Mechano-thermal preparation of zinc sulfide nanoparticles

Abstract

A method for preparing zinc sulfide particles is described which comprising the steps (i) preparing a mixture of zinc mineral and elemental sulfur, (ii) milling the mixture obtained in step (i), and (iii) annealing the mixture obtained in step (ii) at a temperature ranging from 300 to 500° C. to obtain zinc sulfide particles.

Claims

1. A method for preparing zinc sulfide particles comprising the steps: (i) preparing a mixture of zinc mineral and elemental sulfur, (ii) milling the mixture obtained in step (i), and (iii) annealing the mixture obtained in step (ii) at a temperature ranging from 300 to 500° C. to obtain zinc sulfide particles, wherein the zinc mineral is hydrozincite.

2. The method according to claim 1, wherein the milling of step (ii) is carried out until the mixture obtained in step (i) is homogenized.

3. The method according to claim 1, wherein the milling of step (ii) is carried out using a vortex ball mill.

4. The method according to claim 1, wherein weight ratio of zinc mineral to sulfur ranges from 1:0.25 wt. % to 1:1 wt. %.

5. The method according to claim 1, wherein the zinc sulfide particles obtained in step (iii) have an average particle size of 20 nm or below 20 nm.

6. The method according to claim 1, which is carried out in the absence of any solvents including water so that it is a dry method.

7. The method according to claim 1, which is carried out in the absence of any surfactants, stabilizers or other additives.

8. The method according to claim 1, wherein step (iii) is carried out under inert atmosphere.

9. Zinc sulfide particles, obtainable by a method according to claim 1.

10. The method according to claim 1 wherein step (iii) is carried out under nitrogen atmosphere.

11. The method according to claim 1 wherein step (iii) is carried out under argon atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) This invention relates to a method for preparing zinc sulfide nanoparticles, more particularly to a mechano-thermal method for large scale production of zinc sulfide nanoparticles. The method for preparing zinc sulfide particles according to the present invention therefore comprises the steps of (i) preparing a mixture of zinc mineral and elemental sulfur, (ii) milling the mixture obtained in step (i), and (iii) annealing the mixture obtained in step (ii) at a temperature ranging from 300 to 500° C. to obtain zinc sulfide particles. This simple solvent free method uses a naturally occurring zinc precursor and a highly abundant, low cost sulfur source. No toxic solvents and surfactants are used to control the particle size. The method is highly feasible for large scale production with minimal complexity. The present invention also achieves the object to provide a method that uses mechanical and thermal energy to produce zinc sulfide nanoparticles.

(2) The following points underline the advantages of the present invention: 1. Solvent free mechano-thermal synthesis. 2. Use of natural and abundant low cost starting materials. 3. No further purification required for the resulting ZnS nanoparticles. 4. No use of surfactant or capping agent to control the particle size, i.e. the process is operable without the necessity for solvents or surface capping agents to control the particle size. 5. The process can be scaled-up easily.

(3) The use of sulfur in an immediate application as in the present invention, i.e. the use of the element as is, creates a promising use for the abundant chemical sulfur which is increasingly becoming a major issue for the industry.

(4) Additionally, the method according to the invention provides a process of making an existing material via a different and more direct route. In particular, the method according to the invention is a ZnS synthesis method that uses mechanical and thermal energy to produce zinc sulfide nanoparticles. It is believed that companies that would be interested are those that are currently using ZnS in their applications and also companies that are already making ZnS who might find this process more practical to switch to.

(5) In the present invention the term “mineral” is used as common in the art. For example, a mineral can be defined as follows: Minerals are naturally-occurring inorganic substances with a definite and predictable chemical composition and physical properties. According to a preferred embodiment, the used zinc mineral is hydrozincite (Zn.sub.5(CO.sub.3).sub.2(OH).sub.6) because it is a naturally occurring and low cost starting material.

(6) According to a preferred process variant, the milling of step (ii) is carried out until the mixture obtained in step (i) is homogenized which allows for a more predictable and reliable way of obtaining the desired zinc sulfide particles. For the milling of a mixture of a zinc mineral with elemental sulfur the use of a vortex ball mill for the milling of step (ii) has the advantage of a quick milling without loss of substance adhering to mill components.

(7) Particularly good results in terms of process efficiency have been obtained when a preferred weight ratio of zinc mineral to sulfur was in the range of from 1:0.25 to 1:1 wt. %.

(8) In the present invention, it is preferred that the zinc sulfide particles obtained in step (iii) have an average particle size of 20 nm or below 20 nm in order to give the desired nano-sized powder.

(9) It is further preferred that the method according to the invention is carried out in the absence of any solvents including water, i.e. that the method is a dry method. This avoids any formation of clumps and saves costs as no solvents are required which is moreover environmental-friendly.

(10) It is also preferred for the present invention that the inventive method is carried out in the absence of any surfactants, stabilizers or other additives. The present invention does not require any of such additives in order to form ZnS nano-particles in a reliable manner so that no additional costs for such additives are created, and again the method is thereby particularly environmental-friendly.

(11) In order to avoid the formation of any oxides and thus in order to obtain particularly pure zinc sulfide particles it is further preferred according to the invention that step (iii) of the inventive method is carried out under inert atmosphere, preferably under nitrogen atmosphere or argon atmosphere.

(12) The present invention further provides zinc sulfide particles which are obtainable by a method according to the present invention, i.e. zinc sulfide particles which can be produced at low costs in a speedy and reliable manner.

(13) The present invention for the first time allows to prepare zinc sulfide nanoparticles using naturally occurring zinc precursors and to use mechanical and thermal energy, as well as to achieve effective control of nanoscopic particle size, resulting in a uniform dispersion of ZnS nanoparticles. Therefore, advantages of the present invention are: 1) Preparation uses naturally occurring and very cheap raw materials and low toxicity, low cost, safety; 2) Preparation process is solvent free and does not add any surfactant, stabilizer and other additives; 3) Production of zinc sulfide nanoparticles with small and uniform size distribution and good dispersion and stability; 4) Process has high reaction rate and large scale, continuous production can be achieved under industrial production conditions.

Working Example

(14) The following preparation processes and procedures were used: A weighed amount of zinc salt which is naturally occurring zinc mineral (hydrozincite; chemical formula: Zn.sub.5(CO.sub.3).sub.2(OH).sub.6) and elemental sulfur were homogenized under vortex ball mill using different reactants to ball weight ratio, and then the mixture was thermally annealed under inert atmosphere at temperature ranging from 300 to 500° C. The weight ratio of zinc salt and sulfur was varied between 1:0.25 to 1:1 wt-%. to finally obtain the nano-sized zinc sulfide as an off-white powder. This resulted into ZnS particles having an average particle size of sub-20 nm.

(15) This mechano-thermal method of ZnS nanoparticle production does not use any catalyst, surface capping agent, solvent or further purification process and is carried out under effect of moderate thermal energy. The process of the present method is simple, one-step and highly suitable for large scale production of zinc sulfide nanoparticles.