Evaluation for the experiment is done through a worksheet-style lab report on which students report their results and answer several questions to probe their understanding of nanoparticle synthesis and powder diffraction.
I have piloted this experiment four times with a total of 120 students. Every partnership has successfully synthesized ZnO, with crystallite sizes mostly in the range of 20-50 nm. The lab worksheet is graded on a 10-point scale, and class averages have ranged from 8.4-9.0 out of 10. Students usually correctly interpret their results to determine whether or not they have successfully made ZnO. Some students make unit conversion errors in calculating crystallite sizes. Students sometimes get confused between d-spacings and particle sizes and incorrectly say that a structure with a larger unit cell would be expected to have narrower diffraction peaks.
I designed this lab experiment to introduce students to the uses of powder X-ray diffraction in the context of the synthesis of a technologically relevant material. Zinc oxide nanoparticles can be synthesized readily with reagents that are inexpensive and relatively benign with regard to student use and waste disposal. Two experiments described in J. Chem. Ed. feature synthesis of ZnO nanoparticle sols and films and their characterization by UV-vis spectroscopy (see web resources below). This new experiment shows powder X-ray diffraction as a tool for characterization of nanomaterials.
In the experiment, students use the base hydrolysis of zinc acetate in ethanol to prepare a sol of ZnO particles. Students add water to the sol to agglomerate the particles and use centrifugation and drying to isolate a solid for characterization by powder X-ray diffraction. Students identify the phase of their solids by matching to a reference from the Powder Diffraction File, and then estimate crystallite sizes by applying the Debye-Scherrer equation.
This material is based in part upon work supported by the National Science Foundation under grant number DMR-0922588. Any opinions, findings, and conclusions or recommendations are those of the author and do not necessarily reflect the views of the National Science Foundation.
Students should be able to:
1) Explain how solution reaction conditions can be designed to control particle growth
2) Use powder diffraction data to identify phases and approximate crystallite sizes in a sample
3) Apply Bragg’s law to predict how diffraction peak positions compare for isostructural compounds with different unit cell dimensions
4) Explain why X-rays are dangerous and what safety features are built into a modern X-ray diffractometer
Zn(OAc)2·2H2O (0.55 g per reaction)
LiOH·H2O (0.15 g per reaction)
Note: When exposed to humidity and CO2 over time, lithium hydroxide can convert to lithium carbonate. When partially converted material is used in the reaction, students have difficultly fully dissolving the lithium salt in ethanol, and the yield is reduced significantly. Storing the LiOH well sealed and away from humidity (for example, in a desiccator) preserves the salt.
Equipment (for each student or partnership):
Weighing paper or boats
two magnetic stirplates
two magnetic stirbars
two 125-mL Erlenmeyer flasks
two basins for ice baths – at least one will need to be placed on the stirplate and should be of a non-ferromagnetic material
three 13 x 100 mm test tubes
Equipment (for the entire class):
Abderhalden drying pistol or other apparatus for drying under vacuum
Oven (can be a drying oven) capable of holding a temperature of 120 oC
Access to a powder X-ray diffractometer
Students typically work in partnerships for the experiment. I do the experiment over two weeks. Students work full-time on the synthesis during the first week and do the characterization by powder X-ray diffraction in the second week. Because of radiation safety regulations, the students can't run the diffractometer themselves without completing safety training that is prohibitively time-consuming for the number of students involved. The diffractometer is run by a fully trained undergraduate TA or by me, and the students prepare their samples for the experiment, see the instrument working, and process their data files on the computer.
Each partnership spends approximately 15 minutes at the instrument for the characterization, so there is time during the second week for another activity. A good complementary activity that students can easily step in and out of is solid-state model building. I do this lab during the solid-state structures unit in my intermediate-level inorganic course. In the lecture part of the course, students are introduced to X-ray diffraction, with a focus on the Bragg law and the relationship between structures and diffraction peak positions.