STUDENT LABORATORY WORKSHEET EXPERIMENT B: NANOSCALE THIN FILMS

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1 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. 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. 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. Page 1 of 12

2 MATERIALS: NANOPINION EXPERIMENT B- NANOSCALE THIN FILMS 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 lid 1 (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 (FeCl 3) : from stock solution; each thin film synthesis needs 10mL 2 ; 0.05 M Potassium ferricyanide(iii) (K 3Fe(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 cylinder 3 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. Page 2 of 12

3 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 (FeCl 3) and potassium ferricyanide(iii) (K 3[Fe(III)(CN) 6] 3). The synthesis requires an acidic media, hence diluted HCl is added as well. During the deposition, K 3[Fe(III)(CN) 6] 3 is reduced at a glass electrode to produce potassium ferrocyanide (K 4[Fe(II)(CN) 6] 3). The K 4[Fe(II)(CN) 6] 3 at the glass electrode then reacts with the Fe(III)Cl3 in solution to give insoluble Prussian Blue, Fe 4[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 ITO 4 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 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. Page 3 of 12

4 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/cm 2 ; If the area immersed in the solution is different from 1 cm 2, you will need to accordingly adjust the current used for electrodeposition (for example, 80 µa if area is 2 cm 2 ). 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/cm 2. 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: Area that will NOT come into contact with the synthesis solution (you will use an alligator clip to ITO slide: hold the slide from here) 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 Record here your data 5 : SAMPLE 1 SAMPLE 2 SAMPLE 3 WIDTH HEIGHT AREA CURRENT 5 Depending on how your teacher organizes this experiment, you will have one or more samples Page 4 of 12

5 STEP 3. BUILD A GRAPHITE COUNTER ELECTRODE NANOPINION EXPERIMENT B- NANOSCALE THIN FILMS 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) Page 5 of 12

6 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 K 3[Fe(CN) 6] 10 ml of 0.05 M FeCl 3.6H 2O 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 time 6. 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. Page 6 of 12

7 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 facedown 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 Page 7 of 12

8 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: Page 8 of 12

9 A = log (I 0/I ) where log (I 0/I) where I 0 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: A sample = log(i 0/I sample) =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 (nm) THICKNESS Page 9 of 12

10 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 R 2. 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(i 0/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(i 0/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: 7 You can also plot A vs. thickness (in nm) using the data calculated before (Thickness= 0.88 * time). Page 10 of 12

11 Write here the value of R 2 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. Page 11 of 12

12 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). Page 12 of 12