experiment b student document v3 -...

NANOPINION EXPERIMENT B- NANOSCALE THIN FILMS

The research leading to these results has received funding from the European Community's Seventh Framework Programme

(FP7/2007-2013) under Grant Agreement NMP.2011.3-4-290575

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SAFETY NOTE: The chemicals used in this experiment need to be used according to MSDS specifications. Personal

protection must be taken as indicated. As with all chemicals, use precautions. Solids should not be inhaled and contact with

skin, eyes or clothing should be avoided. Wash hands thoroughly after handling. Dispose as indicated. All experiments must

be conducted in the presence of an educator trained for science teaching. All experiments will be carried out at your own

risk. Aarhus University (iNANO) and the entire NANOPINION consortium assume no liability for damage or consequential

losses sustained as a result of the carrying out of the experiments described.

STUDENT LABORATORY WORKSHEET EXPERIMENT B:

NANOSCALE THIN FILMS

Student name:……………………

Date:…………………………………..

AIM: Thin films with nanoscale thickness are interesting novel materials that are being investigated in

“smart” windows (for instance electrochromic and thermochromic thin films) and in biosensors (for

medical detection, food monitoring, etc.). In this experiment you will produce electrochromic thin films

of Prussian Blue having different thickness through electrodeposition. To perform the synthesis, you will

build a graphite counter electrode. In the second part of the experiment, you will study the optical

properties of the thin films (absorbance) and verify a fundamental law in optics, the dependence

between film thickness and magnitude of the absorbance. In the third part of the experiment the you

will verify the well-known electrochromic properties of Prussian blue thin films.

Written by Luisa Filipponi (iNANO)

Interdisciplinary Nanoscience Center

Aarhus University, Denmark

September 2013

Text and Images: L. Filipponi, iNANO, Aarhus University, Creative Commons Share Alike

Non-Commercial 3.0

This document has been created in the context of the NANOPINION project. All information is provided “as is” and no

guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the

information at its sole risk and liability. The document reflects solely the views of its authors. The European Commission is

not liable for any use that may be made of the information contained therein.




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MATERIALS:

The items below are indicated assuming students will work in pairs. Each pair should get:

Several pieces of cleaned ITO conductive glass (your teacher will provide the ITO glass already cut.

Remember to wear gloves when you handle it!)

One graphite electrode (or several HB leads, if students need to make their own graphite electrode)

One food container with plastic lid1 (with a capacity between 50 to 100mL approx.).

One bottle cork

Two metal wires with insulator coating having an alligator clip solded at one end

Electric wires

Galvanostat (or an alternative tool; you might need to share this with the rest of the class, the teacher

will inform you)

0.05 M Ferric chloride hexahydrate solution (FeCl3) : from stock solution; each thin film synthesis needs

10mL2;

0.05 M Potassium ferricyanide(III) (K3Fe(CN)6) : from stock solution ;each thin film synthesis needs 10mL

0.05 M HCl : from stock solution; each thin film synthesis needs 5mL

Three 25 mL or 50 mL cylinder3

A small plastic container (to hold the ITO thin film samples after they are prepared), or closed test tube.

A paper cutter

25mL of the 1.0 M KCl stock solution

One 1.5 V battery

A multimeter (you will probably share this tool with the rest of the class)

Tweezers

Gloves

Paper

Eye protection

1 Prefer tall, rather than shallow, containers. In this protocol, a 70 mL capacity FRIGOVERRE (Bormioli) 12 tall with

blue lid has been used.

2 Your teacher will tell you how many samples you will need to prepare and therefore how much of the stock

solution you need.

3 Or if not available, use less but rinse in between measurements.




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PROCEDURE

1. Electrodeposition of a Prussian Blue thin film

The first part of the experiment consists in the preparation of a thin film with nanometre thickness.

There are numerous methods that researchers use in their laboratories to prepare thin films, for

instance spin coating, sputtering, and dip-coating. Under specific conditions, and using some specific

instrumentation, these methods allow the formation of thin films with controlled morphology,

chemistry and thickness. In a school laboratory those instruments are available; therefore it is suggested

to prepare the thin film by electrodeposition, using a simple electrochemistry cell designed for this

experiment. The advantage of this method is that it allows obtaining thin films with controlled film

thickness just by controlling the synthesis time. This method is interdisciplinary, since it combines

fundamental aspects of chemistry and physics.

In this experiment you will prepare nanoscale thin films of Prussian Blue, a well know electrochromic

material.

Prussian Blue can be deposited as a thin film over a conductive surface by electrodeposition using an

aqueous solutions of ferric chloride (FeCl3) and potassium ferricyanide(III) (K3[Fe(III)(CN)6]3). The

synthesis requires an acidic media, hence diluted HCl is added as well.

During the deposition, K3[Fe(III)(CN)6]3 is reduced at a glass electrode to produce potassium ferrocyanide

(K4[Fe(II)(CN)6]3) . The K4[Fe(II)(CN)6]3 at the glass electrode then reacts with the Fe(III)Cl3 in solution to

give insoluble Prussian Blue, Fe4[Fe(CN)6]3.

STEP 1. FIND THE CONDUCTIVE SIDE OF THE ITO GLASS

In this experiment you will use a piece of conductive glass (called ITO4 glass) as working electrode. The

deposition of the nanoscale this film will occur on this surface. Hence in this part you will determine

which is the conductive side of the ITO-glass using a multimeter to measure resistance; the conductive

side will have a resistance of 12-25 ohm, depending on the product you use. Take your clean ITO

samples by holding them with gloves (to avoid finger prints!) and measure the resistance on both sides.

4 ITO stands for Indium tin oxide; ITO is one of the most widely used transparent conducting oxides because of its

electrical conductivity and optical transparency.




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When you find the side that is conductive, mark it using a permanent marker on the right hand corner.

Repeat for all ITO glass samples the teacher has given you.

Keep cut glass-ITO pieces in a closed vial or closed beaker to avoid dust.

STEP 2. ESTIMATE THE AREA OF ELECTRODEPOSITION

In this experiment you will immerse the ITO glass inside a solution and a nanoscale film will form once

you start the electrosynthesis; the area of the nanoscale thin film depends on the area of the ITO glass

that is in contact with the solution. Therefore you need first to estimate the area of deposition, in order

to decide how much current to give during the electrosynthesis. You want to give 40 µA/cm2; If the

area immersed in the solution is different from 1 cm2, you will need to accordingly adjust the current

used for electrodeposition (for example, 80 µA if area is 2 cm2). You should first measure the area of the

working electrode that will be immersed (you can put a line with a marker to define it), and calculate the

current that should be used to reach 40 µA/cm2.

To do so, take your ITO sample and measure its width; now measure the height of your deposition area,

taking into consideration that you need to leave some space for the alligator clips (which MUST NOT

touch the solution!). See example below:

ITO slide:

Record here your data5:

WIDTH HEIGHT AREA CURRENT

SAMPLE 1

SAMPLE 2

SAMPLE 3

5 Depending on how your teacher organizes this experiment, you will have one or more samples

Area that will be immersed in the synthesis solution

and where the thin film will be deposited. This is the

area that you need to calculate

Area that will NOT come into contact with the

synthesis solution (you will use an alligator clip to

hold the slide from here)




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STEP 3. BUILD A GRAPHITE COUNTER ELECTRODE

If your teacher has already provided you with a graphite (or Platinum) electrode, you can skip this

part.

In the experiment you will use a graphite electrode ad the counter

electrode. A simple way to create a graphite electrode is to use graphite

leads HB 2mm in size. To create the graphite counter electrode, simply

take five graphite leads: one should be longer and placed in the middle

(the alligator will be attached to this lead), and place the remaining four

next to it, two on each side. Wrap them together using a conductive tape

or conductive paste. Then cut the leads at the bottom so they are all the

same length. See Figure 10 for the final product. The dimension of your counter electrode should be

similar to the size of the working electrode. Before using the graphite electrode polish gently its surface

with a mesh. Note that the working and counter electrode should have similar size.

STEP 4. Make a “ready to use” electrochemical cell

If your teacher has given you a “ready to use” electrochemical cell, you can skip this part, and go to

Step 5.

Wash with soap/water the food container and rinse well. Air dry. Cut the wine cork in two pieces using

a paper cutter (careful!!). Open the food container and cut the plastic lid with a cutter to create two

parallel holes where you will insert the electrodes. Make a third hole somewhere else in the lid (safety

hole for letting gases out). Place the two metal wires with the alligator clips through the lid where you

created the holes. Insert the wine cork inside the wire to add stability to it (see Figure 11). Don’t add the

electrodes yet. The cell is “ready for use”.

STEP 5. Electrochemical cell set up

Take a “ready for use” electrochemical cell and place the working

electrode and counter electrodes using the alligators. Push the

wires upwards so that the electrodes are close to the lid.

Figure 1. The electrochemical cell once it is set-up and “ready for use”. (Image credit: L. Filipponi, iNANO, Aarhus University,

Creative Commons Share Alike Non-Commercial 3.0)




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Now measure and mix the synthesis solution directly in the glass food container. NB Open the food

container, remove lid and set aside, add solutions and mix gently with a glass rod or spoon:

10 mL of 0.05 M K3[Fe(CN)6]

10 mL of 0.05 M FeCl3.6H2O

5 mL of 0.05 M HCl

Assemble the cell by placing the lid with the working electrode

parallel to the counter electrode. Electrodes should have similar

size, and the electrical contact (i.e., the alligator clips) should not

touch the solution. The electrodes should NOT touch the solution

at this stage.

Now push gently the working electrode to touch the solution up to

the point you have marked; push down also the counter electrode

of graphite. Remember that the alligators should NOT touch the

solution. Make sure the conductive face of the ITO glass is in front the counter electrode, and that they

are parallel.

STEP 5. Perform the electrosynthesis

This part should be done with the supervision of your teacher.

Connect the working electrode (glass-ITO) to the negative lead of the galvanostat and connect the

positive lead to the counter electrode (Pt or graphite). If you don’t have a galvanostat, your teacher will

provide you with an alternative tool.

Perform the synthesis on different samples for different time6. It is suggested to start with a fresh

solution at each synthesis (especially for long synthesis time, i.e., over 60 seconds)

After each synthesis, rinse the working electrode with distilled water and place on a piece of paper

towel to let it dry. If not used immediately after, store samples in a closed plastic container.

6 The number of synthesis you will perform depends on how many ITO samples your teacher will give you.




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Previous studies have found a correlation between deposition time and estimated thickness, which is

given by the equation: Thickness (nm)= 0.88*time of deposition

Calculate the estimated thickness of your samples.

NB. Your teacher might have given you one or more samples; fill in the table accordingly.

SAMPLE NAME DEPOSITION TIME (seconds) ESTIMATED THICKNESS (nm)

SAMPLE 1

SAMPLE 2

SAMPLE 3

SAMPLE 4

Figure 2. The working electrode after the electrochemical synthesis. (Image

credit: L. Filipponi, iNANO, Aarhus University, Creative Commons Share Alike

Non-Commercial 3.0).

2. Thin film optical properties

In this part of the experiment, you will use an indirect method for obtaining information on the

absorbance of the thin films you have produced. In this method the visible spectra of your samples is

obtained indirectly from the RGB values of their digital colour image (which give the colour intensities of

red, green, and blue for pixels within the selected area in the image). The digital colour image is

collected using a desktop scanner.

For each sample you have made, collect a transparency scanner image by placing the samples face-

down on the central part of a desktop scanner. Place a piece of white paper over the samples, close the

lid of the scanner and scan to collect a JPEG image (300 dpi).

Open ImageJ on your PC and open the image you have just scanned. Using the square selection tool,

select an area of approximately 100x100 pixel and collect the RGB data for the selected area (Analyze




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Histogram). Repeat the same for the different samples by moving with the cursor the selection area to

the next sample (area should not be changed in size). Aim at the same location in each sample, e.g.,

middle part. Repeat for all samples; collect the same information for uncoated ITO.

Values of the “blank” (i.e., ITO uncoated) should be collected in an area of the sample where no film was

grown. ITO changes slightly colour (becomes a bit grey) as a current is passed through it, hence using an

un-used glass- ITO would not be appropriate.

Fill in the table below:

ITO

(BLANK)

SAMPLE 1 SAMPLE 2 SAMPLE 3 SAMPLE 4

R

G

B

TIME

(SECONDS)

How to calculate thin film absorbance from the RGB values?

The image analysis can be used to study the absorbance of your coloured thin films. You can use this

method to verify that the absorbance increases with film thickness. Prussian Blue has a maximum

absorbance peak at 690 nm which is in the red part of the visual spectra.

However the complementary colour of blue (in digital image analysis) is a combination of red and green,

hence you need to study how the sum of the red and green intensity varies with time.

Therefore in the method (which is approximate by definition!) the absorbance of your thin film in its

peak region (around 690nm) is studied by analysing the variation of the red plus green intensity values:




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A = log (I0/I ) where

log (I0/I) → where I0 is the “Red+Green” light intensity by image analysis of ITO (no coating) and I is the

“Red+Green” light intensity by image analysis of the sample with a given thickness.

STEP 1:

For each sample, use the table you have made above to calculate the “R+G” values.

Now for each, calculate A using this equation:

Asample = log(I0/Isample) =log (R+G)ITO/(R+G)sample

Lambert's law states that the absorbance (A) is directly proportional to the thickness of the sample.

You can calculate the thin film thickness using the equation below, which was determined

experimentally by other studies.

Prussian Blue deposited on the working electrode → 0.88 nm/s

Therefore you can calculate the thickness of your samples:

Thickness (nm)= 0.88 * time of synthesis

Do this for each sample you have made, and fill in the table below:

ITO

(BLANK)

SAMPLE 1 SAMPLE 2 SAMPLE 3 SAMPLE 4

R+G

log [(R+G)ITO/(R+G)sample]

TIME (SECONDS)

ESTIMATED THICKNESS

(nm)




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STEP 2:

Now you need to verify that there is a linear correlation between the thickness of the thin films you

have made, and their absorbance values.

Q. DO YOU EXPECT TO HAVE A PERFECT LINEAR CORRELATION BETWEEN FILM THICKNESS AND THE

VALUE OF THE ABSORBANCE YOU HAVE CALCULATED?

…………………………………………………….

Q. WHAT SOURCE OF ERROR(S) DO YOU THINK YOUR DATA COULD HAVE?

…………………………………………………….

The aim of this next part is to perform a data analysis to confirm Lambert’s Law which states that there

is a linear correlation between thickness and thin film absorbance. In a perfect linear dependency,

each value of thickness in X should be directly proportional to a value in Y, following a linear equation of

type Y=mX. However, these are experimental data, therefore each data carries an error, hence it is

expected that there will be some deviation from this perfect linear equation. A measure of this deviation

can be obtained performing a regression to find the linear function that fits the data best. In this case,

we use a software to find the “best linear correlation” between the data: the software finds an equation

but we need a parameter to decide how good this fit is (are data really linked through a linear

equation?). This is verified by a parameter called R2. This value should be as close as possible to “1”; with

experimental data, any value above 0.95 is considered very good.

Using the data in the table above, use a software (like Microsoft Excel) to produce a plot “A vs. time” 7.

For each sample → A= log(I0/I) =log [(R+G)ITO/(R+G)sample] (from table above)

Time= time of synthesis of sample (in seconds)

To do so, select the values of time as X values, and the log(I0/I) values associated at each time as Y

values; then select the two columns and ask the software to create a plot and find a linear regression

fit. The program also shows the linear regression equation.

You should get a plot similar to the one shown in Figure below:

7You can also plot A vs. thickness (in nm) using the data calculated before (Thickness= 0.88 * time).




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Write here the value of R2 you have obtained for your data:…………………………………………………

Q. BASED ON THE RESULTS YOU HAVE OBTAINED, CAN YOU SAY THERE IS A LINEAR CORRELATION

BETWEEN THE TIME OF SYNTHESIS OF YOUR SAMPLES AND THEIR OPTICAL ABSORBANCE?

……………………………………………………………………………………………………..

3. Testing Prussian Blue electrochromic properties

Prussian Blue shows electrochromism, which has attracted significant scientific attention in research, for

instance in the development of smart windows and biosensors.

The main application of the Prussian Blue thin films in the electrochromic devices is based on the

electroreduction that yields Prussian white (PW) also known as Everitt’s salt (and this is the process we

will be focusing on in this part of the experiment). During this process Prussian Blue thin films become

colourless.

In this part of the experiment you will use the Prussian Blue thin film you have prepared in Part 1 and

study their electroreduction to Prussian white (WP): you will apply a positive voltage using a 1.5 V

battery to the film and see the colour variation from blue to transparent.




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STEP 1

Wash and dry the food container you have used in Part 1; set-up the cell as you did in Part 1, using one

of the thin film Prussian Blue samples you have made as your working electrode.

NOTE. If you have more than one sample, it is suggested you use the one with higher thickness (150

seconds of synthesis), so you can better see the colour switch.

Wear gloves. Add 25 mL of KCl 1 M to the glass food container and add few droplets of HCl 0.05 M to

make the solution acidic. Mix with the glass rod. Gently place the plastic food container lid: electrodes

should be parallel and the alligators should not touch the solution.

STEP 2

To stabilize the film, it is suggested to first apply a negative voltage to the film: glass electrode to battery

(-) and graphite electrode to battery (+). Now bring voltage to zero, by connecting the glass electrode

directly to the graphite electrode: the film turns blue. After doing this a couple of times, you can test

the electrochromism from Prussian blue (PB) to Prussian white (PW): connect glass electrode (PB film)

to battery (+) and graphite electrode to battery (-): a positive voltage is applied and the film goes from

blue to transparent. To switch the film back to blue, bring voltage to zero, by connecting the glass

electrode directly to the graphite electrode (See Figure below)

Figure 3. Testing a sample of Prussian Blue thin film for electrochromism. (Image credit: L. Filipponi, iNANO, Aarhus

University, Creative Commons Share Alike Non-Commercial 3.0).

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