Intercalation Compounds:WO3 and

CHROMIC MATERIALS

Prof. Antonella Glisenti - Dip. Scienze Chimiche - Università degli Studi di Padova

Laurea Magistrale in Scienza dei MaterialiMateriali Inorganici Funzionali

BIBLIOGRAPHYBIBLIOGRAPHY

Properties, requirements and possibilities of smart windows for dynamicdaylight and solar energy control in buildings: A state-of-the-art review

R. Baetens, B.P. Jelle, A. GustavsenSolar Energy Materials & Solar Cells 94 (2010) 87-105

Electrochromic tungsten oxide films review of progress 1993-1998Solar Energy Materials & Solar Cells 60 (2000) 201-262

C.G. Granqvist

Oxide electrochromics: An introduction to devices and materialsC.G. Granqvist

Solar Energy Materials & Solar Cells 99 (2012) 1

Progress in chromogenics: New results for electrochromic qanthermochromic materials and devices

C.G. Granqvist et al. Solar Energy Materials & Solar Cells 93 (2009) 2032

Switching sequence of an electrochromic laminated glass

Dynamic tintable or smart windows: devices that can change properties such as the solar factor and the transmission of radiation in the solar spectrum in response to an electric current or to the changing environmental conditions themselves.

Drastic reduction of the energy consumption of highly glazed buildings by reducing cooling loads, heating loads and the demand for electric lighting

Information, circuitry

Electric lighting energy and cooling energy for different types of fenestration.

External triggering:Chromic materialsLiquid crystals Electrophoretic or suspended-particle devices.

Chromic devices may be:ElectrochromicGasochromic,PhotochromicThermochromic

Parameters to judge:Transmittance modulation range in the visible and whole solar

spectrumLifetime and number of achieved cycles (without or only minor

degradation)Switching time (connected to the size)Energy consumptionOperating voltageOperating temperature range

Liquid crystals When the electrical supply is switched on, the liquid crystal

molecules align and incident light passes through and the LC Glass panel instantly clears.When the power is switched off the liquid crystal molecules are randomly oriented scattering light and the LC Glass becomes opaque (private).

Suspended Particles Glasses When the power supply is switched on, the rod shaped

suspended particle molecules align, light passes through and theSPD Glass panel clears. When the power supply is switched off the rod shaped suspended particle molecules are randomly oriented blocking light and the SPD Glass becomes dark blocking up to 99.4% of light.

Electrochromic Glasses

ElectrochromismIt is the property of a device to change its optical properties reversibly if an external potential is applied, associated with ion insertion and extraction processes.

Electroactive layersOften denoted as electrochromics, change their optical properties by switching between their oxidized and reduced form.

Most favourable are electrochromics that are reflecting in their coloured state instead of absorbing.

Electrochromism may be seen as a device characteristicinstead of material property

The basis is a glass or plastic covered by a transparent conducting film, i.e. mostly ITO, on which one (or multiple) cathodic electroactive layer(s) are affixed. These are followed by a layer of ion conductor, on its turn followed by an ion-storage film or one (or multiple) complimentary anodic electroactive layers and another transparent conducting film.

by combining different type of electrochromics, ion-storage films and ion conductors, different properties can be obtained for the device, where the modulation range, durability and switching speeds can be optimized.

Requirements:

high transparencyhigh electronic conductivity

Must widely used:

Tin-doped indium oxide In2O3(Sn) or ITOheavily doped conductors: SnO2:F (FTO), ZnO:Al,

ZnO:Ga,

TRANSPARENT CONDUCTORS

TC UNDER INVESTIGATION

PEDOT or poly(3,4-ethylenedioxythiophene)Transmittance above 0.90, electrical resistivity currently

below 400 Ω. Long-term stability remains a problem (degrades over time if exposed to light or heat)

Carbon nanotubes (CNT)High transparency in the VIS and IR spectrum, resistances

around 10-4 Ω. CNT networks are p-type conductors, whereas traditional transparent conductors are exclusively n-type (p-type could allow a new and cheaper cell design). They are easier and cheaper than ITO to deposit on glass and plastic surfaces, sincethey can be formed into a solution, compared with ITO which has to be sputtered onto a surface in a vacuum

12CaO ● 7Al2O3Electrochromic material that turns green by a process

involving H0 H+ +e- after incorporation of hydrogen inside the cages.

DEVICES:DEVICES:The central layer is a pure ionic conductor for small ions (H+, Li+, Na+). The voltage needs to be applied only when optical changes are to take

place, i.e., the device has open-circuit memory.Electrochromic layer is located on

one side of the ion conductor, an ion storage layer serves as counter electrode on the other side of the ion conductor. The optical modulation takes place when ions are moved back and forth between the electrochromic layer and the counterelectrode. This movement is induced by a static electric field applied between electrically conducting layers, one being on the outer side of the electrochromicmaterial and the other on the outer side of the counterelectrode. Both of these electrical conductors must be transparent if optical transmittance modulation is desired.

MATERIALS CHALLENGES:MATERIALS CHALLENGES:EC and counter electrode = well defined nanoporosity over large areas

(non conventional deposition technologies).

TC = excellent electrical conductivity and optical transparency, which is demanding especially for T sensitive substrates (PET). Expensive part

Charge/insertion and charge balancing are vital processes

Electrolyte = good ion conductyivity end minimun electrical conductivity; high stability under UV irradiation, good adhesive

Long term cycling durability

Large scale manufacturing: no time-consuming production steps (post treatments, …)

DEVICES:DEVICES:Data for inorganic electrochromic devices showing materials, sample size, modulation range for the transmittance (T), number of color/bleach (c/b) cycles, and switching time (τSW). G denotes glass

DEVICES:DEVICES:Data for laminated electrochromic devices showing materials, sample size, modulation range for the transmittance (T), number of color/bleach (c/b) cycles, and switching time (τSW). G denotes glass, P denotes polymer,

OtherOther electrochromicelectrochromic metal metal oxidesoxides

Bi2O3, CeO2, CoO, CuO, FeOOH, Fe2O3, Fe3O4, FeO, MnO2, MoO3, P2O2-y, RhO3, RuO2, SnO2, Ta2O5, TiO2and V2O5

nickel oxide, NiIIO(1-y)Hziridium dioxide, IrO2niobium pentoxide, Nb2O5.

Electrochromic Metal Oxides

‘‘cathodic’’ (coloring under ion insertion) ‘‘anodic’’ (coloring under ion extraction). Oxides based on vanadium can be viewed as a ‘‘hybrid’’. The standard EC device incorporates two EC films, and it is obviously

advantageous to combine one ‘‘cathodic’’ oxide (for example based on W, Mo, or Nb) and one ‘‘anodic’’ oxide (for example based on Ni or Ir).

Electrochromic Metal OxidesCrystalline structure,

The EC oxides can be categorized as (defect) perovskites, rutiles, and having layer/block structures. All of these structures can be treated within a framework of MeO6 octahedra connected by sharing common corners and/or edges.

Vanadium pentoxide (V2O5) can be represented as consisting of heavily distorted VO6 octahedra or, alternatively and perhaps more adequately, asSquare pyramidal VO5 units.

Hydrous nickel oxide — which is the pertinent EC material rather than pure NiO — is thought to contain layers of NiO6 octahedra sharing edges.

Schematic band structures for different types of EC oxides,

WOWO33

WO3 + xM+ + xe- MxWO3

M = H+, Li+, Na+, K+

W5+

W oxide is transparent as a thin film

It can be reversibily transformed to a material with differentproperties (absorbents if heavily disordered, IR reflectent ifsufficiently crystalline) if it incorporates electrons and charge-balancing ions

Real thin films are hydrous and may deviate from WO3stoichiometry

Deep blueTransparent

W6+ W4+ W6+ W4+ W5+

Films based on NiOreasonable cost electrochromic properties, which even can be improved by

mixing NiO with wide band gap oxides such as MgO or Al2O3NiO:X, i.e. where X is Mg, Al, Si, V, Zr, Nb, Ag, Ta, Li, Al or B,

has been found complementary with WO3:X in the visible and near IR: pairing them in a complete device results in a very dark colour neutral system.The main effect of electrochromic phenomenon takes place in the UV and VIS spectra and reaches a very high colouration UV and VIS spectra and reaches a very high colouration efficiency 800 nmefficiency 800 nm. A 200 nm layer has been defined by a Tvis of 0.80–0.10

(transparent) Ni(OH)2 2 NiOOH + H+ + e- (grey)

(transparent) NiOH + Ni(OH)2 2 Ni2O3 + 3H+ + 3e-(brownish)

Nb2O5Pure Nb2O5 and doped Nb2O5:X, i.e. where X is Sn, Zr, Ti, Li, Mo, WO3 or TiO2, layers change colour by insertion of H+ or Li+ ions from transparent to brown, blue or grey depending on the crystallinity of the layer.

(transparent) Nb2O5 + xM+ + xe- 2 MxNb2O5 (blue, brown or grey)

Undoped Nb2O5 has a high transmittance of 0.80–0.92 in the visible region for the bleached state and transmittances between 0.10 and 0.30 are obtained in the coloured state, with relatively slow colouring and bleaching times. The disadvantage of the Nb2O5 layers is their small colouration efficiency CE of about 12–27 cm2/C compared with the CE of 37–50 cm2/C of tungsten oxide.

Transmittance spectra of undoped and Mo- and Li-doped Nb2O5:X sol–gel double layers on K-glass in

�the coloured and bleached state at 2.2 and +1 V

Lithiated niobium oxide LixNb2Oy films exhibit a much higher electrochromicreversibility, where bleaching is accomplished after a few seconds, while colouring times remain the same

Films based on IrO2 and Ir2O3

(transparent) Ir2O3 ● xH2O 2 Ir2O4(x-1)H2O + 2H+ + 2e- (brown)While IrO2-based films are excessively expensive, good

electrochromic properties are obtained after dilution with the much cheaper Ta2O5.

Ir2O3

Other inorganic electrochromics

Prussian blue (PB), i.e. K3Fe(CN)6, which colour reaction may be described as

(transparent) M2FeII[Fe(CN)6]2 MFeIII[Fe(CN)6] + M+ + e- (blue)

M+ = cation, e.g. K+.

A cycle life of 105 cycles has been found in solutions of pH 2–3, PB film on a polyaniline coating yields a superior cycling lifetime

compared with a PB film deposited directly onto ITO substrate.

MIXED EC OXIDESMIXED EC OXIDES

To obtain a more neutral color

To widen the optical band gap in order to provide higherbleached-state transmittance in nickel-oxide-based and iridium-oxide-based films

To dilute expensive iridium oxide films without major changes of the electrochromism

Coloration efficiency (= change in the optical absorption per unitof inserted charge) can be increased by mixing suitable oxides, cathodic tungsten oxide and anodic hydrous nickel oxide films

Spectral coloration efficiency of Ni–W oxide films

THIN FILM PREPARATION AND THIN FILM PREPARATION AND CHARACTERIZATIONCHARACTERIZATION

W mainly as (IV) and (VI)As-deposited films = heavily disordered; porosity can be

increased allowing deposition in presence of residual gas (N2, O2, H2O)

Evaporated electrochromic W oxide films tend to be hydrous(even if water is not deliberately added)

WOx x = 3.00 bulk, 3.10 surface; H/W decreases with annealing(0.8 in the as-deposited)

Deposition at 200°C followed by annealing at 430°C in O2 = monoclinic WO3

Triclinic structure for annealing at 400-500-600°C; epitaxialtetragonal phase on sapphire at 600°C

Densification with ion beam irradiation

EvaporationEvaporation

THIN FILM PREPARATION AND THIN FILM PREPARATION AND CHARACTERIZATIONCHARACTERIZATION

Large scale, large areaDeposition rate = 2-3 nm/sColumnar morphology with void content depending on PO2 (5% pO2

10 mTorr, 12% PO2 = 30 mTorr)WOx x = 3.10Crystallization between 300 and 350°C (Raman O-W-O, W=O)Ti = stbilized the disordered structure to higher temperature

SputteringSputtering: DC magnetron : DC magnetron sputteringsputtering

3-, 4-, 6-membered ring of octahedra sharing corners as in hexagonal crystals

> PO2 = tetragonal phase stabilized (< oxygen > hexagonal)

SputteringSputtering: RF magnetron : RF magnetron sputteringsputtering

THIN FILM PREPARATION AND THIN FILM PREPARATION AND CHARACTERIZATIONCHARACTERIZATION

Very high porosity (density between 0.4 and 0.7)Mixed oxides with additions of Mo, Ni, Co, Cr, Fe, Ru, Zn, Co+Ni, Co+Zn,

Ni+Zn, … the substrate adherence depend strongly on the metal additive (good results for Ni, Fe, Zn)

ElectrodepositionElectrodeposition

W carbonyl, W alkoxideAmorphous and smooth films at 0.5 nm/s onto substrates at 300-350°C

CVDCVD

H2WO4 in aqueous ammonia sprayed onto substrates at 150°C (amorphous)

Monoclinic WO3 upon annealing at 400°C

Spray Spray pyrolysispyrolysis

Acidification of aqueous salt, hydrolysis of organometallic compounds(Na2WO4 2H2O, WOCl4, W2[OC(CH3)3]6)

Relative density = 0.57 (as deposited) to 0.64 (after annealing at 190°C) Microcrystallites with a hexagonal-like structure,Mixed oxides

SolSol--gelgel

EC FILMS: INFLUENCE OF STRUCTUREEC FILMS: INFLUENCE OF STRUCTUREMeO6 octahedral based atomic

arrangements allow spaces—tunnelsin three dimensions that are largeenough for the transport of smallions.

Many different types of crystallinity depending on T, P, preparation procedure … , by meansof combination of corner and edgesharing for the octahedral building blocks.

Cubic structure = exists onlyunder high pressure, Tetragonal structure = usualHexagonal structure = seems toform in thin films made by differenttechniques. Favorable since it leavesparticularly generous spacesbetween the octahedra

Atomic arrangement in Woxide with (a) cubic, (b) tetragonal, and (c) hexagonal structure. Dots indicate sites available for ion intercalation in the open spaces between the WO6 octahedra. Dashed

lines indicate unit cells.

EC FILMS: INFLUENCE OF MORPHOLOGYEC FILMS: INFLUENCE OF MORPHOLOGY

refers to sputter deposition, The nanostructures that emerge depending on P, T during the sputtering.

‘‘ZoneT’’ = compact structure, frequently desired for sputterdeposited films with gooddurability

‘‘zone1’’ = advantageous for EC films. The columnarnanostructure allows easy iontransport over the cross-sectionof the film.

Schematic picture (‘‘Thornton diagram’’) showing nanostructures of

films deposited by sputtering at different argon pressures and

substrate temperatures. The melting point of the material is denoted Tm.

FLEXIBLE ELECTROCHROMIC FOILSFLEXIBLE ELECTROCHROMIC FOILS

Sketch of an EC foil-type device and the unit for supplying charge.

Transmittance, reflectance, and absorptance as a function of wavelength for an electrochromic foil-type device of the design shown and the orientation as measured in colored and bleached states.

Photovoltaic integrated electrochromic devices

EC windows with no external wiring are most desirable in the building industry. An integrated photovoltaic powered window is an obvious

choice: PV and EC technology have compatible operational characteristics

(National Renewable Energy Laboratory of Golden, USA)

Data for photovoltaic integrated electrochromic devices found in literature showing materials, sample size, modulation range, the performed number of cycles and the switching time for colouration and bleaching τc/b.

Gasochromic windows, the principle:

An gasochromic device is switched between a bleached and coloured state by hydrogen gas H2 instead of applying a voltage.

Not all electrochromic materials can be coloured with hydrogen gas

simple and inexpensive (only a single electrochromic layer is sufficient, and transparent electrically conducting layers are no longer necessary)

The transmittance modulation of the gasochromic devices exceed these of most solid-state EC windows

The effect has been studied in devices based on NiO, MoO3 andV2O5, WO3. Usually, a thin catalytic layer of Pt or Pd is incorporated to facilitate the gasochromic effect, but the oxides remain electrically nonconducting.

WO3-based gasochromic windowsOptically active material = sputtered, highly porous, columnar film of (doped) WO3, thickness around 400 nm, coated by a 1–5nm thin catalytic layer of platinum Pt. Exposed to a low concentration of H2 in a carrier gas, it colours blue.A more neutral colour, i.e. grey–blue, can be obtained by using a mixture of WO3 with molybdenum oxide WxMoyO3, (inferior transmittances).On exposure to O2, the layer bleaches back to its original state

(transparent)WO3 + H2 WO3 + 2H H2WO3 WO2+H2O (coloured)(coloured) 2WO2+O2 2WO3 (transparent)

Optical density and rate of coloration = film thickness and/or gas concentration

Mechanism of colouration by H2

Transmittance for the coloured and bleached state

of a gasochromic WO3

WO3-based gasochromic windows

Data for gasochromic devices found in literature showing materials, sample size, modulation range for the transmittance or reflectance, the performed number of cycles and the switching time for colouration and

bleaching.

THERMOCHROMIC DEVICESIn principle,theTC approach can be simple and a single film is all

that is needed. Superimposed EC andTC thin-film stacks, devised in order to

enhance the optical modulation

VO2 below Tc the material is monoclinic, semiconducting and relatively IR transparent, and above Tc it is tetragonal, metallic

and IR reflecting. Pure bulk-like VO2 TC = 68°C (W-doping) Spectral normal transmittance for TiO2/VO2/TiO2(left) and TiO2/VO2/TiO2/VO2/TiO2 films (lower panel). The data are compared with those for corresponding VO2films.

Crystal Crystal StructureStructure

WO3 and related materials can be found in a large variety of crystal structures: WO6 octahedra arranged in various corner-sharing or edge-sharing configurations

Pure bulk-type = monoclinic phaseThin films = octahedra hexagonally ordered (depending on

fabrication route and deposition temperature)

ReO3 structure Crystallographic shear in the ReO3structure on three orientations

Crystal Crystal StructureStructure of of intercalatedintercalated W W oxideoxide

Intercatalion leads to intricate structural changes (not fullyinvestigated)

H0.5WO3, LixWO3 (0.1<x<0.4), NaxWO3 (0.3<x<1) = Cubicstructure

Li and Na = center of the perovskite units; H = off-centeredattached to oxygen atoms in –OH groups

Unit cells for cubic (Li,Na)WO3(a) and HWO3 (part b) W = largesolid circles, O = open circles, Li, Na = large dashed circle, H = small solid circles located on a line between an oxygen site and the central position.

DENSITY OF STATESDENSITY OF STATESWO3 = Semiconductor (band gap

= 2.6-2.7 eV)VB = dominated by the O 2p

states while W 5d states dominate the conduction band

Hydrated hexagonal phase: hexagonal layers of WO6octahedra shifted from one layerto another Dehydrated for whichthe hexagona layers are stackeddirectly on top of each other

Bottom of the CB = smoothonset above the gap in the hexagonal structure while the cubic phase has a peak at thisposition

DOS for cubic WO3 (a) and comparisonhexagonal/cubic (b). Filled states at energies belowzero and empty states above zero. Also shownare the O 2p and W 5d projected state densities

DOS IN INTERCALATED W OXIDEDOS IN INTERCALATED W OXIDE

Li 2s band centered at 8.2 eV above the top of the valence band; the Fermi energy lies at 2.9 eV in the conduction band: very low occupation of the Li band

Li is ionized, charge balancing electron occupies the bottom of the conduction band

Bandwidth is reduced from 7.1 to 6.4 eVNa very similar to Li

DOS for LiWO3 (a), NaWO3 (b). Dashed curves:

Projectedstates;

Vertical lines: Fermi levels

DOS IN H INTERCALATED W OXIDEDOS IN H INTERCALATED W OXIDE

H in central position = large H-projected peak in DOS at the Fermi energy

< dOH = H peak moves away from the Fermi level towards higherenergies and a asecond H peak originating from the hybridizationwith oxygen atoms in the OH grows at low energies

Oxygen states in the hydroxide pair is much lowered in energy(broadening of the valence band)

1H = 2.6 eV total energy gain; 2H (H2O) = 2.4 eV total energygain; 2 H (OH) = 3.9 eV – hydroxide is favored.

DOS for HWO3 (c). Dashed

curves: projected states

Vertical lines: Fermi levels

Total energy for HWO3 as a function of the O-H distance

OPTICAL PROPERTIES:OPTICAL PROPERTIES:ReflectanceReflectance modulationmodulation in in WW--oxideoxide basedbased crystalscrystals

Crystalline ion-containing W oxidecan exhibit a high reflectance, expecially in the infrared

Amorphous systems = absorption

α = absorption coefficientη depends on the type of otpical transition in the gap region; ½, 3/2, 2, and 3 fortransitions being direct and allowed, direct and forbidden, indirect and allowed, indirectand forbiddenO 2p-W 5d = allowed

ħωα ; (ħω – Eg)η

OPTICAL PROPERTIES:OPTICAL PROPERTIES:AbsorbanceAbsorbance modulationmodulation in in disordereddisordered WW--oxideoxide--

basedbased filmsfilmsCE is a function of λCE can be dependent on film preparation conditions

CE vs oxygen content in the sputter plasma for W oxide filmsmade by reactive DC magnetron sputtering.

Oxygen presence: Ar/O2 gas mixture ratio > 10%: CE is about 30 cm2/C;

Ar/O2 = 5% CE exhibitslarger values (W4+)

SPUTTER DEPOSITED WSPUTTER DEPOSITED W--OXIDEOXIDE--BASED BASED FILMS: CASE STUDYFILMS: CASE STUDY

Deposition rate and durabilityFluorine in the sputter plasmaElectron bombardment of the growing filmDeposition of heated targetsOblique incidence of sputtered flux

C.G. Granqvist – Solar energy materials and solar cells 60 (2000) 201-263

SPUTTER DEPOSITED WSPUTTER DEPOSITED W--OXIDEOXIDE--BASED BASED FILMS: CASE STUDYFILMS: CASE STUDY

In presence of CF4 inthe sputter plasma the deposition rate depends on bias

Surface roughness = 0.1 μm between the protrusions (Ar+O2, Ar+O2+CF4); electron bombardment: 0.5 μm between the protrusions

Deposition rate vs. applied substrate bias for films made by sputtering of W in Ar+O2 (+CF4).

Filled and open symbols denote experimental data, and curves were drawn for convenience

SPUTTER DEPOSITED WSPUTTER DEPOSITED W--OXIDEOXIDE--BASED BASED FILMS: CASE STUDYFILMS: CASE STUDY

Electron bombarded and annealed = long range order; asdeposited and oxyfluoride = no long range order

IR600 cm-1 = disorderedW-O framework700 and 800 cm-1 = phonon spectrum in WO3 crystalsRaman780 cm-1 = disorderedW-O framework+ 950 cm-1 = W=Oasymmetric stretching vibration

Spectral IR absorptance (a) and Ramanintensity (b) of films made by sputtering ofW in the presence of Ar+O2 (+CF4).

SPUTTER DEPOSITED WSPUTTER DEPOSITED W--OXIDEOXIDE--BASED BASED FILMS: CASE STUDYFILMS: CASE STUDY

r is affected bychemical factors (CF4content – reactive ionetching) but physicalsputtering (ion impact) isalso significant(different rates fordifferent Ar content)

Ar content isirrelevant at 50° and sputtering is dominatedby chemical effects

Deposition rate for W oxyfuoride films madeby sputtering of W in (Ar)+O2+CF4. Filledandopen circles = experimental data, curvesdrawn for convenience.

SPUTTER DEPOSITED WSPUTTER DEPOSITED W--OXIDEOXIDE--BASED BASED FILMS: CASE STUDYFILMS: CASE STUDY

The preparation procedures strictlyinfluence the film durability in increasingcycles and the performance

Cyclic voltammograms for W oxyfluoride, W oxide prepared under electron bombardmentand a tandem with a W oxyfluoride film coveredwith a thin layer of electron bombarded W oxide

Spectral transmittancefor an electron

bombarded W oxide

Transmittance at 550 nm during coloration

and bleaching

OPTICAL PROPERTIES:OPTICAL PROPERTIES:Case Case studystudy on on sputtersputter depositeddeposited WW--oxideoxide--basedbased

filmsfilmsRule-of-thumb: haze become visible when Td exceeds ~ 10-2. Electron bombarded film is not ideal, whereas both the

conventionally prepared W-oxide film and the W oxyfluoride film have a clear appearance.

Spectral total and diffuse transmittance (Tt and Td, respectively) for filmsin the as-deposited state and after voltammetric cycling. (a), (b), and (c)

refer to W oxide, W oxyfluoride, and electron bombarded W oxide

Td = Tt [2π(1-n)(δ/λ)]2δ: Root-mean-square roughness

(Scalar scattering theory)