**Meet the editor**

Dr. Y. Fedorenko graduated from Saratov State University, Russia, in 1998. Since 2001 she works as a post-doctoral researcher and scientist for companies and universities in Belgium, the UK and Finland. Her scientific interests include thin film synthesis, semiconductor processing and investigation of optical, structural and electronic properties of thin films and nanostructures.

Contents

**Preface VII**

**Highlights 1** Yanina Fedorenko

Chapter 1 **Introductory Chapter: Electron Microscopy - Research**

Chapter 2 **Molecular Simulation Study of Water Transport through Aquaporin-Inspired Pore Geometry 5** Daejoong Kim and Majid Shahbabaei

Liliana Rozemarie Manea and Andrei-Petru Bertea

**Influence of Hydrodynamic Conditions 41**

Chapter 5 **Nanostructured Perovskites for Catalytic Combustion 75**

Chapter 6 **Proton Exchange Membrane Water Electrolysis as a Promising**

Chapter 4 **Fabrication of Ordered and High-Performance Nanostructured Photoelectrocatalysts by Electrochemical Anodization:**

Bianca Lucas-Granados, Rita Sánchez-Tovar, Ramón M. Fernández-

**Technology for Hydrogen Production and Energy Storage 95**

Chapter 3 **Sensors from Electrospun Nanostructures 23**

Domene and José García-Antón

Radenka Maric and Haoran Yu

Chapter 7 **Nanostructured Si/SiO2 Quantum Wells 119** Toshio Takeuchi and Yoshiji Horikoshi

**Metal-Mesogenic Systems 135**

Tatyana I. Shabatina

Chapter 8 **Self-Assembled Nanostructures Formation in Hybrid**

Corneliu Doroftei

## Contents

#### **Preface XI**

Preface

ter treatments.

structure peculiarities.

seemingly disparate materials systems.

and results in an improved desalination performance.

materials such as ceramics and carbon in a nanofiber form.

fibers in the field of chemiresistive, optical, and other sensors.

I hope that this book, composed as a volume of self-contained research articles, appeals to a wide readership that is interested in phenomena occurring at the nanoscale in a variety of

New water treatment technologies are indispensable in the supply of safe water for domes‐ tic and industrial needs. Desalination is a method of choice to address the demands of water consumption. Membrane processes in water purification technologies are versatile and eco‐ nomically beneficial for drinking water production, desalination, water reuse, and wastewa‐

In research written by M. Shahbabaei and D. Kim, the authors consider water transport mechanisms in multilayer nanoporous graphene, which features the asymmetric hourglass nanopore structure of aquaporin protein water channels that facilitates fast water transport

Because of the unique capability to tune nanostructure geometry, surface morphology and charge, nanomaterials are exploited in virtually any kind of sensor devices because of the ability to tailor the sensing functions. The chemical composition, surface conditions, and crystal structure of nanomaterials are critical in adjusting transduction mechanisms. In this book, cost-effective approaches to the fabrication of nanomaterials are represented by the electrospinning technique, which is explored to prepare electrospun nanostructures. Electro‐ spinning is a versatile method that, in addition to polymers, is suitable to synthesize other

In the following chapter, L. R. Manea and A.-P. Bertea review electrospinning techniques for producing polymer nanofibers and elucidate a variety of applications of electrospun nano‐

In the chapter presented by B. Lucas-Granados, R. Sánchez-Tovar, R. M. Fernández–Do‐ mene, and J. García-Antón we can see research results on novel nanostructured metal oxide photoanodes synthesized by electrochemical anodization of transition metals under hydro‐ dynamic conditions. In electrochemical water splitting, upon illumination, the electric field generated by the surface band bending of a wide-bandgap n-type semiconductor drives the minority charge carriers toward the surface to oxidize water and produce electricity. Photo‐ electrochemical properties of the metal oxide anodes are discussed pertaining to the nano‐

Catalysis is ubiquitous in almost any industrial chemical process and can be utilized to aid in the reduction of air pollution and generation of energy from hydrogen. The catalysts cur‐ rently employed for environmental control are either noble metals or metal oxides, which

## Preface

I hope that this book, composed as a volume of self-contained research articles, appeals to a wide readership that is interested in phenomena occurring at the nanoscale in a variety of seemingly disparate materials systems.

New water treatment technologies are indispensable in the supply of safe water for domes‐ tic and industrial needs. Desalination is a method of choice to address the demands of water consumption. Membrane processes in water purification technologies are versatile and eco‐ nomically beneficial for drinking water production, desalination, water reuse, and wastewa‐ ter treatments.

In research written by M. Shahbabaei and D. Kim, the authors consider water transport mechanisms in multilayer nanoporous graphene, which features the asymmetric hourglass nanopore structure of aquaporin protein water channels that facilitates fast water transport and results in an improved desalination performance.

Because of the unique capability to tune nanostructure geometry, surface morphology and charge, nanomaterials are exploited in virtually any kind of sensor devices because of the ability to tailor the sensing functions. The chemical composition, surface conditions, and crystal structure of nanomaterials are critical in adjusting transduction mechanisms. In this book, cost-effective approaches to the fabrication of nanomaterials are represented by the electrospinning technique, which is explored to prepare electrospun nanostructures. Electro‐ spinning is a versatile method that, in addition to polymers, is suitable to synthesize other materials such as ceramics and carbon in a nanofiber form.

In the following chapter, L. R. Manea and A.-P. Bertea review electrospinning techniques for producing polymer nanofibers and elucidate a variety of applications of electrospun nano‐ fibers in the field of chemiresistive, optical, and other sensors.

In the chapter presented by B. Lucas-Granados, R. Sánchez-Tovar, R. M. Fernández–Do‐ mene, and J. García-Antón we can see research results on novel nanostructured metal oxide photoanodes synthesized by electrochemical anodization of transition metals under hydro‐ dynamic conditions. In electrochemical water splitting, upon illumination, the electric field generated by the surface band bending of a wide-bandgap n-type semiconductor drives the minority charge carriers toward the surface to oxidize water and produce electricity. Photo‐ electrochemical properties of the metal oxide anodes are discussed pertaining to the nano‐ structure peculiarities.

Catalysis is ubiquitous in almost any industrial chemical process and can be utilized to aid in the reduction of air pollution and generation of energy from hydrogen. The catalysts cur‐ rently employed for environmental control are either noble metals or metal oxides, which are less expensive and readily processable. In the next chapter, C. Doroftei describes a series of nanostructured transition metal oxide compounds possessing perovskite structure that is applicable for catalytic combustion of volatile organic compounds at low and moderate tem‐ peratures. The synthesis method of the perovskite metal oxides is the precursor method of self-combustion with polyvinyl alcohol as a colloidal medium. The author discusses struc‐ tural properties, morphology, and chemical composition of as-prepared perovskites in rela‐ tion to the catalytic activity.

Authors R. Maric and H. Yu review in their chapter future perspectives of hydrogen as an energy carrier, classify water electrolysis technologies, and outline challenges in the devel‐ opment of membrane and catalyst materials. The oxygen evolution reaction at the anode is strongly influenced by the chemical properties and surface structure of the nanostructured catalyst. The study reveals that the co-presence of iridium oxides comprising Ir in III and IV oxidation states supports a higher oxygen evolution reaction activity due to the presence of electrophilic oxygen species, which promote the formation of O–O bonds. The size effects of bimetallic oxides and nanoparticles on the electronic structure of the Ir-based catalysts and the role of metal support to enhance oxygen evolution reaction activity and the catalyst sta‐ bility are outlined.

Light emission and manipulation at the nanoscale is a long-standing research interest of the interdisciplinary scientific community. Although silicon dominates in present-day electron‐ ics, it is a poor emitter of light due to the indirect bandgap.

The following chapter presented by authors T. Takeuchi and Y. Horikoshi shows research results on the fabrication of amorphous Si/SiO2 quantum wells, which incorporate silicon light-emitting nanocrystals and reports on the optical properties of these nanostructures. The light-emitting Si/SiO2 nanostructures are nanolayers of amorphous Si, SiO2, and Si/SiO2 films grown by means of ultrahigh vacuum radio-frequency magnetron sputtering. The ten‐ tative absorption coefficients are evaluated for different thicknesses of silicon layers in Si/SiO2 nanostructures. Quantum confinement effects are found to modify optical properties of the Si/SiO2 nanostructures as inferred from the optical transmittance and reflectance spec‐ tra. The rapid-thermal-annealed Si/SiO2 multilayer films demonstrate strong photolumines‐ cence at ~1.45 eV originating from nanocrystalline silicon. The temperature dependence of the nanocrystalline luminescence intensity is discussed invoking the Kapoor model.

The last chapter by T. I. Shabatina reviews methods of supramolecular chemistry and cryo‐ chemistry applied to the synthesis and morphological and structural investigation of hybrid nanostructures and nanosized aggregates comprising plasmonic metals and several meso‐ genic compounds, which can reveal at certain temperatures different liquid crystalline mes‐ ophases—nematics, smectics, and cholesterics. The chapter also considers the directed formation of metal nanoparticles by applying thermal treatments. New hybrid nanosystems were characterised by infrared and UV-Vis spectroscopy, transmission electron microscopy, and selected area electron diffraction.

Finally, I wish to express my sincere gratitude to Ms. Romina Skomersic for support in the course of preparing this volume.

> **Yanina Fedorenko** The University of Liverpool, UK

**Chapter 1**

**Provisional chapter**

**Introductory Chapter: Electron Microscopy - Research**

**Introductory Chapter: Electron Microscopy - Research**

Scientific knowledge begins with observation. Scientists use observation to obtain data and verify theories and hypotheses. The most straightforward method of observation consists in imaging an object under study. Therefore, microscopy is an integral part of modern research in natural sciences. Here, I highlight recent innovations of transmission electron microscopy such as the development of new techniques of sample preparation, application of electron diffraction to study atomic displacements in disordered materials, determination of local structural variations in glassy materials, and imaging dopants atoms in quantum dots. The highlights may interest researchers working on nanoscale phenomena in solid state physics,

**2. A novel method of sample preparation for transmission electron**

Improved instrumentation and optics significantly contributed to the development of transmission electron microscopy (TEM) with considerable effort put into innovative techniques for preparation of samples and introduction of new equipment, which progressed from the ion-beam polishers to tripod polishing and focused-ion-beam systems. Even though the latter can damage a specimen, the method is almost universally used for the sample preparation in electron microscopy. To prevent sputtering damage, a photoresist or a combination of a

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.80729

**Highlights**

**Highlights**

Yanina Fedorenko

Yanina Fedorenko

**1. Introduction**

**microscopy**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.80729

technology, materials physics, and engineering.

photoresist and an inorganic film may be used.

#### **Introductory Chapter: Electron Microscopy - Research Highlights Introductory Chapter: Electron Microscopy - Research Highlights**

DOI: 10.5772/intechopen.80729

#### Yanina Fedorenko Yanina Fedorenko

are less expensive and readily processable. In the next chapter, C. Doroftei describes a series of nanostructured transition metal oxide compounds possessing perovskite structure that is applicable for catalytic combustion of volatile organic compounds at low and moderate tem‐ peratures. The synthesis method of the perovskite metal oxides is the precursor method of self-combustion with polyvinyl alcohol as a colloidal medium. The author discusses struc‐ tural properties, morphology, and chemical composition of as-prepared perovskites in rela‐

Authors R. Maric and H. Yu review in their chapter future perspectives of hydrogen as an energy carrier, classify water electrolysis technologies, and outline challenges in the devel‐ opment of membrane and catalyst materials. The oxygen evolution reaction at the anode is strongly influenced by the chemical properties and surface structure of the nanostructured catalyst. The study reveals that the co-presence of iridium oxides comprising Ir in III and IV oxidation states supports a higher oxygen evolution reaction activity due to the presence of electrophilic oxygen species, which promote the formation of O–O bonds. The size effects of bimetallic oxides and nanoparticles on the electronic structure of the Ir-based catalysts and the role of metal support to enhance oxygen evolution reaction activity and the catalyst sta‐

Light emission and manipulation at the nanoscale is a long-standing research interest of the interdisciplinary scientific community. Although silicon dominates in present-day electron‐

The following chapter presented by authors T. Takeuchi and Y. Horikoshi shows research results on the fabrication of amorphous Si/SiO2 quantum wells, which incorporate silicon light-emitting nanocrystals and reports on the optical properties of these nanostructures. The light-emitting Si/SiO2 nanostructures are nanolayers of amorphous Si, SiO2, and Si/SiO2 films grown by means of ultrahigh vacuum radio-frequency magnetron sputtering. The ten‐ tative absorption coefficients are evaluated for different thicknesses of silicon layers in Si/SiO2 nanostructures. Quantum confinement effects are found to modify optical properties of the Si/SiO2 nanostructures as inferred from the optical transmittance and reflectance spec‐ tra. The rapid-thermal-annealed Si/SiO2 multilayer films demonstrate strong photolumines‐ cence at ~1.45 eV originating from nanocrystalline silicon. The temperature dependence of

the nanocrystalline luminescence intensity is discussed invoking the Kapoor model.

The last chapter by T. I. Shabatina reviews methods of supramolecular chemistry and cryo‐ chemistry applied to the synthesis and morphological and structural investigation of hybrid nanostructures and nanosized aggregates comprising plasmonic metals and several meso‐ genic compounds, which can reveal at certain temperatures different liquid crystalline mes‐ ophases—nematics, smectics, and cholesterics. The chapter also considers the directed formation of metal nanoparticles by applying thermal treatments. New hybrid nanosystems were characterised by infrared and UV-Vis spectroscopy, transmission electron microscopy,

Finally, I wish to express my sincere gratitude to Ms. Romina Skomersic for support in the

**Yanina Fedorenko**

The University of Liverpool, UK

ics, it is a poor emitter of light due to the indirect bandgap.

and selected area electron diffraction.

course of preparing this volume.

tion to the catalytic activity.

VIII Preface

bility are outlined.

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.80729

## **1. Introduction**

Scientific knowledge begins with observation. Scientists use observation to obtain data and verify theories and hypotheses. The most straightforward method of observation consists in imaging an object under study. Therefore, microscopy is an integral part of modern research in natural sciences. Here, I highlight recent innovations of transmission electron microscopy such as the development of new techniques of sample preparation, application of electron diffraction to study atomic displacements in disordered materials, determination of local structural variations in glassy materials, and imaging dopants atoms in quantum dots. The highlights may interest researchers working on nanoscale phenomena in solid state physics, technology, materials physics, and engineering.

## **2. A novel method of sample preparation for transmission electron microscopy**

Improved instrumentation and optics significantly contributed to the development of transmission electron microscopy (TEM) with considerable effort put into innovative techniques for preparation of samples and introduction of new equipment, which progressed from the ion-beam polishers to tripod polishing and focused-ion-beam systems. Even though the latter can damage a specimen, the method is almost universally used for the sample preparation in electron microscopy. To prevent sputtering damage, a photoresist or a combination of a photoresist and an inorganic film may be used.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The sample geometry for TEM analysis in a majority of studies is the thin-foil. A new technique of sample preparation has been developed to enable transmission microscopy in most basic equipment with the use of usual imaging modes [1]. As an example of TEM by specimen design, the strain in thin film planar devices has been studied. The strain is one of the important parameters for understanding nanoscale phenomena in physics because it controls the mechanical and electronic properties of materials. The new measurement technique of electron microscopy proposes an original design of the specimen geometry to apply the technique in basic conventional TEM, *i.e*. by exploiting the moiré imaging phenomenon. The Moiré patterns are analyzed for the strain, the lattice parameters, orientation, and distortions. A new method of two-sample preparation is proposed to utilize the properties of moiré patterns and improve the spatial resolution of strain maps. The method explores cutting a thin lamella sample into two pieces to provide precise control of parallelism of the two superimposed surfaces. The misorientation of the two surfaces is only 0.05° that is smaller by one order of magnitude than typical Bragg angles in the range of 0.3–0.6° and the zone, which needs to be imaged, is orientated almost in the same direction as the reference lattice. The spatial resolution of TEM imaging obtained by applying this sample preparation procedure is about one nanometer that is achieved by controlling rotations of the samples during preparation.

**4. Spatial heterogeneity in metallic glasses studied by high-angle**

Glasses have been considered as liquids frozen by quenching of high-temperature melts. Metal glasses are hard to prepare because compared to organic materials and silicates metals readily crystallize during solidification that introduces heterogeneities in the amorphous materials structure. The heterogeneous nature of metal glasses has long escaped an experimental evidence since nanoscale crystal-like aggregates in amorphous solids appear at a very short length scale, which is not detectable by high-resolution electron diffraction methods.

Introductory Chapter: Electron Microscopy - Research Highlights

http://dx.doi.org/10.5772/intechopen.80729

3

To reveal crystal-like aggregates at nano-scale and study the local atomic structure of spatial heterogeneous metallic glasses, researchers use advanced experimental methods of angstrom-beam electron diffraction (ABED) and aberration-corrected scanning transmission electron microscopy (STEM) [3]. The crystalline phase is detected by the technique of highangle annular dark field (HAADF) imaging in STEM. In HAADF, the scattering angles are large and incoherent thermal diffuse scattering surpasses the coherent Bragg scattering. Being sensitive to the atomic numbers, HAADF provides contrast images of compositional nonuniformities in materials. The structurally different regions in the metallic glass are observed as dark and bright areas in HAADF STEM images and analyzed by using ABED to detect the local atomic configurations. The structural variations within the glassy material result in spatial heterogeneity of metallic glasses that manifest itself in a fluctuation of density occurring without fluctuation of chemical composition. The structural heterogeneity has important implications in glass science and technology because many of the material properties such as

the glass-forming ability and the elastic behavior are structurally dependent.

**5. Ultra-thin graphene oxide support film for TEM observations of**

Over the last five decades, SiO2 has been intensively studied as the basic ingredient in field-effect transistors, silicon photovoltaics, and nanoscale light-emitting sources. Metaloxide-semiconductor technology is dominating for very large scale integration applications, enabling smaller dimensions, a faster switching speed, and a lower power dissipation. Ability to control the oxide thickness with an angstrom precision facilitates the fabrication of ultrasmall devices operating at a higher drive current. Despite the successful implementation

electronic and optoelectronic devices is still hindered due to low carrier concentration and low charge carrier mobility. Although the optical and electronic properties of silicon nanostructures are remarkably dependent on the modification of the defect ensemble at the Si nanocrystal-SiO2 interface, direct imaging of the amorphous structure surrounding silicon nanocrystals has been found difficult due to the insufficient contrast of electron microscopy. The use of atomically thin graphene oxide supports in electron microscopy of colloidal silicon quantum dots reveals the amorphous structure of the surrounding core with the core

/Si nanostructures in transistors, the device application of Si nanocrystals in

**annular dark field microscopy**

**core-shell nanocrystals**

of planar SiO2

#### **3. Disordered materials studied by electron transmission microscopy**

The atoms vibrate in the matter at room temperature. The effect of vibrations is observable from diffraction patterns of electron microscopy. Vibrating atoms introduce a diffuse background of low intensity superimposed with the diffraction peaks. Without the thermal diffuse scattering (TDS) diffraction angles of stationary atoms would give a set of discrete lines. Electrons, as well as x-rays and neutrons, are characterized by a wavelength, which is dependent on an accelerating voltage applied between the cathode and the anode. A typical wavelength of an electron in a TEM instrument is 0.04–0.02 Å, *i.e.* by a few orders of magnitude smaller than the wavelength of x-rays. Therefore, much smaller Bragg angles can be achieved that enables crystalline order to be studied with higher precision. Also, x-ray analysis requires rotation of a sample, whereas electron diffraction does not. A range of disordered lattices can be studied by using electron transmission microscopy − flexible framework structures, disordered solid solutions, stacking fault structures, and materials susceptible to structural instabilities such as novel perovskite metal-halide semiconductors.

Diffuse scattering has been studied in the molecular crystals, which experience large molecular displacements as a result of thermal molecular vibrations [2]. The dynamic disorder in molecular systems contributes to fluctuations of the transfer integrals, *i.e*. to charge carrier transport in disordered molecular solids and the energy distribution of transport sites. The lattice vibrational modes of a range of organic semiconductors have been studied from electron diffraction patterns in TEM. The research illuminates the interrelation of thermal lattice fluctuations and molecular structure of organic semiconductors.

## **4. Spatial heterogeneity in metallic glasses studied by high-angle annular dark field microscopy**

The sample geometry for TEM analysis in a majority of studies is the thin-foil. A new technique of sample preparation has been developed to enable transmission microscopy in most basic equipment with the use of usual imaging modes [1]. As an example of TEM by specimen design, the strain in thin film planar devices has been studied. The strain is one of the important parameters for understanding nanoscale phenomena in physics because it controls the mechanical and electronic properties of materials. The new measurement technique of electron microscopy proposes an original design of the specimen geometry to apply the technique in basic conventional TEM, *i.e*. by exploiting the moiré imaging phenomenon. The Moiré patterns are analyzed for the strain, the lattice parameters, orientation, and distortions. A new method of two-sample preparation is proposed to utilize the properties of moiré patterns and improve the spatial resolution of strain maps. The method explores cutting a thin lamella sample into two pieces to provide precise control of parallelism of the two superimposed surfaces. The misorientation of the two surfaces is only 0.05° that is smaller by one order of magnitude than typical Bragg angles in the range of 0.3–0.6° and the zone, which needs to be imaged, is orientated almost in the same direction as the reference lattice. The spatial resolution of TEM imaging obtained by applying this sample preparation procedure is about one nanometer that is achieved by controlling rotations of the samples

2 Nanostructures in Energy Generation, Transmission and Storage

**3. Disordered materials studied by electron transmission microscopy**

The atoms vibrate in the matter at room temperature. The effect of vibrations is observable from diffraction patterns of electron microscopy. Vibrating atoms introduce a diffuse background of low intensity superimposed with the diffraction peaks. Without the thermal diffuse scattering (TDS) diffraction angles of stationary atoms would give a set of discrete lines. Electrons, as well as x-rays and neutrons, are characterized by a wavelength, which is dependent on an accelerating voltage applied between the cathode and the anode. A typical wavelength of an electron in a TEM instrument is 0.04–0.02 Å, *i.e.* by a few orders of magnitude smaller than the wavelength of x-rays. Therefore, much smaller Bragg angles can be achieved that enables crystalline order to be studied with higher precision. Also, x-ray analysis requires rotation of a sample, whereas electron diffraction does not. A range of disordered lattices can be studied by using electron transmission microscopy − flexible framework structures, disordered solid solutions, stacking fault structures, and materials susceptible to structural instabilities such as novel perovskite metal-halide

Diffuse scattering has been studied in the molecular crystals, which experience large molecular displacements as a result of thermal molecular vibrations [2]. The dynamic disorder in molecular systems contributes to fluctuations of the transfer integrals, *i.e*. to charge carrier transport in disordered molecular solids and the energy distribution of transport sites. The lattice vibrational modes of a range of organic semiconductors have been studied from electron diffraction patterns in TEM. The research illuminates the interrelation of thermal lattice

fluctuations and molecular structure of organic semiconductors.

during preparation.

semiconductors.

Glasses have been considered as liquids frozen by quenching of high-temperature melts. Metal glasses are hard to prepare because compared to organic materials and silicates metals readily crystallize during solidification that introduces heterogeneities in the amorphous materials structure. The heterogeneous nature of metal glasses has long escaped an experimental evidence since nanoscale crystal-like aggregates in amorphous solids appear at a very short length scale, which is not detectable by high-resolution electron diffraction methods.

To reveal crystal-like aggregates at nano-scale and study the local atomic structure of spatial heterogeneous metallic glasses, researchers use advanced experimental methods of angstrom-beam electron diffraction (ABED) and aberration-corrected scanning transmission electron microscopy (STEM) [3]. The crystalline phase is detected by the technique of highangle annular dark field (HAADF) imaging in STEM. In HAADF, the scattering angles are large and incoherent thermal diffuse scattering surpasses the coherent Bragg scattering. Being sensitive to the atomic numbers, HAADF provides contrast images of compositional nonuniformities in materials. The structurally different regions in the metallic glass are observed as dark and bright areas in HAADF STEM images and analyzed by using ABED to detect the local atomic configurations. The structural variations within the glassy material result in spatial heterogeneity of metallic glasses that manifest itself in a fluctuation of density occurring without fluctuation of chemical composition. The structural heterogeneity has important implications in glass science and technology because many of the material properties such as the glass-forming ability and the elastic behavior are structurally dependent.

## **5. Ultra-thin graphene oxide support film for TEM observations of core-shell nanocrystals**

Over the last five decades, SiO2 has been intensively studied as the basic ingredient in field-effect transistors, silicon photovoltaics, and nanoscale light-emitting sources. Metaloxide-semiconductor technology is dominating for very large scale integration applications, enabling smaller dimensions, a faster switching speed, and a lower power dissipation. Ability to control the oxide thickness with an angstrom precision facilitates the fabrication of ultrasmall devices operating at a higher drive current. Despite the successful implementation of planar SiO2 /Si nanostructures in transistors, the device application of Si nanocrystals in electronic and optoelectronic devices is still hindered due to low carrier concentration and low charge carrier mobility. Although the optical and electronic properties of silicon nanostructures are remarkably dependent on the modification of the defect ensemble at the Si nanocrystal-SiO2 interface, direct imaging of the amorphous structure surrounding silicon nanocrystals has been found difficult due to the insufficient contrast of electron microscopy. The use of atomically thin graphene oxide supports in electron microscopy of colloidal silicon quantum dots reveals the amorphous structure of the surrounding core with the core thickness being affected by the concentration of doping atoms [4]. The insufficient doping might stem from the aggregation of the dopant and the silicon atoms in a partially oxidized amorphous shell, which forms on the top of a crystalline silicon core.

**Chapter 2**

**Provisional chapter**

**Molecular Simulation Study of Water Transport**

**Molecular Simulation Study of Water Transport**

DOI: 10.5772/intechopen.77394

As the solid-state nanochannels are hourglass-shaped as opposed to having a straight structure, little effort has been made on the transport phenomena across aquaporin-mimicking ultra-fine pores. Moreover, to the best of the authors' knowledge, the existing studies on the water transportation through hourglass-shaped pore structures are limited to the symmetric geometries. In order to benefit from the advantage of combining the properties of nanoporous graphene and the unique structure of the aquaporin water channels, this chapter aims to study designing a highly effective permeable membrane for water transportation through nanopores with hourglass shape in multilayer structure using molecular dynamics (MD) simulation. Keeping the layer separation constant at 6 Å, we intend to examine the geometry effect on the pressure-induced transport of water molecules through hourglass-shaped nanopores. Three models of hourglass-shaped nanopores are selected: a symmetric structure α = 0 and asymmetric structure of α = 1/3 and α = −1/3. Our MD findings indicate that the permeability of water molecules highly increases across hourglassshaped pores with an asymmetric structure of α = 1/3 because of the increased number of hydrogen bonds resulting from the length effect. Thus, we can conclude that an hourglassshaped pore with a proper design can remarkably improve the rate of water transportation. **Keywords:** water transport, aquaporin water channel, hourglass-shaped pore, molecular

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

The growing demand for water in everyday human activities makes producing fresh water a great challenge. Using membrane separation technology in the areas of water treatment is widely implemented. Producing portable water from existing resources such as surface water,

**through Aquaporin-Inspired Pore Geometry**

**through Aquaporin-Inspired Pore Geometry**

Daejoong Kim and Majid Shahbabaei

Daejoong Kim and Majid Shahbabaei

http://dx.doi.org/10.5772/intechopen.77394

**Abstract**

dynamics simulation

**1. Introduction**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

## **Author details**

#### Yanina Fedorenko

Address all correspondence to<EMAIL_ADDRESS>
Stephenson Institute for Renewable Energy, University of Liverpool, Liverpool, UK

## **References**

#### **Molecular Simulation Study of Water Transport through Aquaporin-Inspired Pore Geometry Molecular Simulation Study of Water Transport through Aquaporin-Inspired Pore Geometry**

DOI: 10.5772/intechopen.77394

Daejoong Kim and Majid Shahbabaei Daejoong Kim and Majid Shahbabaei

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.77394

#### **Abstract**

thickness being affected by the concentration of doping atoms [4]. The insufficient doping might stem from the aggregation of the dopant and the silicon atoms in a partially oxidized

Stephenson Institute for Renewable Energy, University of Liverpool, Liverpool, UK

[1] Cherkashin N, Denneulin T, Hÿtch MJ. Electron microscopy by specimen design: Application to strain measurements. Scientific Reports. 2017;**7**:12394. DOI: 10.1038/s41598-

[2] Eggeman AS, Illig S, Troisi A, Sirringhaus H, Midgley PA. Direct measurement of molecular motion in organic semiconductors. Nature Materials. 2013;**12**:1045-1049. DOI:

[3] Zhu F, Hirata A, Liu P, Song S, Tian Y, Han J, Fujita T, Chen M. Correlation between local structure order and spatial heterogeneity in a metallic glass. Physical Review Letters.

[4] Sugimoto H, Yamamura M, Sakiyama M, Fujiia M. Visualizing a core–shell structure of heavily doped silicon quantum dots by electron microscopy using an atomically thin

support film. Nanoscale. 2018;**10**:7357-7362. DOI: 10.1039/C7NR09474D

amorphous shell, which forms on the top of a crystalline silicon core.

4 Nanostructures in Energy Generation, Transmission and Storage

Address all correspondence to<EMAIL_ADDRESS>
2017;**119**:215501. DOI: 10.1103/PhysRevLett.119.215501

**Author details**

Yanina Fedorenko

**References**

017-12695-8

10.1038/nmat3710

As the solid-state nanochannels are hourglass-shaped as opposed to having a straight structure, little effort has been made on the transport phenomena across aquaporin-mimicking ultra-fine pores. Moreover, to the best of the authors' knowledge, the existing studies on the water transportation through hourglass-shaped pore structures are limited to the symmetric geometries. In order to benefit from the advantage of combining the properties of nanoporous graphene and the unique structure of the aquaporin water channels, this chapter aims to study designing a highly effective permeable membrane for water transportation through nanopores with hourglass shape in multilayer structure using molecular dynamics (MD) simulation. Keeping the layer separation constant at 6 Å, we intend to examine the geometry effect on the pressure-induced transport of water molecules through hourglass-shaped nanopores. Three models of hourglass-shaped nanopores are selected: a symmetric structure α = 0 and asymmetric structure of α = 1/3 and α = −1/3. Our MD findings indicate that the permeability of water molecules highly increases across hourglassshaped pores with an asymmetric structure of α = 1/3 because of the increased number of hydrogen bonds resulting from the length effect. Thus, we can conclude that an hourglassshaped pore with a proper design can remarkably improve the rate of water transportation.

**Keywords:** water transport, aquaporin water channel, hourglass-shaped pore, molecular dynamics simulation

#### **1. Introduction**

The growing demand for water in everyday human activities makes producing fresh water a great challenge. Using membrane separation technology in the areas of water treatment is widely implemented. Producing portable water from existing resources such as surface water,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

seawater, brackish water, or groundwater can be done by using membrane processes. The membrane acts as a barrier with which two phases are separated, and transport of different chemical species are restricted selectively. It is basically referred to as a thin layer which separates materials depending on their chemical/physical properties when an external force (e.g., gradient pressure, electrical force, etc.) is applied across the membrane. Nanoporous graphene has gained great attention as promising membrane material for RO desalination because of its exceptional properties such as being mechanically strong, chemically robust, and extremely thin. Furthermore, using graphene—with such an extremely thin thickness—in desalination plants can remarkably yield higher water fluxes with significant reduction in the operating costs, for example, cost in terms of energy and cost in terms of capital and infrastructure requirements [1]. Considering the graphene-based membrane as highly selective and permeable, the generation of pore with various pore shape and size can be achieved in graphene by using electron beam irradiation [2]. Recent studies in both theory and experiment revealed that graphene [3–8] and graphene oxide [9, 10] were highly selective and permeable and could be used as filtration membranes. Though single-layer graphene has demonstrated to be a promising candidate as a reverse osmosis membrane, synthesizing multilayer graphene is economically affordable [11–14]. Considering parameters including pore size and chemistry, a wide range of studies have considered transport phenomena through single-layer graphene. However, less effort has been made on transport properties through nanoporous multilayer graphene. Experimentally, the studies on graphene oxide have revealed that constructing carbon-based material in molecular-scale porous structure could be achieved with tuned pore sizes and interlayer spacing [15–18]. A better selectivity and higher permeability for artificial membranes could be achieved mimicking bio-inspired membranes such as aquaporins. Aquaporins are pore-forming proteins and ubiquitous in living cells, which provide fast water transportation in a selective manner resulting from their unique structure of hourglass shape [19]. Recent study [20] showed that the desalination performance could be significantly improved incorporating the properties of aquaporin water channel into desalination membranes. Revealing an hourglass-shaped structure for the solid-state pores [21], transport phenomena pores in hourglass-shaped structure is not fully understood. Although the existing studies on water transportation properties across hourglass-shaped pore systems are limited to the symmetrical structures, to the best of our knowledge [22–24], further molecular-level studies on the transport phenomena across asymmetrical geometries are still needed [25–27]. The water transportation phenomena in confined environments (e.g., nanochannels), where the critical dimension can be a few molecular diameters, differ from macroscopic scales which explains the transport phenomena based on continuum theories (e.g., Navier-Stokes equations). Accordingly, a muchneeded molecular-level understanding of the water transportation phenomena at the nanoscale is necessary. Molecular dynamics (MD) has become a powerful tool for investigating the effects of confinement on fluid properties and calculating phenomenological transport coefficients for use in multi-scale fluid flow models. A significant effort has been devoted, often with the help of MD simulation, to understand the water transportation properties in carbon nanotubes (CNTs) and graphene [28–31]. Several researchers have shown a freezing state for water confined in CNTs at low temperature and the appearance of ice phases stabilizing by confinement [32, 33]. Using MD simulations, Koga and colleagues [32] showed that water molecules inside CNTs with diameters varying from 11 to 14 Å indicate phase transitions with applied pressure of 50–550 MPa. Experimentally, Kolesnikov and colleagues [33] observed a combination of water-chain and ice-shell structure for water molecules confined in CNTs having a liquid phase state even at low temperature. The appearance of single-layer and bi-layer ice structures were also disclosed for water molecules confining in between graphene slits under applied pressure [34, 35]. Another study revealed that water confined in graphene slits freezes at ambient temperature while no pressure is applied [36]. Chakraborty and colleagues [37] have shown a diversity of results on the structure, dynamics, and thermodynamics of confined water molecules in CNTs and graphene oxide sheets. They found the appearance of the single-file ordered structure for water molecules confined in narrow CNTs and the appearance of the n-gonal ice structure in larger CNTs. Their findings also revealed the appearance of two-dimensional ordered structures for water molecules confined in slit-pore geometries. Considering the graphene properties combining with aquaporin water channel structure, this study aims to design an optimal and effective permeable membrane for water transportation across hourglass-shaped pores in multilayer nanoporous graphene using MD technique.

Molecular Simulation Study of Water Transport through Aquaporin-Inspired Pore Geometry

http://dx.doi.org/10.5772/intechopen.77394

Molecular dynamics (MD) is used to investigate the transport of water molecules across hourglass-shaped nanopores in nanoporous multilayer graphene. The schematic of the simulation models are given in **Figure 1**. Three pore models of hourglass shape of asymmetric (α = 1/3 and α = −1/3) and symmetric (α = 0) are chosen. Seven layers of graphene (31.9 × 34.3 Å<sup>2</sup>

with a constant interlayer separation of d = 6 Å with total length of 36 Å is represented as the multilayer structure. Considering the single-file water arrangement across the narrowest part and a bulk structure of water molecules in the largest segments, we considered being the narrowest (a) and the largest diameters (D) of hourglass-shaped structure in 4 and 7 Å, respectively, embedding in a periodic box of over 2000 SPC/E water molecules. To generate the pore in graphene layers, we removed adjacent carbon atoms in the center and considered the pores with no sharp edges (for more information, see Ref. [12]). To generate water flow in the models, we applied the pressure difference along –z direction across α = 1/3, α = −1/3, and α = 0 systems. A pressure-induced flow, ΔP = nf/A (denoting n and A as the number of water molecules and the membrane surface area, respectively), is achieved by applying a constant force, f, on water molecules inside the reservoir [38]. All the simulations are performed using large-scale atomic/molecular massively parallel simulator (LAMMPS) simulation package [39], implementing the constant number (N), volume (V), and energy (E) (NVE) ensemble with dissipative particle dynamic (DPD) thermostat [40] aiming at maintaining the temperature constant at 300 K. We have chosen a time step of 0.5 fs in order for ensuring that water molecules between the bulk region and layers are accurately simulated. Considering a periodic boundary condition in three directions, a cutoff radius of 10 Å is selected for the Lennard-Jones (LJ) potentials. To reduce the computational cost, we have considered the carbon atoms of the graphene to be frozen. We have modeled only the interaction between water molecules (oxygen) and the graphene (C) by using the Lennard-Jones parameters as σO─<sup>O</sup> = 3.169 Å, ƐO─<sup>O</sup> = 0.1515 kcal/mol, σO─<sup>C</sup> = 3.190 Å, and ƐO─<sup>C</sup> = 0.0956 kcal/mol. Using the Ewald summation method [41] for the coulomb interactions, the SHAKE algorithm is applied to hold the water molecules rigid [42]. Conducting more than 15 ns, MD runs for every three cases (α = 1/3, α = −1/3, and α = 0), the last

)

7

**2. Simulation setup**

12 ns trajectory was considered for data analysis.

observed a combination of water-chain and ice-shell structure for water molecules confined in CNTs having a liquid phase state even at low temperature. The appearance of single-layer and bi-layer ice structures were also disclosed for water molecules confining in between graphene slits under applied pressure [34, 35]. Another study revealed that water confined in graphene slits freezes at ambient temperature while no pressure is applied [36]. Chakraborty and colleagues [37] have shown a diversity of results on the structure, dynamics, and thermodynamics of confined water molecules in CNTs and graphene oxide sheets. They found the appearance of the single-file ordered structure for water molecules confined in narrow CNTs and the appearance of the n-gonal ice structure in larger CNTs. Their findings also revealed the appearance of two-dimensional ordered structures for water molecules confined in slit-pore geometries. Considering the graphene properties combining with aquaporin water channel structure, this study aims to design an optimal and effective permeable membrane for water transportation across hourglass-shaped pores in multilayer nanoporous graphene using MD technique.

### **2. Simulation setup**

seawater, brackish water, or groundwater can be done by using membrane processes. The membrane acts as a barrier with which two phases are separated, and transport of different chemical species are restricted selectively. It is basically referred to as a thin layer which separates materials depending on their chemical/physical properties when an external force (e.g., gradient pressure, electrical force, etc.) is applied across the membrane. Nanoporous graphene has gained great attention as promising membrane material for RO desalination because of its exceptional properties such as being mechanically strong, chemically robust, and extremely thin. Furthermore, using graphene—with such an extremely thin thickness—in desalination plants can remarkably yield higher water fluxes with significant reduction in the operating costs, for example, cost in terms of energy and cost in terms of capital and infrastructure requirements [1]. Considering the graphene-based membrane as highly selective and permeable, the generation of pore with various pore shape and size can be achieved in graphene by using electron beam irradiation [2]. Recent studies in both theory and experiment revealed that graphene [3–8] and graphene oxide [9, 10] were highly selective and permeable and could be used as filtration membranes. Though single-layer graphene has demonstrated to be a promising candidate as a reverse osmosis membrane, synthesizing multilayer graphene is economically affordable [11–14]. Considering parameters including pore size and chemistry, a wide range of studies have considered transport phenomena through single-layer graphene. However, less effort has been made on transport properties through nanoporous multilayer graphene. Experimentally, the studies on graphene oxide have revealed that constructing carbon-based material in molecular-scale porous structure could be achieved with tuned pore sizes and interlayer spacing [15–18]. A better selectivity and higher permeability for artificial membranes could be achieved mimicking bio-inspired membranes such as aquaporins. Aquaporins are pore-forming proteins and ubiquitous in living cells, which provide fast water transportation in a selective manner resulting from their unique structure of hourglass shape [19]. Recent study [20] showed that the desalination performance could be significantly improved incorporating the properties of aquaporin water channel into desalination membranes. Revealing an hourglass-shaped structure for the solid-state pores [21], transport phenomena pores in hourglass-shaped structure is not fully understood. Although the existing studies on water transportation properties across hourglass-shaped pore systems are limited to the symmetrical structures, to the best of our knowledge [22–24], further molecular-level studies on the transport phenomena across asymmetrical geometries are still needed [25–27]. The water transportation phenomena in confined environments (e.g., nanochannels), where the critical dimension can be a few molecular diameters, differ from macroscopic scales which explains the transport phenomena based on continuum theories (e.g., Navier-Stokes equations). Accordingly, a muchneeded molecular-level understanding of the water transportation phenomena at the nanoscale is necessary. Molecular dynamics (MD) has become a powerful tool for investigating the effects of confinement on fluid properties and calculating phenomenological transport coefficients for use in multi-scale fluid flow models. A significant effort has been devoted, often with the help of MD simulation, to understand the water transportation properties in carbon nanotubes (CNTs) and graphene [28–31]. Several researchers have shown a freezing state for water confined in CNTs at low temperature and the appearance of ice phases stabilizing by confinement [32, 33]. Using MD simulations, Koga and colleagues [32] showed that water molecules inside CNTs with diameters varying from 11 to 14 Å indicate phase transitions with applied pressure of 50–550 MPa. Experimentally, Kolesnikov and colleagues [33]

6 Nanostructures in Energy Generation, Transmission and Storage

Molecular dynamics (MD) is used to investigate the transport of water molecules across hourglass-shaped nanopores in nanoporous multilayer graphene. The schematic of the simulation models are given in **Figure 1**. Three pore models of hourglass shape of asymmetric (α = 1/3 and α = −1/3) and symmetric (α = 0) are chosen. Seven layers of graphene (31.9 × 34.3 Å<sup>2</sup> ) with a constant interlayer separation of d = 6 Å with total length of 36 Å is represented as the multilayer structure. Considering the single-file water arrangement across the narrowest part and a bulk structure of water molecules in the largest segments, we considered being the narrowest (a) and the largest diameters (D) of hourglass-shaped structure in 4 and 7 Å, respectively, embedding in a periodic box of over 2000 SPC/E water molecules. To generate the pore in graphene layers, we removed adjacent carbon atoms in the center and considered the pores with no sharp edges (for more information, see Ref. [12]). To generate water flow in the models, we applied the pressure difference along –z direction across α = 1/3, α = −1/3, and α = 0 systems. A pressure-induced flow, ΔP = nf/A (denoting n and A as the number of water molecules and the membrane surface area, respectively), is achieved by applying a constant force, f, on water molecules inside the reservoir [38]. All the simulations are performed using large-scale atomic/molecular massively parallel simulator (LAMMPS) simulation package [39], implementing the constant number (N), volume (V), and energy (E) (NVE) ensemble with dissipative particle dynamic (DPD) thermostat [40] aiming at maintaining the temperature constant at 300 K. We have chosen a time step of 0.5 fs in order for ensuring that water molecules between the bulk region and layers are accurately simulated. Considering a periodic boundary condition in three directions, a cutoff radius of 10 Å is selected for the Lennard-Jones (LJ) potentials. To reduce the computational cost, we have considered the carbon atoms of the graphene to be frozen. We have modeled only the interaction between water molecules (oxygen) and the graphene (C) by using the Lennard-Jones parameters as σO─<sup>O</sup> = 3.169 Å, ƐO─<sup>O</sup> = 0.1515 kcal/mol, σO─<sup>C</sup> = 3.190 Å, and ƐO─<sup>C</sup> = 0.0956 kcal/mol. Using the Ewald summation method [41] for the coulomb interactions, the SHAKE algorithm is applied to hold the water molecules rigid [42]. Conducting more than 15 ns, MD runs for every three cases (α = 1/3, α = −1/3, and α = 0), the last 12 ns trajectory was considered for data analysis.

value of water occupancy inside the pores is more affected by the pore structure compared to the applied pressure (e.g., α = 1/3 > α = 0 > α = −1/3). The number of water flux is calculated by counting the number of water molecules that cross the pore thoroughly, as shown in **Figure 2b**. Qualitatively, the results indicate that the water flux increases linearly with applied pressure, which is consistent with the recent study [4]. Moreover, the MD results show that the number of water flux is higher in the case of α = 1/3 compared to α = 0 and α = −1/3 systems. Interestingly, the symmetric hourglass-shaped pore (α = 0) shows a lower flux of water molecule compared to the asymmetric pore structure (e.g., α = 1/3), which attributes to the entrance effects. Therefore, it is understood that the entrance effects have a significant contribution to flux variation inside pores with the structure of hourglass shapes. The permeability of water molecules through pores in hourglass shape is significantly restricted due to the energy barriers at the entrances. In case of α = 1/3 for example, a large amount of water molecules entering the long cone can contribute to reduce the energy barriers caused by the narrowest section of pore (of which accommodates water molecules only in single-file configuration), resulting from the increase in the number of hydrogen bonds. It is also inferred that the decrease in the cone angle would enhance the permeability of water molecules across hourglass-shaped pores in a multilayer structure (indeed, this implication very recently discussed for the permeability of water molecules in solid-state nanochannels of hourglass shape [22]). To assess the transport capacity of a membrane, one could calculate an important parameter named osmotic permeability. By drawing a best-fit slope through flux versus applied pressures, the osmotic permeability of a membrane can be achieved as shown in **Figure 2b**. In agreement with the results reported for flux, the corresponding magnitudes are (4 ± 1) × 10−14, (7.52 ± 9.76) × 10−15, and (2.3 37 ± 4.33) × 10−15 cm3 Pa−1 s−1 for the pore systems of α = 1/3, α = 0, and α = −1/3, respectively. It is also inferred that achieving high permeability of water molecules across the pore of hourglass shape would strongly depend on the structure of the pore. Therefore, a design of hourglassshaped pore with which adsorbs bulk water molecules can be a promising geometry aimed

**Figure 2.** (a) Average water occupancy inside the pore systems and (b) value of water flux through the pore models

Molecular Simulation Study of Water Transport through Aquaporin-Inspired Pore Geometry

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shown as a function of the pressure difference.

**Figure 1.** Schematic diagram of the asymmetric (α = 1/3 and α = −1/3) and symmetric (α = 0) hourglass-shaped nanopore in multilayer structure with a constant interlayer spacing of d = 6 Å. The hourglass-shaped pores with a total length of 36 Å, the largest diameter D = 7 Å, and the narrowest diameter a = 4 Å are considered as the pore models. A pressure difference is applied along −z direction to drive water flow.

#### **3. Results and discussion**

In our simulations, we applied the pressure difference, ΔP, ranging from 365 to 635 MPa across the multilayer nanopore systems. Three pore models of α = 1/3, α = −1/3, and α = 0 with the long, short, and equal entrance lengths are considered. **Figure 2a** shows the average number of water occupancy with the applied pressures inside each of the three models. The water occupancy is calculated by counting the number of water molecules, which could diffuse through the 36-Å-long multilayer systems during the simulations. It has been found that the average water occupancy in the case of α = 1/3 is higher than that of α = −1/3, and α = 0 systems. Since the pore volume increases, at α = 1/3, for example, the influx of water molecules could be encouraged stemming from an increased cone length. Though a significant variation of the number of water occupancy in relevance to applied pressure is found in α = −1/3 model, generally, the

Molecular Simulation Study of Water Transport through Aquaporin-Inspired Pore Geometry http://dx.doi.org/10.5772/intechopen.77394 9

**Figure 2.** (a) Average water occupancy inside the pore systems and (b) value of water flux through the pore models shown as a function of the pressure difference.

value of water occupancy inside the pores is more affected by the pore structure compared to the applied pressure (e.g., α = 1/3 > α = 0 > α = −1/3). The number of water flux is calculated by counting the number of water molecules that cross the pore thoroughly, as shown in **Figure 2b**. Qualitatively, the results indicate that the water flux increases linearly with applied pressure, which is consistent with the recent study [4]. Moreover, the MD results show that the number of water flux is higher in the case of α = 1/3 compared to α = 0 and α = −1/3 systems. Interestingly, the symmetric hourglass-shaped pore (α = 0) shows a lower flux of water molecule compared to the asymmetric pore structure (e.g., α = 1/3), which attributes to the entrance effects. Therefore, it is understood that the entrance effects have a significant contribution to flux variation inside pores with the structure of hourglass shapes. The permeability of water molecules through pores in hourglass shape is significantly restricted due to the energy barriers at the entrances. In case of α = 1/3 for example, a large amount of water molecules entering the long cone can contribute to reduce the energy barriers caused by the narrowest section of pore (of which accommodates water molecules only in single-file configuration), resulting from the increase in the number of hydrogen bonds. It is also inferred that the decrease in the cone angle would enhance the permeability of water molecules across hourglass-shaped pores in a multilayer structure (indeed, this implication very recently discussed for the permeability of water molecules in solid-state nanochannels of hourglass shape [22]). To assess the transport capacity of a membrane, one could calculate an important parameter named osmotic permeability. By drawing a best-fit slope through flux versus applied pressures, the osmotic permeability of a membrane can be achieved as shown in **Figure 2b**. In agreement with the results reported for flux, the corresponding magnitudes are (4 ± 1) × 10−14, (7.52 ± 9.76) × 10−15, and (2.3 37 ± 4.33) × 10−15 cm3 Pa−1 s−1 for the pore systems of α = 1/3, α = 0, and α = −1/3, respectively. It is also inferred that achieving high permeability of water molecules across the pore of hourglass shape would strongly depend on the structure of the pore. Therefore, a design of hourglassshaped pore with which adsorbs bulk water molecules can be a promising geometry aimed

**3. Results and discussion**

difference is applied along −z direction to drive water flow.

8 Nanostructures in Energy Generation, Transmission and Storage

In our simulations, we applied the pressure difference, ΔP, ranging from 365 to 635 MPa across the multilayer nanopore systems. Three pore models of α = 1/3, α = −1/3, and α = 0 with the long, short, and equal entrance lengths are considered. **Figure 2a** shows the average number of water occupancy with the applied pressures inside each of the three models. The water occupancy is calculated by counting the number of water molecules, which could diffuse through the 36-Å-long multilayer systems during the simulations. It has been found that the average water occupancy in the case of α = 1/3 is higher than that of α = −1/3, and α = 0 systems. Since the pore volume increases, at α = 1/3, for example, the influx of water molecules could be encouraged stemming from an increased cone length. Though a significant variation of the number of water occupancy in relevance to applied pressure is found in α = −1/3 model, generally, the

**Figure 1.** Schematic diagram of the asymmetric (α = 1/3 and α = −1/3) and symmetric (α = 0) hourglass-shaped nanopore in multilayer structure with a constant interlayer spacing of d = 6 Å. The hourglass-shaped pores with a total length of 36 Å, the largest diameter D = 7 Å, and the narrowest diameter a = 4 Å are considered as the pore models. A pressure

**Figure 3.** Density of water molecules along the axial direction of the layer with the narrowest diameter of the pore inside α = 1/3 and α = −1/3. The inset shows the layer with the narrowest pore (dashed line).

at obtaining high permeability. Within this context, the water molecules in bulk-like manner contribute to remarkably reduce the energy barriers inside the pores because of the size effects. Next, we investigate the viscosity dissipation inside asymmetric pore systems at the smallest section of the pores. Very recently, a numerical study [22] has shown that the entrance effects (defined as viscous dissipation) would dramatically restrict the water permeability inside solid-state pores of hourglass shape. Basically, the molecules of water confined in graphene nanopores show different viscosity compared to the carbon nanotubes (CNTs). Considering a similar characteristic of pore size, a recent study has shown that the molecules of water confining in a graphene pore pose higher viscosity compared to the water molecules confining in CNTs. Increasing the pore diameter leads to the increase in the viscosity of confined water molecules in CNTs approaching the bulk viscosity [29, 43]. In graphene nanopores, the viscosity of water molecules increases, while decreasing the pore diameter in accordance with the structure of water molecules [38]. The structure of water molecules along the flow direction inside graphene nanopores shows different behavior compared to CNTs' nanochannels. Recent MD study [44] has shown that the viscosity of water molecules across graphene nanopore is linearly increased in accordance to the layering effect. In fact, the viscosity of water molecules through graphene nanopores is in relation to the layering effect of water molecules. Through the density distribution, one can obtain the water layering effect along the flow direction in the graphene system. Here, we measured the water layering factor as (ρmax − ρmin)/ρbulk, to quantify the water layering effect inside only the asymmetric pore structures, noting that ρmax and ρmin are the maximum and minimum density numbers in the vicinity of the layer with the narrowest pore diameter, respectively, as shown in **Figure 3**. The ρbulk is estimated to be in the range of bulk fluid densities reported, 0.8σO─<sup>O</sup> −3 ≤ ρbulk ≤ 1.1σO─<sup>O</sup> −3 [45]. When α = −1/3, the density profile near the layer with the smallest pore diameter shows greater distribution compared to α = 1/3, suggesting a significant effect of water layering inside α = −1/3. Within

this context, the water layering effect increases the viscosity of water molecules while crossing the smallest diameter in single-file arrangement. Accordingly, a significant reduction in the

**Figure 4.** Probability distribution of dipole orientation of water molecules inside the (a) α = 1/3 and (b) α = −1/3 systems,

Molecular Simulation Study of Water Transport through Aquaporin-Inspired Pore Geometry

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and (c) dipole orientation of water molecules in relation to time inside these two pore systems.

Furthermore, in case of α = 1/3, large amount of water molecules (bulk-like) approaching the narrowest diameter of the pore should contribute to diminish highly the water viscosity of single-file configuration because of the increase in hydrogen bonds. Although the dipole orientation of water molecules inside CNTs maintains oversimulation time, in graphene nanopores, the dipole orientation of water molecules is frequently flipped [46]. Here, we intend to show how dipole orientation of water molecules may behave across pores in multilayer system. Taking into account all the water molecules inside pores, the averaged dipole orientation was estimated by computing the angle between water dipole and the axial direction (perpendicular to the surface of the multilayer). **Figure 4a** and **b** shows the profiles of the averaged dipole orientation of water molecules inside asymmetric pores α = 1/3 and α = −1/3, respectively. Over 4000 calculated values are used to plot the average dipole orientation of water molecules inside asymmetric structures (α = 1/3 and α = −1/3). In case of α = 1/3, the average

local permeability inside α = −1/3 is expected.

Molecular Simulation Study of Water Transport through Aquaporin-Inspired Pore Geometry http://dx.doi.org/10.5772/intechopen.77394 11

at obtaining high permeability. Within this context, the water molecules in bulk-like manner contribute to remarkably reduce the energy barriers inside the pores because of the size effects. Next, we investigate the viscosity dissipation inside asymmetric pore systems at the smallest section of the pores. Very recently, a numerical study [22] has shown that the entrance effects (defined as viscous dissipation) would dramatically restrict the water permeability inside solid-state pores of hourglass shape. Basically, the molecules of water confined in graphene nanopores show different viscosity compared to the carbon nanotubes (CNTs). Considering a similar characteristic of pore size, a recent study has shown that the molecules of water confining in a graphene pore pose higher viscosity compared to the water molecules confining in CNTs. Increasing the pore diameter leads to the increase in the viscosity of confined water molecules in CNTs approaching the bulk viscosity [29, 43]. In graphene nanopores, the viscosity of water molecules increases, while decreasing the pore diameter in accordance with the structure of water molecules [38]. The structure of water molecules along the flow direction inside graphene nanopores shows different behavior compared to CNTs' nanochannels. Recent MD study [44] has shown that the viscosity of water molecules across graphene nanopore is linearly increased in accordance to the layering effect. In fact, the viscosity of water molecules through graphene nanopores is in relation to the layering effect of water molecules. Through the density distribution, one can obtain the water layering effect along the flow direction in the graphene system. Here, we measured the water layering factor as (ρmax − ρmin)/ρbulk, to quantify the water layering effect inside only the asymmetric pore structures, noting that ρmax and ρmin are the maximum and minimum density numbers in the vicinity of the layer with the narrowest pore diameter, respectively, as shown in **Figure 3**. The ρbulk is estimated to

**Figure 3.** Density of water molecules along the axial direction of the layer with the narrowest diameter of the pore inside

α = 1/3 and α = −1/3. The inset shows the layer with the narrowest pore (dashed line).

10 Nanostructures in Energy Generation, Transmission and Storage

the density profile near the layer with the smallest pore diameter shows greater distribution compared to α = 1/3, suggesting a significant effect of water layering inside α = −1/3. Within

−3 ≤ ρbulk ≤ 1.1σO─<sup>O</sup>

−3 [45]. When α = −1/3,

be in the range of bulk fluid densities reported, 0.8σO─<sup>O</sup>

**Figure 4.** Probability distribution of dipole orientation of water molecules inside the (a) α = 1/3 and (b) α = −1/3 systems, and (c) dipole orientation of water molecules in relation to time inside these two pore systems.

this context, the water layering effect increases the viscosity of water molecules while crossing the smallest diameter in single-file arrangement. Accordingly, a significant reduction in the local permeability inside α = −1/3 is expected.

Furthermore, in case of α = 1/3, large amount of water molecules (bulk-like) approaching the narrowest diameter of the pore should contribute to diminish highly the water viscosity of single-file configuration because of the increase in hydrogen bonds. Although the dipole orientation of water molecules inside CNTs maintains oversimulation time, in graphene nanopores, the dipole orientation of water molecules is frequently flipped [46]. Here, we intend to show how dipole orientation of water molecules may behave across pores in multilayer system. Taking into account all the water molecules inside pores, the averaged dipole orientation was estimated by computing the angle between water dipole and the axial direction (perpendicular to the surface of the multilayer). **Figure 4a** and **b** shows the profiles of the averaged dipole orientation of water molecules inside asymmetric pores α = 1/3 and α = −1/3, respectively. Over 4000 calculated values are used to plot the average dipole orientation of water molecules inside asymmetric structures (α = 1/3 and α = −1/3). In case of α = 1/3, the average dipole orientation fell into two range groups of 149–153° and 158–162°, while in the case of α = −1/3, the value of average dipole orientation fell only in the range of 44–47°. This implies a degree of water molecule ordering inside the pore in the case of α = 1/3, which enhances the formation of hydrogen bonds between water molecules. Moreover, the variation of the dipole orientation tends to strengthen the water-water interaction resulting in higher hydrogen bonds. This is responsible for fast transport of water molecules in monolayer graphene [13]. We believe that this implication could be also valid in multilayer graphene systems.

molecules, as shown in **Figure 6b**. Furthermore, these results of the RDF are in agreement with the results of the free energy fluctuations. The transport mechanism of molecules in confined spaces could be monitored by using the potential mean force (PMF). Using the RDF which illustrates how density varies as a function of distance from a reference particle, one can calculate the PMF since the density would be a key index to explain probability [48]. **Figure 6b** illustrates the profiles of the potential mean forces (PMFs) for water molecules inside α = 1/3 and α = −1/3 pore systems. Because there is a weak interaction between the molecules of water inside α = −1/3 pore model, a large energy barrier caused by vacant spaces between the molecules of water is understood. We believe that the large energy barrier could be attributed to the increased viscosity caused by the pore size at the smallest section, which dramatically reduces the passage of molecules of water. Encouraging the water molecules in bulk structure to get through the pore of α = 1/3, they can significantly contribute overcoming the energy barrier. Therefore, water molecules with a tight hydrogen bonding network can pass the smallest section of the pore, which contributes strongly improving the entire passage of molecules of water across the pore. To further understand the mechanism of water transportation in confined

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13

**Figure 5.** Free energy of occupancy as a function of the number of water molecules inside the (a) α = 1/3 and (b) α = −1/3 systems, and (c) free energy of occupancy as a function of the number of hydrogen bonds inside these two pore models.

Although, the average dipole orientation inside α = 1/3 and α = −1/3 systems is only flipped during the course of few picoseconds, it maintains for whole the simulation time showing a higher magnitude of dipole inside α = 1/3, as given in **Figure 4c**. At constant layer spacing (d = 6 Å), the obtained results show that the dipole orientation of water molecules inside the multilayer graphene membrane is different from that across single-layer graphene, in which the dipole orientation has frequently flipped. This implies that every single layer in multilayer system provides the energy barrier which allows the water molecules to freely flip. It would be expected that by increasing the layer spacing up to several angstroms, this phenomenon changes. The fluctuation of free energy occupancy in relation to the number of water molecules across asymmetric pores is given in **Figure 5a** and **b**. We computed the free energy using the equation, G(N) = −Ln[P(N)]/b, where P(N) is the probability of finding the exact N molecules of water in the system, b = (kBT)−1, KB and T stand for the Boltzmann constant and temperature, respectively. Ranging from 896 to 926 water molecules, the most probable of water molecules is being with 904 molecules inside α = 1/3. However, considering 722–756 water molecules inside α = −1/3, the 728 is the most probable. In comparison to α = −1/3, the water molecules are increasingly accommodated inside α = 1/3 because of the strong interplay between molecules, which results in forming the stabilized chain of water molecules. In comparison to α = −1/3, a wider distribution of probability with higher number of water molecules are found inside α = 1/3. As a result, a high rate of water conductivity can be found inside fully filled asymmetric model of α = 1/3. In addition, the probability of existence of higher numbers of water molecules is scarcely observed inside α = −1/3 due to the strong water-wall interplay.

The free energy of occupancy in relation to the hydrogen bonds number is given in **Figure 5c**. The hydrogen bond was calculated in accordance to the method reported in Ref. [47]. As the figure shows, the average number of hydrogen bonds varies from 1.48 to 1.61 inside α = −1/3, whereas it ranges from 1.56 to 1.73 for the case of α = 1/3. Then, a wider distribution for probability of formation of higher numbers of hydrogen bonds inside α = 1/3 suggests an enhancement in the water density distribution. In other word, the establishment of the stable chains of water molecules inside α = 1/3 can support formation of the long-lasting network of hydrogen bonds which is significantly contributed to achieving higher flux. Radial distribution function (RDF) is an important parameter to analyze the structure of molecules in confined environments. **Figure 6a** depicts the profile of the RDFs between the water molecules (oxygen atoms) inside asymmetric pore models. A rapid decay for the RDFs can be seen inside both pore models of α = 1/3 and α = −1/3. According to the high magnitudes of the RDFs at low interatomic distances, the formation of solid clustering between water molecules inside α = 1/3 is predicted, which triggers potential reduction in the energy barrier caused by the vacant spaces between molecules, as shown in **Figure 6b**. Furthermore, these results of the RDF are in agreement with the results of the free energy fluctuations. The transport mechanism of molecules in confined spaces could be monitored by using the potential mean force (PMF). Using the RDF which illustrates how density varies as a function of distance from a reference particle, one can calculate the PMF since the density would be a key index to explain probability [48]. **Figure 6b** illustrates the profiles of the potential mean forces (PMFs) for water molecules inside α = 1/3 and α = −1/3 pore systems. Because there is a weak interaction between the molecules of water inside α = −1/3 pore model, a large energy barrier caused by vacant spaces between the molecules of water is understood. We believe that the large energy barrier could be attributed to the increased viscosity caused by the pore size at the smallest section, which dramatically reduces the passage of molecules of water. Encouraging the water molecules in bulk structure to get through the pore of α = 1/3, they can significantly contribute overcoming the energy barrier. Therefore, water molecules with a tight hydrogen bonding network can pass the smallest section of the pore, which contributes strongly improving the entire passage of molecules of water across the pore. To further understand the mechanism of water transportation in confined

dipole orientation fell into two range groups of 149–153° and 158–162°, while in the case of α = −1/3, the value of average dipole orientation fell only in the range of 44–47°. This implies a degree of water molecule ordering inside the pore in the case of α = 1/3, which enhances the formation of hydrogen bonds between water molecules. Moreover, the variation of the dipole orientation tends to strengthen the water-water interaction resulting in higher hydrogen bonds. This is responsible for fast transport of water molecules in monolayer graphene [13]. We believe that this implication could be also valid in multilayer graphene systems.

12 Nanostructures in Energy Generation, Transmission and Storage

Although, the average dipole orientation inside α = 1/3 and α = −1/3 systems is only flipped during the course of few picoseconds, it maintains for whole the simulation time showing a higher magnitude of dipole inside α = 1/3, as given in **Figure 4c**. At constant layer spacing (d = 6 Å), the obtained results show that the dipole orientation of water molecules inside the multilayer graphene membrane is different from that across single-layer graphene, in which the dipole orientation has frequently flipped. This implies that every single layer in multilayer system provides the energy barrier which allows the water molecules to freely flip. It would be expected that by increasing the layer spacing up to several angstroms, this phenomenon changes. The fluctuation of free energy occupancy in relation to the number of water molecules across asymmetric pores is given in **Figure 5a** and **b**. We computed the free energy using the equation, G(N) = −Ln[P(N)]/b, where P(N) is the probability of finding the exact N molecules of water in the system, b = (kBT)−1, KB and T stand for the Boltzmann constant and temperature, respectively. Ranging from 896 to 926 water molecules, the most probable of water molecules is being with 904 molecules inside α = 1/3. However, considering 722–756 water molecules inside α = −1/3, the 728 is the most probable. In comparison to α = −1/3, the water molecules are increasingly accommodated inside α = 1/3 because of the strong interplay between molecules, which results in forming the stabilized chain of water molecules. In comparison to α = −1/3, a wider distribution of probability with higher number of water molecules are found inside α = 1/3. As a result, a high rate of water conductivity can be found inside fully filled asymmetric model of α = 1/3. In addition, the probability of existence of higher numbers of water molecules

is scarcely observed inside α = −1/3 due to the strong water-wall interplay.

The free energy of occupancy in relation to the hydrogen bonds number is given in **Figure 5c**. The hydrogen bond was calculated in accordance to the method reported in Ref. [47]. As the figure shows, the average number of hydrogen bonds varies from 1.48 to 1.61 inside α = −1/3, whereas it ranges from 1.56 to 1.73 for the case of α = 1/3. Then, a wider distribution for probability of formation of higher numbers of hydrogen bonds inside α = 1/3 suggests an enhancement in the water density distribution. In other word, the establishment of the stable chains of water molecules inside α = 1/3 can support formation of the long-lasting network of hydrogen bonds which is significantly contributed to achieving higher flux. Radial distribution function (RDF) is an important parameter to analyze the structure of molecules in confined environments. **Figure 6a** depicts the profile of the RDFs between the water molecules (oxygen atoms) inside asymmetric pore models. A rapid decay for the RDFs can be seen inside both pore models of α = 1/3 and α = −1/3. According to the high magnitudes of the RDFs at low interatomic distances, the formation of solid clustering between water molecules inside α = 1/3 is predicted, which triggers potential reduction in the energy barrier caused by the vacant spaces between

**Figure 5.** Free energy of occupancy as a function of the number of water molecules inside the (a) α = 1/3 and (b) α = −1/3 systems, and (c) free energy of occupancy as a function of the number of hydrogen bonds inside these two pore models.

small and the water molecules confined in between the layers would provide very limited spaces for molecules to move due to the strong interaction between water layer surfaces. Considering states with higher densities, the large number of hydrogen bonds would contribute to form the larger clusters yielding larger movement. In case of α = 1/3 pore, by increasing the applied pressure, the diffusion coefficient decreases. We believe that the water density does not have a significant effect on the mechanism of diffusion across α = 1/3 pore, as can be realized through the result reported for the water occupancy (see **Figure 2a**). The decreased diffusion coefficient with applied pressure could be ascribed to the formation of stabilized hydrogen bonding networks resulting from weak interaction between water layer surfaces. In comparison with α = 0 and α = 1/3 pore systems, a sharp slope for diffusion coefficient is observed inside α = −1/3 which suggests a density-dependent effect. Since the applied pressure increases, a collective diffusion mechanism can be understood inside α = 1/3 because of the formation of the stabilized hydrogen bonding networks. A discreet mechanism for filling behavior of water molecules with applied pressure can be realized across α = −1/3 and α = 0 systems due to existence of the large energy barrier. At the overlap pressure (444 MPa), the similar magnitudes for the diffusion coefficient of water molecules inside each of the three pore systems are obtained. The average residence time of water molecules inside α = 1/3 system as a function of the pressure difference is given in **Figure 7b**. Considering a molecule entering from one side and leav-

Molecular Simulation Study of Water Transport through Aquaporin-Inspired Pore Geometry

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15

ing the pore from the other side, one can calculate the residence time tres = t

we can see that the residence time inside α = 1/3 shows a negligible variation when the applied pressure increases from 365 to 444 MPa, whereas it highly decreases while the applied pressure increases from 444 to 635 MPa which suggests the complete passage of a molecule across the pore in a short course of simulation time. Accordingly, it is inferred that the time residence is dependent on the applied pressure. Since the applied pressure increases, water molecules in bulk structure are encouraged to the cone and exit the pore in a fast behavior. This suggests that the structure of the pore has less contribution to the movement of water molecules. The profiles of the axial and total velocities of water molecules inside pores are shown in **Figure 8a** and **b** as the function of applied pressure. By increasing the applied pressure, the axial velocity of water molecules is strongly increased inside α = 1/3, whereas a slight increase is found for the axial velocity inside α = 0 system. In case of α = −1/3, the increase in the applied pressure results in the reduction in the axial velocity inside the pore. It can be found that by increasing the applied pressure, the occupancy number of water molecules inside α = −1/3 is increased. Due to the strong interaction between water layer surfaces it should be difficult for the large displacement of water molecules across the pore model of α = −1/3, which results in producing low axial velocity. In addition, the figure describes high speed of transport of water molecules with applied pressure across α = 1/3 and α = 0 systems because of the weak interaction of water layer surfaces and the strong water–water interaction. Accordingly, the formation of tight hydrogen bonding networks is being expected inside α = 1/3 and α = 0 pores. It is realized that the pore structure plays a dramatic role on the transport phenomena across pores in hourglass structure. **Figure 8b** indicates the profile of the total velocities of water molecules in relation to applied pressure inside each of the three pore models. Generally, the total velocity (Vx + Vy + Vz) is higher inside α = −1/3 compared to α = 1/3 and α = 0 systems. At applied pressure of 365 MPa, our simulation results show the same value for the total velocity inside symmetric and asymmetric pore systems. It is understood that at given applied pressure, the velocity in the x and y

out − tin. In **Figure 7b**,

**Figure 6.** (a) Profiles of RDF between the oxygen of water molecules inside whole systems of α = 1/3 and α = −1/3 pore models and (b) profiles of the potential mean force (PMF) for water molecules across the whole systems of α = 1/3 and α = −1/3.

**Figure 7.** (a) Axial diffusion coefficient of water molecules inside symmetric (α = 0) and asymmetric (α = 1/3 and α = −1/3) pore systems and (b) average residence time for water molecules inside α = 1/3, as a function of the pressure difference.

geometries, an important parameter known as diffusion coefficient can be used. We computed the axial diffusion coefficient by calculating the mean square displacement (MSD), as given in Ref. [49]. The diffusion rate is dependent on density because it stems from the collision between molecules and the interplay between molecules and the pore surface [50]. The axial diffusion coefficient of water molecules in the −z coordinate of the symmetric and asymmetric pore systems is given in **Figure 7a**. The figure shows that the axial diffusion coefficient decreases with applied pressure inside α = 1/3; however, it is increased inside α = −1/3 and α = 0 pore systems indicating a larger slope for diffusion inside α = −1/3. It can be found that the increase in the density with applied pressure could be the responsible for the increase in the diffusion coefficient of water molecules inside α = −1/3 and α = 0 pore models. Looking at the low density states (in relation to low applied pressures), we believe that the water layering would be extremely small and the water molecules confined in between the layers would provide very limited spaces for molecules to move due to the strong interaction between water layer surfaces. Considering states with higher densities, the large number of hydrogen bonds would contribute to form the larger clusters yielding larger movement. In case of α = 1/3 pore, by increasing the applied pressure, the diffusion coefficient decreases. We believe that the water density does not have a significant effect on the mechanism of diffusion across α = 1/3 pore, as can be realized through the result reported for the water occupancy (see **Figure 2a**). The decreased diffusion coefficient with applied pressure could be ascribed to the formation of stabilized hydrogen bonding networks resulting from weak interaction between water layer surfaces. In comparison with α = 0 and α = 1/3 pore systems, a sharp slope for diffusion coefficient is observed inside α = −1/3 which suggests a density-dependent effect. Since the applied pressure increases, a collective diffusion mechanism can be understood inside α = 1/3 because of the formation of the stabilized hydrogen bonding networks. A discreet mechanism for filling behavior of water molecules with applied pressure can be realized across α = −1/3 and α = 0 systems due to existence of the large energy barrier. At the overlap pressure (444 MPa), the similar magnitudes for the diffusion coefficient of water molecules inside each of the three pore systems are obtained. The average residence time of water molecules inside α = 1/3 system as a function of the pressure difference is given in **Figure 7b**. Considering a molecule entering from one side and leaving the pore from the other side, one can calculate the residence time tres = t out − tin. In **Figure 7b**, we can see that the residence time inside α = 1/3 shows a negligible variation when the applied pressure increases from 365 to 444 MPa, whereas it highly decreases while the applied pressure increases from 444 to 635 MPa which suggests the complete passage of a molecule across the pore in a short course of simulation time. Accordingly, it is inferred that the time residence is dependent on the applied pressure. Since the applied pressure increases, water molecules in bulk structure are encouraged to the cone and exit the pore in a fast behavior. This suggests that the structure of the pore has less contribution to the movement of water molecules. The profiles of the axial and total velocities of water molecules inside pores are shown in **Figure 8a** and **b** as the function of applied pressure. By increasing the applied pressure, the axial velocity of water molecules is strongly increased inside α = 1/3, whereas a slight increase is found for the axial velocity inside α = 0 system. In case of α = −1/3, the increase in the applied pressure results in the reduction in the axial velocity inside the pore. It can be found that by increasing the applied pressure, the occupancy number of water molecules inside α = −1/3 is increased. Due to the strong interaction between water layer surfaces it should be difficult for the large displacement of water molecules across the pore model of α = −1/3, which results in producing low axial velocity. In addition, the figure describes high speed of transport of water molecules with applied pressure across α = 1/3 and α = 0 systems because of the weak interaction of water layer surfaces and the strong water–water interaction. Accordingly, the formation of tight hydrogen bonding networks is being expected inside α = 1/3 and α = 0 pores. It is realized that the pore structure plays a dramatic role on the transport phenomena across pores in hourglass structure. **Figure 8b** indicates the profile of the total velocities of water molecules in relation to applied pressure inside each of the three pore models. Generally, the total velocity (Vx + Vy + Vz) is higher inside α = −1/3 compared to α = 1/3 and α = 0 systems. At applied pressure of 365 MPa, our simulation results show the same value for the total velocity inside symmetric and asymmetric pore systems. It is understood that at given applied pressure, the velocity in the x and y

**Figure 7.** (a) Axial diffusion coefficient of water molecules inside symmetric (α = 0) and asymmetric (α = 1/3 and α = −1/3) pore systems and (b) average residence time for water molecules inside α = 1/3, as a function of the pressure difference.

**Figure 6.** (a) Profiles of RDF between the oxygen of water molecules inside whole systems of α = 1/3 and α = −1/3 pore models and (b) profiles of the potential mean force (PMF) for water molecules across the whole systems of α = 1/3 and

α = −1/3.

14 Nanostructures in Energy Generation, Transmission and Storage

geometries, an important parameter known as diffusion coefficient can be used. We computed the axial diffusion coefficient by calculating the mean square displacement (MSD), as given in Ref. [49]. The diffusion rate is dependent on density because it stems from the collision between molecules and the interplay between molecules and the pore surface [50]. The axial diffusion coefficient of water molecules in the −z coordinate of the symmetric and asymmetric pore systems is given in **Figure 7a**. The figure shows that the axial diffusion coefficient decreases with applied pressure inside α = 1/3; however, it is increased inside α = −1/3 and α = 0 pore systems indicating a larger slope for diffusion inside α = −1/3. It can be found that the increase in the density with applied pressure could be the responsible for the increase in the diffusion coefficient of water molecules inside α = −1/3 and α = 0 pore models. Looking at the low density states (in relation to low applied pressures), we believe that the water layering would be extremely

with α = −1/3, the water molecules crossing the smallest pore diameter inside α = −1/3 system have shown a larger layering effect which strongly reduce the water transport rate. The findings revealed that the asymmetric model of α = 1/3 is able to transfer a larger number of fluxes in comparison to the symmetric pore of hourglass shape (α = 0). This could be ascribed to the small viscosity in relation to small water layering which comes from the length effects. It is also understood a proportionality between the permeability and the pore structure with a higher value for the case of α = 1/3. In comparison with α = −1/3, the MD trajectory indicated that the dipole orientation is widely distributed inside α = 1/3, falling in the ranges of 149–153° and 158–162°. Basically, a large distribution of dipole orientation triggers the fast flow of water inside the pore. It can be seen that the diffusion coefficient decreases with the pressure difference inside α = 1/3 pore because of the strong clustering of water molecules.  Finally, our MD results described that the contribution of the velocity perpendicular to

Molecular Simulation Study of Water Transport through Aquaporin-Inspired Pore Geometry

http://dx.doi.org/10.5772/intechopen.77394

17

the layers (z-axis) exposes the influx ability of molecules of water inside the pores.

Research Center Program (Grant no. NRF-2012-0009578).

\*Address all correspondence to<EMAIL_ADDRESS>
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean government (MEST) (no. NRF-2014R1A2A2A01003618) and the Pioneer

Department of Mechanical Engineering, Sogang University, Seoul, Republic of Korea

[1] Cohen-Tanugi D, McGovern RK, Dave SH, Lienhard JH, Grossman JC. Quantifying the potential of ultra-permeable membranes for water desalination. Energy & Environmental

[2] Fischbein MD, Drndi M. Electron beam nanosculpting of suspended graphene sheets.

[3] Nair RR, Wu HA, Jayaram PN, Grigorieva IV, Geim AK. Unimpeded permeation of water through helium-leak-tight graphene-based membranes. Science. 2012;**335**:442-444 [4] Cohen-Tanugi D, Grossman JC. Water desalination across nanoporous graphene. Nano

**Acknowledgements**

**Author details**

**References**

Daejoong Kim\* and Majid Shahbabaei

Science. 2014;**7**:1134-1141

Letters. 2012;**12**:3602-3608

Applied Physics Letters. 2008;**93**:113107

**Figure 8.** (a) Axial velocity and (b) total velocity of water molecules across α = 1/3, α = 0, and α = −1/3 systems.

directions has a significant impact on total velocity inside α = −1/3 model compared to α = 1/3 and α = 0 structures. The results reported in **Figure 8a** and **b** together suggest that the contribution of velocity in the z direction (Vz) should indicate the permeation ability in our simulation models, though a negligible effect of the velocities in x and y directions on the transport mechanism could be understood.

#### **4. Concluding remarks**

Graphene with one-atom thickness of sp<sup>2</sup> carbon atoms organizing a honeycomb structure possess an excellent mechanical strength, supreme electrical and thermal conductivity, and other amazing properties. Lately, the application of nanoporous graphene in desalination and water purification processes has been greatly increased. Using experimental techniques [51–54], one can generate nano-/micro-scale pores of various diameters and shapes on graphene sheets. Considering the properties of nanoporous graphene and taking advantage of aquaporin water channel structure, this computer simulation study aims to design an optimal and effective permeable membrane for water transportation across hourglass-shaped pores in multilayer nanoporous graphene.

In this study, by conducting MD simulations, we investigated the mechanism of water transportation across hourglass-shaped nanopores in symmetric and asymmetric structures constructed by multilayer nanoporous graphene. Considering a constant interlayer spacing of 6 Å, we particularly focused on the applied pressure effects on the transport properties through symmetric (α = 0) and asymmetric (α = 1/3 and α = −1/3) pores with hourglass shape in multilayer graphene. The results show a higher occupancy of water molecules in relation to applied pressure inside α = 1/3 compared to α = −0 and α = −1/3. In case of α = 1/3, because of the increase in the volume of the pore (the increase in the cone length), the flow of molecules of water is fostered. Then, the occupancy of water molecules inside the pore with hourglass shape is geometry dependent compared to pressure effect. The number of flux of water increases linearly as a function of applied pressure inside the pores, suggesting higher flux inside α = 1/3 pore. In comparison with α = −1/3, the water molecules crossing the smallest pore diameter inside α = −1/3 system have shown a larger layering effect which strongly reduce the water transport rate. The findings revealed that the asymmetric model of α = 1/3 is able to transfer a larger number of fluxes in comparison to the symmetric pore of hourglass shape (α = 0). This could be ascribed to the small viscosity in relation to small water layering which comes from the length effects. It is also understood a proportionality between the permeability and the pore structure with a higher value for the case of α = 1/3. In comparison with α = −1/3, the MD trajectory indicated that the dipole orientation is widely distributed inside α = 1/3, falling in the ranges of 149–153° and 158–162°. Basically, a large distribution of dipole orientation triggers the fast flow of water inside the pore. It can be seen that the diffusion coefficient decreases with the pressure difference inside α = 1/3 pore because of the strong clustering of water molecules.  Finally, our MD results described that the contribution of the velocity perpendicular to the layers (z-axis) exposes the influx ability of molecules of water inside the pores.

## **Acknowledgements**

directions has a significant impact on total velocity inside α = −1/3 model compared to α = 1/3 and α = 0 structures. The results reported in **Figure 8a** and **b** together suggest that the contribution of velocity in the z direction (Vz) should indicate the permeation ability in our simulation models, though a negligible effect of the velocities in x and y directions on the transport mecha-

**Figure 8.** (a) Axial velocity and (b) total velocity of water molecules across α = 1/3, α = 0, and α = −1/3 systems.

an excellent mechanical strength, supreme electrical and thermal conductivity, and other amazing properties. Lately, the application of nanoporous graphene in desalination and water purification processes has been greatly increased. Using experimental techniques [51–54], one can generate nano-/micro-scale pores of various diameters and shapes on graphene sheets. Considering the properties of nanoporous graphene and taking advantage of aquaporin water channel structure, this computer simulation study aims to design an optimal and effective permeable membrane for

In this study, by conducting MD simulations, we investigated the mechanism of water transportation across hourglass-shaped nanopores in symmetric and asymmetric structures constructed by multilayer nanoporous graphene. Considering a constant interlayer spacing of 6 Å, we particularly focused on the applied pressure effects on the transport properties through symmetric (α = 0) and asymmetric (α = 1/3 and α = −1/3) pores with hourglass shape in multilayer graphene. The results show a higher occupancy of water molecules in relation to applied pressure inside α = 1/3 compared to α = −0 and α = −1/3. In case of α = 1/3, because of the increase in the volume of the pore (the increase in the cone length), the flow of molecules of water is fostered. Then, the occupancy of water molecules inside the pore with hourglass shape is geometry dependent compared to pressure effect. The number of flux of water increases linearly as a function of applied pressure inside the pores, suggesting higher flux inside α = 1/3 pore. In comparison

water transportation across hourglass-shaped pores in multilayer nanoporous graphene.

carbon atoms organizing a honeycomb structure possess

nism could be understood.

**4. Concluding remarks**

Graphene with one-atom thickness of sp<sup>2</sup>

16 Nanostructures in Energy Generation, Transmission and Storage

This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean government (MEST) (no. NRF-2014R1A2A2A01003618) and the Pioneer Research Center Program (Grant no. NRF-2012-0009578).

## **Author details**

Daejoong Kim\* and Majid Shahbabaei

\*Address all correspondence to<EMAIL_ADDRESS>
Department of Mechanical Engineering, Sogang University, Seoul, Republic of Korea

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**Chapter 3**

**Provisional chapter**

**Sensors from Electrospun Nanostructures**

**Sensors from Electrospun Nanostructures**

DOI: 10.5772/intechopen.78289

Nanotechnology exerts a significant influence on materials science, providing new insights into the design of functional materials. One of the most studied areas of nanotechnologies is that of nanofibres, characterised by high specific volume, chemical activity and volume-dependent physical processes. The most promising method of producing nanofibres with various morphologies and functionalities from different materials is electrospinning, where high voltage is applied between the spinneret and the collector to the charged polymer solution (or melt) to draw polymer filaments. This chapter reviews the main electrospinning techniques for producing nanofibres from polymers, provides an overview on the influence of the spinning solution characteristics, the process parameters and the working environment on the process and highlights the many applications of electrospun nanofibres in the field of sensors. Latest advances in this field and the

prospects for obtaining new electrospun nanofibre sensors are discussed.

**Keywords:** nanostructures, electrospinning, electrospun polymers, electrospinning

According to the definition adopted by the National Nanotechnology Initiative, nanotechnology is the manipulation of matter with at least one dimension in the range of 1–100 nanometres [1]. At the moment nanotechnology, an interdisciplinary field having confluence of physics, chemistry and engineering [2] is one of the most attractive areas of science [3], with applications in various domains, which radically effected progress in the field of material science [4]. Nanofibre technology, which comprises the synthesis, processing, fabrication and application of nanoscale fibres [5], is one of the main progressions in nanotechnology [6].

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Liliana Rozemarie Manea and Andrei-Petru Bertea

Liliana Rozemarie Manea and Andrei-Petru Bertea

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.78289

**Abstract**

techniques, sensors

**1. Introduction**

#### **Sensors from Electrospun Nanostructures Sensors from Electrospun Nanostructures**

DOI: 10.5772/intechopen.78289

Liliana Rozemarie Manea and Andrei-Petru Bertea Liliana Rozemarie Manea and Andrei-Petru Bertea

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.78289

**Abstract**

Nanotechnology exerts a significant influence on materials science, providing new insights into the design of functional materials. One of the most studied areas of nanotechnologies is that of nanofibres, characterised by high specific volume, chemical activity and volume-dependent physical processes. The most promising method of producing nanofibres with various morphologies and functionalities from different materials is electrospinning, where high voltage is applied between the spinneret and the collector to the charged polymer solution (or melt) to draw polymer filaments. This chapter reviews the main electrospinning techniques for producing nanofibres from polymers, provides an overview on the influence of the spinning solution characteristics, the process parameters and the working environment on the process and highlights the many applications of electrospun nanofibres in the field of sensors. Latest advances in this field and the prospects for obtaining new electrospun nanofibre sensors are discussed.

**Keywords:** nanostructures, electrospinning, electrospun polymers, electrospinning techniques, sensors

#### **1. Introduction**

According to the definition adopted by the National Nanotechnology Initiative, nanotechnology is the manipulation of matter with at least one dimension in the range of 1–100 nanometres [1]. At the moment nanotechnology, an interdisciplinary field having confluence of physics, chemistry and engineering [2] is one of the most attractive areas of science [3], with applications in various domains, which radically effected progress in the field of material science [4]. Nanofibre technology, which comprises the synthesis, processing, fabrication and application of nanoscale fibres [5], is one of the main progressions in nanotechnology [6].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **2. Electrospinning: principles and main techniques**

Among the technologies that can be used to obtain nanofibres (phase separation, template synthesis, self-assembly, melt blowing) [7], electrospinning is one of the most promising technologies, being commonly used for producing nanosize filaments from both organic polymers and inorganic materials [8], as it is uncomplicated, versatile and capable of producing controlled size fibres with high surface to volume ratio, hence more capable to interrelate with the surrounding environment. In addition, the electrospun nanofibre structures are characterised by complex three-dimensional open porous assembly, facilitating even more the interactions with the adjacent environment. The fact that nanofibres have only one dimension at nanoscale, since the others are at macroscopic one, makes it possible to mingle the advantages of nanostructures (high chemical and biological reactivity and electroactivity) with those of conventional solid membranes, such as comfortable manipulation and easy applicability.

A polymer solution or melt can be electrospun if it is able to carry an electric charge and has enough viscosity to be strained without breaking up into droplets. Up to now, more than 200 types of materials have been electrospun into nanofibres, among which natural and synthetic polymers and many hybrid blends [6].

The principle of electrospinning is quite simple: by applying high voltages electrostatic field (e.g. 10–50 kV) to a polymer solution or a polymer melt, the surface of the fluid elongates and first forms a conical shape, known as the Taylor cone. Then, when the electric voltage reaches a threshold value, the jet of charged liquid overcomes the surface tension, leaves the Taylor cone and is drawn to the collector of different potential, forming a nanofibre net [9].

*2.1.1. Polymer solution characteristics that influence the solution electrospinning process*

volatility characteristics of the solvent.

**Figure 1.** Typical solution electrospinning setup.

the solution viscosity.

electrospun.

concentration of the polymer solution and its viscosity.

The main characteristics of the polymer solution with a significant role in the result obtained after electrospinning are concentration, viscosity, conductivity, superficial tension and the

Sensors from Electrospun Nanostructures http://dx.doi.org/10.5772/intechopen.78289 25

The solution viscosity is considered to be the dominant variable that determines the fibre diameter. To make electrospinning possible, the viscosity of the electrospun polymer solution must be within a precisely defined range. A too low viscosity leads to interrupted polymer filaments and the appearance of polymer droplets [14], while too high viscosity makes polymer extrusion impossible. The optimum values of viscosity differ according to the molecular weight of the polymer and the solvent used. Of course, there is a close correlation between the

An innovative approach to reducing the viscosity of the polymer solution is to apply lowfrequency vibrations during the process [15, 16], when vibrations disrupt some of the van der Waals interchain interactions, disordering the polymer chains, which leads to a decrease in

Electrospinning fundamentally requires the transfer of electrical charge from the electrode to the polymer droplet at the end point of the syringe needle, so a minimum electrical conductivity in solution is therefore essential for the process; solutions lacking conductivity cannot be

In addition to the classic electrospinning process, many variants have been developed, capable of producing nanofibres with special features: core/sheath nanofibres [10], nanofibres with hollow structures, nanofibres with porous structures [11] and necklace-like and ribbon nanofibres [12].

The elementary electrospinning setup has four parts: a reservoir of polymer in melt or solution form, a spinneret (in the simplest cases, a syringe), the high voltage power supply and the collector, which acts as a counter electrode (**Figure 1**).

Developments in electrospinning technology have made it possible to electrospin, besides polymers, polymers loaded with nanoparticles and functional molecules, ceramic materials and metal oxides. In addition, there have been developed fibres with new special structures [13].

#### **2.1. Factors that influence the properties of 3D nanostructures obtained by electrospinning**

In addition to this basic setup showed in **Figure 1**, there are relatively many other approaches to the process, but in all cases the electrospinning process is influenced by the following three categories of factors: the particularities of the spinning polymer solution/melt, the parameters of the electrospinning process and the environment factors.

**Figure 1.** Typical solution electrospinning setup.

**2. Electrospinning: principles and main techniques**

24 Nanostructures in Energy Generation, Transmission and Storage

applicability.

nanofibres [12].

**electrospinning**

polymers and many hybrid blends [6].

collector, which acts as a counter electrode (**Figure 1**).

of the electrospinning process and the environment factors.

Among the technologies that can be used to obtain nanofibres (phase separation, template synthesis, self-assembly, melt blowing) [7], electrospinning is one of the most promising technologies, being commonly used for producing nanosize filaments from both organic polymers and inorganic materials [8], as it is uncomplicated, versatile and capable of producing controlled size fibres with high surface to volume ratio, hence more capable to interrelate with the surrounding environment. In addition, the electrospun nanofibre structures are characterised by complex three-dimensional open porous assembly, facilitating even more the interactions with the adjacent environment. The fact that nanofibres have only one dimension at nanoscale, since the others are at macroscopic one, makes it possible to mingle the advantages of nanostructures (high chemical and biological reactivity and electroactivity) with those of conventional solid membranes, such as comfortable manipulation and easy

A polymer solution or melt can be electrospun if it is able to carry an electric charge and has enough viscosity to be strained without breaking up into droplets. Up to now, more than 200 types of materials have been electrospun into nanofibres, among which natural and synthetic

The principle of electrospinning is quite simple: by applying high voltages electrostatic field (e.g. 10–50 kV) to a polymer solution or a polymer melt, the surface of the fluid elongates and first forms a conical shape, known as the Taylor cone. Then, when the electric voltage reaches a threshold value, the jet of charged liquid overcomes the surface tension, leaves the Taylor

In addition to the classic electrospinning process, many variants have been developed, capable of producing nanofibres with special features: core/sheath nanofibres [10], nanofibres with hollow structures, nanofibres with porous structures [11] and necklace-like and ribbon

The elementary electrospinning setup has four parts: a reservoir of polymer in melt or solution form, a spinneret (in the simplest cases, a syringe), the high voltage power supply and the