| Potassium dihydrogen phosphate,KH2PO4(KDP),is a widely concerned non-linear optical material with large electro-optical and nonlinear optical coefficients and high laser damage threshold.What's more,large-scale KDP crystal which meets the need of high power laser devices for large size frequency doubling crystal can be easily grown from its saturated aqueous solution.Thus it has wide practical applications in laser frequency converter,electro-optic modulators,optical switches and other high technology field due these unique properties.However during the producing,machining and application process,the adsorption of water could have significant influences on the surface electronical and physical properties of KDP crystal.Therefore,to control the influences of water adsorption,it's essential to understand the interaction between water and KDP crystal surfaces.The study of the interaction between a water molecule and the KDP surfaces could help us locate the functional atoms on the KDP surfaces which pull the water molecule toward the surfaces and understand how exactly the water molecule is adsorbed on the two surfaces of KDP crystal,which might be the theoretical evidence of hydrogen bonding between water and the KDP surfaces observed in the experiment and help us understand the nature of the liquid order at the water-KDP interface.This would lay a good foundation for the deeper understanding of the complex interaction between KDP crystal and water.The adsorption of single water molecule on external surfaces of KDP crystal,including(100)and(101)surfaces,was theoretically investigated based on density functional theory(DFT)method.The most favorable adsorption sites on both surfaces were considered based on the adsorption energy.The electron density difference and partial density of states were also calculated to analyze the bond formation during the adsorption process.The main findings of our work are listed below:1.The O,H and K atoms on the surfaces are the functional atoms which pull the water molecule toward the surfaces.The 0 atom of the water molecule is pulled toward the surfaces by interacting with the H and K atoms on the surfaces.Meanwhile the H atom of the water molecule is pulled toward the surfaces through interacting with the O atom exposed on the surfaces.These functional atoms play a dominating role in the adsorption process.2.The most favorable adsorption sites on(100)surface are determined as P top site(position e)which is actually the H-O bridge position within a phosphate group and P-K bridge position(position h)which is actually the middle site between the O atom of the phosphate group and the K atom on the surface.While on the(101)surface,the sites are both O-K bridge positions(position o and p).All the most favorable adsorption sites are the middle sites between two functional atoms.3.On the KDP(100)surface,the water molecule may form two hydrogen bonds with the neighbor phosphate group(position e)or form an O-K ionic bond with the neighbor K atom and a hydrogen bond with the neighbor phosphate group(position h)during the adsorption progress.While on the KDP(101)surface,the water molecule can only be adsorbed by forming an O-K ionic bond and a hydrogen bond.This difference is caused by the different surface structure of the(100)and(101)surfaces of KDP crystal. |
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