Nanostructured electrochromic and photoelectrochemical materials and devices

• Main Author: Jiao, Zhihui
• Other Authors: Ke Lin
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/61039
• Institution: Nanyang Technological University

Nanostructured tungsten oxide (WO3) and hydrated tungsten oxide (WO3•H2O) films were assembled on fluorine doped tin oxide (FTO) coated glass substrates by seed-assisted hydrothermal methods. Plate-like, brick-like and stick-like nanostructured WO3 or hydrated WO3 films were obtained by using sodium sulfate (Na2SO4), ammonium sulfate ((NH4)2SO4) and ammonium acetate (CH3COONH4)as capping agents, respectively.The electrochromic (EC) characteristics of the as-prepared films were investigated in single EC layer devices and complementary EC devices with Prussian blue (PB) as the counter electrode. Compared to the films without capping agent, the plate-like nanostructured 3WO3•H2O film grown with Na2SO4 shows significantly improved EC performance. A high coloration efficiency value of 112.7 cm2C-1 at 632.8 nm is obtained, and the response time for 90% transmittance changes is found to be 4.3 sfor coloration and 1.4 s for bleaching for a sample size ~2 cm2 (FTO sheet resistance:15Ω/□), respectively, while the brick-like monoclinic WO3 film displays a comparable switching response of tc,90%= 272 s and tb, 90%= 364 s (sample size ~2 cm2, FTO sheet resistance: 15Ω/□)and a coloration efficiency of 38.2 cm2C-1 at 632.8 nm. Both films show adjustable transmittance modulation under different voltages and a good reversible coloration/bleaching stability. The single layer EC devices based on these films demonstrate promising applications in smart windows and large-area information displays. Complementary EC devices based on nanostick/nanoparticle and nanobrick 3WO3•H2O film and PB film were also fabricated. The orthorhombic nanostick/nanoparticle 3WO3•H2O film grown with CH3COONH4 as the capping agent shows better EC performances than the nanobrick film without capping agent. A complementary EC device based on the nanostick/nanoparticle 3WO3•H2Ofilm and PB was assembled and shows higher optical modulation (54% at 754 nm), a larger coloration efficiency (151.9 cm2C-1 at 754 nm)and faster switching responses with a bleaching time of 5.7 sand a coloring time of 1.3 s (sample size ~2 cm2, FTO sheet resistance: 15Ω/□) than a single 3WO3•H2O layer device. Orthorhombic nanobrick 3WO3•H2O films were obtained by using (NH4)2SO4 as the capping agent, and a complementary device made up of the nanobrick 3WO3•H2O film and PB film was also fabricated and demonstrate smuch faster coloration/bleaching response (tc,90%= 1.8 s, tb,90%= 3.7 s for a sample size ~1.7cm2, FTO sheet resistance: 15Ω/□), larger coloration efficiency value (164.6 cm2C-1 at 632.8 nm) and increased color contrast. Moreover, a fast-switching negative photoelectrochromic (PEC) cell composed of dye-sensitized nanocrystalline TiO2 and PB electrodes was fabricated. Adjustable grayscales were obtained by illuminating the cell with different light intensity and time. The photo bleaching response of the cell at 100 mWcm-2 is 6.2 s, and the EC response with a 2 V bias is about 600 ms for 70% transmittance change at 780 nm with coloration efficiency of 103.4 cm2C-1 (sample size ~1.5 cm2, FTO sheet resistance: 15Ω/□). This hybrid PEC cell shows good stability and can be potentially applied in erasable displays. The photoelectrochemical(PE) properties of the porous WO3 thin films consisted of sheet-like, wedge-like and plate-like nanostructures with the capping agents of CH3COONH4, (NH4)2SO4 and Na2SO4 were also investigated. The sheet-like film grown using CH3COONH4 could generate anodic photocurrents of 1.15mA cm-2 for oxidization of methanol and 0.5 mA cm-2 for water splitting, the highest photoconversion efficiency of about 0.3% can be obtained for water splitting under simulated solar illumination. In summary, this dissertation addresses the fabrication and characterization of WO3or 3WO3•H2O films grown with various capping agents using the hydrothermal methods. Single layer EC devices and complementary layer EC devices based on these films were assembled and their EC performances were investigated. The photoelectrochemical properties of the porous WO3filmswere also studied. And a PEC cell based on PB electrodes was fabricated and investigated. The work done demonstrates the promising applications of the nanostructured WO3 or 3WO3•H2O films and PB films in EC and photoelectrochemical devices.