis reaction in a toxic system and minimizes environmental pollution. Limitations of hydrothermal method: This method is only applicable to the preparation of oxides or a few water- insensitive sulfi de semiconductor nanomaterials. In this context, researchers design solvent thermal synthesis in new solvent system, expanding the application of hydrothermal method. (2) Advantages and disadvantages of solvent thermal synthesis method In hydrothermal synthesis, water is the medium to transfer pressure and also played the role of mineralizer. Replacing water with a non-aqueous solvent also has the same effect and additional effects. At high temperature and high pressure, the solvent of the synthesis reaction is in critical or near-critical state. The physical and chemical properties of the reactants in the solvent have a great change, and the thermochemical reaction of the solvent is much diff erent from normal. The functional materials and crystal has its own excellent characteristics in terms of its nature. Therefore, the solvent thermal reaction is an eff ective method for the synthesis of chalcogenides. Yajia Ding, et al 33 Solvent thermal methods have the following advantages over other methods: (a) Solvent thermal method may replace the solid phase reaction and the difficult synthesis reaction due to the change in the reactivity of the reactants under solvent thermal conditions and the improvement of the activity. (b) As intermediate state, steady state, and specific phase is relatively easy to be generated, the synthesis and development of a series of special metastable structure and special condensed products can be produced. (c) Able to crystallize compound with low melting point, substance that is non-liquid in high vapour pressure condition, and high temperature decomposition phase. (d) Low temperature, isobaric, are solution conditions of the solvent thermal method, are beneficial due to less defects, perfect crystals with good orientation, synthesis of high crystallinity and easy to control the crystal particle size. (e) As the atmosphere under solvent thermal method is easily adjustable, it is beneficial for the formation of compounds in low valence state, intermediate state and special valence state, doping is uniformed, and benefi cial to the synthesis of new structures. Although the solvent thermal synthesis of nano-materials has the above advantages, this method has some shortcomings as following: (a) The solubility of the solvent and the rate of crystal growth are unpredictable. (b) It takes a long time to grow crystals of the appropriate size. (c) the infl uencing factors are more complex and have no mature theoretical causality. 1.2.3 Advances in hydrothermal / solvent thermal processes Reaction systems in hydrothermal synthesis are usually the following: water, inorganic ammonia, inorganic strong complexing agent system, organic amine, organic alcohol, organic alkane, other organic solvent system. Various reaction systems have their own characteristics and properties. Over the past decade, hydrothermal (solvothermal) synthesis has made considerable progress in the preparation of chalcogenide nanomaterials. The conditions of synthesis of chalcogenide nanomaterials under hydrothermal (solvothermal) were studied in detail, and various binary and ternary chalcogen compounds were synthesized. The results show that the hydrothermal (solvothermal) method is simple and effective for nanocrystalline morphology control and phase control. 1.3. Nano lead sulfi de Lead sulfi de is a cubic rock salt structure semiconductor material with narrow band gap (0,41 eV) and a large Bohr exciton radius (18 nm) [16]. PbS nanocrystals have a strong quantum confinement effect, and their cubic nonlinear optical properties are about 30 times that of GaAs and 1000 times of CdSe. They are potential application in infrared communication, photon switch, thermal and biological imaging, optoelectronic devices and solar cells. Recently, the generation of effective polychromes has been detected in PbS quantum dots, making it the most promising highly effi cient photovoltaic conversion material [17-23]. As an important member of the IV-VI semiconductors, lead sulfi de has a small band gap and a larger Bohr radius, and the nanoscale PbS band moves from near-infrared blue to the visible region, showing special optical properties and electrical properties [24], and thus, it has been extensively studied in nonlinear optical devices, infrared detectors and the application of solar receivers. The properties of PbS nanomaterials are directly related to their microstructure. 2. Experimental Part 2.1. Experimental reagents and instruments 2.1.1 Experimental reagents Table 2-1-1 List of reagents used in the experiment Drug name Product purity Manufacturer High - purity lead acetate Analytical No. 230 Shuguan Dao, Hebei District, Tianjin City Sublimation of Sulphur Chemical Chongqing Oriental Reagent Factory Ethylene glycol Analytical Tianjin Ke Miou Chemical Reagent Co., Ltd Solvent Thermal Method to control Lead Sulfi de Nano/Micron Crystal and Its Ni/PbS Composites 34 Glacial acetic acid Analytical Kaifeng Chemical Reagent Factory Lead Nitrate Analytical Tianjin Chemical Reagent Factory Anhydrous Ethanol Analytical Anhui Ante Biochemical Co., Ltd Distilled water Analytical University of Information Materials Research Institute Hydrated Nickel Chloride Analytical Tianjin Ke Miou Chemical Reagent Co., Ltd Hydrate Hydrazine Analytical Tianjin Zhiyuan Chemical Reagent Co., Ltd. Sodium Hydroxide Analytical Zhengzhou Paini Technology Cetyltrimethylammonium Bromide Analytical Tianjin Ke Miou Chemical Reagent Co., Ltd Thioacetamide Analytical Tianjin Ke Miou Chemical Reagent Co., Ltd Note: Of all reagents used in the experiment, sublimation of sulfur as pure chemical and the others are analytical. Glass equipment used are washed with tap water, rinsed with distilled water, and rinsed with industrial alcohol and dried before use. Reactor lining is soaked in water, washed with tap water, rinsed with distilled water, and rinsed with industrial alcohol and dried before use. 2.1.2 Experimental apparatus Table 2-1-2 List of experimental instruments Instrument Name and Model Manufacturer Electric Mixer Shanghai Pudong Physics Optical Instrument Factory Electronic Balance Beijing Sartorius Instrument System Co., Ltd Electric Constant Temperature Blast Drying Oven Shanghai Yiheng Instruments Co., Ltd. D/max-2200PC X-ray Diff raction (XRD) Japanese Science DGG-9246A Electric Constant Temperature Blast Drying Oven Shanghai Qi Xin Scientifi c Instrument Co., Ltd DMS-653 Optical Microscope Boyu Instrument 85-2 Digital Thermostat Magnetic Stirrer Shanghai Pudong Physics Optical Instrument Factory KH 2200 Ultrasonic Cleaner Kunshan Wo Chong Ultrasonic Instrument Co., Ltd 800B Centrifuge Shanghai Anting Scientifi c Instrument Co., Ltd. 2.2. Experimental steps 2.2.1 Preparation of PbS nanocrystals / micrometer crystals (1) 1 mmol (0.379 g) of lead acetate was added to a polytetrafl uoroethylene lined stainless steel kettle, and 10 ml of ethylene glycol was added as a solvent to stir the mixture on a magnetic stirrer. (2) 1.5 mmol of the recovered sulfur powder was added to the kettle of (1), and the mixture was suffi ciently stirred to disperse. (3) 1.5 ml of glacial acetic acid was added to the solution using a pipette and stirring was continued. (4) When the sulfur powder was dispersed uniformly, 4 ml of the ethylene glycol solution was fi nally added, and the mixture was stirred till suffi ciently dispersed. (5) Kettle was sealed and placed into the oven. In the constant temperature of 240°C for 24h, after the end of the reaction, let the reactor naturally cooled to room temperature. When cooled to room temperature, the black product is poured out by fi ltration, repeatedly washed with alcohol until the fi ltrate become colorless. A small amount of product is observed under optical microscope and recorded. All the black powder was then dried in a vacuum oven at 60°C. The product is collected for further characterization. 2.2.2 Preparation of Ni-coated PbS heterogeneous materials (1) 0.2 mmol (0.048 g) of lead sulfi de, 0.4 mmol of nickel chloride hydrate and 8 mmol of hydrazine hydrate were added to a mixed solvent of ethylene glycol / water (30 ml of ethylene glycol, 6 ml of water). (2) 1 ml, 2 ml, and 4 ml of 1 M NaOH were each added to the above solution and stirred at room temperature for 30 min on a magnetic stirrer. Yajia Ding, et al 35 (3) Kettle was sealed and placed into the oven, at 140°C for 12h. After the reaction has completed, let the reactor naturally cooled to room temperature. After cooled, the product is poured out, fi ltered, and repeatedly washed with alcohol. A small amount of the product is observed under optical microscope and recorded. Use magnet to verify whether the outside of the sulfur has been coated with metal Ni, and use the magnetic size to judge the coverage condition under diff erent variables. The black powder was then dried in a vacuum oven at 60°C. The product is collected for further characterization. 2.3. Characterization method The phase and purity of the product were measured by a Japanese Rigalcu D/Max-3c X-ray powder deff ractometer (XRD) using Cu Kα (λ = 1.5418Å), a test voltage of 40 kV, and a measurement angle of 2θ 10 to 80°; the morphology and size of the product were observed using an optical microscope and a Quanta 200 Environmental Scanning Electron Microscope (ESEM, FEI Company). 3. Results and discussion 3.1. Characterization of PbS nanocrystals / micrometers 3.1.1 Product phase and purity analysis The phase and purity of the product were examined by XRD. Figure 3-1-1 shows the X-ray diff raction spectrum of the product obtained when the molar ratio of lead acetate and sulfur powder is 2: 3, 14ml ethylene glycol is used as solvent and 1.5ml acetic acid is added. It can be seen from the fi gure: the purity of lead sulfi de is relatively high. The diff raction peak is strong and sharp, and all the diff raction peaks can be indexed into cubic phase PbS with a unit cell parameter a = 5.936Å which is consistent with the reported literature (JCPDS card value 05-0592). XRD does not detect impurity peaks such as S, PbSO4, PbO, which indicates that the purity and crystallinity of the product are relatively high. Figure 3-1-1 shows the XRD pattern of the product at 140°C for 24 hours 3.1.2 Morphological analysis of products The morphology of the product was observed by SEM. Figure 3-1-1 shows the molar ratio of lead acetate and sulfur powder of 2: 3, with 14ml of ethylene glycol as solvent, adding 1.2ml of acetic acid, at 140°C for 24 hours. Figure 3-1-1a is the overall topography of the product at low magnifi cation. As can be seen from the fi gure, the product is mostly tree-like six-arm symmetrical structure and some crystal arm has small bifurcation and some did not. Its size is relatively uniform, arm length of about 5 µm. In addition, some six-arm structure is broken, indicating that the tree-like PbS is not easy for long-term ultrasonic dispersion. Figure 3-1-1b is a SEM image of a single tree structure at high magnifi cation. The structure of the lead sulfi de micrometer crystal can be clearly seen from the fi gure. A single lead sulfi de tree structure consists of six diff erent orientations of arms, each arm size is more uniform, the orientation distribution is also more uniform, but they are not in the same plane. In addition, each arm is distributed with fine bifurcation, they are arranged more uniform, the overall structure is like a symmetrical branch. By the fi gure, each small bifurcation is about 1µm, each arm is about 5µm, each crystal is about 10µm, so we consider it more suitable for micron crystal. Figure 3-1-1c is another six-arm lead sulfi de structure we have obtained. Compared to the six-arm structure in Figure 3-1-1b, its surface is relatively smooth, there is no small bifurcation, but its overall shape and the former is basically the same, the size is similar with the former. Solvent Thermal Method to control Lead Sulfi de Nano/Micron Crystal and Its Ni/PbS Composites 36 Figure 3-1-1 PbS sample obtained from molar ratio of lead acetate to sulfur powder of 2: 3, reaction at 140°C for 24 h 3.1.3 Eff ect of sulfur content on product morphology Experiments show that the amount of sulfur powder on the product morphology has a greater impact. In this regard, we analyze them through SEM image. Figure 3-1-3 is the SEM image of the product with diff erent amounts of sulfur powder. Figure 3-1-3 SEM images of PbS with diff erent amounts of sulfur powder (Figure a, b, c uses 1mmol, 1.5mmol, 2mmol of sulfur powder, respectively) It can be seen from the fi gure, when the other variables are the same (lead acetate 1mmol, acetic acid 1.2ml, ethylene glycol 14ml, reaction time 24 hours, temperature 140°C), when the sulfur powder is 1mmol, the product morphology is more messy, there are fl ower-like structure, there are square-shaped structure, but less non-planar six-arm structure; when the sulfur powder is 1.5mmol, the product morphology is very uniform, more than 80% non-planar six-arm structure, the size is relatively uniform, the symmetry is also very good, and the results are ideal; when the sulfur powder is 2mmol, although there are still non-planar six-arm structure, but there are also other structures, and the six-arm structures are not very regular, the size is not very uniform. This shows that the amount of sulfur powder has a certain role on the product structure, morphology of the regularity, and the size of uniformity. Sulfur powder dosage should not be too less or too much, from the experimental results we know that sulfur powder 1.5mmol is more appropriate. 3.1.4 The eff ect of acetic acid on the morphology of the product Experiments show that the amount of acetic acid also has a great impact on the appearance of lead sulfi de. Likewise, we analyze the impact through SEM image. Figure 3-1-4 is the SEM image of lead sulfide obtained with different amounts of acetic acid. It can be seen from the fi gure, when the other variables are the same (lead acetate 1mmol, sulfur powder 1.5mmol, ethylene glycol 14ml, reaction time 24 hours, temperature 140°C), when acetic acid dosage is 1ml, its morphology is more messy and the size of each petal is not uniform; when the amount of acetic acid is 1.2ml, the product morphology is very uniform, more than 80% of non-planar six-arm structure, the size is also more uniform, the symmetry is also very good, and the results are ideal; when the amount of acetic acid is 1.5ml, the product is mostly six-arm structure, there are fl ower structure and other cluttered structure, and size is also diff erent; when the amount of acetic acid is 2ml, there are very few six-arm structure, arm lengths are diff erent, mostly are fl ower structure and cubic block structure, the shape and size is not regular. It can be seen that the preparation of lead sulfi de microstructure requires appropriate acidic conditions, where the increase in the amount of acetic acid is conducive to the product to the uniform morphology changes. However, the acidity cannot be too strong; otherwise it is not conducive to the formation of uniform morphology products. From the results, a usage of 1.2ml acetic acid is more appropriate. Yajia Ding, et al 37 3.1.5 Eff ect of lead source on product morphology When diff erent lead sources are used, the morphology of the resulting products is also diff erent. Figure 3-1-5 is an image observed under optical microscope of the product obtained using lead acetate and lead nitrate, respectively, as lead sources. Figure 3-1-5 Image observed under optical microscope of product obtained under diff erent lead sources (Figure a and b are lead acetate and lead nitrate, respectively) It can be seen from the fi gure that when the other variables are the same (acetic acid amount, solvent, sulfur source, reaction time, reaction temperature), when lead acetate is used as lead source, the product has a non-planar six-arm structure with uniform size, looks more regular, and has a good symmetry; and when the use of lead nitrate as a lead source, the product has a large size four corner spindle structure, its shape is also more structured but the size is not uniform. When observed by the naked eye: PbS products made with lead acetate are darker, and is of black powder, and PbS prepared by using lead nitrate is of gray powder. At the same time, we tried to use lead chloride as a lead source but failed to get the product. The specifi c reason is unclear. 3.2. Characterization of Ni / PbS Composites 3.2.1 Product phase and purity analysis The phase and purity of Ni/PbS composites were observed by XRD. Figure 3-2-1 shows the X-ray diffraction pattern of the product obtained by coating 0.2 mmol of PbS with nickel at 140°C for 12 hours with polyethylene glycol and water as a composite solvent. In this case, we compare the spectra of lead sulfi de with the products obtained with diff erent amounts of 1 mol/L NaOH in the same spectrum. It can be seen from the fi gure: the resulting PbS (05-0592) diff raction peak is strong and sharp, indicating that its purity and crystallinity are relatively high. When NaOH is added (under alkaline conditions, the reduction of hydrazine hydrate is enhanced and nickel chloride is more easily reduced to elemental nickel), the strength of the peak varies with nickel, and with the addition of diff erent amount of NaOH, the change is not the same. Secondly, the product is not of pure Ni/PbS composite, from the fi gure we can clearly see the diff raction peak of Ni3Pb2S2 (26-1287) indicating that in the coating process, bimetallic sulfi de is produced. In addition, with the increase in the amount of NaOH, the peak of bimetallic sulfi de has weakened, while the peak of metal nickel initially enhanced and then weakened. When adding 2ml of NaOH, nickel peak is the strongest, indicating that at this time there is more Ni/PbS heterogeneous materials. Figure 3-2-1 XRD pattern of the product obtained at 140°C for 12h (a is the XRD pattern of PbS, b, c, d is the composite product obtained by adding 1, 2, 4ml 1mol/L NaOH, respectively) Solvent Thermal Method to control Lead Sulfi de Nano/Micron Crystal and Its Ni/PbS Composites 38 3.2.2 Morphological analysis of products The morphology of the composite product was observed by SEM. Figure 3-2-2 shows SEM image of the product obtained by coating 0.2 mmol of PbS with nickel at 140°C for 12 hours with ethylene glycol and water as a complex solvent (Vethylene glycol: Vwater = 5: 1). Figure 3-2-2 SEM image of products obtained at 140°C for 12 h (Figure a, b, c were added with 1, 2, 4ml 1mol/L NaOH, respectively) It can be seen from the fi gure: When NaOH is added to the lead sulfi de, the six-arm structure of lead sulfi de can still maintain its original state. When adding 1ml of NaOH, the six-arm structure surface is coated with many small particles, while the surrounding is also scattered with some particles, they are not evenly distributed, and in addition to six-arm structure, there are other clutter structure; when adding 2ml of NaOH, the distribution of small particles is more uniform, well coated on the surface of lead sulfi de, it has similar properties as a ‘binder’; when adding 4ml of NaOH, the are very thick layer of small particles stuck to the six-arm structure, surrounded by a large number of scattered particles, and the overall phenomenon of agglomeration is very serious, which may be due to slightly stronger alkaline and reaction was too fast. It can be seen, when coating lead sulfi de, 2ml of NaOH is more appropriate. When NaOH is less, there were more impurities, more reactions which is too fast, and not conducive to dispersion. 4. Conclusions In this experiment, the nano-crystal materials with uniform morphology (non-planar six-arm structure) were successfully synthesized at the appropriate temperature by the compound solvent method, and the eff ects of various factors (acetic acid dosage, sulfur content and lead source) are studied. The XRD, SEM and microscopy were used to characterize them. Finally, nickel-coated lead sulfi de heterogeneous materials were prepared. (1) The amount of sulfur powder on the product morphology has a greater impact. When the amount of sulfur powder is too much or too little, it will affect the regularity and uniformity of the product morphology. Different amounts of sulfur powder may be produce diff erent products. The appropriate amount of sulfur powder is 1.5mmol. (2) The amount of acetic acid is another factor aff ecting the product morphology. The preparation of lead sulfi de requires proper acidity, and the increase in acetic acid is benefi cial to the uniformity of the product. However, the acidity shall not be too strong; otherwise it is not conducive to the formation of uniform morphology. From the results: 1.2ml acetic acid is more appropriate. (3) Diff erent lead sources also aff ect the product. When using diff erent lead sources, not only the product's color and sedimentation capacity is diff erent, the product also shows a completely diff erent morphology and the size is also very diff erent. (4) When coating with nickel, the amount of NaOH used gives a great impact. Its amount will not only aff ect the speed of the coating, but also aff ect the purity of the composite material, and product dispersion. The results show that the amount of 2ml is more appropriate. In this experiment, diff erent variables were studied and observed using XRD, SEM and microscopy. The target products were the non-planar six-arm structure. The optimal reaction conditions were as follow: 1.2ml acetic acid, 1mmol lead acetate, 1.5mmol sulfur powder, reaction temperature 140°C, reaction time 24 h, 14ml ethylene glycol as solvent. For coating, 2ml of NaOH is more appropriate. References 1. D.K. Kim, Y. Zhang, W. Voit, et al. Synthesis and characterization of surfactant coated superparamagnetie monodispersed iron oxide nanoparticles [J]. J. Magn. Mater, 2001, 225: 30-32. Yajia Ding, et al 39 2. Xiang Hang. 'Functional Materials and Nanotechnology' [M]. Chemical Industry Press, 2002. 3. Zhang Yulong, Li Changde. 'Nanotechnology and Plastics' [M]. China Light Industry Press, 2002. 4. Cavicchi P E, Silsbee R H. Coulomb Suppression of Tunneling Rate from Small Metal Particles [J]. Phys. Rev. Lett, 1984, 52: 1453-1456. 5. Zhu X, Birribger R, Herr U, et al. X-ray diff raction structure of nanometer-sizedeen materials [J] Phys. Rev. Lett, 1987, 35: 9085-9090. 6. Ball P, Garwin L. Science at the Atomic Scale [J]. Nature, 1992, 35: 57-61. 7. Tabagi H, Ogawa H. Quantum size eff ects on photoluminescence in ultrafi ne Si particles [J]. Appl Phys Let, 1990, 56 (24): 2379-2380. 8. Q.Qiu, Y.W. Du, H. Tnag, et al. A mossbuaer study of fi neiron Partieles [J]. APPI. 9. Phys, 1988, 63: 4100-4104. 10. Alivisatos A P. Semiconductor clusters, nanocrystals, and quantum dots [J]. Science, 1996, 271: 933-936. 11. Zhang Lide, Mu Jimei. 'Nanomaterials and nanostructures' [M]. Science Press, 2001. 12. Xu Hong, Liu Jianhong, Chen Pei, et al. Preparation of nanometer iron oxide and its catalytic eff ect on thermal decomposition of absorbent drugs [J]. Journal of Explosives \u0026 Propellants, 2002, 3 (65): 51-52. 13. Chinese Journal of Experimental Surgery, 2000, 17 (3): 257-258. [J]. Chinese Journal of Experimental Surgery, 2000, 17 (3): 257-258. 14. Cai Yurong, Zhou Lian. Nano-Ceramics for Biological Materials [J]. Rare Metal News, 2002, 2: 1-3. 15. Li Y D, Duan X F, Qian Y T, et al. Solvothermal Co-reduction Route to the 16. Nanocrystalline Ⅲ-ⅤSemiconductor InAs [J]. 1997, 119 (33): 7869-7870. 17. Murray C B, Norris D J, Bawendi M G. Synthesis and characterization of nearly 18. Monodisperse CdE (E = sulfur, selenium, tellurium) semiconductor nanocrystallitesites [J]. J. Am. Chem. Soc, 1993, 115 (119): 8706-8715. 19. Zhang C, Kang Z H, Shen, et al. Synthesis and evolution of PbS nanocrystals through a surfactant-assisted solvothermal route [J]. Phys. Chem, B, 2006, 110 (1): 184-189. 20. Lee SM, Jun YW, ho, et al. Single crystalline star-shaped nanocrystals and their evolution programming the geanetry of nano- building blocks [J]. Am. Chem. Soc, 2002, 124 (38): 11244- 11245. 21. M Donald S A, Konstantatos C, Zhang S C, et al. Solution processed PbS quantum dot infrared photo-detedtors and photovoltaics Nat Mater, 2005, 4 (2): 138-142. 22. Levina L, Sukhovatk in W, Musikhin S, et al. Effi cient infrared emmitting PbS quantun dots grown on DNA and stable in aqueous solution and blood plasm [J]. Ad, Master, 2005, 17 (15): 1854- 1857. 23. Choudhury KR, SahooY, Jang SJ, et al. Effi cient photosensitization and high optical gain in a novel quantum dot sensitized hybrid photorefractive nanocomposite at a telecommunication 's wavelength [J]. Adr, Funct Mater, 2005, 15 (5) : 751-756. 24. Ge JP, Wang J, Zhang H X, et al. Orthogonal nanowire arrays and networks and their scattering behavior [J]. Chem. Eur, 2005, 11 (6): 1889-1894. 25. Kuang D, Xu A, Fang Y, et al. Surfactant assisted growth of novel PbS dendritic nanostructures via facile hydrothermal process [J]. Adc, Mater, 2003, 15 (20): 1747-17. 26. Ellingon R J, Beard M, Johnson J C, et al. Highly effi cient multiple exciton generation in colloidal PbSe and PbS quantum dots [J], Nana. Lett, 2005, 5 (5): 865-871. 27. Li, D.Liang, C.-J .; Liu, et al. The optical properties of nanocrystals [J]. Lumin, 2007, 122: 549-550. 28. Ricolleau C, Gandais M, Gacoin I, et al. Correlation between structural and optical properties of PbS nanocrystals [J]. Crystal Growth, 1996, 166: 769-773. 29. Gao F, Lu Q, Liu X, et al. Controlled synthesis of semiconductor PbS nanocrystals and nanowires inside mesoporous silica SBA-15 phase [J]. Nano Lett, 2001, 1 (12): 743-748. 30. Wang S, Yang S. Preparation and characterization of oriented PbS crystalline nanorods in polymer fi lms[J]. Langmuir, 2000, 16: 389-397. 31. Yu D B, Wang D B, Meng Z, et al. Synthesis of closed PbS nanowires with regular geometric morphologies[J]. Mater Chen, 2002, 12: 403-405. 32. Zhang Z, Zhang J, Xue Q. Crystal Growth[J]. J. Phys. Chem, 1994, 98: 12973-12974. 33. Xue Q, Liu W and Zhang Z. Wear[J]. Phys. Chem, 1997, 213: 29-30. 34. A.A. Rempel, N. S. Kozhevnikova, A. J. G. Leenacrs, et al. Towards particle size regulation of chemically deposited lead sulfi de (PbS)[J]. Journal of Crystal Growth, 2005, 280: 300-302. 35. G. P. Michell, C. A. Mirkin, R. L. Letsinger. Programmed Assembly of DNA Functionalized Quantum Dots [J]. Journal of Crystal Growth, 2005, 280: 300-304. 36. ZHAO Jia-long, ZHANG Ji-sen. Quantum size effect and optical properties of PbS semiconductor ultrafine particles [J]. Chinese Journal of Luminescence, 1993,14 (1): 11-13. 37. Chen Renhou, Feng Gang, Ma Xiaodong. Quantum size eff ect of semiconductors [J] .Journal of Laser and Infrared, 2000, 49 (2): 2-5. 38. Chen Shuang, Liu Weimin. TEM study of surface modifi ed PbS nanoparticles [J]. Journal of Lanzhou University, 1998, 17 (5): 565-566. 39. Han Jianzhong. Optoelectronic Materials Technology [J]. New Military Materials Technology, 1996, 38 (5): 90-92. 40. YU Bao-long, ZHU Cong-shan.Three-order nonlinear optical properties of semiconductor lead sulfi de nanoparticles [J] .Acta Physica Sinica, 2000,49 (2): 324-327.