Structures and Dynamics of Interfacial Water: Input from Theoretical Vibrational Sum-frequency Spectroscopy

107 Pages·2019·3.844 MB·English

by  Fujie Tang

Springer Theses
Recognizing Outstanding Ph.D. Research
Fujie Tang
Structures and
Dynamics of
Interfacial Water
Input from Theoretical Vibrational
Sum-frequency Spectroscopy
Springer Theses
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Fujie Tang
Structures and Dynamics
of Interfacial Water
Input from Theoretical Vibrational
Sum-frequency Spectroscopy
Doctoral Thesis accepted by
Peking University, Beijing, China
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Author Supervisors
Dr. Fujie Tang Prof. Limei Xu
International Center International Center
for QuantumMaterials for QuantumMaterials
Schoolof Physics Schoolof Physics
PekingUniversity PekingUniversity
Beijing,China Beijing,China
Dr. YukiNagata
MaxPlanckInstitute
for Polymer Research
Mainz,Germany
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-981-13-8964-1 ISBN978-981-13-8965-8 (eBook)
https://doi.org/10.1007/978-981-13-8965-8
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To my parents
’
Supervisors Foreword
Itisagreatpleasureandhonorfor ustowrite aforewordtothegreatachievement
onthetheoreticalmodelinginterfacialwatersummarizedinthisthesisbookwritten
by Dr. Fujie Tang. Interfacial water is of great importance for many physical and
chemicalprocesses.Variousphenomena,suchasthereorganizationofquasi-liquid
layer on ice surface due to the temperature change, water split at the water–TiO
2
interfaces, segregation of the water and oil layer and formation of micelle and
reverse micelle are all driven by the molecular interaction of the interfacial water.
However, capturing the microscopic structure and dynamics of interfacial water is
extremely difficult, due to the limitation of most conventional experimental meth-
ods; for example, when we probe the interfacial water with light, it has been very
challenging to distinguish the bulk signals the contribution of the interface. Other
techniques such as atomic force microscopy (AFM) may change or damage the
structures of the interface during measurement. Therefore, non-contact and
surface-specific technique has been suited for probing the interfacial water.
Sum-frequency generation (SFG) spectroscopy, a technique with high surface
specificity, is ideal for exploring the interfacial water. However, how to connect
microscopic information with the SFG spectroscopic features to gain a better
understanding of the molecular structures and dynamics of the interfacial water
remains challenging.
Based on the combination of molecular dynamics (MD) simulation and
sum-frequency generation spectroscopy, this thesis developed a novel approach,
linking the SFG features and macroscopic properties at different interfaces. Fujie’s
thesismainlyfocusedonthestructureanddynamicsofwatermoleculesatwater–air
interface, ice–air interface, and TiO –water interface. Through these studies, Fujie
2
proposed a unique way to extract the behaviors of water molecules from these
variousinterfaces.Inparticular,heprovidedanewdefinitionofthefreeO−Hgroup
of the interfacial water molecules commonly found at the water–air and water–
hydrophobic medium interface. This definition is defined by using the contrast
of the free O−H SFG peak to the SFG spectra from the selected O−H groups via
MD simulations. By using this new definition, Fujie investigated the free O−H
groupsorientationbehaviorsinmoredetails,hefoundtheorientationdistributionof
vii
viii Supervisors’Foreword
free O−H groups of interfacial water is exponential, and the broad orientational
distribution indicates the presence of the free O−H group pointing down to the
bulk. Then,Fujieascribedthe origin ofthedown pointingfree O−Hgroups tothe
presence of capillary waves on the water surface. Applying this new definition to
difference interfaces, Fujie found that the topmost monolayer of ice–water surface
hasanomalyofthehydrogen-bonding.Thehydrogen-bondingnumberisincreased
with increasing temperature, probably against the thermodynamic expectation. He
successfully explained that such counterargument can be understood based on the
ordering–disordering transition of the interfacial water molecules. Knowledge
ofthesurfacestructureoficeiscriticalforunderstandingreactionsoccurringonice
surfaces and ice nucleation.
Thecontentofthethesisishighlyoriginalandisimportantforthefundamental
understanding of water structures and dynamics at interfaces, such as water–hy-
drophobic medium interfaces, like ice surface, and water–TiO interface. We are
2
pleasedthatthisthesisnotonlyprovidesaninsightintophysicsandchemistry,but
also a fun of science to the readers.
Beijing, China Prof. Limei Xu
Mainz, Germany Dr. Yuki Nagata
August 2018
Abstract
By combining the molecular dynamics (MD) simulation and sum-frequency gen-
eration(SFG)spectroscopy,thisthesishassystematicallystudiedthestructuresand
dynamicsoftheinterfacialwater,especiallyitsvibrationalspectroscopy,changesin
the structures, absorption of the water–air interface, ice–air interface, and TiO –
2
water interface.
Interfacial water is of great importance for the understanding of phenomenon
observedinatmospherescience,earthchemistry,biology,etc.Duetothelimitation
of the experimental methods, it is very difficult for conventional experimental
methods to distinguish the difference of the signal from interface region and bulk.
SFGisasurface-specifictool,whichruledoutthebulkcontributionsfromtheSFG
signal, thanks to the second-order nonlinear process. However, it is not straight-
forward to connect the information on the microscopic structure and dynamics at
the interfaces with the SFG signals. MD simulation together with SFG spectra
calculation can make a breakthrough for linking the microscopic structure at
interfacesandSFGsignal.Thefocusofthisthesisistostudytheinterfacialwaterat
different interfaces by using both MD simulation and SFG experiment.
1. Water–air interface by using theoretical SFG calculation. I have formulated a
new definition for capturing the sub-ensemble of the surface free O−H groups
using the intermolecular distance and the angle formed by the water dimer,
through the comparison of the *3700 cm−1 SFG response and the responses
from the selected free O−H groups at the HOD−air interface. Using these
optimized free O−H group definitions, I infer the fraction of interfacial water
moleculeswithfreeO−Hgroupsof28%,avibrationallifetimeofthefreeO−H
groupsof1.3ps,andtheangleformedbythefreeO−Hgroupsandthesurface
normal of 63° at the water−air interface. I expect that this improved free O−H
group definition can be helpful in exploring the structure and dynamics of the
interfacial water.
2. Water–air interface by using MD simulation and SFG measurement: Together
withexperiment,IfindtheaverageangleofthefreeO−Hgroups,relativetothe
surface normal, is found to be *60°, substantially larger than previous
ix
x Abstract
estimates of 30–40°. This discrepancy can be traced to erroneously assumed
Gaussian–stepwiseorientational distributionsoffree O−Hgroups.Instead, MD
simulationrevealsabroadandexponentiallydecayingorientationaldistribution.
The broad orientational distribution indicates the presence of the free O−H
grouppointingdowntothebulk.IascribetheoriginofsuchfreeO−Hgroupsto
the presence of capillary waves on the water surface.
3. Ice–air interface: The topmost monolayer, interrogated through its
non-hydrogen-bonded, free O−H stretch peak, exhibits a maximum in surface
H-bond density around 200 K. This maximum results from two competing
effects: Above 200 K, thermal fluctuations cause the breaking of H bonds;
below200 K,theformationofbulk-likecrystallineinterfacialstructuresleadsto
H-bond breaking. Knowledge of the surface structure of ice is critical for
understanding reactions occurring on ice surfaces and ice nucleation.
4. TheinterfacialmolecularstructureofwateratthesurfaceofanataseTiO ,using
2
phase-sensitive SFG spectroscopy together with spectra simulation using
ab initio molecular dynamic trajectories. I identify two oppositely oriented,
weakly and strongly hydrogen-bonded sub-ensembles of O−H groups at the
superhydrophilicUV-irradiatedTiO surface.Thewatermoleculeswithweakly
2
hydrogen-bonded O−H groups are chemisorbed, i.e., form hydroxyl groups, at
the TiO surface with their hydrogen atoms pointing toward bulk water. The
2
strongly hydrogen-bonded O−H groups interact with the oxygen atom of the
chemisorbed water. Their hydrogen atoms point toward the TiO . This strong
2
interaction between physisorbed and chemisorbed water molecules causes
superhydrophilicity.
(cid:1) (cid:1)
Keywords Interfacialwater Sum-frequencygenerationspectroscopy Molecular
(cid:1) (cid:1) (cid:1) (cid:1)
dynamic Water–air interface Ice surface Free O−H groups TiO –water
2
interface