Oriented zinc oxide hexagonal rods are grown by aqueous thermal decomposition of hexamethylenetetramine serving as a kinetic pH buffer,
+ 10 H2O → 6 H2CO + 4 NH4+ + 4 OH-,
and subsequent formation of ZnO on F-doped SnO2 glass (FTO),
Zn+2 + 2 OH- → Zn(OH)2 → ZnO + H2O.
The tin oxide surface acts as seed crystals for the ZnO growth. Alternatively ZnO nanoparticles can be added as smaller seeds for more reproducible results.
ZnO is a semiconductor and these undoped materials are not very conductive. Shining UV light on ZnO excites electrons from the filled valence band to the empty conduction band where they can travel to the conductive FTO glass. Solution photoelectrochemistry (PEC) can demonstrate ZnO conduction in the presence of UV light.
The excited electron and/or its empty hole can also be used for photoremediation or treatment of wastewater. In this experiment methylene blue, a non-toxic dye that is not degraded by ultraviolet light, is used to simulate an organic pollutant in water. Absorbance spectroscopy can be used to measure the rate of methylene blue degradation.
Orientation of the ZnO crystals can be measured by powder x-ray diffraction (XRD) in comparison with the XRD of randomly-oriented ZnO powder.
You will do the synthesis ahead of time, preparing two (hopefully identical) ZnO-coated FTO samples. To do the analysis in one lab period we will work in groups of four. One pair of students will do the SEM and PEC experiments on one sample and the other pair will do the absorption spectroscopy, XRD, and decolorization of methylene blue experiments on the matched sample. You will need the results from both pairs to answer the report questions.
Synthesis (15 minutes operator time, Groups A and B together)
Clean two FTO glass samples with water and ethanol.
Find the conducting side of each clean FTO glass.
Preferred option: Add a few drops of ZnO seed solution in ethanol to the conducting side of each sample. Wait for the ethanol solvent to evaporate.
Use a permanent marker to write a small label in one corner of the seeded conducting side. (You should label the glass after the seed solvent evaporates since the ethanol will dissolve the label.)
Add 17 mL of 0.20 M Zn(NO3)2.6H2O solution and
13 mL of 0.20 M hexamethylenetetramine solution, , to an autoclavable 50 mL polypropylene centrifuge tube. Mix well.
Add the two FTO glass samples to stack at right angles.
Place the tightly-capped centrifuge tube in a 90°C oven for at least 3 hours (or a week.) This is a hydrothermal synthesis so the temperature is important; try to maintain a constant temperature.
Rinsing (Groups A and B together)
After sufficient heating remove the bottle from the oven.
Has the solution changed appearance? Discard the solution while still hot. Remove the glass from the centrifuge tube. Which side has more ZnO? Rinse the sample thoroughly with water several times to remove precipitates. The ZnO will be quite firmly attached. Blasting from a syringe is one way to rinse thoroughly.
The FTO conducting side can be examined without coating if carbon tape is used to connect the conducting side to the metal stage. Examine samples by SEM using a magnification of 1000 to 5000x.
Photoelectrochemical Experiments (PEC, Group A)
Place a Ag/AgCl reference electrode and a platinum or graphite auxiliary electrode in a 30 mL beaker. Use the ZnO-coated FTO sample as the working electrode. Check the volume needed to cover the ZnO-coated FTO electrode without immersing the aligator clip. The ZnO coating should face the UV light source (you labeled that side earlier.)
Add 0.010 M KOH up to the desired depth. When you are ready to proceed with the next step, insert the ZnO electrode.
Illuminate the electrode with a UV light source. For stable readings the light source must not move so clamp it in position. Record the current while alternately exposing and blocking the UV light source. (Run bulk electrolysis with 0.0 applied volts vs Ag/AgCl for a constant voltage with varying illumination. See Pine WaveNow directions below.)
Obtain the cyclic voltammogram without illumination. (Record current while scanning from 0 V to 1 V to -0.5 V to 0 V at 100 mV/sec for a constant illumination with varying voltage. See Pine WaveNow directions below.)
Then illuminate the electrode with a UV light source and repeat the cyclic voltammogram. Did illumination change the scan? If so, why?
Rinse the sample with water when finished.
Measure the powder x-ray diffraction of the sample, scanning 2θ from 30-40° (stepwidth 0.03, count time 1s, total time 6 minutes). It is important that the height of the sample be correct so a special holder is used. Compare the scan with that for powdered ZnO (shown at right) and for a clean FTO sample (FTO shows small peaks).
Absorption Spectroscopy (UV, Group B)
Measure the UV-visible absorption spectrum by using double stick tape to fasten the sample in front of the cuvet holder in the HP spectrometer and clicking on sample. Click on a spectrum, File/Export Selected Spectrum As/CSV Format. Choose a filename and later use the excel template to find the band gap.
Measure the initial absorbance of 10 µM methylene blue solution. Remove and discard the solution.
Place the sample with the zinc oxide side up in the bottom of a 50 mL beaker. Also place a plain FTO sample with the conductive side up in another beaker as a control. Add 5 mL of 10 µM methylene blue solution to each beaker.
Place the beakers under a UV light source. Cover the whole experiment with a box (not shown) to protect you from UV. Start timing when you turn on the UV light.
Do not remove the box unless the power to the lamp is off.
Six times over a 60 minute period, turn off UV-light source and stop timing. Measure the absorbance of the solution. What wavelength will you graph as a function of time? (The data for both samples should be recorded in one data file.) Return the solution to beaker. Cover with a box and turn on the UV-light and resume timing. Repeat. Plot absorbance versus exposure time.
At the end of the experiment rinse the sample with water.
For a first-order degradation reaction the change in methylene blue concentration is proportional to the methylene blue concentration:
d[A]/dt = -k [A]. Integrating this expression gives [A] = [A]o exp(-k t). Taking the ln of the integrated equation gives a linear form, ln[A] = ln[A]o- k t, where a plot of ln[A] versus t is a straight line with slope -k and the half life is t1/2 = (ln 2)/k.
Conclusions (Groups A and B together)
What evidence do you have for preferential growth along a particular crystal face?
What is the wavelength for the band gap of your sample?
While applying 0 V vs Ag/AgCl, by what factor did the current change in the presence of a UV light source? (Which UV light source did you use?)
What is the resistance of the sample with and without illumination? Hint: Apply Ohm's Law to your cyclic voltammetry results and show your calculations. (Which UV light source did you use?)
Based on your measurements of the absorbance as a function of time, what is the rate constant and how long would it take to remove half the methylene blue using UV light?
Summarize the evidence that ZnO is a semiconductor.
0.20 M Zn(NO3)2.6H2O, Aldrich 228737. Dissolve 5.76 g in 100 mL water.
0.20 M Hexamethylenetetramine, C6H12N4, Aldrich H11300 or 398160. Dissolve 2.93 g in 100 mL of water. Use within a few days.
ZnO seed crystals prepared from Zn(CH3CO2)2.2H2O and NaOH in ethanol rather than isopropanol following the procedure from J. Chem. Educ. (2005) 82, 775-778 or Synthesis of ZnO Nanoparticles (See http://chemistry.beloit.edu/edetc/nanolab/ZnO). Prepared ethanol seed solutions kept in a freezer have been successfully used after storage for 4 months.
0.010 M KOH. Dissolve 0.56 g KOH in a liter of water.
10 µM methylene blue, Aldrich M9140. Dissolve 0.019 g in 250 mL water to make a stock solution, then dilute 5 mL of stock to 100 mL for the sample solution.
Photodegradation equipment (visible absorbance spectrometer, 50 mL beakers, cuvets, plastic pipets for changing samples)
Pine WaveNow Electrochemistry Directions
Connect the computer to the WaveNow Potentiostat using the USB cable.
Connect the power cord to the potentiostat and turn it on.
Connect electrodes to the aligator clips coming from the WaveNow instrument. Be sure to keep the aligator clips out of the solution. The metal clips should not touch each other or any clamps.
Red: Working Electrode (ZnO-coated FTO glass).
Green: Auxiliary Electrode (graphite rod).
White: Reference Electrode (Ag/AgCl).
The orange lead should be plugged into the red banana plug.
Run the AfterMath software.
Wait for the potentiostat to appear in the AfterMath Instrument List (0).
Create an empty archive to store your data (1).
Choose the Bulk Electrolysis method or the Cyclic Voltammetry method from the Perform an experiment list or from the Experiments menu. (2)
For bulk electrolysis, set electrolysis potential 0V, duration 120s, and number of intervals 1200.
For cyclic voltammetry, use 3 segments from 0 V to 1 V (rising direction) to -0.5 V to 0 V at 100 mV/sec.
Click on the perform button. (If the button is gray, make sure that the basic parameters are filled in and that an instrument has been selected.) The current will be plotted as a function of time and the results are placed in the archive.
Click on BE Parameters or CV Paramaters in the Archive to run again or make changes.
To overlay scans, right-click on the name of the archive and select new plot. Then copy-and-paste "current vs potential" under "voltammogram" onto the plot for each of the scans to be overlayed.