Category: Chemistry

 

 

The procedure shown here was adapted from an adaption by Paul Hansen and George Lisensky from Kurt Winkelmann, Thomas Noviello, and Steven Brooks, “Preparation of CdS Nanoparticles by First-Year Undergraduates,” J. Chem. Educ. (2007) 84, 709-710, which was based on M. L. Curri, A. Agostiano, L. Manna, M. D. Monica, M. Catalano, L. Chiavarone, V. Spagnolo and M. Lugarà, J. Phys. Chem. B, (2000) 104, 8391-8397.

Preamble

Zinc sulphide is a very important II-VI semiconductor material with a wide direct band gap (Eg = 3.68eV for bulk ZnS). It has been studied due to its wide applications as phosphors and catalysts as well as electro-luminescent devices, solar cells and other opto-electronic devices. ZnS is also of interest as small biomolecular probes fro fluorescence and laser scanning microscopy. It is currently used as a shell or capping layer in nanoprobes such as CdSe/ZnS core structures.

Aim

You will prepare bulk and nano-sized ZnS particles, measure their absorbance spectrum, calculate the diameter of the nanoparticles and observe differences between nanoparticles
and the analogous bulk material.

Introduction

We are going to make zinc sulphide through the following chemical reaction:

ZnCl2 (aq) + Na2S(aq) ? ZnS (s) + 2NaCl(aq)

(you need to complete the balanced chemical equation)

Of course, what we want to do is make nano-sized particles of this material. A problem that exists is that nanoparticles tend to agglomerate to form larger, non-nanosized (bulk) particles. In order avoid this we need to use special methods to limit their growth. So how are we going to do this? Well what we need is some type of nano -sized reaction vessel. One in which the size of the vessel limits the growth of the sulphide particles. What could we use as a nano-reaction vessel?

 

 

 

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In this experiment we will use micelles to control the size of the zinc sulphide particles formed. What do we mean by the term micelle?

The answer is as close as the nearest sink. You use soap to remove grease from your hands – water by itself does not work. Oils contain molecules called hydrocarbons which consist of long chains of carbon atoms with hydrogen atoms attached. Hydrocarbons do not mix with water and are considered hydrophobic (“afraid” of water) . Soap contains molecules, called surfactants, with a hydrophobic end and an ionic charged group at the other end. The ionic charge makes that portion of the molecule hydrophilic (“likes” water). Water dissolves hydrophilic compounds, so water will dissolve soap. Hydrophobic substances tend to mix well, so the hydrophobic ends of many soap molecules will form a shell around a few hydrocarbon molecules. These shells, called micelles, consist of 50 – 100 soap molecules. The hydrophilic end of the surfactant molecule is located on the outer surface of the micelle and continues to interact with. You will prepare solutions containing micelles that limit the growth of ZnS particles.

One key difference between the example of soap described above and this experiment is that the solvent you use will be hexane – an organic, hydrophobic liquid – and small amounts of aqueous solutions will be added. The charged end of the surfactant will be pointed towards the centre of the micelle and the nonpolar portion of the molecule will be exposed to the nonpolar solvent. Such a structure is called a reverse micelle. Figure 1 illustrates the difference between micelles and reverse micelles.

 

 

 

 

 

 

 

 

 

(a) (b)

Figure 1. (a) micelle, (b) reverse or inverse micelle

In this experiment we shall try to produce different sized nanoparticles. As the particle size is a function of micelle size, we could look at making different sized micelles.

 

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Hexadecyltrimethyl ammonium bromide or cetyltrimethyl ammonium bromide (CTAB) has a long hydrophobic chain and a polar head group. (Figure 2 (a)). The molecule does not dissolve well in either aqueous or organic solvents. In an organic solvent containing a small amount of water the hexadecyltrimethylammonium bromide traps the aqueous portion in a micelle sphere with the polar heads facing in and the non-polar tails facing out

Mixing hexadecyltrimethylammonium bromide pentanol micelles of ZnCl2 with similar micelles containing Na2S produces nanoparticle ZnS because the aqueous solution serves as a nanoreactor and the particles cannot grow bigger than the micelle. The pentanol also acts as a capping agent to stabilize the ZnS particles. The formation of ZnS nanoparticles can be detected by spectroscopy since quantum size effects make the visible absorption spectra different than that of bulk ZnS.

One way to do this would be to use a different surfactant that has a bigger head or a shorter/longer tail:

• Use dodecyltrimethyl ammonium bromide (shorter tail) – DTAB (Figure 2(b))
• Use dioctyl sodium sulfosuccinate (double tail) – AOT (Figure 2(c))

 

 

 

 

 

 

(a)

 

 

 

 

 

(b)

 

 

 

 

 

 

 

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(c)

Figure 2. The anionic surfactants a) CTAB, b) DTAB, c) AOT

Another way would be to use different amounts on pentanol. (Figure 3). The relative amount of pentanol co- surfactant controls the size of the micelle. More pentanol would make the diameter bigger.

 

surfactant

pentanol

 

 

 

 

 

 

 

 

Figure 3. A water-in-oil microemulsion droplet. This static picture does not properly convey “the dynamic reality of the aggregates.”

Figure based on J. Phys. Chem. 100, 3190-3198 (1996).

 

 

 

 

 

 

 

 

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Wear eye protection Chemical gloves
and lab coat
recommended

Experimental Procedure

*Clean your test tubes before starting!

Note: The reagents have been tested by adding a drop of aqueous Zn2+ to a drop of aqueous S2-. A bluey-white colour should appear if the Na2S solution is good. If the mixture remains clear, remake the Na2S solution.

Dissolve surfactant:

• Add 0.20 g of the surfactant, e.g. hexadecyltrimethylammonium bromide, to a test tube. The weight can be 0.19 to 0.21g, as long as it is in excess.

• Add 4.0 mL heptane and 1.0 mL pentanol to the surfactant. (Some groups may be directed to use 0.5 ml of pentanol instead). Stir to give a suspension. A vortex mixer is found to do this best. Press test tube into rubber cup to activate – hold on tight! Immediately transfer half the suspension to a second tube. Stir both solutions to maintain the suspension. *Be quick! One person should stir the solution while the other person should gather ZnCl2 and Na2S reagents for the next part.

• To one test tube, add 0.1 mL of 0.012 M ZnCl2. The solution will be clear as hexadecyltrimethylammonium bromide micelles containing ZnCl2 form.

• To the second test tube, add 0.1 mL of 0.012 M Na2S. The solution will be clear as hexadecyltrimethylammonium bromide micelles containing Na2S form.

Precipitate ZnS nanoparticles:
• Mix the two solutions. The solution should go clear again. Record the UV absorption spectrum in a quartz cuvette. Note: Use a pentanol/heptane mixture of the same concentration as above in the reference cell. This removes the contribution of the solvents to the measured absorption spectrum. *Do not vortex beyond this point.

• In a quartz cuvette, add an equal amount of aqueous 0.012 M Zn2+ and aqueous 0.012 M S2-. Record your observations and immediately obtain the UV absorption spectrum (before the solution becomes too opaque). This is the bulk ZnS particle suspension.

 

 

 

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Calculations

The x-intercept of the linear portion of the absorbance as a function of wavelength graph is a measure of Eg. It can be shown that:
h
@ =

Where is taken from the x-intercept of the linear portion of the absorbance-wavelength graph.

When the absorbance spectra of ZnS particles of different size are measured it is found that there is a blue shift (towards a shorter wavelength) of the spectra as the particle size decreases. (Figure 1) This indicates a strong quantum size effect. This shift can be utilized in determining the crystal radius using the effective mass model of Yoffe (A.D Yoffe, Adv. Phys., 42, 172 (1993))

 

 

 

 

 

 

 

 

 

 

Figure 4. Effect of particle size on the absorbance spectra.

 

 

 

 

 

 

where r is the radius of the nanoparticle.
The D + D I part represents the reduced mass of electron hole effective

C9E 9H
* *

mass, where me* is the effective mass of the electron = 0.34m0 and mh* is the effective mass of the electron hole = 0.23m0. mo is the electron mass (9.106 x 10-28g).