What is the first thing that comes to your mind when you think about nanotechnology? Group of bobbling spheres going through a gummy liquid, nanobot cutting through a cell in a vividly red background or like in my wife’s case two fat rabbits eating grass. For me it’s a picture of two serious looking scientists inside a clean room, suited from top to bottom, trying to figure out what’s going inside the flask that they are holding. I honestly believe, at least for most of us, picture would be like something similar to mine. But it doesn’t have to be. There are many types of nanoparticles that can be prepared at your home, with easy to find chemicals and with the items available with you. If you have access to a community science center, the process would become whole lot easier. If you are still going to school or college, you can make lot of types of nanoparticles at your lab and experience some of the interesting properties they exhibit. As a first step, today, let’s look at a nanomaterial that is among the most easiest to produce; Zinc oxide.
Although, making zinc oxide nanoparticles is easy, it has few very interesting properties. These properties have been exploited in some of the products that are already in the market today. The most common are zinc oxide sunscreens, anti-odor socks, anti-bacterial textiles, self-cleaning coats, food packaging sheets, UV blocking window sheets, etc. Zinc oxide sunblocks and UV blocking window sheets take use of very high UV blocking ability by Zinc oxide nanoparticles. The self-cleaning coatings and the materials are exploiting the photocatalytic ability of the ZnO nanoparticles. Products like, anti-odor socks, anti-bacterial textiles and food packaging sheets relies on anti-bacterial and fungicidal effects of Zinc oxide nanoparticles.
To make Zinc oxide nanoparticles, you will need the following
Two small glass beakers or flaks
Zinc sulfate or other zinc salt (it’s a very common chemical, get it from your local chemical store or science supply store , or you can make it like in here)
Urea (common fertilizer)
Sodium hydroxide or caustic soda solution (0.5 g Sodium hydroxide dissolved in a 20 ml of water)
Microwave oven (This will help you to get smaller particles. Normal heating will also work but will take longer time periods and bigger particles, The one at your community science center is the best, we do not recommend using the one you use at home for obvious reasons)
pH papers (Optional)
Necessary safety items like gloves and eye protection
Zinc oxide nanoparticle synthesis
The method of Zinc oxide nanoparticle synthesis is quite straightforward. Dissolve 0.7 grams of Zinc sulfate and 0.2 g of urea in 100 ml of water. Slowly shake or stir the solution until the crystals completely disappear. At this point the solution will be completely transparent. Then add few drops (3-5) of sodium hydroxide solution to the above solution or if you have pH papers, adjust the pH to 12 using the sodium hydroxide solution. Just as you add the sodium hydroxide solution, the solution will become turbid, producing a whitish cloudy precipitate. Then put the solution inside the microwave oven, turn the power by half and keep it running for 5 minutes. If everything went well you will have a nice white dispersion with slightly bluish color. This bluish color usually indicates the presence of very small particles.
If you don’t have a microwave oven or do not intend to use it, you can try just heating the solution. Just make sure you keep the solution boiling for at least 15 minutes to prepare ZnO nanoparticles.
One of the most interesting Zinc oxide property is photocatalytic activity. Zinc oxide nanoparticles can absorb UV light from the sunlight or from other light source to produce electrons and holes in its structure. These electrons and holes, migrate in to the surface of the particles and react with water and oxygen in the surroundings of the nanoparticles to make highly unstable radicals. These radicals will then go and destroy any impurities like organic stains, dyes, bacteria and toxic organic substances. You can demonstrate the photocatalytic activity of the prepared nanoparticles simply by adding an organic stain like a dye to the diluted ZnO nanoparticles dispersion made according to the above method and keep it under sunlight. You can see that the color intensity of the dye in the Zinc oxide nanoparticle dispersion slowly reduces with the time and completely disappear subsequently.
What happens in the process?
In very simple terms, Zinc sulfate in this reaction system detonates Zinc and Urea donates Oxygen to make Zinc Oxide (ZnO). Zinc sulfate is a salt, and when added in to the water it dissociate in to Zinc (II) ion and sulfate ion. Urea however, is not a salt and simply get dissolved in the solution and float around. When small amount of sodium hydroxide is added, hydroxyl groups in the solution shoots up. These hydroxyl groups will react with Zinc(II) ion to make Zinc hydroxide (Zn(OH)2). This is the white precipitate we observed when we add few drops of sodium hydroxide in to the solution. When the solution is subjected to microwave, it heats up rapidly and make things even more interesting. First, Zinc hydroxide will be converted in to Zinc oxide and first clusters of ZnO nanoparticles are born. Secondly, and more interestingly, urea starts to decompose at higher temperatures giving out ammonia and carbon dioxide. Thus produced ammonia (weak base), dissolves back in to the water and make more hydroxyl groups. These newly made hydroxyl groups will find free Zinc (II) ions in the solution to make Zinc hydroxide. Like in the previous case, Zinc hydroxide will be converted in to ZnO under thermal conditions, providing more clusters of Zinc oxide crystals.
When microwave technique is used, large bursts of energy is given to the solution, increasing its temperate at very high rate. This results in large number of very small nanocrystals called nuclei which will self-assemble to make nanoparticles. However, if general heating is used, the decomposition of urea is relatively slow producing small amount of nuclei for longer durations. These will self-assemble and grow in to bigger particles due to continues supply of growth materials. This is the reason why we can expect bigger particles of ZnO if general heating is used.
Few projects ideas with the ZnO nanoparticles
Incorporate in to a fabric and see whether it becomes self-cleaning
Observe how ZnO nanoparticles work on a fungus
Improve the UV blocking ability of a fabric
Spray in to a plant fungal infection
See how ZnO coating would help extend the shelf life of tomatos