Band Structure of Tungsten(VI) Oxide

Band Diagram of Tungsten (VI) oxide

Tungsten (VI) Oxide

- One of the do-transition metal oxides; has many interesting optical, electrical, structural, and defect properties.

-Forms tungsten bronzes with alkali metals (e.g. Li, Na and K) which have unusual metal-insulator phase transition and are superconductors at low temperatures.

-exhibits electrochromism and photochromism (used in smart windows)

Generating the Band Structure

Obtain the reciprocal lattice

TRINIDAD, L.J.P.L.

Determine the first Brillouin Zone

Analyze the Orbital Interactions

Generate the band structure for each orbital interaction

Structure of Tungsten (VI) Oxide

- adopts an empty perovskite structure.

- corner shared WO6 octahedra, W in primitive cubic while O bisects the unit cell edges.

Analyzing Orbital Interactions

p tangential interactions:

The conduction band

Comparing with the computational band structure

dxy orbital interactions:

Pi interactions of Oxygen atoms

dxz orbital interactions:

X

M

R

dyz orbital interactions:

X M R

--> due to p tangential and p perpendicular

-the dashed line indicates the fermi level

p perpendicular interactions:

d(x2-y2) orbital interactions:

X M R

Last Step:

d(z2) orbital interactions:

Sum up all the spaghetti curves generated into one band diagram

X M R

Density of States

Conduction Band

-generated from the 5d orbitals of W

Sigma Intetactions in the Valence Band

--> Mostly due to O orbitals

What type of inteactions are possible?

in the x-axis:

In the z- axis:

Technique:

Which orbital should interact?

in the y axis:

Partially determine the orbital interaction for both valence and conduction. Do this by also determining the sigma and pi interaction separately.

in the z axis:

Partial Band Structure of Tungsten (VI) Oxide

TIP: Be Consistent in using the axes when determining orbital interactions. Use the first Brillouin zone.

-generated band from the sigma interaction of the Oxygen 2p-orbitals

Obtaining the Reciprocal Lattice

-The Reciprocal lattice is obtained by drawing a plane that bisects through adjacent lattice points

Real Space Lattice

Reciprocal Space Lattice

Sigma Interactions of Oxygen

The First Brillouin Zone

The boundaries of the first Brillouin zone are the planes normal to the six reciprocal lattice vectors ( +/-) b1,( +/-) b2, and (+/-) b3 at their midpoints:

+/- (πpi/a)

--> The most crucial and by far the most difficult part:)

The First Brillouin Zone

http://lamp.tu-graz.ac.at/~hadley/ss1/bzones/sc.php

Generated from:

Checklist

Obtain Reciprocal Lattice

Determine the First Brillouin Zone

Analyze Orbital Interactions

Generate Band Structure

Molecular Orbital Diagram for Tungsten (VI) oxide

Tungsten Bronzes

-HOMO is t2g and LUMO is eg

-Large separation between HOMO and LUMO.

-greatest interaction in 2p - O and 5d -W

-bonding orbitals have greater O character and antibonding orbitals have greater W character

Typical Perovskite

Cubic Structure

Two Form of Tungsten (VI) Oxide

Insertion of Alkali metal ion

Hexagonal Structure

Oxygen Vacancies

-Hexagonal structure is sometimes observed in film deposition.

-Other forms other than the hexagonal structure are observed in thin films due to rotation of the octahedra and deposition conditions

-When a defect is created, the defect levels are expected to be near or inside the valence band holding two electrons.

-When one electron is removed from this level, repulsions occur between W-ions and displacement of W occurs which result to an upward shift in the band gap(=color center).

-the transition from the first transition to the second results to coloration.

Electrochromism and Photochromism

-various excitation sources; same spectrum with little variation in peak position and half-width.

-Structure sensitive (amorphous having greater band gap, 1.2 eV and 0.7 eV for crystalline)

-Fully oxidized film shows no coloration except in high T and reducing atmosphere

-optical coloration upon irradiation of light within band gap energy (max 3.6 eV)

-doping can affect coloration mechanics

-optically induced coloration cannot be bleached by light but can be bleached electrically

-color-center induced photoconductivity is observed at high electric field and elevated T

-coloration is associated with an increase in electrical conductivity (semicon->metallic)

-electrical coloration produces an internal emf that opposes the external

-loss of oxygen is observed in the coloration process

APPLICATIONS

Photocatalyst

-Hydrogen Economy

Smart Windows and PEC devices

Other Applications:

-gas sensor

-biosensor

-image and optical recording devies

-superconductor

References

S.K. Deb, Opportunities and Challenges in Science and Technology of WO3 for Electrochromic and Related Applications, Solar Energy Materials and Solar Cells 92(2008) 245-258.

L. Kopp, H. N. Harmon and S. H. Liu, Solid State Commun. 22 (1977) 677.

S. K. Deb, Appl. Opt. Suppl. 3 (1969) 192.

C. G. Granqvist, Handbook of inorganic Electrochromic Materials, (Elsevier, Amsterdam, 1995).

L. Seguin, M. Figlarz, A novel supermetastable WO3 phase, Solid State Ionics, 63-65 (1993) 437-441.