Synthesis of Hierarchical ZnO/CdSSe Heterostructure Nanotrees

The overall goal of this experiment is to synthesize a tree-like nanostructure with zinc oxide stems and cadmium sulphoselenide branches. And characterize its morphology, crystal structure, and optical properties. This method can help answer key questions in the application of nanocomposite materials in optical and electro-optical applications.

The main advantage of this technique is that chemical vapor deposition is a simple and cost-effective way to produce tree-like nanostructures with well-defined interfaces. The cadmium sulphoselenide branches absorb visible light and form of type-II heterojunction with a zinc-oxide stem. The heterojunction facilitates efficient electric transfer from the branches to the stem.

The optical properties indicate that the nano trees are promising materials for applications that benefit from a z-scheme charge transfer mechanism, like solar cells and other optoelectronic devices. Though this matter can provide insight into zinc-oxide cadmium sulphoselenide nanotrees, it can also be applied to other systems, such as nanowires, nanotubes, nanobots and so on. Generally, individuals new to this method will struggle because each parameter, such as emulsification, temperature and fluoride will affect the morphology of the nanotrees.

First, clean a sapphire slide by sonication in 99.5%ethanol for five minutes. Then using a sputter coater, deposit a 10-nanometer thick gold film onto the slide. Next, stir one gram of a one-to-one mixture by mass of zinc oxide nanopowder and activated carbon into 10 milliliters of 99.5%ethanol.

Sonicate the mixture in a water bath at 20 degrees Celsius for 30 minutes. This step insures that no air is present in the mixture. And zinc oxide and carbon are in close contact.

Dry the sonicated mixture at 80 degrees Celsius for five hours. Then place the dry mixture in an alumina combustion boat and compact it with a spatula to exclude air from the mixture. Place a gold-coated slide on top of the combustion boat, gold-side down.

Place the boat in the center of the quartz tube of a horizontal tube furnace. Then set the heating and cooling parameters on the furnace. Purge the tube with argon gas at a flow rate of 40 standard cubic centimeters per minute at room temperature for one hour.

Then increase the temperature from room temperature to 900 degrees Celsius at 80 degrees Celsius per minute and remain at that temperature for two hours. Then, open the tube to air at both ends to provide oxygen for the reaction. Continue heating the sample at 900 degrees Celsius with the tube open to air for three hours.

Cool the furnace down to room temperature at 10 degrees Celsius per minute and remove the boat and slide. The white film on the slide consists of the zinc oxide nanowires. Next, thoroughly mix 0.25 grams each of cadmium sulfide and cadmium selenide powder and place the mixture in another alumina combustion boat.

Compact the mixture well. Place the zinc oxide nanowire-coated slide about 10 centimeters downstream from the boat. Place the boat in the center of the quartz furnace tube.

On the slide, ensure that the zinc oxide nanowires face up. Again, set the parameters on the furnace. Purge the quartz tube of the furnace with argon gas at 40 standard cubic centimeters per minute at room temperature for one hour.

Then heat the furnace to 820 degrees Celsius at 80 degrees Celsius per minute and hold the temperature at 820 degrees for 30 minutes. A different temperature will give rise to a different composition and morphology. Then cool the furnace down to room temperature at 10 degrees Celsius per minute and remove the boat and slide from the furnace.

The zinc oxide cadmium sulphoselenide nanotrees are obtained as an orange-yellow film on the slide. Prepare control samples of zinc oxide and cadmium sulphoselenide nanowires on clean, gold-coated sapphire slides using the same procedures. Characterize the nanostructures with scanning electron microscopy, X-ray powder diffraction, and energy-dispersive X-ray spectroscopy.

Obtain photoluminescence emission spectra and measure the time result photoluminescence and fluorescence decay lifetimes. cadmium sulphoselenide nanowires were grown by chemical vapor deposition on zinc oxide nanowires to form tree-like nanostructures. The stems were capped with cadmium sulphoselenide.

The nanotrees showed X-ray diffraction peaks characteristic of both the pure zinc oxide nanowires and the pure cadmium sulphoselenide nanowires. An additional peak observed in the XRD spectrum is assigned to a different phase of cadmium sulphoselenide forming on point defects on the zinc oxide stem initiating branch growth. The mole percentage ratio of sulfur to selenium in the branches was determined from EDS and XRD to be about 54 to 46.

The mole ratio affects the band gap of the branches and can be tuned by altering the branch growth temperature. The photoluminescence lifetime of the nanotrees was shorter than the lifetimes of the zinc oxide and cadmium sulphoselenide nanowires. This suggests that fast electronic transfer occurs across the interface of the nanostructure providing an alternate relaxation pathway.

Once mastered, this technique can be done in eight hours if it is performed properly. Six hours for zinc oxide preparation and two hours for cadmium sulphoselenide growth. And don't forget that working with cadmium sulfur and cadmium selenide can be extremely hazardous.

And precautions such as wearing gloves, lab coat, and closed shoes should always be taken while performing this procedure.