J. Roland Pitts

Electrochromic and optical properties of mesoporous tungsten oxide films

Standard and mesoporous sol–gel tungsten oxide thin films were prepared by a spin-coating technique from an ethanolic solution of tungsten hexachloride. A block copolymer (BASFk Pluronic p123) was employed as a template to generate the mesoporous structure. An ultraviolet (UV) illumination method was employed to remove the polymer templates at room temperature. The electrochromic and optical properties of the mesoporous films are described and compared to standard sol–gel tungsten oxide films. Results are also presented on the samples prepared by thermal treatment. We demonstrate that the UV illumination/ozone treatment is a superior method to remove templates which enables us to more effectively investigate the effect of mesoporosity on the electrokinetics of ion insertion into tungsten oxide films. D 2002 Elsevier Science B.V. All rights reserved.

Alternating current impedance and Raman spectroscopic study on electrochromic a-WO3 films

The chemical diffusion of lithium ions in a-LixWO3 films is investigated using alternating current impedance spectroscopy and Raman scattering measurements. The diffusion coefficients increase with increasing x in a-LixWO3 up to x=0.072 and then decrease. Raman measurements show that the W6+=O/O–W6+–O ratio also increases at the early stage of lithium insertion and then decreases with further lithium insertion. We conclude that the diffusion kinetics of lithium ions in a-LixWO3 films is very closely related to the W6+=O/O–W6+–O ratio.

Stand-alone photovoltaic-powered electrochromic smart window

Three different, innovative approaches have been taken to develop photovoltaic (PV) integrated electrochromic (EC) devices for smart-window applications. These are: (i) a stand-alone, side-by-side PV-powered EC window; (ii) a monolithically integrated PV-EC device; and (iii) a novel photoelectrochromic device based on a dye-sensitized TiO2 solar cell. The compatibility of PV-EC devices has been analyzed, and the potential for large energy savings for building applications has been suggested. The first monolithic, amorphous-silicon based, PV-powered electrochromic window is described in detail. The device employs a wide bandgap a-Si1−xCx/H n–i–p PV cell as a semitransparent power source, and a LiyWO3/LiAlF4/V2O5 EC device as an optical-transmittance modulator. The EC device is deposited directly on top of a PV cell that coats a glass substrate. The a-Si1−xCx/H PV cell has a gap of 2.5 eV and a transmittance of 60–80% over a large portion of the visible light spectrum. Our prototype 16-cm2 PV-EC device modulates the transmittance by more than 60% over a large portion of the visible spectrum. The coloring and bleaching times of the EC device are approximately 1 min under normal operating conditions (±1 V). A brief description of photoelectrochromic windows based on a combination of dye-sensitized TiO2 and WO3 EC-layer is also given.

Comparison of electrochromic amorphous and crystalline tungsten oxide films

Abstract A detailed systematic study of the tungsten oxide thin films has been carried out using WO 3 films after they were annealed at progressively increasing temperatures ranging from 350°C to 450°C in oxygen environments. The structural properties of the films were characterized using X-ray diffraction and Raman spectroscopy. The amorphous WO 3 films remain as an amorphous phase up to 385°C and begin to crystallize at 390°C and then are completely crystallized at 450°C. Absorption peaks of the films are found to shift to a higher energy side with increasing annealing temperature up to 385°C and then shift abruptly to a lower energy as the films begin to crystallize at 390°C. Deconvolution of the absorption spectra shows that there are two different polaron transitions in the amorphous WO 3 films.

Raman spectroscopic studies of electrochromic a-MoO3 thin films

We report the effects of electrochromic coloration on the Raman spectra of sputtered a-MoO3 films. The Raman spectra of as-deposited films show two strong peaks at 828 and 951 cm−1 due to vibrations of the OMo6+O and Mo6+O bonds, respectively. A shoulder on the right side of the Raman peak at 951 cm−1 is observed, and we attribute it to the Mo6+O bonds which originate from the singly coordinated oxygen atoms in α phase layered MoO3. When lithium (or hydrogen) ions and electrons are intercalated into a-MoO3, the overall Raman intensity decreases due to electrochromic coloration and an extra Raman peak due to the bonds between Mo5+ and oxygen atoms appears at ∼400 cm−1. In addition, the force constant for the OMo6+O bonds increases due to the compressive stress induced by lithium (or hydrogen) ion insertion. We conclude the Mo5+ states are generated as a result of the reduction of the Mo6+ states with ion/electron insertion and the optical absorption in the colored state is caused by transitions between the Mo6+ and Mo5+.

Raman spectroscopic studies of gasochromic a-WO3 thin films

The gasochromic mechanism in amorphous tungsten oxide films has been investigated using Raman scattering measurements and optical cycling tests. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm 1 due to vibrations of the OW 6 O and W 6 O bonds, respectively. When the Pd:a-WO3 films are gasochromically colored by exposure to diluted hydrogen gas, extra Raman peaks due to OW 5 Oa nd W 5 O bonds appear at 330 and 450 cm 1 , respectively. Optical cycling tests confirm that the Pd:a-WO3 films can not be bleached without oxygen. The Raman spectra of a-WO3 films bleached with isotopic heavy oxygen ( 18 O) show that the gasochromic coloration in a-WO3 films is directly related to the double injection of hydrogen ions and electrons.

Gasochromic mechanism in a-WO3 thin films based on Raman spectroscopic studies

The gasochromic mechanism in amorphous tungsten oxide films has been investigated using Raman scattering measurements. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm−1 due to vibrations of the O–W6+–O and W6+=O bonds, respectively. When the Pd/a-WO3 films are gasochromically colored by exposure to diluted hydrogen gas, extra Raman peaks due to O–W5+–O and W5+=O bonds appear at 330 and 450 cm−1, respectively. The Raman spectra of a-WO3 films bleached with isotopic heavy oxygen (18O) show that the gasochromic coloration in a-WO3 films is directly related to the double injection of hydrogen ions and electrons.

Microstructure study of amorphous vanadium oxide thin films using raman spectroscopy

Microstructural changes of amorphous V2O5 films with lithium intercalation are studied using Raman-scattering measurements. The Raman spectra of as-deposited films show two broad peaks around at 520 and 650 cm−1, due to the stretching modes of the V3–O and V2–O bonds, respectively, and a relatively sharp peak at 1027 cm−1 due to the V5+=O stretching mode of terminal oxygen atoms. In addition, there is a peak at 932 cm−1 that we attribute to the V4+=O bonds. Comparison of the Raman spectra of V2O5 films with different oxygen deficiencies confirms this assignment. This Raman peak due to the stretching mode of the V4+=O bonds develops and shifts toward lower frequencies with increasing lithium concentration. Comparison to results from gasochromic hydrogen insertion indicates that the 932 cm−1 Raman peak is not a result of vibrations which involve Li or H atoms. We propose that the V4+=O bonds are created by two different mechanisms: a direct conversion from V5+=O bonds and the breaking of the single oxygen b...

Nanocomposite Counter Electrode Materials for Electrochromic Windows

In this paper, we describe our development of a counter electrode material that shows promise in terms of both durability and performance and is grown using common vacuum deposition methods and materials. The counter electrode films are formed by sputter deposition from a composite ceramic lithium, nickel, and tungsten oxide target. Consistent with previous reports, NiO nanocrystallites are observed to form. 12,13 Secondary phases from the other elements present in the sample are not observed, which implies that they act as substitutional or interstitial dopants or segregate to the grain boundaries and form an amorphous layer between the NiO crystallites. Experimental

Effect of Nonstoichiometry of Nickel Oxides on Their Supercapacitor Behavior

How electrochemical capacitance is affected by the non-stoichiometric properties of nickel oxide (NiO x ) films is reported. Nonstoichiometric NiO x thin-film electrodes were prepared for use in a supercapacitor by reactive radio-frequency (rf) sputtering of a nickel metal target. The oxygen partial pressure in the sputtering gas was systematically varied to fabricate various nonstoichiometric NiO x films. As the oxygen partial pressure increased in the sputtering gas, hole concentration increased from 4.7 X 10 1 6 to 1.0 X 10 1 8 cm - 3 while the specific capacitance of the NiO x films decreased from 148 to 80 F/g. A mechanism to explain the relationship between the specific capacitance and nonstoichiometry is proposed. The specific capacitance values of these NiO x films are determined by the number of Ni 2 + states that can be oxidized to Ni 3 + as a result of the double injection of OH - ions and holes.