Miguel A. Arvizu

Galvanostatic Ion Detrapping Rejuvenates Oxide Thin Films.

Ion trapping under charge insertion-extraction is well-known to degrade the electrochemical performance of oxides. Galvanostatic treatment was recently shown capable to rejuvenate the oxide, but the detailed mechanism remained uncertain. Here we report on amorphous electrochromic (EC) WO3 thin films prepared by sputtering and electrochemically cycled in a lithium-containing electrolyte under conditions leading to severe loss of charge exchange capacity and optical modulation span. Time-of-flight elastic recoil detection analysis (ToF-ERDA) documented pronounced Li(+) trapping associated with the degradation of the EC properties and, importantly, that Li(+) detrapping, caused by a weak constant current drawn through the film for some time, could recover the original EC performance. Thus, ToF-ERDA provided direct and unambiguous evidence for Li(+) detrapping.

Galvanostatic Rejuvenation of Electrochromic WO3 Thin Films: Ion Trapping and Detrapping Observed by Optical Measurements and by Time-of-Flight Secondary Ion Mass Spectrometry

Electrochromic (EC) smart windows are able to decrease our energy footprint while enhancing indoor comfort and convenience. However, the limited durability of these windows, as well as their cost, result in hampered market introduction. Here, we investigate thin films of the most widely studied EC material, WO3. Specifically, we combine optical measurements (using spectrophotometry in conjunction with variable-angle spectroscopic ellipsometry) with time-of-flight secondary ion mass spectrometry and atomic force microscopy. Data were taken on films in their as-deposited state, after immersion in a Li-ion-conducting electrolyte, after severe degradation by harsh voltammetric cycling and after galvanostatic rejuvenation to regain the original EC performance. Unambiguous evidence was found for the trapping and detrapping of Li ions in the films, along with a thickness increase or decrease during degradation and rejuvenation, respectively. It was discovered that (i) the trapped ions exhibited a depth gradient; (ii) following the rejuvenation procedure, a small fraction of the Li ions remained trapped in the film and gave rise to a weak short-wavelength residual absorption; and (iii) the surface roughness of the film was larger in the degraded state than in its virgin and rejuvenated states. These data provide important insights into the degradation mechanisms of EC devices and into means of achieving improved durability.

Electrochemical Rejuvenation of Anodically Coloring Electrochromic Nickel Oxide Thin Films

Nickel oxide thin films are of major importance as anodically coloring components in electrochromic smart windows with applications in energy-efficient buildings. However, the optical performance of these films degrades upon extended electrochemical cycling, which has hampered their implementation. Here, we use a potentiostatic treatment to rejuvenate degraded nickel oxide thin films immersed in electrolytes of LiClO4 in propylene carbonate. Time-of-flight elastic recoil detection analysis provided unambiguous evidence that both Li+ ions and chlorine-based ions participate in the rejuvenation process. Our work provides new perspectives for developing ion-exchange-based devices embodying nickel oxide.

Rejuvenation of degraded electrochromic MoO3 thin films made by DC magnetron sputtering : Preliminary results

Molybdenum oxide thin films were deposited by reactive DC magnetron sputtering and were subjected to voltammetric cycling in an electrolyte comprised of lithium perchlorate in propylene carbonate. The films were heavily degraded during 20 voltammetric cycles in an extended voltage range. The films were subsequently rejuvenated by use of potentiostatic treatments under different voltages during 20 hours. Optical changes were recorded during the electrochemical degradation and ensuing rejuvenation.

Sputter deposited W1–x–yNixTiyO3 thin films: Electrochromic properties and durability

Previous research demonstrated that a small amount of nickel enhances the coloration efficiency of tungsten-nickel oxide electrochromic (EC) thin films with respect to that of pure tungsten oxide (WO3) films. Furthermore the incorporation of titanium gives an improvement in the durability of tungsten-titanium oxide EC thin films. In this work we investigated the EC performance of tungsten-nickel-titanium oxide (W1–x–yNixTiyO3) EC thin films with emphasis on durability. The films were deposited on indium tin oxide covered glass by reactive dc sputtering from tungsten, tungsten-titanium alloy and nickel targets. Cyclic voltammetry was performed using 1 M LiClO4 in propylene carbonate as electrolyte. The voltage window was chosen to induce fast degradation of the samples within 80 cycles. Elemental compositions were obtained by Rutherford Backscattering Spectroscopy.

Electrochromism in sputter deposited W1-y MoyO3 thin films

Electrochromic (EC) properties of tungsten-molybdenum oxide (W1-y MoyO3) thin films were investigated. The films were deposited on indium tin oxide covered glass by reactive DC sputtering from tungsten and molybdenum targets. Elemental compositions of the W1-y MoyO3 films were determined by Rutherford back scattering. Voltammetric cycling was performed in an electrolyte of 1 M LiClO4 in propylene carbonate. An increase in molybdenum content in the EC films caused both a shift towards higher energies and a lowering of the maximum of the optical absorption band, as compared with WO3 EC films. Durability under electrochemical cycling was diminished for W1-y MoyO3 EC films.

Optical, structural and electrochromic properties of sputter-deposited W-Mo oxide thin films

Thin metal oxide films were investigated by a series of characterization techniques including impedance spectroscopy, spectroscopic ellipsometry, Raman spectroscopy, and Atomic Force Microscopy. Thin film deposition by reactive DC magnetron sputtering was performed at the Angstrom Laboratory. W and Mo targets (5 cm diameter) and various oxygen gas flows were employed to prepare samples with different properties, whereas the gas pressure was kept constant at about 30 mTorr. The substrates were 5×5 cm2 plates of unheated glass pre-coated with ITO having a resistance of 40 ohm/sq. Film thicknesses were around 300 nm as determined by surface profilometry. Newly acquired equipment was used to study optical spectra, optoelectronic properties, and film structure. Films of WO3 and of mixed W- Mo oxide with three compositions showed coloring and bleaching under the application of a small voltage. Cyclic voltammograms were recorded with a scan rate of 5 mV s-1. Ellipsometric data for the optical constants show dependence on the amount of MoOx in the chemical composition. Single MoOx film, and the mixed one with only 8% MoOx have the highest value of refractive index, and similar dispersion in the visible spectral range. Raman spectra displayed strong lines at wavenumbers between 780 cm-1 and 950 cm-1 related to stretching vibrations of WO3, and MoO3. AFM gave evidence for domains of different composition in mixed W-Mo oxide films.