Electrochromism in Sputtered  WO 3 Thin Films

Abstract

The aim of this project was to engineer the materials studied to enhance the so-called 3S criteria: Sensitivity, Selectivity, and Stability, by using the advantage of controlling structure and properties at nanometer dimensions. It targeted sensor materials that are able to detect poisonous gases resulting from coal-gasification processes, especially sulfur containing emissions. Research findings based on this award demonstrate that doping tungsten oxide (WO 3) with a small amount of Ti (e.g. 5% in our work) results in a new material that has a higher structural symmetry (e.g. tetragonal morphology) as well as narrower crystalline particle size distribution. As high quality materials with excellent ordered structure and narrower particle-size distributions (which can also withstand high-temperature technological environments such as those encountered in furnaces and coal gasification systems without their structure being affected by phase transformations) are needed for developing new, more sensitive sensor materials, W-Ti-O thin films grown by RF sputtering are valuable candidates for such roles. It is well known that pure WO 3 will change its structure at elevated temperatures. Our work indicates that, Ti doping not only increases the stability of the resultant material by promoting structural phase modifications, but also increases its sensitivity by increasing the effective surface area exposed to the poisonous gas (fine microstructure and uniform distribution were observed). Study the characteristics of undoped WO 3 thin films synthesized by RF sputtering……………..5 Explore the stabilization and structural modifications of Ti-doped WO 3 thin films……………….6 Determine computationally the amount of Ti doping that will maximize material structural stability………………………………………………………………………………………………… 8 Investigate sensitivity and stability of WO 3 thin films under gas testing………………………….11 Promoting research and education in the area of sensors and controls………………………....12 CONCLUSIONS……………………………………………………………………………………...13 5 EXECUTIVE SUMMARY The overall efforts proposed were to develop high-quality new sensor materials for achieving improved response time, controlled microstructure for long-term stability, and narrow particle size distribution for improved sensor characteristics and performance. The aim was to identify methods to enhance the so-called 3S criteria: sensitivity, selectivity, and stability, by utilizing the advantages of controlling structure and properties at nanometer dimensions. The milestones of the proposed research were: (1) to study the characteristics of undoped WO 3 thin films synthesized by RF sputtering, (2) to explore the stabilization and structural modifications of Ti-doped WO 3 thin films, (3) to determine computationally the amount of Ti doping that will maximize material structural stability, (4) to investigate sensitivity and stability of WO 3 thin films under gas testing, and (5) to promote research and education in the area of sensors and controls. The effect of processing conditions on the ...

Electrochromic properties of nanostructured tungsten trioxide (hydrate) films and their applications in a complementary electrochromic device

“…For electrochromic applications, WO 3 nanostructures need to be assembled as a thin film on conductive substrates and the microstructures of the film concerning the electrochromic performance largely depend on the film assembling techniques and processing conditions. Such thin films of WO 3 have been grown by vacuum deposition [21], electrodeposition [22], sol-gel [23] and hydrothermal method [24,25]. Hydrothermal approach is one of the most promising methods for fabricating WO 3 film because of its merits of low cost, low reaction temperature, flexible substrate selection and easy scalingup for production.…”

Large cation model of dissociative reduction of electrochromic WO3−x films

“…Therefore, it is not unlikely that the intercalation process comprises proton jumps from one water molecule to another without actual transfers of hydronium ion through the (WO) 4 gates. Indeed, the electrochemically deposited electrochromic WO 3 films show the fastest response of any WO 3 films grown by other techniques [4,5,7,9,10]. This mechanism is also supported by the fact that water molecule diameter (2r = 3.33 Å) is smaller than that of H 3 O + (2r = 3.48 Å) and hence, separate jumps of water molecules and of protons (between intercalated water molecules) should be more facile than transport of undissociated H show that the cation length is 25.7 Å and the size of the head group is 6.1 by 4.9 Å (as determined from the distances between farthest H-atoms in various directions plus van der Waals radius of H, r = 1.2 Å).…”

“…The optical and electrochemical properties of WO 3 films have been extensively explored in view of scientific interests [1][2][3] and promising applications for electrochromic films for optical switching and modulation [2][3][4][5][6][7][8][9][10][11][12][13][14], solar energy conversion [15][16][17][18], photocatalytic detoxification of pollutants [19][20][21][22][23], smart windows [24], gas sensors [25][26][27][28], and others [24,[29][30][31][32]. Due to the efficient photoconversion, the nanostructured n-WO 3 films have been utilized as photoanodes in solar cells and in photoelectrocatalytic reactors.…”

“…Due to the efficient photoconversion, the nanostructured n-WO 3 films have been utilized as photoanodes in solar cells and in photoelectrocatalytic reactors. The change in optical density of WO 3 films controlled by the applied electrode potential has been studied for electrooptical modulation [3][4][5][6][7][8][9][10][11][12][13][14]. These electrochromic properties of WO 3 films have potential applications for optical communication and electronic display devices.…”

“…In previous works [3,13,14], we have investigated fast optical switching in WO 3-x films formed by controlled potentiostatic electrodeposition which provided better film properties for optical electromodulation than other techniques, such as magnetron sputtering [5], chemical vapor deposition [9], photochemical vapor deposition [10], vacuum evaporation [4], or constant current anodization [7]. Switching speeds of 20 to 50 ms have been observed.…”