Musialowicz 4. Temperature ( C) SE1 Measurement (nm) Silicon oxide film thickness (nm) Time (min) BSD Measurement (nm)

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Esther Sanders
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1 Musialowicz 1 Introduction: The development of renewable energies, specifically solar power, has come to a critical bypass. Despite increased interest in alternative forms of energy, the wide-spread and commercial use of solar power has stagnated. First generation solar cells employ crystalline silicon doped with small quantities of other elements. Although these cells make up 80% of the commercial market, the production of the high purity silicon necessary for these cells is expensive, making them less cost-competitive with fossil fuel energy sources. 1,2 From a broader perspective, the challenge of producing low-cost and efficient solar cells is also complicated by the fact that energy production from solar devices is transient. Electricity is only efficiently produced during clear, sunny days. Therefore, complete adoption of solar cells would require an efficient means for storing solar energy for use at night or on overcast days. The only means for efficiently storing electrical energy is using battery technology, which further increases the cost of widespread use of solar energy relative to fossil fuels. Photoelectrochemical cells (PECs), offer a promising alternative to previous forms of capturing solar energy. A crucial advantage of PECs is that they have the potential to provide a platform for achieving artificial photosynthesis, a process in which light energy is converted to a fuel that can be collected and stored for use when there is no sunlight. In a PEC, light energy is absorbed creating an electron-hole pair which can be used to split water into hydrogen and oxygen gas. 3 Another class of PECs, dye sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) convert the electron-hole pair directly to electricity. These solar cells are advantageous because of their potential for relatively high energy conversion efficiencies (greater than 40%), and cost efficiency as compared to previous generations of solar cells. 2 Both classes of PECs are constructed in a similar manner, being made up of a thin-film metal oxide nanoparticles, most notably titanium dioxide. Light passes through the glass exterior of the cell and photons are absorbed by either an organic dye in DSSCs or a perovskite material in PSCs. The absorption of a photon excites the photosensitizer, which transfers the electron to the titanium dioxide nanoparticles. Within the titanium dioxide, the electron is transported to the thin-filmed titanium dioxide creating an electrical current. Using an iodide/tri-iodide redox couple shuttle, DSSCs transport electrons in the current back to the dye, thus returning the system to its initial state. 1, 4 In a cell performing artificial photosynthesis, the electron would be transferred to water producing hydrogen at the anode and the hole or positive charge would be transferred to water at the cathode producing oxygen. 5

2 Musialowicz 2 The goal of this research project is to optimize the efficiency of PECs, ultimately allowing for cost-effective construction and commercial viability of these devices. Our main objective is to study the effect of doping the metal oxide nanoparticle film with silica coated gold nanowires. Based on prior literature, gold nanowires are known to scatter light because of the surface plasmon resonance (SPR) effect. 6, 7 The SPR effect is due to the collective movement of electrons in the gold wires when they are exposed to visible light. Within the solar cell, the pathlength is distance that light travels as it passes through the active layer. In a standard cell, the pathlength (Figure 1, top) is equivalent to the thickness of the metal oxide nanoparticle film. When gold nanowires are added, some of the light is scattered within the nanoparticle film and this leads to a much longer average pathlength for the light 8. In effect, the scattering effects of the gold wires concentrates light within the film allowing for better conversion of photons to current by the photosensitizer. Since the efficiencies of PECs is proportional to the current, we expect that the gold nanowires will increase the efficiency of a working PEC device. We coat the wires with silicon dioxide to electronically isolate them from the cell 9. This allows the gold wires to scatter the light without causing negative effects such as recombination. Figure 1. Light (yellow lines) passing through a film of TiO 2 nanoparticles (black square) has a pathelength equivalent to the thickness of the film (top). Adding gold nanowires (red lines) scatter the light. This effect increases the pathlength of light through the film (bottom). Preliminary Results: I joined Dr. Koenigsmann s research project in September of 2016, and have continued as a member of this project, registered in the course CHEM2999 Sophomore Research. I was awarded a Spring Undergraduate Research Grant for the Spring Semester 2017 and this has greatly helped my work over the semester. Since joining the group, I have been trained by Dr. Koenigsmann and Josephine Jacob-Dolan on the synthesis and characterization of the gold nanowires. Additionally, I have been trained to coat these nanowires with thin silicon oxide shells and subsequently image the product using a scanning electron microscope (SEM). In the fall semester, we have optimized the production of the gold nanowires using the U-tube method developed by Dr. Koenigsmann 10. During a U-tube synthesis, gold nanowires grown in the tubular pores of a 50-nm polycarbonate filter membrane that is constricted between two halves of a U-tube. One half is filled with tetrachloroauric acid (HAuCl4) dissolved in ethanol,

Musialowicz 3 which is the source of the gold ions. The other half is filled with the reducing agent sodium borohydride (NaBH4) that has been dissolved in ethanol.

The metallic gold fills the tubular pores forming nanowires of the same size and shape as the pore.

3 Musialowicz 3 which is the source of the gold ions. The other half is filled with the reducing agent sodium borohydride (NaBH4) that has been dissolved in ethanol. The reaction begins within the pores membranous template where the gold ions are reduced to gold metal. The metallic gold fills the tubular pores forming nanowires of the same size and shape as the pore. The gold nanowires are then isolated from the polycarbonate template over a series of 8 washings using dichloromethane, which dissolves the polycarbonate freeing the nanowires into solution. In the spring semester, we have focused on coating the wires with silica using a method developed by Yin and co-workers 11. The gold nanowires are dispersed in isopropanol, water, ammonia, and tetraethyl orthosilicate (TEOS) solution. The ammonia catalyzes the reaction of the TEOS with water resulting the formation of silicon oxide. Over the last semester, we have optimized the concentration of TEOS, ammonia, and DMA catalyst to produce films with thicknesses of approximately 10 nm. We measure the thickness of the silicon oxide film by comparing the thickness of the wires measured from images taken by the secondary electron and backscatter detectors (Figure 2) of the scanning electron microscope. These detectors operate differently and are sensitive to the thickness of the wire and to the thickness of the film, respectively. After running multiple reactions, we have determined that the ideal concentration for the catalyst is 0.2 M, which gives a reproducible thickness of ~20 nm. Figure 2. Scanning electron microscope images taken with the secondary electron detector of two replicate samples prepared with 0.2 M DMA. The analysis of the images is summarized in Table 1. Additionally, Dr. Koenigsmann is teaching me the process of assembling DSSC devices. Doctor-Blading is the technique used to create the essential dye films and it requires the careful manual spreading of TiO2 across the surface of the glass substrate using a glass pipet. Using Scotch tape, a small portion of the glass substrate is boxed off. The TiO2 paste is then applied

Musialowicz 4 directly above the exposed glass substrate and spread in one quick motion using a glass pipet. Afterwards, the film is heated on a hot plate for 8 minutes.

4 Musialowicz 4 directly above the exposed glass substrate and spread in one quick motion using a glass pipet. Afterwards, the film is heated on a hot plate for 8 minutes. The process is repeated twice more to ensure a film with a sufficient thickness 12. Temperature ( C) Time (min) SE1 Measurement (nm) BSD Measurement (nm) Silicon oxide film thickness (nm) ± ± ± ± ± ± 13 Table M DMA concentration offers consistent silica oxide coating thickness (difference between SE1 and BSD wire diameters is equal to combined silica oxide coating on each side of nanowire) Proposed Work: Our first major objective over the course of the summer will be to design thin dye films necessary for the construction of our DSSC devices. This semester I will also continue to be trained in the process of assembling DSSC devices. Our second objective will be to prepare a set of control devices without gold nanowires and characterize their performance. To determine the effect of the gold wires, we must make sure that we can assemble devices with reproducible efficiency and performance. The scattering spectrum of the gold nanowires is well matched with Figure 3. Image of solar simulator that will be used to test the current, voltage, and efficiency output of DSSC devices. the absorption spectrum of the MK-2 photosensitizer, which we will use to prepare the DSSCs. Our laboratory is outfitted with a solar cell testing station that can generate simulated sun light. We use this light source to shine light on the cells and measure their current and voltage output and their efficiency. Now that we have optimized the synthesis of the silica coated gold nanowires, we will incorporate them into the active layer by mixing the gold wires with the TiO2 nanoparticle paste that is used to form the active layer 13. Dr. Koenigsmann has successfully used this method to introduce gold nanoparticles into the active layer in prior research. We will compare the cell current and efficiency between the control and the doped-devices to determine the effect of the gold wires. Our long-term objective is to measure the efficiency of the photosensitizer in the presence of the gold nanowires utilizing spectroscopic techniques known as time-resolved THz spectroscopy and transient absorption spectroscopy. We will access these instruments through a collaboration with Dr. Koenigsmann s colleagues at Yale University s Energy Sciences Institute.

5 Musialowicz 5 References: 1. Bella, F.; Gerbaldi, C.; Barolo, C.; Gratzel, M., Aqueous dye-sensitized solar cells. Chem. Soc. Rev. 2015, 44 (11), Nayak, P. K.; Bisquert, J.; Cahen, D., Assessing Possibilities and Limits for Solar Cells. Adv. Mater. 2011, 23 (25), Daeneke, T., Uemura, Y., Duffy, N. W., Mozer, A. J., Koumura, N., Bach, U., & Spiccia, L. Aqueous Dye-Sensitized Solar Cell Electrolytes Based on the Ferricyanide-Ferrocyanide Redox Couple. Advanced Materials, 2012, 24 (9), (a) Halme, J.; Vahermaa, P.; Miettunen, K.; Lund, P., Device Physics of Dye Solar Cells. Adv. Mater. 2010, 22 (35), E210-E234; (b) Hara, K.; Arakawa, H., Dye-Sensitized Solar Cells. In Handbook of Photovoltaic Science and Engineering, John Wiley & Sons, Ltd: 2005; pp ; (c) O regan, B.; Grfitzeli, M., A low-cost, high-efficiency solar cell based on dye-sensitized. nature 1991, 353 (6346), (a) Martineau, D., Solaronix: Dye Solar Cells For Real, The Assembly Guide for Making Your Own Solar Cells. Solaronix 2012, 1; (b) Ito, S.; Chen, P.; Comte, P.; Nazeeruddin, M. K.; Liska, P.; Péchy, P.; Grätzel, M., Fabrication of screen-printing pastes from TiO2 powders for dyesensitised solar cells. Progress in Photovoltaics: Research and Applications 2007, 15 (7), ; (c) Nemes, C. T.; Koenigsmann, C.; Schmuttenmaer, C. A., Functioning Photoelectrochemical Devices Studied with Time-Resolved Terahertz Spectroscopy. J. Phys. Chem. Lett. 2015, 6 (16), Kobayashi, Yoshio, Hironori Katakami, Eiichi Mine, Daisuke Nagao, Mikio Konno, and Luis M. Liz-Marzán. Silica coating of silver nanoparticles using a modified Stöber method. Journal of Colloid and Interface Science 2005, 283.2, Atwater, H. A.; Polman, A., Plasmonics for improved photovoltaic devices. Nat Mater 2010, 9 (3), Jung, I.; Jang, H.-J.; Han, S.; Acapulco, J. A. I.; Park, S., Magnetic Modulation of Surface Plasmon Resonance by Tailoring Magnetically Responsive Metallic Block in Multisegment Nanorods. Chem. Mater. 2015, 27 (24), (a) Zhou, H.; Zhou, W.-p.; Adzic, R. R.; Wong, S. S., Enhanced Electrocatalytic Performance of One-Dimensional Metal Nanowires and Arrays Generated via an Ambient, Surfactantless Synthesis. J. Phys. Chem. C 2009, 113 (14), ; (b) Zhen, W., Plasmonresonant gold nanoparticles for cancer optical imaging. SCIENCE CHINA Physics, Mechanics & Astronomy 2013, 56 (3), Duan, L.; Tong, L.; Xu, Y.; Sun, L., Visible light-driven water oxidation-from molecular catalysts to photoelectrochemical cells. Energy Environ. Sci. 2011, 4 (9), Santulli, A. C.; Koenigsmann, C.; Tiano, A. L.; DeRosa, D.; Wong, S. S., Correlating titania morphology and chemical composition with dye-sensitized solar cell performance. Nanotechnology 2011, 22 (24), Wang, H., Chen, J., Xiao, F., Zheng, J., & Liu, B. Doping-induced structural evolution from rutile to anatase: formation of Nb-doped anatase TiO2nanosheets with high photocatalytic activity. J. Mater. Chem. A, 2016, 4 (18), Yin, Y.; Lu, Y.; Sun, Y.; Xia, Y., Silver Nanowires Can Be Directly Coated with Amorphous Silica To Generate Well-Controlled Coaxial Nanocables of Silver/Silica. Nano Lett. 2002, 2 (4),

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