Water Adsorption on Graphene: A First principles study

Mihir Ranjan Sahoo

IIT, Bhubaneswar

Graphene as Gas Sensors: • Graphene: highly sensitive gas sensor

• Increase in Graphene charge carrier/concentration induced by adsorbed gas molecule leads to highly sensitive sensors, even possibility of detecting individual molecules.

• Sensor property based on changes in resistivity due to molecules adsorbed on graphene sheet.

• Excellent sensor properties of graphene due to 2 important facts:

i) 2-D crystal structure with only surface and no volume, maximizes the effect of surface dopant.

ii)Highly conductivity and shows metallic conductance in the limit of zero conductance.

Water Molecule on 4X4 Graphene Supercell

Water on Graphene Position Orientation Height (in A) Adsorption Energy

(in meV)

Centre Up 3.70 36.22

Centre Down 4.02 30

Centre Parallel 3.55 36.80

Top Up 3.70 33.87

Top Down 4.05 28.95

Bridge Up 3.70 32.45

Bridge Down 4.05 29.87

Centre Pointing 3.50 54.26

The Adsorption Energy has weaker Water-Orientation dependence.

Down Pointing Parallel

Up

Band Structure

Pristine Graphene Water on Graphene with O atom Up

Water on graphene with one O-H is pointing

Band Structure

• Band Structure of water on graphene is almost identical to that of Pristine Graphene due to weak Interaction of water with Graphene.

• HOMO of water molecule is located at -2.62 eV relative to the Fermi level

• LUMO of water molecule is located at 3.31eV relative to the Fermi level.

Density of States a)- HOMO of H2O b) – LUMO of H2O Main panel- DOS of H2O on Graphene

Charge Transfer

Orientation Vs Charge transfer

Orientation Charge Transfer(e)

Up 0.021

Down -0.009

Parallel 0.015

Pointing -0.025

Adsorption of water on Graphene

• In Up Orientation , the HOMO plays dominant role, through a small mixing of graphene orbitals above the fermi level, and donates some charge to Graphene. (Donor)

• In Down and Pointing orientation,the LUMO of water interacts stronger with the surface, through a small mixing with the graphene orbitals below Dirac point, to accept some charge from graphene (Acceptor)

• Experimentally, water acts as acceptor, which is also energetically favorable condition in Center-Pointing position predicted in theoretical calculation.

Thank You