Surface-Functionalized Ceramics E-Book
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- Herausgeber: Wiley-VCH GmbH
- Kategorie: Fachliteratur
- Sprache: Englisch
This book gives a comprehensive overview of ceramic surface functionalization approaches for ceramic materials including alumina, zirconia, titania and silica and their use as sensors, chemical and biological probes, chromatographic supports for (bio)molecule purification and analysis, and as adsorbents for toxic substances and pollutants. It provides a broad picture of the enormous possibilities offered by surface functionalization and, in addition, addresses the current challenges regarding surface analysis, characterization and stability.
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Table of Contents
Cover
Title Page
Copyright
Dedication
Dedications and Acknowledgments
Personal Dedications
Preface
Foreword
1 Introduction to Ceramic Materials
1.1 Introduction: Ceramics for Biotechnological and Environmental Applications
1.2 What are Ceramic Materials?
1.3 Oxide Ceramics
1.4 Nonoxide Ceramics
1.5 Carbon‐based Materials
1.6 Conclusions
References
2 Processing Methods for Advanced Ceramics
2.1 Introduction
2.2 Powder Synthesis and Preparation
2.3 Shaping Methods
2.4 Additive Manufacturing
2.5 Conclusions
References
3 Surface Modification of Ceramic Materials
3.1 Introduction
3.2 Chemical Activation Strategies for Inert Ceramic Surfaces
3.3 Derivatization Strategies by Wet Chemistry Functionalization
3.4 Ceramic Surface Decoration for Biotechnological and Environmental Applications
3.5 Summary and Outlook
References
4 Atomic Force Microscopy for Imaging and Chemical Analysis of Ceramic Surfaces
4.1 Introduction
4.2 The Basic AFM Modes of Operation
4.3 Dry AFM vs. Liquid‐cell AFM
4.4 Technical Details About the AFM Imaging Process
4.5 Application of AFM for Biotechnological and Environmental Purposes
4.6 Conclusions
References
5 Surface Chemical Analysis of Ceramics and Ceramic‐Enhanced Analytics
5.1 Introduction
5.2 Methods for Surface Chemical Analysis of Ceramics: An Overview
5.3 Using Ceramic Colloids and Nanomaterials for Advanced Surface Chemical Analysis
5.4 Concluding Remarks and Outlook
References
6 Methods for Electrokinetic Surface Characteristics
6.1 Introduction
6.2 The Electric Double‐layer
6.3 Electrokinetic Phenomena‐theory
6.4 Experimental Evidences, Applications
6.5 Concluding Remarks
Acknowledgments
References
7 Functionalized Surfaces and Interactions with Biomolecules
7.1 Introduction
7.2 Fundamentals of Biomolecule Interactions with Functionalized Material Surfaces
7.3 Influence of Surface Functionality, Multifunctionality, and Heterogeneous Surface Chemistry
7.4 Conclusions and Outlook
Acknowledgments
References
8 Interactions Between Surface Material and Bacteria: From Biofilm Formation to Suppression
8.1 Introduction
8.2 Biofilm Formation
8.3 Theoretical Models of Bacteria–Surface Interactions
8.4 Detrimental Effects of Biofilms: Some Examples
8.5 Prevention of Biofilm Formation
8.6 Characterization of Antimicrobial Materials and Coatings
8.7 Conclusions and Outlook
References
9 Carbon Nanomaterials for Antibacterial Applications
9.1 Introduction
9.2 Inherent Antibacterial Properties of Carbon Nanomaterials
9.3 Functionalization of Carbon Nanomaterials for Tailoring Antibacterial Properties
9.4 Summary and Outlook
References
10 Mesoporous Silica and Organosilica Biosensors for Water Quality and Environmental Monitoring
10.1 Introduction
10.2 Mesoporous Silica Materials for Biosensor Development
10.3 Functionalization of Mesoporous Silica and Organosilica‐Based Biosensors
10.4 Applications of Mesoporous Silica and Organosilica‐Based Biosensors
10.5 Conclusions and Outlook
Acknowledgments
Abbreviations
References
11 Ceramic‐Based Adsorbents in Bioproduct Recovery and Purification
11.1 Introduction
11.2 Chromatography and Chromatography Support
11.3 Functionalization of Ceramic‐Based Adsorbents
11.4 Characterization of Ceramic Adsorbent Particles
11.5 Fundamentals of Bioproduct Adsorption onto Ceramic Beads
11.6 Application of Ceramic‐Based Adsorbents
11.7 Conclusions and Outlook
References
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Crystal structure and physicochemical properties of aluminas.
Table 1.2 Structural information and properties of most common titania poly...
Table 1.3 Crystal structure and physicochemical properties of silicas.
Chapter 2
Table 2.1 Representative colloidal forming routes classified by consolidatio...
Table 2.2 Overview of ceramic 3D‐printing technologies and relevant features...
Chapter 4
Table 4.1 Overview of AFM mode properties.
Chapter 5
Table 5.1 Synoptic table showing the essential features of X‐ray photoelect...
Chapter 6
Table 6.1 Screening length
Le
=
κ
−1
(nm), for various electrolyt...
Table 6.2 The double layer thickness (
L
e
), electrophoretic mobility (
μ
Table 6.3 Electrophoretic mobilities (
μ
e
), zeta potentials (ζ) and cha...
Table 6.4 Zeta potential
ζ
i
and the electrokinetic surface charge
σ
...
Chapter 7
Table 7.1 General material and media properties that influence biomolecule ...
Chapter 8
Table 8.1 Signal and molecular mechanism involved in biofilm dispersal proc...
Table 8.2 Main antibacterial compounds in release‐based coatings.
Table 8.3 Overview of common microscopy techniques for biofilm investigatio...
Table 8.4 Overview of different techniques for biofilm analysis.
Table 8.5 Industrial standard evaluation tests of antimicrobial surface des...
Chapter 9
Table 9.1 Application of carbonaceous nanomaterials' unique properties in e...
Chapter 10
Table 10.1 Characteristics and comparison of different enzyme immobilizatio...
Chapter 11
Table 11.1 Commercially used chromatographic supports.
Table 11.2 A list of commercially available ceramic adsorbents.
Table 11.3 Application of ceramic adsorbents.
List of Illustrations
Chapter 1
Figure 1.1 Historical timeline of development of materials.
Figure 1.2 Traditional and advanced ceramics: classification and application...
Figure 1.3 Types of chemical bonding of different ceramic materials.
Figure 1.4 Overview of the most relevant advanced ceramics: properties and a...
Figure 1.5 Thermal transformation sequence of hydrated alumina.
Figure 1.6 Structures of main titania polymorphs (unit cells): rutile, anata...
Figure 1.7 (a) Photocatalytic mechanism of titania. (b) Decomposition of gas...
Figure 1.8 (a) Sol–gel porous titania microspheres for HPLC packing. (b) Tit...
Figure 1.9 Phase diagrams of magnesia (MgO) and yttria (Y
2
O
3
)‐stabilized zir...
Figure 1.10 Phase diagram for the silica system.
Figure 1.11 Silica transformation at different temperatures.
Figure 1.12 (a) Schematic of the structure of functionalized mesoporous sili...
Figure 1.13 A scheme of functionalized nanosized mesoporous silica particles...
Figure 1.14 Schematic illustration for synthesis of fluorescent silica NPs w...
Figure 1.15 (a) Schematic illustration of a fully functionalized iron oxide ...
Figure 1.16 (a) Schematic diagram of sensor device using vertically aligned ...
Figure 1.17 Forecast of ceramics by type and application field for the next ...
Chapter 2
Figure 2.1 Relationships between chemical composition, atomic structure, fab...
Figure 2.2 Application areas of some ceramic‐shaping technologies in depende...
Figure 2.3 Schematic overview and comparison between powder‐based and precer...
Figure 2.4 Examples of some materials obtained by mechanical and chemical me...
Figure 2.5 Examples of extruded ceramic materials. (a) Ag‐doped zirconia cap...
Figure 2.6 Example of injection‐molded ceramics parts. (a) Ceramic microgear...
Figure 2.7 Sketch of the slip‐casting process.
Figure 2.8 Modified microemulsion process (m‐μM). (a) Scheme of m‐μM process...
Figure 2.9 Flowchart of gel‐casting process of ceramics with synthetic and b...
Figure 2.10 Overview of a gel‐casting process, the alginate cross‐linking me...
Figure 2.11 (a) Schematic flow chart of the freeze‐cast process. (b–c) Chara...
Figure 2.12 (A) Schematic overview of the manufacturing process of hydroxyap...
Figure 2.13 Processing routes to materials using sol–gel methods.
Figure 2.14 Example of particles by the sol–gel process. (A) (left) SEM imag...
Figure 2.15 Schematic of a tape‐casting device (Doctor blade).
Figure 2.16 Freeze tape casting. (a) Modified freeze tape casting apparatus ...
Figure 2.17 Examples of ceramics fabricated by additive manufacturing techni...
Chapter 3
Figure 3.1 Examples of oxide ceramic substrate morphologies serving as basis...
Figure 3.2 Schematic overview of the functionalization levels for the surfac...
Figure 3.3 Reaction scheme for silanization of ceramic oxide surfaces.
Figure 3.4 Chemical structures of mostly used silane precursors for surface ...
Figure 3.5 Main linkages for coupling of phosphonates (a) and carboxylates (...
Chapter 4
Figure 4.1 Schematic representation of an AFM. The sharp AFM probe is locate...
Figure 4.2 (a) Schematic representation of AFM imaging modes: contact mode, ...
Figure 4.3 Dynamic force spectroscopy for understanding mineral‐organic inte...
Figure 4.4 Applications of AFM techniques. Nowadays AFM applications are not...
Figure 4.5 Examples of different approaches for AFM tip functionalization. (...
Figure 4.6 Schematic of chemical force microscopy (CFM). CFM is similar to A...
Figure 4.7 AFM apparatus and typical atomic force curves: (a) Illustration o...
Figure 4.8 AFM images of PM
2.5
particles and soot aggregates. (a, b) Three‐d...
Figure 4.9 Adhesion and deformation of PM
2.5
particles. (a–c) Topographic, c...
Figure 4.10 (a) AFM images from samples taken from tributaries of the river ...
Figure 4.11 AFM tapping mode images of microbial biofilms: (a)
Candida tropi
...
Figure 4.12 Single‐cell force spectroscopy method to decipher bacterial adhe...
Figure 4.13 AFM images of alumina–zirconia–titania (AZT) ceramic membranes c...
Chapter 5
Figure 5.1 Scheme of a generic photoexcitation process at the basis of XPS....
Figure 5.2 Escape depth as a function of kinetic energy. Data refer to silve...
Figure 5.3 General scheme of the Auger process.
Figure 5.4 Example of XPS spectrum of TiO
2
nanoshells analogous to those rep...
Figure 5.5 Comparison of Auger and XPS sensitivity as a function of the atom...
Figure 5.6 SIMS depth profiling of a TiO
2
/ZnO multilayer thin film deposited...
Figure 5.7 Schematic view of different possible applications of microRaman i...
Figure 5.8 Schematic view of light propagation at the interface between two ...
Figure 5.9 Working principle of WGM‐based sensing. See the main text for det...
Figure 5.10 Light concentration by micron‐sized spheres in air (
n
= 1). Depe...
Figure 5.11 Raman response as a function of sample morphology for planar, mo...
Figure 5.12 Schematic view of recyclability tests. The same photocatalytic b...
Figure 5.13 Scheme of combined MALDI/SERS analysis using T‐rex substrates.
Chapter 6
Figure 6.1 The dependence of the normalized potential drop across the diffus...
Figure 6.2 The dependence of the effective surface potential
of a spherica...
Figure 6.3 The
function (curve 1, calculated from Eq. (6.38)) and the Henr...
Figure 6.4 A schematic representation of the shear flow past a planar solid ...
Figure 6.5 A schematic representation of the streaming current in the case o...
Figure 6.6 The dependence of electrophoretic mobility on pH for silica, silv...
Figure 6.7 The SEM micrographs of latex particles, (a) the positive A100 lat...
Figure 6.8 The dependence of the zeta potential of particles calculated from...
Figure 6.9 The dependence of the zeta potential of S800 latex on the A100 la...
Figure 6.10 Electrophoretic mobility of polystyrene sulfate latex particles ...
Figure 6.11 Crystallographic structure of three plasma proteins: Fibrinogen,...
Figure 6.12 The dependence of the zeta potential of latex particles (L800) o...
Figure 6.13 Zeta potential versus the coverage of rHSA on the negatively cha...
Figure 6.14 The dependence of the electrophoretic mobility of polystyrene la...
Figure 6.15 The dependence of the zeta potential of mica on pH determined by...
Figure 6.16 The dependence of the zeta potential of silicon covered by silic...
Figure 6.17 The dependence of the zeta potential of the plasma deposited flu...
Figure 6.18 The dependence of electrophoretic mobility,
μ
e
, on pH, dete...
Figure 6.19 The dependence of the zeta potential of mica
ζ
on the nomin...
Figure 6.20 The dependence of the zeta potential of PAH monolayers (
ζ
)...
Figure 6.21 The dependencies of the zeta potential of bare and polyelectroly...
Figure 6.22 The dependence of the zeta potential of mica on the coverage of ...
Figure 6.23 The dependence of the normalized zeta potential of hematite‐cove...
Figure 6.24 The zeta potential determined by streaming potential measurement...
Figure 6.25 The dependence of the zeta potential of mica on the coverage of ...
Figure 6.26 The dependence of the zeta potential of mica on the coverage of ...
Figure 6.27 The universal plot dependence of the coverage of silver particle...
Figure 6.28 The kinetics of silver particle desorption from mica/PAH support...
Figure 6.29 The dependence of zeta potential of silver monolayers
Θ
s
on...
Figure 6.30 Formation of the silver nanoparticle monolayer on hematite‐cover...
Figure 6.31 The dependence of the zeta potential of mica on the surface conc...
Figure 6.32 The desorption kinetics of rHSA under flow conditions expressed ...
Figure 6.33 The dependence of the zeta potential of the rHSA monolayer on pH...
Figure 6.34 The dependence of the zeta potential of mica on the coverage of ...
Figure 6.35 The kinetics of fibrinogen adsorption at mica under the diffusio...
Chapter 7
Figure 7.1 (a) Proteins, saccharides, nucleic acids, and peptides as example...
Figure 7.2 (a) Distribution of
and
groups on the 14.3 kDa protein lysozy...
Figure 7.3 Events on the material surface may occur during or after protein ...
Figure 7.4 (a) Functional groups that establish a charged state, dependent o...
Figure 7.5 (a) Length, molecular weights, structure, and surface confirmatio...
Figure 7.6 (a) Illustrations of mixed, multifunctional, and nanostructured s...
Chapter 8
Figure 8.1 Steps of the biofilm formation process. The substrate becomes cov...
Figure 8.2 Composition of the EPS matrix and the functions of its major cons...
Figure 8.3 Major actors involved in biofilm dispersal. In response to a broa...
Figure 8.4 Schematic presentation of the interfacial free energies
γ
ij
...
Figure 8.5 The Lifshitz–van der Waals G
LW
(d) and electrostatic G
EL
(d) intera...
Figure 8.6 Possible profiles of the system energy components according to th...
Figure 8.7 Classes of antibacterial coatings. Surface repelling bacterial at...
Figure 8.8 (a) Dual‐functional, superhydrophobic coatings with simultaneous ...
Figure 8.9 Schematic of stimuli‐responsive coatings made of soft matter on i...
Figure 8.10 An example of antibiotic‐containing bead chains for the treatmen...
Figure 8.11 (a) Near‐net‐shape hydroxyapatite scaffolds and parts fabricated...
Figure 8.12 Overview of methods to grow and characterize biofilms, which inc...
Figure 8.13 Schematics of agar zone of (a) inhibition and (b) suspension met...
Figure 8.14 Pros and cons of
in vitro
standardized biofilm models.
Chapter 9
Figure 9.1 The different allotropes of nanocarbons. (A) nanocarbon ordered b...
Figure 9.2 Effects of CNMs on microorganisms. Different CNMs have different ...
Figure 9.3 (A) (a) Viability of
E. coli
after incubation with graphite (Gt),...
Figure 9.4 (A) Scanning electron microscopy (SEM) images of
E. coli
cells ex...
Figure 9.5 Mechanism of ROS production by fullerenes.
Figure 9.6 (A) DLC film.
Chapter 10
Figure 10.1 Illustrative TEM views of typical mesoporous silica materials ob...
Figure 10.2 Schematic representation of a poly(lactic acid)‐coated MCM‐41‐ty...
Figure 10.3 Schematic representation of the various ways to modify/functiona...
Figure 10.4 Beneficial effects of enzyme immobilization. (a) Inhibition of e...
Figure 10.5 Surface modification of mesoporous silica materials with nickel(...
Figure 10.6 Scheme of an electrochemical biosensor. Biological sensing eleme...
Figure 10.7 TEM images of (a) amino‐functionalized mesoporous silica nanopar...
Figure 10.8 (a) Schematic representation of the glucose‐responsive MSN‐based...
Figure 10.9 (A) Photograph of the ethidium bromide‐bridged, fluorescent meso...
Figure 10.10 (a) Scheme of a generic sensor element; most common gas sensors...
Figure 10.11 (a) Schematic of HFIP‐sensing group modification on the inner w...
Figure 10.12 Schematic representation of the fabrication of the immunosensor...
Chapter 11
Figure 11.1 Generalized downstream processing flow sheet for purification of...
Figure 11.2 (a) Scanning electron microscopy micrographs of chromatographic ...
Figure 11.3 Overview of immobilization strategies for (a) organic monoliths,...
Figure 11.4 Schematic of simple chromatographic set‐up and process flow.
Guide
Cover
Table of Contents
Title Page
Copyright
Dedication
Preface
Foreword
Begin Reading
Index
End User License Agreement
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Surface‐Functionalized Ceramics
For Biotechnological and Environmental Applications
Edited by Laura Treccani and Fabian Meder
Editors
Dr. Laura TreccaniCSMT Gestione S.c.a.r.l.Polo Tecnologico – Innovative Contamination HUBVia Branze 4525123 BresciaItaly
Dr. Fabian MederIstituto Italiano di Tecnologia (IIT)Bioinspired Soft RoboticsViale Rinaldo Piaggio 3456025 PontederaItaly
Cover Image: Courtesy of Fabian Meder and Laura Treccani
All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No.: applied for
British Library Cataloguing‐in‐Publication DataA catalogue record for this book is available from the British Library.
Bibliographic information published by the Deutsche NationalbibliothekThe Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d‐nb.de>.
© 2023 WILEY‐VCH GmbH, Boschstr. 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978‐3‐527‐33835‐1ePDF ISBN: 978‐3‐527‐69806‐6ePub ISBN: 978‐3‐527‐69805‐9oBook ISBN: 978‐3‐527‐69804‐2
Dedications and Acknowledgments
We would like to thank all the authors and contributors who have helped in completing this book. We thank them for providing great content and giving us the chance to learn something new.
We would like to thank Wiley for their support, and in particular Dr. Martin Preuss, for trusting us and giving us the time to finish this project
To all a great thanks for your contribution and your patience; without them, this book would not have been possible.
Personal Dedications
Laura Treccani …to my grandmother Emma, my first scientific mentor
I should thank countless people who in different ways have contributed to this work. A particular thanks to my former team at the University of Bremen. Thanks to Prof. Kurosch Rezwan and Prof. Georg Grathwohl for giving me the chance to join their Department as PostDoc. Thanks to Kurosch for believing in me and encouraging me. Thank you to my PhD students, students, colleagues, and in particular Tina Kuehn that contributed to amazing ideas and taught me new. I also thank Fabian Meder for joining me in this project.
A deep thanks to my family for supporting me through trying times, always believing in me, and for giving me strength.
Fabian Meder …to my children, my wife, and family
I particularly express thanks to Laura Treccani and Kurosch Rezwan for the opportunity to join the team and assist in realizing this book as well as for their guidance during my first steps in the world of ceramics and their surface functions as a PhD student. A special thanks to all supervisors, students, and colleagues that supported and inspired me in discovering materials, functionalizing, and applying them. Thanks to all friends and people close to me, especially to my wife for her patience and support of my work and a particular thanks to my children for their fantastic questions, a great motivation to always find out more about this fascinating world.
Preface
The surface determines the reactions of materials and essentially qualifies them for specific applications, in particular in biological environments. Ceramic materials – here the high‐tech advanced ceramics – can be fabricated in numerous shapes with tailored composition. Since a long time, ceramics are crucial in most scientific and industrial applications. Still, in the past two decades, surface functionalization technologies of ceramics have been revolutionized, allowing a combination of inorganic, organic, and biomolecular matter. This not only creates novel functionalities but also qualifies the final “advanced ceramics” for many new and advanced applications.
This book starts with an overview of the properties and manufacturing of ceramic materials and their surface functionalization – with a focus on biotechnological and environmental applications. Chapters 1 and 2 give an extended summary of compositions and structures of ceramics as well as the fabrication and processing techniques. This provides the necessary background in terms of materials science and processing in combination with selected application examples.
Techniques to vary physicochemical properties such as surface charges, chemical functions, and nanostructures are presented in Chapter 3. Chapter 3 focuses on the most important chemical surface functionalization techniques ranging from surface activation strategies to small molecule and large biomolecule attachment to create functionalities that do not exist in bare ceramics. The result is a new class of functional materials that combines the intrinsic versatility of ceramics with novel and moreover very useful functionalities.
It is, in the context of this book, absolutely necessary to discuss methods to access information on the quality and success of a surface functionalization process by characterization techniques spanning from topographical investigations, such as atomic force microscopy and surface chemical analysis, to the electrokinetic surface characteristics, and Chapters 4–6, respectively, are dedicated to surface analytics, including exiting relevant applications and theory.
Controlling and knowing the surface functionality is essential to understand and predict potential interactions of the functionalized ceramics in a biological environment such as the surface adhesion of biomolecules (Chapter 7) and bacterial interactions (Chapter 8). Such interactions essentially influence the success of the envisioned application, in a positive or negative manner, and the two chapters will introduce the essential processes of these biology–material interactions on the nano‐ and molecular scales.
The overview of potential applications in a book about materials so versatile as functionalized ceramics can only be incomplete. Nevertheless, we aimed to give a focus on important – if not some of the most important – examples for the biotechnological and environmental fields such as novel carbon‐based antibacterial surfaces (Chapter 9), biosensing and water quality monitoring (Chapter 10), and ceramic adsorbents for bioproduct recovery and purification (Chapter 11) among others.
Overall, with this book, we intend to supply the reader with sufficient fundamentals and examples on the opportunities of surface‐functionalized ceramics to provide a general understanding and to provoke creative thinking and novel ideas for designing surface‐engineered materials.
This book is for researchers (experimental or theoretical), undergraduate and graduate students of disciplines like materials science, biotechnology, and environmental sciences and related fields that want to get introduced in the pacing developing, interdisciplinary field of functionalized material surfaces, particularly ceramics. A secondary market are biotechnological industries and environmental sectors and their Research and Development (R&D) sections.
October, 2022
Fabian Meder
Istituto Italiano di Technologia, Genova, Italy
Laura Treccani
CSMT Gestione S.c.a.r.l., Brescia, Italy
Foreword
Against the background of increasing population, we see the immense impact on the environmental pollution and associated climate changes. In this combination, we face today probably the biggest challenge to humankind in its history. In addition, everyone is or will be affected, without exceptions.
As a scientist and scholar, I do teach and educate young adults every day with my team at a university. Through our role at the university, we do shape the minds of these young people. Hence, we do shape tomorrow's society. However, what if there is no tomorrow? Having said this, we should all feel naturally obliged to make our own contribution to the best of our ability.
This is where I see the major and indispensable contribution of this book, showing an important area, where materials scientists can make a great impact toward tomorrow: By having a closer look at the materials interface to the biotechnological and environmental applications world, it gives a review of what is known today and pins down open challenging questions to be tackled at the same time.
Focusing on ceramic materials, it elucidates the strengths of this class of material toward biotechnological and environmental materials. By its inertness and hardness foremost, it is an ideal class of material to interact in a controlled manner with biological entities. At the same time, the inertness can be overcome by surface chemistry tailoring directed to a specific bifunctionality. This step, however, requires a great knowledge about details of ceramic surfaces and the targeted biological environment. By a sophisticated materials design, it can be achieved that biotechnological and environmental applications can reduce significantly energy consumption and environmental pollution. With a focus on antibacterial modifications and ceramic‐based adsorbents for bioproduct recovery/purification, this book arches, in an excellent manner, fundamentals to applications, giving the reader a complete picture.
I personally believe that by arching the materials gap to the bioworld, this book helps materials scientists to contribute not only to a better tomorrow but more importantly to make sure that there will be a tomorrow.
With these words, I wish you many new insights and much pleasure in reading this book.
January 2022
Bremen
Prof. Dr.‐Ing. Kurosch Rezwan
University of Bremen, Germany
1Introduction to Ceramic Materials
Laura Treccani
CSMT Gestione S.c.a.r.l., Polo Tecnologico – Innovative Contamination HUB, Via Branze 45, 25123 Brescia, Italy
1.1 Introduction: Ceramics for Biotechnological and Environmental Applications
Ceramic materials have been a part of human life for close to 30 000 years, and because of their superior properties, functionalities, and aesthetic characteristics, have become indispensable for a wide range of technical applications, consumer products, and everyday items [1].
Ceramics were the first man‐made materials [2], and they played a very important role until about 1500 C.E., when metal technology took over (Figure 1.1). A second ceramic age started in the mid‐twentieth century that witnessed the development and use of “new advanced ceramics,” overcoming age‐old limitations, such as poor mechanical strength and brittleness [1, 4]. A fast technical evolution supported by innovative processing methods enabled the development of multifunctional and smart materials with outstanding characteristics and opened new unimaginable possibilities. This has gone along with new approaches and prospective in material design and, generally speaking, there has been a shift from “homogeneity to controlled inhomogeneity.” As the environments where materials are used become more complex, demanding, and severe, there are frequent cases in which the conventional homogeneous ceramics are inadequate and defective. “Inhomogeneous materials,” or more precisely functionally graded materials, are characterized by smooth varying structure or composition. This means that their properties change continuously, and different characteristics can be achieved on separate surfaces, layers or parts. A gradation of properties across the volume, performing a set of specific and diverse functions within the given material, overcomes the inherent limit of homogeneous ceramics and leads to highly customizable multifunctional materials (see, e.g. Chapter 16 “Functionally Graded Materials” in [5]).
Now, the importance of ceramics is stronger than ever, and global trends show that different ceramics with multiple functionalities, along with sustainable, long‐term innovative technologies, are tackling some of the major global challenges such as healthcare, environmental pollution, and energy shortages among others. For the sake of completeness, it must be stated that the production volume of advanced ceramics falls strongly behind that of the classic ceramics (bricks, tiles, cement/concrete, etc.), but owing to their high value‐added nature, their sales volume approaches those of the classic ceramics [1].
Figure 1.1 Historical timeline of development of materials.
Source: Adapted from Ashby et al. [3].
In comparison to other materials, ceramics feature unique structural and functional properties and are well suited to being combined with other materials. This makes them the ideal choice for long‐life components for thermally, mechanically, and chemically demanding applications, such as chemical and pharmaceutical processes, biomedicine, environmental monitoring, remediation, and food processing.
Examples of the application of ceramics to relevant technical fields are here briefly introduced. Ceramic membranes were developed more than 30 years ago and are becoming an important technology in the emerging areas of separation, purification, and water management for the upstream oil and gas industry, as well as in refineries [6–13]. Ceramic membranes outdo their polymeric counterparts because of their capacity to withstand harsh operating conditions in terms of pH, temperature, pressure, and chemical stability. These peculiarities make ceramic membranes usable several times without any loss in performance. Therefore, they can be considered potential candidates for targeting worldwide goals aimed at improving environmental remediation, alternative energy, and raw material‐saving. Ceramic membranes are easy to clean and sterilize, and they can contribute to the production of high‐quality foods as well as chemical and pharmaceutical compounds, thus allowing this industrial sector to satisfy more demanding quality standards. This makes them suitable for filtration of biological fluids, foodstuff and water purification, recovery and recycling of organic solvents, dye separation, filtration of acids and caustics, and so on [8, 14]. Ceramic membranes represent an inexpensive option for building microbial fuel cells (MFCs), a highly promising technology to produce electricity, and these can be used to treat wastewater [15]. Ceramic membranes are proposed as suitable candidates for the purification and recycling of chemicals like boric acid from radioactive waste water, thus reducing the emission of radioactive wastewater as well as enabling the reuse of high‐value 10B‐rich nuclear‐grade boric acid from nuclear power plants [16]. Ceramic filters can be effectively employed to reduce gaseous and volatile organic compound (VOC) emissions from industrial complexes and foster the Clean Air Act, implemented and enforced by the Environmental Protection Agency (EPA), to safeguard public from exposure to harmful toxic gases [17]. Different forms of ceramics are widely used for air quality, environmental protection and fight climate change [18], by e.g. capturing or decomposing particulate matter (PM), nitrogen oxides (NOX), and other VOC, which are mainly responsible for heavy pollution. A striking example is titania, which is largely used in photocatalytic, self‐cleaning, and “antipollution” systems [19]. Continuous growth of the global ceramic filter market is expected because of more stringent environmental regulations in developed and developing countries with regards to water management (both drinking and disposal water) and air quality. However, some issues regarding performance and manufacturing costs still need to be overcome to meet the economic feasibility.
Another area where ceramic materials are widely used is in sensing and monitoring. Ceramics are considered a high‐potential alternative to silicon for the fabrication of thick films for microelectromechanical systems (MEMS) and microdevices [20, 21]. Ceramic sensors feature mechanical strength and resistance to chemicals, and they are thermally and physically stable in hostile liquid or gaseous environments. MEMS devices are used in different sectors, such as automotive, aerospace, medical, industrial process control, instrumentation, and telecommunications for products such as airbag crash sensors, pressure sensors, biosensors and ink jet printer heads [20], humidity sensors [22], and high‐temperature sensors [23]. For example, negative temperature coefficient of resistance (NTCR) ceramic sensors occupy a respected market position because they afford the best sensitivity and accuracy at the lowest price [24].
These examples illustrate, indeed, only some of the application potential of ceramics for biotechnological and environmental purposes. Due to the rapidly changing technological, environmental, social, and industrial trends and requirements, a constant increase in the demand for customized, unconventional materials exists. In response to the changing needs, several approaches have been pursued to create or add and combine new functionalities in materials. A particularly important approach is surface functionalization, which can be roughly defined as the modification of the surface by physical, chemical, or biological methods, or a combination of them. New characteristics can be given to the surface, which can largely differ from the original one. Surface functionalization is extremely versatile and can be applied to particles, thick and thin films, or bulky parts with varying compositions.
The choice of a particular material and a functionalization strategy is a rather delicate issue. Not only the properties of the single components must be considered but also their interaction, the whole manufacturing process, and the final application setting (e.g. chemical and environmental conditions). Functionalized oxide and nonoxide ceramics have been developed and contributed to revolutionizing many technical sectors as well as everyday life. For example, by wet chemistry approaches, also known as chimie douce, organic and inorganic components can be brought together and offer new opportunities for the scientist's imagination and the synthesis of innovative, multifunctional hybrid, or bioinspired materials. Surface functionalization can be achieved by other methods, for example, based on physical approaches, and these are not less important. Despite this, the scientific and industrial communities are still actively working to develop innovative and better performing materials and developing strategies for the design of new materials, characterization, and understanding of their structure and properties. Surface functionalization will be thoroughly treated in Chapter 3.
In this chapter, a general definition of ceramics is given, and their main features will be described. In addition, the main application fields of ceramic materials together with some examples, with a main focus on biotechnological and environmental purposes, will be introduced. Some of the most relevant advanced ceramic materials for these applications, which are later described in the following chapters, will be highlighted.
1.2 What are Ceramic Materials?
In material science, materials are commonly divided into three primary classes: ceramics, metals, and polymers. This classical designation is based on the types of atoms involved and the bonding between them. The other two widely recognized classes are semiconductors and composites. Composites are combinations of more than one primary material class and often consist of ceramics. Semiconductors are materials with intriguing electrical properties, and most of them are classified as ceramics.
The concept of ceramics derives from the Greek term, κɛραμoσ, and it is historically related to terracotta and pottery [25]. Ceramics have so profoundly evolved over time, and today's advanced ceramics bear little resemblance to their origins, which makes it difficult to provide a unique and straightforward definition. Perhaps, the most widely accepted definition of ceramics is that “ceramics are nonmetallic inorganic solids” [26]. However, this definition is limiting it provides little information about chemical composition, the nature of bonding forces, or structure and it does not reveal other important properties [27]. For each application, e.g. semiconductor packages, fuel cells, gas sensors, laser host materials, piezoelectric ceramics, electronic devices, biomaterials, filters and adsorbents, sensors, machining tools, and rocketry parts, ceramic materials have very diverse chemical and physical characteristics, which are unique and specifically designed for a particular function [5, 28, 29].
1.2.1. Advanced and Traditional Ceramics
For the sake of completeness, some standard definitions of set terminology are here briefly introduced. Ceramics can be divided into two major categories: traditional and advanced, which have very diverse applicative sectors as schematically shown in Figure 1.2.
Traditional ceramics can be considered the oldest ceramics, having developed since the earliest civilizations. These are based almost exclusively on naturally occurring raw materials, most commonly silicaceous minerals such as clay, mica, quartz, and feldspars. The term traditional could be misleading, and it must not be associated with low technology. Today, advanced manufacturing techniques, complex tooling, and computer‐assisted processes are often used for the manufacturing of traditional ceramics. Evolution is pushed by several new requirements, such as customized technical properties, known and defined chemical and physical characteristics, and low contaminant amounts. Nowadays, traditional ceramics are largely employed for high‐volume items such as construction products (bricks and tiles, clay‐based refractories, cements and concretes, and glasses) and consumer products (whitewares and pottery). Traditional ceramics share the largest proportion of ceramic sales worldwide, and the market is clearly dominated by glass, for example, glass window production [25, 28].
Figure 1.2 Traditional and advanced ceramics: classification and applications.
Source: Adapted from Tanzi et al. [30].
Advanced ceramics have been developed more recently, approximately within the last 100 years, and are also referred to as “special,” “technical,” "fine," or “engineered” ceramics. The terminology for defining this type of ceramic differs from continent to continent. For example, “advanced” or “technical” ceramics are used in American literature, and “fine” ceramics are used in Japanese literature. The term “technical” ceramics is more frequent in the European context. A further classification, common in the UK, is related to use. Functional ceramics refer to electronic applications, whereas structural ceramics typically refer to mechanically loaded components. According to ISO 20507:2014, these can be described as “materials produced with precisely controlled chemical compositions, microstructures, configurations, and production processes to fulfill intended functions and which are composed mainly of nonmetallic, inorganic substances.” In comparison to traditional ceramics, these exhibit superior and very diverse properties and are included in electrical, magnetic, electronic, and optical applications (sometimes referred to as functional ceramics) and ceramics for structural applications at ambient as well as at elevated temperatures (structural ceramics). Advanced ceramics typically consist of metals and nonmetals and form different classes, such as oxides, nitrides, carbides, and silicides. Other materials can be classified as ceramics. For example, glasses are a special type of ceramic. Glass‐ceramics, which can be produced through the controlled crystallization of base glass (known as “ceramming”), share many properties of both glasses and ceramics. Forms of carbon, like diamond and graphite, and all inorganic semiconductors can be considered ceramics [25]. Diamond is an example of elementary ceramic, and it is often used as a reinforcing component. More recently, dispersed primary diamond particles (e.g. detonation nanodiamonds) have been considered for several attractive applications like nanospacer lubrication in water, fluorescent bioprobes, drug‐delivery systems, and catalyst carriers [31]. Further information about carbonaceous ceramics will be given inSection 1.5, and its potential for antibacterial applications will be discussed in Chapter 9.
1.2.2. Properties of Advanced Ceramics
Advanced ceramics are known for their outstanding properties, such as high melting points, electric insulation, and transparency in the visible range, which are imparted by the ionic–covalent bonds, poor thermal and electrical conductivity due to the tied up valence electrons in bonds, which are not free like in metals; brittleness because of the ionic–covalent bonds; high degree of hardness; and high moduli of elasticity [5, 25, 28, 29].
However, this lists the most common properties of ceramics, and it gives only a relatively simple and limited categorization. Ceramic materials are more complex and do not have uniform characteristics or behaviors, although there are several exceptions. A striking feature of ceramic materials is their possibility to form different types of chemical bonds, including ionic, covalent, and metallic (Figure 1.3).
For example, some ceramics are semiconducting, like barium titanate (BaTiO3), or superconductive like cuprates (YBa2Cu3O7), titanates (SrTiO3), or BaPbO3 [5, 25]. Diamond is the best thermal conductor at ambient temperature, and certain ceramics (AlN, BeO, or SiC) perform better than metals like copper. Most ceramics are brittle at room temperature but not necessarily at elevated temperatures. For example, at high temperatures, glass behaves as a viscous liquid and no longer in a brittle manner [25]. The chemical inertness of ceramics is usually taken for granted; however, it can severely differ when ceramics are used at high temperatures, as in the chemical and metallurgical industries [28]. Oxides are stable in oxidizing atmospheres, but nonoxidic ceramics can oxidize even at relatively low temperatures. Silicon compounds (e.g. silicon carbide and silicon nitride) can self‐protect from oxidation at high temperatures thanks to the formation of a silica layer [32]. Graphite and carbon compounds are ultrarefractory, but they can only be used in a protective environment. Magnesium oxide and aluminum carbide are unstable in the presence of moisture.
Figure 1.3 Types of chemical bonding of different ceramic materials.
Source: Adapted from Heimann [1].
Figure 1.4 Overview of the most relevant advanced ceramics: properties and applications.
In Figure 1.4, an overview of the most relevant advanced ceramics, properties and applications is given. Usually, for a given application, one property may be of particular importance, but in fact, all relevant properties need to be considered [28].
From this point onwards, some general properties of the most relevant ceramic materials for biotechnological and environmental applications and materials considered in other chapters are briefly described. For more exhaustive information that goes beyond the scope of this book, some selected references will be highlighted.
1.3 Oxide Ceramics
All oxides that can be formed from the periodic table elements belong to ceramic materials. Oxide materials provide a plethora of possible applications and offer ample opportunity for different applications, including biotechnology, biomedicine, and environmental purposes (e.g. remediation, monitoring, and energy harvesting).
1.3.1 Alumina
Aluminum oxide, generally referred to as alumina (Al2O3), is one of the most important technical ceramic materials, both pure or as a component, and perhaps the most mature high‐technology ceramic in terms of quantity produced and variety of industrial uses [29].
1.3.1.1 Alumina Structure and Properties
Alumina features an excellent combination of intrinsic properties and versatility: high melting point (>2000 °C), hardness, strength, abrasion resistance, high electrical resistance, excellent optical properties, chemical stability, and inertness.
Alumina is the only oxide formed by the metal aluminum and occurs in nature as the mineral corundum (Al2O3), with several metastable forms, and numerous hydrated phases. There are three trihydroxide forms of aluminum: bayerite [α‐Al(OH)], nordstrandite [β‐Al(OH)3], and gibbsite or hydrargillite [γ‐Al(OH)]. Bauxite is an impure form of gibbsite containing 40–60% of equivalent alumina in the form of hydrargillite, boehmite, diaspore, and silicoaluminous minerals; 10–20% iron oxide; and approximately 5% quartz sand and other various impurities. The two oxyhydroxides are: diaspore (α‐AlOOH) and boehmite (γ‐AlOOH) [33, 34].
At all temperatures and up to at least 78 GPa pressure, corundum or α‐Al2O3 is the most stable phase of alumina. The considerably high enthalpy of formation (∼1600 kJ/mol1) makes alumina one of the most tightly bonded compounds, resulting in very high hardness (9 on the Mohs hardness scale) and high melting and boiling temperatures (2050 and 3500 °C, respectively). The density of corundum is close to 4 g/cm3, and in single‐crystal form, corundum is a bright white solid [26]. Trace amounts of impurities, such as iron, titanium, chromium, copper, or magnesium, can give corundum blue, yellow, pink, purple, orange, or greenish colors [25]. Corundum has a crystal structure consisting of oxygen ions in a slightly distorted close‐packed hexagonal (rhombohedral) lattice (space group R3c). The aluminum ions occupy two‐thirds of the octahedral sites in the oxygen lattice. Alumina has many metastable polymorphs, which are generally divided into two structural categories depending on the O anion arrangement. Metastable aluminas, which are all less dense than corundum, have O packings that are near to close‐packed cubic. Eta or gamma aluminas are typically formed at low temperature, and as temperatures rise, alumina transforms in the sequence γ → δ → θ → α alumina (Figure 1.5) [33, 36]. α‐Al2O3 is usually prepared as medium–low surface area powder by thermal decomposition of diaspore (α‐AlOOH) at temperatures above 450 °C or by calcination of gibbsite (Al(OH)3) at ∼1000 °C [36]. Many other variants are also possible. For example, γ‐Al2O3 can form at higher temperatures and transform directly to α. The kinetics of transformation and the sequence of phases are influenced by several factors, like particle size, heating rate, impurities, and atmosphere. At temperatures above 1200 °C, only the α phase is typically present. At relatively low temperatures (nearly 300 °C), alumina hydrates decompose to Al2O3 and water [34].
Alumina in the α and γ forms is a common choice as a catalyst carrier, and corundum is highly important in mineral processing and adsorption of heavy metal elements [33].
Gamma‐Al2O3 becomes especially advantageous when it is relatively stable against γ → α conversion, and among all polymorphs, it is the most important for applications, such as catalysts, adsorbents, or coatings [33, 34]. Purities and grain sizes can vary considerably from one product to another. Typically, fine powders, with a grain size of about one micrometer and “reactive” powders, like low‐temperature‐calcined alumina, are necessary to produce sintered parts [34, 37]. An increase in the calcination temperature increases the particle and crystal sizes, which disadvantages the sintering process. For example, reductions in the average particle size, tighter control of its distribution, and improved purity characteristics have been highly beneficial in obtaining finer microstructures in alumina implants and thus better performance [38]. Refractory materials require coarse grains (several hundred micrometers) to limit creep and reduce reactions with the environment, and therefore prefer tabular alumina. It must be noted that alumina reactivity is crucial for many applications, requiring highly densified ceramics with a well‐controlled microstructure obtained at sintering temperatures as low as possible [37].
Figure 1.5 Thermal transformation sequence of hydrated alumina.
Source: Adapted from Wefers and Misra [35].
The structures of all forms of alumina (stable, unstable, and hydrated) are listed in Table 1.1.
The Al2O3 surface is positively charged at pH 7 because of the alkaline isoelectric point (IEP). The IEP, or the pH where the ξ‐potential is zero, of alumina is 8–9 [28, 39, 40]. Alumina solubility in water is finite and depends on the type of solution species, solution pH, temperature, other ionic species, and the solid phase. Generally, alumina is stable in an aqueous environment between pH values of 3 and 12, where Al3+ and AlO2− ion release concentrations sum up to a value smaller than 10−2 mol l−1. Alumina has a low solubility in solutions with pH ranging from 4 to 9; at 25 °C, it is less than 10−7 mol−1 at pH 6, but it dissolves readily in strong acids (HCl, HNO3, H2SO4) and strong bases (NaOH, KOH) at temperatures well above ambient (e.g. >90 °C). Solubility, generally, increases with temperature, as it is common for most materials. Alpha‐Al2O3 is much less reactive than γ‐Al2O3, but it dissolves in heated alkaline media or eutectics with low melting points. Highly reactive metals (calcium or magnesium) can reduce alumina beyond ∼900 °C. Transition aluminas (γ‐Al2O3), with a metastable structure and very high specific surface area, are soluble in aqueous, acid, or basic solutions but not in neutral aqueous solutions, thus being regarded as an amphoteric oxide [33, 41]. The catalytic activity of transitional aluminas (γ ‐, η‐, δ‐, θ‐Al2O3) is undoubtedly mostly related to the Lewis acidity of a small number of low‐coordination surface aluminum ions, as well as to the high ionicity of the surface Al—O bond. Characterization studies determined that the transition aluminas also present significant surface basicity. In fact, CO2 adsorption sites on γ‐Al2O3 are quite strong, although the density of sites for strong adsorption (ΔH ∼ 50–180 kJ/mol) is small (0.06 site/nm2) [36].
Table 1.1 Crystal structure and physicochemical properties of aluminas.
Lattice parameters (Angstrom/angle)
Designation
Structure
A
b
c
Density (g/cm
3
)
Corundum (alpha)
Hexagonal (rhombohedral)
4.758
12.991
3.96
Eta
Cubic (spinel)
7.90
Gamma
Tetragonal
7.95
7.79
3.2
Delta
Tetragonal
7.97
23.47
3.2
Theta
Monoclinic
5.63
2.95
11.86/103° 42′
3.56
Kappa
Orthorhombic
8.49
12.73
13.39
3.3
α‐Al(OH)
3
gibbsite
Monoclinic
8.64
5.07
9.72/85°26′
2.42
AlOOH diaspora
Orthorhombic
4.40
9.43
2.84
3.44
α‐AlOOH bohemite
Orthorhombic
2.87
12.23
3.70
3.01
β‐Al(OH)
3
Bayerite
Monoclinic
4.72
8.64
5.06/90°7′
2.53
Adapted from Ganguli and Chatterjee [34].
1.3.1.2 Applications of Alumina: Some Examples
Alumina can be considered the foremost basic compound for “advanced ceramics” because it exhibits exceptional versatility. Applications include: high‐temperature systems; catalyst substrates; filters; biomedical implants; optical systems; abrasive materials; bearings; cutting tools; electrical insulators; refractory materials; jewelry; filler for plastics and toothpaste; desiccant; and a wide variety of other industrial products [25, 33, 42–45]. Pressed powders are employed in electrical insulators, windows or radomes transparent to microwaves, envelopes for lamps, and electrical devices [5, 28]. As a single crystal, it is used for optical purposes, electronic applications (e.g. as a semiconductor), in watch bearings, and pressure‐resistant windows. Polycrystalline alumina is the basis of refractory bricks, crucibles, sparkplug insulators, and biomedical implants [25, 26]. Alumina is the first choice for the ball joint in a hip prosthesis, owing to its hardness and characteristic low friction [25, 46]. Porous alumina membranes are used for medical treatments [47], wastewater treatment, energy and environmental remediation [48], and other applications [49]. Additionally, alumina is used as a component in ceramics and glasses, mullite components, electrical insulators, porcelain, and durable glasses.
Pure alumina has been used for nearly 20 years in dental and orthopedic applications, and in addition to its advantageous chemical and mechanical properties, it exhibits excellent biological tolerance (allergy‐free and biocompatible), an absence of wear, and the rarity of osteolysis. Alumina is mostly used as the bearing surface in total hip joint endoprostheses [50, 51]. The usage of alumina for dental applications is limited to abutment materials because of its lower fracture toughness and bending strength compared with zirconia [40]. To overcome the limitation, a new material consisting of alumina doped with polycrystalline metastable tetragonal yttria–partially stabilized zirconia has been developed, and it is currently being exploited for femoral heads for the hip endoprosthesis. Such materials have outstanding mechanical properties, like high fracture strength greater than 700 MPa and fracture toughness up to 12 MPa m½. In this material, cracks introduced by external loads tend to move toward the zirconia particle aggregates; the crack energy will force the transformation toughening of tetragonal zirconia into monoclinic zirconia [52]. This mechanism will be described later in the section dedicated to zirconia.
Alumina finds large usage as a stationary phase in chromatography. The activity of alumina is affected by two main factors, namely the chemical nature of the surface, which involves the number of hydroxyl groups and acid sites on the surface, as well as the texture of the solid, and the availability of various reactants to reach an active site, which depends on the pore sizes and their distribution. Particular advantages are high selectivity, thermal stability, and resistance to different external factors [53]. Columns packed with alumina combined with anion‐exchange columns can be used for the determination of sulfate in brines and biological fluids and for the trace determination of iodide down to the low ppb range in mineral waters and fruit juice samples. Alumina proved to be a highly selective stationary phase for the preconcentration of sulphate from complex matrices. Thanks to its selectivity, it is well suited for online column‐coupling techniques [54].
Other well‐known applications of alumina are in drug‐delivery systems [55, 56] and medical biosensing devices [57]. Alumina membranes features porous structures, large surface area, tunable pore size and pore size distribution, low fabrication costs, and ease of processing [40]. Alumina membranes are extensively used for protein separation and purification because of their moderate hydrophilic character (contact angle of 40–50°) and the lowest cost among other oxide materials. One of the main concerns about the applications of alumina membranes and other ceramics is membrane fouling, which occurs because of the deposition and adsorption of proteins or other biological species on the surface and pore walls. Extensive work is aimed at minimizing membrane fouling using several approaches, like enhancing the hydrophilic character of the membranes [58], creating a coating of SiO2 by sol–gel [59], or well‐organized mesoporous TiO2 layers on poly(vinyl pyrrolidone) precoated porous support [60], as well as linking bactericidal silver nanoparticles encapsulated in positively charged polyethyleneimine [61]. Surface functionalization with organic modifiers has been widely applied to enhance long‐term stability and operability, as well as to control biomolecule adsorption and release [40].
Sensor devices based on Al2O3 are excellent sensing materials for humidity. These devices operate at near room temperature and show good stability, as no surface chemical reactions take place when exposed; they have a relatively long service time and can be regenerated. Moreover, in comparison to other humidity‐sensing materials, devices based on Al2O3 show a very rapid response time of the order of one minute. Al2O3‐based sensors are volume effect devices, whereas the porous Al2O3 film acts as a dielectric with pores whose conductivity varies as a function of moisture content trapped in micropores [62]. At relative higher temperatures (about 450 °C), aluminum oxide‐based sensors can detect different gases like H2, O2, and CO2 [62]. Theoretical calculations of gas adsorptions on α‐Al2O3 (0001) surface indicate that the adsorption of a monolayer of —OH on top of the surface aluminum ions and hydrogen atoms bonded to the surface oxygen ions generates a new bond that is shifted a few eV above the top of the valence band in the bulk bandgap [62]. Nanoporous alumina films can be used as moisture sensors for the detection and discrimination of several cyclic VOC like cyclohexanes, cyclohexene, benzene, toluene, and xylene isomers [63].
1.3.2 Titania
Titanium dioxide (TiO2), also known as titanium (IV) oxide, finds use as a pigment in paints (called Titanium white or Pigment White 6), coatings, cosmetics, foodstuff, dye‐sensitized solar cells, self‐cleaning windows, whose production is affected by gross domestic product worldwide [64–67]. The first industrial production of TiO2 dates back to 1918. Now, several megatons of TiO2 are produced per year by precipitation from oversaturated solutions according to the sulfate process, by hydrolysis or pyrolysis of titanium tetrachloride, or by several other, highly sophisticated methods [67]. In the recent decades, particular attention has been given to nanoscale titania because of its photocatalytic properties, which have found a great number of applications in many areas such as material science, physics, biology, chemistry, and medicine. Nanoscale titania is now available in different forms, including spheroidal crystallites and particles, along with tubes, sheets, and fibers. When compared with other nanomaterials, it is abundant and potentially of low cost [64].
1.3.2.1 Titania Structure and Properties
Titania exists in three different forms: rutile, anatase, and brookite. Rutile is the thermodynamically stable form within a wide range of temperatures and oxygen partial pressures. Anatase and brookite are metastable and readily transformed to rutile when heated [64]. TiO2 also exhibits high‐pressure forms, including the monoclinic titanium dioxide (B) or TiO2(B) [64, 68] and the orthorhombic α‐PbO2‐like form [69].
Each of the TiO2 structures consists of Ti cations in the center of the oxygen octahedra. Rutile, anatase, and brookite show a decreasing symmetry (Figure 1.6).
Rutile has tetragonal symmetry, and the structure is constructed by linking octahedra. An octahedron is placed at each of the eight corners such that two actually share an apex. The six points of these octahedra are then connected by one rotated octahedron at the center of the unit cell. The edges of the octahedra are linked together to form chains along the z‐axis. Each Ti4+ ion is surrounded by six O2− anions, and each O2− anion is surrounded by three Ti4+ ions. The structure is primitive tetragonal, with a = 0.459 nm, c = 0.296 nm, and two formula units per unit cell (Table 1.2). The rutile structure has been described as a distorted hexagonal close‐packed oxide array, alternate rows of octahedral sites are full and empty, and it is regarded as an ionic structure [25, 62, 71, 72]. Rutile is the simplest of the titanates with high dielectric constants (κ) of ∼100 and is more thermally stable than anatase [25].
Figure 1.6 Structures of main titania polymorphs (unit cells): rutile, anatase, and brookite.
Source: Adapted from Esch et al. [70].
Table 1.2 Structural information and properties of most common titania polymorphs.
Lattice parameters (Angstrom)
Designation
Structure
a
b
c
Density (g/cm
3
)
Rutile
Tetragonal
4.584
2.953
4.240
Anatase
Tetragonal
3.758
9.514
3.830
Brookite
Orthorhombic
5.456
9.182
5.143
4.170
