FINAL REPORT - Functionalizing Diatoms with TiO2 for Solar Cell Applications

Florida Polytechnic Sustainability Innovation Competition Submitted: April 5, 2016

Functionalizing Diatoms with TiO2 for Solar Cell Applications

Chris Dowdy – [email protected]

Dalton Reith – [email protected]

Samuel Trappen – [email protected]

Dr. Melba Horton – [email protected]

Dr. Chris Coughlin – [email protected]

Dr. Sesha Srinivasan – [email protected]

Florida Polytechnic University - 4700 Research Way, Lakeland, FL 33868

Dr. Sarah Spaulding – [email protected]

University of Colorado - Campus Box 450, University of Colorado, Boulder, CO 80309

Executive Summary Diatoms are microscopic eukaryotic algae that obtain energy through photosynthesis. They have a very unique physiological mechanism of obtaining nutrients from the aquatic environment to create a porous silica shell around themselves called frustules. After the cells die, these frustules which vary in size (2 to 500 microns) and shapes (centric or pennate) can be harvested for several unique applications. The world today is in dire need of a more sustainable source of energy. One of the most readily available is solar cells. Nevertheless, the cost of production outweighs the benefit. This project is conceptualized with a long term goal of enhancing the efficiency of the current forms of solar cells by incorporating diatom frustules modified by physico-chemical processes to serve as photoelectrodes. These functionalized diatoms will increase the surface area of the solar cell and consequently enhance the total electrical output. In order to attain the overall goal, the first object of this project is to functionalize diatom cells. Three different species (Cyclotella, Synedra, Didymosphenia) of varying size and shape are explored and titanium dioxide is the semiconductor being used. Living cells produce organic materials of which the degree of chemical attraction with titanium dioxide is not yet established. Therefore, a bleach cleaning procedure was developed and tested. Further, as a proof of concept, titanium dioxide nanoparticles were successfully inserted onto a diatom frustule. This outcome is very promising and is pointing to the next steps of testing the functionalized diatoms on actual solar cells. The research team envisions pursuing the work by submitting a research proposal to funding agencies/industry partners.

Page 2 of 17 Functionalizing Diatoms with TiO2 for Solar Cell Applications

Table of Contents

Page

Executive Summary ......................................................................................... Title

Objective ......................................................................................... 3

Methodology ......................................................................................... 7

Results and Discussion

......................................................................................... 8

Future Research ......................................................................................... 13

Bibliography ......................................................................................... 15

Appendix A: Media Presentation

......................................................................................... 17


Objective

The Problem Modern day solar panels are unfortunately considered inefficient. Although solar energy has garnered much media attention, the average solar panel actually only has an efficiency of about 11-15% (Toothman 2000). Scientists have been able to produce advanced solar panels that can reach about 25% efficiency, however, they are far too expensive to produce commercially (Green 2009). To increase solar cell efficiency, researchers have looked to the natural process of photosynthesis as a guide. Unfortunately, plants also have some efficiency issues. During the light dependent reactions in the Calvin cycle, photon energy is transferred into mechanical energy. When a plant produces six G3Ps, also known as the plant’s potential energy, it uses around four of those potential energy G3Ps to produce more. To increase efficiency, improvements in the current state of the art for photovoltaic systems are sought. This project aims to use principles from both plants and mechanics. The union of these two means can potentially boost the efficiency of modern photovoltaic systems. Current Research Titanium dioxide (titania) is a semiconductor compound with photosensitive and photocatalytic characteristics. When titanium dioxide is struck by a photon carrying an adequate amount of energy, a valence electron is knocked out of its orbit (Khataee 2012). This free electron will be looking for something else to which to bond. Inside a solar cell, the functionalized diatoms will sit on top of a layer of silicon and trace phosphorous elements. This will create a

positively charged layer with many open spaces attracting the free electrons. The free electrons will start moving towards this positive layer and will adhere to some of these open spaces. As more electrons attempt to cross to this positive layer, it will create an electric field and keep more electrons from coming in (Toothman 2000). A grid made out of carbon nanotubes (CNT), a highly conductive material, will be placed under the silicon phosphorus layer and will conduct the electrons out for useful work. There are currently no commercially available bio devices that utilize diatoms in a similar manner. Currently, the only devices that use diatoms have been used for the detection of antibodies (Jeffryes 2011). However, there have been applications and research on the matter on energy harvesting and solar applications such as diatom dye-sensitized solar cells (DSSC) (Gratzel 2003). The most typical DSSC configuration has a ruthenium based dye bound to a photo anode comprised of anatase TiO2 nanoparticles. To produce current, a dye molecule absorbs a photon and in the photo

Figure 1. Diagram of traditional DSSC (A) and DSSC with diatom addition (Jeffryes 2011).


excited state injects an electron into the conduction band of the semiconducting TiO2, creating an electron-hole pair (EHP). The electron diffuses through the anatase titania layer to a transparent conductive glass front electrode and then performs work when passed through a load. An electron from the electrolyte replenishes the hole (Jeffryes et al. 2011). Figure 1 shows a traditional DSSC and an improved DSSC exploiting the light-scattering properties of a diatom frustule-TiO2 composite material. Project Goals This project has two primary goals. The first is to establish the best method for ‘cleaning’ diatoms to optimize the bonding between the diatom frustule and titanium dioxide. The second objective is to functionalize different shapes of cleaned diatoms with titanium dioxide, mimicking the thylakoid inside the chloroplast of plants, for solar cell application. Goal 1 – Diatom Cleaning The most common method for cleaning diatoms involves boiling the frustules in acid (usually nitric or sulfuric). In some laboratory procedures peroxide is used as a cleaning agent (Sterrenburg 2006). Originally, this project sought to use both cleaning agents. However, due to the current laboratory set-up of the university, the group ended up utilizing commercial bleach as an alternative. In many ways, bleach can be regarded as a better option for the following reasons; 1), commercial bleach is a much safer chemical agent with which to work, and 2) if this process works and is ventured for commercial production, the expenditure will be much lower than using acid or peroxide. Bleach works as a cleaning agent because sodium hypochlorite (NaOCl) is a strong oxidizing agent. The bleach oxidizes the

remaining organic material inside dead diatoms, destroying the cell walls and structure (Sandoval 2009). Furthermore, the hypochlorous acid produced when bleach is dissolved in water has been shown to alter proteins in single celled organisms which leads to their death and eventually dissolution (Winter et al. 2008). While placing diatoms in a strong base concentration can dissolve the frustule structures (Carr 1986), the dilutions used in this experiment will alleviate that concern. The cleaning protocol employed in this experiment using bleach is safe, mass producible and cost effective. Deeper knowledge and research of the cleaning process alone could be another meaningful step toward understanding the functionalization of diatoms for a more affordable and highly efficient solar cell production. Goal 2 – Diatom Functionalization Within the chloroplast of green leaves, there are tiny organelles called grana. These grana are made out of stacks of little disks called thylakoids which are where the light dependent reactions in photosynthesis take place. Figure 2 shows a diagram of grana and

Figure 2. Grana and thylakoid in chloroplast

(Cogdell 2013).


thylakoids in chloroplasts. When the grana are struck by sunlight, it causes a reaction to take place where water is separated into its individual hydrogen and oxygen atoms (similar to electrolysis). The hydrogen then moves on to the light independent reaction where it is used to make glucose. The oxygen is released into the air as a beneficial waste product. Because these thylakoids are so adept at capturing light, this project is attempting to replicate thylakoids mechanically using diatoms and implement them into a photovoltaic system. Research has shown that functionalized diatoms can boost the overall light collection efficiency (quantum yield) of a solar cell (Jeffryes et al. 2011). Three species of diatoms were used for functionalization and testing in this project: a centric disk-shaped (Cyclotella sp.), a pennate symmetrical rod-shaped (Synedra sp.), and another pennate bilaterally symmetrical biraphid hourglass-shaped (Didymosphenia geminata). Cyclotella grows a very flat and strong frustule that averages around 20 μm in diameter (Lowe 2011). The round centric shape is a close mimic of the natural

thylakoid. A conductive percolation network should be achievable and similar to a natural network. Also, the species is hardy and available. Figure 3 is a scanning electron micrograph of Cyclotella distinguenda. Synedra sp. has a distinct thin rod shape that could be an ideal candidate to construct a percolative network inside a solar cell. These frustules are long, slender and sturdy. They should be able to hold up well in the mechanical process required for placement inside a photovoltaic cell. The average length is 30 to 40 μm and 3 μm in width (Bahls 2012). An SEM image of Synedra famelica is shown in Figure 4.

Didymosphenia geminata is commonly called ‘Didymo’ in the scientific community and ‘Rock Snot’ to naturalists because of the mucus resembling mats it can leave in streams. D. geminata is native to the waters of the far north. However, this invasive species has been on the move and causing problems. It is perhaps the most invasive species in North America. The species is creeping to the south and clogging up numerous waterways along its route. It is, therefore, readily available for samples. D. geminata is a relatively large diatom, with an average aspect ratio of 120 μm long and 40 μm wide (Spaulding 2010). This particular aspect ratio should provide an efficient percolation network. The frustule is a strong silica shell with varying sizes of pores. Figures 5 and 6 show scanning electron

Figure 3. SEM image of Cyclotella distinguenda. Scale bar equals 2 μm (Lowe 2011).

Figure 4. SEM image of Synedra famelica. Scale bar equals 10 μm (Bahls 2012).


microscope images of the D. geminata frustule.

These three diatom species, once functionalized and tested, will provide good data on solar cell efficiency related to shape. This data should help the scientific community move forward in a pursuit of more proficient solar energy technology. Long-term, the goal is to lessen the need for fossil fuels and increase the ability to meet those needs with renewable, sustainable energy from the sun, mechanically mimicking a process plants have been using since their origin. Opportunities for Community Sustainability Clearly, the most notable societal benefit of this research would be greater efficiency from solar cells. Every improvement makes solar energy a more viable alternative to fossil fuel based power. Moreover, every new solar cell that goes online eliminates more carbon emissions in the atmosphere.

Perhaps the most likely candidate to buy this technology are the manufacturers of solar panels. According to Forbes Magazine, the biggest producers of solar panels in the world are Trina Solar, Yingli Green Energy, Canadian Solar, Dyesol and Jinko Solar (Wang 2014). The overwhelming majority of these companies and their vast market share are foreign. This technology could potentially shift the eco market to the USA. Another important societal benefit to this project could be the useful harvesting of D. geminata from the streams of North America. It is no exaggeration that this species is choking out many freshwater streams and water bodies on the continent. Environmental agencies across the country are undergoing massive efforts to remove mats of D. geminata remains (Jellyman et al.

Figure 6. SEM image of Didymosphenia geminata taken from a 1 to 1 cleaned sample.

Figure 5. SEM image of Didymosphenia geminata. External view, apical pore field at

foot pole. Scale bar equals 20 μm (Spaulding 2010).


2011). If this organism could be usefully employed and valued, it would incentivize both government and private organizations to help eliminate its impact to the environment while putting it to better use.

Methodology Diatom Sample Acquisition The three species of diatoms used in this experiment were: Didymo geminata, acquired from the University of Colorado, hour-glass shape. Synedra sp., currently grown in the Florida Poly Lab (1031), rod shape. Cyclotella sp., currently grown in the Florida Poly Lab (1031), disk shape. Cleaning the Diatom Samples Cleaning the diatoms is critically important to the work of this project. Diatoms with organic matter still attached might or might not be as receptive to the titanium dioxide for functionalization and eventual insertion into a solar cell. Also, if organic matter remains inside the diatom, this might shade the light collecting ability of the silica frustule. Popular methods of cleaning diatoms require boiling samples in highly corrosive acids, which sometimes create an explosion risk, or in strong peroxide, which is also highly corrosive. Because of these safety concerns, a protocol for cleaning diatoms with commercial bleach was developed using previous work from the Department of Forestry (Carr et al. 1986). All three species of diatoms in this project were cleaned on February 15, 2016. Controls and tests were gathered and performed in triplicate for each diatom species in standard test tubes with rubber

stoppers. Samples were cleaned by adding varying levels of commercial bleach to the test tubes (purchased from Publix, 8.25% sodium hypochlorite). The control samples were prepared with 5 mL of diatom sample mixed with 5 mL of DI water, in three replicates for each species. The highest concentration of bleach used to clean the samples were prepared in a 1:1 ratio (5 mL sample for 5 mL commercial bleach), resulting in a 4.125% bleach concentration. The other concentrations were prepared in 2:1 (5 mL sample to 2.5 mL commercial bleach, resulting in a 2.75% bleach concentration), and 4:1 (5 mL sample to 1.25 mL commercial bleach, resulting in a 1.65% bleach concentration) ratios. Each treatment condition for every diatom species used was prepared in three replicates. The bleach solution was allowed to interact with the diatom samples for 60 minutes. During this time, the test tubes were agitated (20 seconds) twice. The bleach was rinsed from the diatoms by filling the test tubes with DI water, allowing the diatoms to settle to the bottom of the test tube, decanting off the top liquid and refilling the test tube with new DI water three full times. After three rinses, the test tubes were left undisturbed in the lab (1031) for seven days. Functionalizing the Diatoms with Titania The solution to gelation (sol-gel) method was chosen to obtain titanium dioxide particles (Jeffryes 2008). This method is a simple, repeatable and successful way of obtaining the titanium dioxide needed to functionalize the diatoms. This method will allow the shape, size, and the crystallinity of the resultant nanoparticles to be controlled throughout the process. The reaction is a controlled sol–gel hydrolysis of solutions of


titanium isopropoxide. The protocols below were performed on March 25, 2016. One mL of titanium isopropoxide was mixed with 3 mL of ethanol in a flask (Flask A). In another flask, 50 mL of deionized water, .500 mL of sulfuric acid (58%), and a sample of 1 to 1 cleaned D. geminata diatom frustules (~0.1 g) were mixed together (Flask B). The contents of Flask B were then added to Flask A. The reaction in the flask quickly formed a white, milky dispersion, characteristic of a titanium dioxide suspension. The contents in Flask A were set on a hot plate and stirred at 80 °C for 30 minutes, then lowered to 60 °C for another 30 minutes. The solution in the flask was left to continue stirring for about ~72 hours. Soon after turning off the stir bar, the particles settled into a white powdery bed in the flask. Six separate samples (1.5 mL each) were pipetted into microcentrifuge tubes. The samples were centrifuged for 2 minutes at 14.5 x 103 rpm. Then, 1.25 mL was decanted from each tube. To rinse the samples, 1 mL of DI water was placed back into each microcentrifuge tube and shaken for 30 seconds to re-disperse the substance. The tubes were centrifuged again at the aforementioned settings and 1 mL of liquid was decanted from each tube, leaving a white pellet of titania and frustules.

Results and Discussion Tests on Cleaned Diatoms The cleaned diatom samples were first viewed under a compound light microscope for qualitative observation of the cleaning procedure. Figures 7 and 8 show images of D. geminata at 1:1 bleach to sample cleaning and 1:2 bleach to sample cleaning, respectively.

Visual comparison of the images obtained of the diatoms subjected to higher and lower concentrations of bleach shows much cleaner frustule in the former than in the latter concentration. Diatom samples cleaned at 4.125% concentration has noticeable green organic masses remaining in the frustules while those treated with 2.75% bleach concentration has little to

Figure 7. 400x optical magnification of Didymosphenia geminata taken from a 1 to 1 (4.125%) cleaned sample.

Figure 8. 400x optical magnification of Didymosphenia geminata taken from a 1 to 2 (2.75%) cleaned sample.


none. These results are in agreement with previous studies indicating the most effective concentration powers of bleach (Winter 2008). For a closer view of the cleaned diatoms, samples were also investigated under scanning electron microscope (SEM) using the back-scattered electrons (BSE) detector for strong topographic contrast. Figures 9-12 show the SEM images of D. geminata that were not cleaned and served as control, the 1:4, 1:2, and the 1:1 bleach:sample ratio, respectively.

As shown in fig. 9, the diatoms have a ‘muddy’ look from the organic materials associated with the organism. This image serves as a good baseline for gauging the cleaning efficacy of the different bleach concentrations. The 1:4 bleach cleaned sample shown in Fig. 10 was noticeably cleaner than the control. The pores in the silica frustule are beginning to become visible. However, there is the appearance of a ‘filmy’ organic layer on the surface of the diatom.

The 1:2 bleach cleaned sample shown in Fig. 11 indicates qualitative improvement over the lower concentration. The frustule pores are more visible and the ‘film’ that appeared in the 1:4 sample is no longer visible.

Figure 9. SEM BSE image of uncleaned D. geminata. Scale bar = 50 μm.

Figure 10. SEM BSE image of 1:4 bleach cleaned D. geminata. Scale bar = 50 μm.



The 1:1 bleach cleaned sample is similar to the 1:2 bleach cleaned sample, but a marked improvement over the control as shown in Fig. 12. The pores and structure of the frustule are now clearly observed. Synedra sp. was also examined in the SEM. This rod-shaped diatom frustule appears like the child’s game of “Pick Up Sticks” under the microscope. This configuration, even inside the SEM shows the value this particular diatom design could yield in forming a percolative network in a solar cell. Figures 13-16 show SEM-BSE images of Synedra sp. samples: control, 1:4, 1:2, and 1:1 bleach:sample ratio, respectively. The control sample that was not cleaned shows the organic matter still inside the frustules, registering as the dark shades inside the rods. However, the structures of the rods and some pores are still visible, even without any cleaning done (Fig. 13).

The 1:4 bleach cleaned sample of Synedra sp. in Fig. 14 appears to have lost the organic matter. However, this SEM image is quite hazy and difficult to interpret.


Figure 13. SEM BSE image of uncleaned Synedra sp. Scale bar = 40 μm.

Figure 14. SEM BSE image of 1:4 bleach cleaned Synedra sp. Scale bar = 40 μm.


The 1:1 (Fig. 16) and 1:2 (fig. 15) bleach cleaned samples of Synedra sp. appear very similar, as was the case for D. geminata. They are both well cleaned with the frustule pores and the structures are thoroughly defined.

The team hypothesizes that the haziness in all of the Synedra sp. samples is due to bleach that has settled and dried on the surface of these cells. Further testing and additional rinsing procedures should be employed in future experiments. No Cyclotella sp. samples were found or viewable in the SEM. It is believed that since Cyclotella sp. is a salt water species, the cells imploded along with their frustules once they were placed in fresh water for cleaning and testing. Further investigation will have to be conducted on this aspect. The topographic option of the BSE was chosen for the SEM imaging because this mode allows observation of any layering on the surface of the samples. Therefore, this provides a better picture as to whether or not the surface of the diatom frustules are cleaned of organic materials. The controls are covered with an obvious organic layer, which would most likely impede the functionalization of the diatoms. However, the 1:1 and 1:2 bleach cleaned samples appear to be free of organic matter barriers. Cleaned diatom frustules were also tested via Fourier transform infrared spectroscopy (FTIR) to determine what organic matter may still be present after cleaning. A sample of uncleaned control D. geminata and a sample of 1:1 bleach cleaned D. geminata were both mixed with KBr salt and pressed into pellets. These pellets were then examined in the FTIR using transmission mode. KBr was used for the pellet base because in a pure, anhydrous form, KBr is practically transparent to infrared light. A sample of pure silica and KBr was tested against the 1:1 cleaned sample. Figure 17 is a side by side of the FTIR plot of pure silica (A) and the cleaned sample (B).




To determine how well the diatoms were cleaned, spectra were obtained from pure KBr, a cleaned sample, and an uncleaned sample. These are shown in Figure 18. The lower two spectra are the pure KBr and the

1:1 cleaned D. geminate. The broad silica peak at around 1100 cm-1 is clearly present in the D. geminata spectrum. The top spectrum, from the uncleaned sample, shows several additional peaks as shoulders on the broad peaks present in all samples.

(A) Pure silica.

(B) 1:1 Cleaned D. geminata.

Figure 17. Side by side FTIR plots of (A) pure silica and (B) 1:1 cleaned D. geminata.

Figure 18. FTIR plot of KBr and uncleaned D. geminata, KBr and 1:1 cleaned D. geminata and KBr alone.


These are suggestive of amine and C-H stretch modes which we would expect to be present if significant organic matter were still present. From this data, it does appear that the bleach cleaning protocol is effective in removing organic matter from the D. geminata samples. Tests on Functionalized Diatoms After functionalization, the samples were observed using the SEM and elemental data was taken using the Energy Dispersive X-ray Spectroscopy function (EDS). EDS is a secondary function of the SEM. When the beam of the SEM is turned on, the energy from the beam is so intense that it can knock electrons in the inner shells of atoms from their outer orbits. This creates a hole and makes the atom unstable. To remedy this situation, an electron from an outer shell will fall into the hole left behind and will release energy in the form of X-rays as they descend to the inner shell. These released X-rays are characteristic to each atom and are absorbed by a detector inside of the SEM. The detector then gives the data as to the elemental makeup of the specimen (Haffner 2013). Using EDS, the presence of elements in a given sample such as titanium can be detected. Figures 19 and Figure 20 show EDS

images of D. geminata with deposits of titania on its frustule. This paves the way for future work in testing its electrical conductivity and solar capabilities now that titanium dioxide has been placed on the surface of a diatom.

One important observation that can be noted on the images shown in the figures is the cracked and brittle appearance of the frustule. This can be attributed to the use of a heat gun to evaporate the liquid associated with the sample prior to SEM observation. To avoid such condition that might influence the property of the functionalized diatom, samples will be allowed to dry in an oven held at a relatively low temperature in order to let the moisture evaporate slowly, avoiding cracks in the sample.

Future Research While the current results obtained from the experiments indicated that the cleaning procedure applied works, and that the diatoms are successfully indicating the presence of titania, the group acknowledges that more tests need to be done to replicate the evidence observed. With the remaining

Figure 19. EDS image of functionalized D. geminata. Scale bar = 10μm.

Figure 20. Close up EDS image of ribs of a functionalized D. geminata. Scale bar = 25μm.


funds available from the Sustainability Competition grant, the group hopes to continue this project throughout the remainder of 2016. Further refining of the cleaning process alone could lead to a standalone publication. Moreover, with the proof of concept for functionalization in hand, the project can now move to functionalize all three diatom species. Eventually, the group intends to pursue the testing of functionalized diatoms inside solar cells for maximum energy production through a research venture with funding support from agencies and industry partners.


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Appendix A: Media Presentation The media presentation for this project was performed in a Biology 1 class on Tuesday, September 29. This sustainability project was born out of a smaller scale class project (BSC1010GENS04). The presentation linked below was given in front of peers and two professors, Dr. Melba Horton and Dr. Chris Coughlin. Both of which have provided valuable expertise in putting this project together. Because this was a class presentation, a non-group member is present. However, that presenter is no longer associated with this sustainability project or the class project.

Functionalizing Diatoms with TiO2 For reference, the web address of the video is: https://www.youtube.com/watch?v=inSKl4cSXAk&list=PLy-UzF5VyamnfDu4DhkCj86FEhVSVQyuI&index=6

https://www.youtube.com/watch?v=inSKl4cSXAk&list=PLy-UzF5VyamnfDu4DhkCj86FEhVSVQyuI&index=6

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FINAL REPORT - Functionalizing Diatoms with TiO2 for Solar Cell Applications