Carbon-based hybrid materials: growth, characterization and investigation of properties
Arshad, Muhammad
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1
Chapter 1
Introduction
This chapter highlights the phenomenology of carbon nanostructures both as an isolated material as well as in hybrid form. The carbon nanotubes, carbon nanofibers, graphene and their hybrid nanostructures offer a wide range of applications. However, challenges remain in the controlled application specific synthesis as well as in the integration of versatile carbon structures into hybrids, and in how their properties are influenced when they become building blocks of a composite material. This dissertation is aimed at shedding light on these prospects and challenges and here we introduce the rationale and motivation behind this work. A brief outline of thesis is also given at the end of this chapter.
2 1.1
Carbon nanostructures
Carbon present in nature has various crystalline and amorphous phases. Since prehistoric times carbon in its miscellaneous forms has been employed in art and technology, for example the cave paintings at Lascaux, and Altamira were produced with a mixture of charcoal and soot. [1]
In the periodic Table, carbon has smallest atomic number of the column IV elements and forms covalent bonds with its neighbouring elements. A free carbon atom in ground state has six electrons with an atomic orbital configuration 1s2, 2s2, 2p2. To form covalent bonds, one of the 2s electrons
can be excited to the 2p level and as a consequence, the overlap of 2s and 2p atomic orbitals can result in hybridization. The spn hybridization with
n=1, 2, 3 results in equivalent bonds that stem from the 2s electron and one, two, or three 2p electrons. Carbon is very special because it shows sp, sp2 and sp3 hybridization in different molecules and solids.
In its allotropes, carbon exhibits different chemical structures like diamond, graphite, lonsdaleite, carbon nanotubes (CNTs), carbon nanofibers (CNFs), graphene, whiskers, and fullerene. Diamond, lonsdaleite and graphite are the only three naturally occurring crystalline forms of carbon. Diamond and lonsdaleite have an sp3 three-dimensional structure, the first cubic, the
second hexagonal; whereas graphite is made of sp2 bonded planes, held
together by Van der Waals bonds. Diamond is the hardest crystalline structure but compression of glassy carbon transforms the latter into an even harder amorphous solid. [2]
In 1985, H. Kroto et al. synthesized molecular carbon C60 and after that
intense research attention was paid towards synthesizing carbon structures made of graphitic sheets.[3] A tubular form of carbon was discovered by S.
Iijima[4], employing the arc discharge method and using iron and cobalt as
catalysts. The obtained structures were subsequently named multiwalled carbon nanotubes (MWCNTs) and single-walled nanotubes (SWNTs), depending on the number of carbon layers present in the structures. The structural characterization of MWCNTs and SWCNTs by transmission
3
electron microscopy (TEM) added new excitement in the field of carbon science. [4-7]
The structure of SWCNTs is described by its one-dimensional unit cell, defined by chiral vector Ch and translational vector T as shown in figure 1.1
a. The chiral vector Ch can be described by unit vectors a1 and a2 of the
hexagonal honeycomb lattice of single graphite sheet, which can be given as Ch=na1+ma2 Where n and m are the integers which specifies the chiral
vector Ch. [8]
Three distinguishable SWCNT structures can be generated by rolling up single graphite sheets into cylinders: armchair nanotubes occur when n=m, that is Ch = (n,n); zigzag nanotubes correspond to the case of m=0, or Ch =
(n,0); all other Ch = (n,n) chiral vectors describe chiral nanotubes, with
n m
0≤ ≤ due to the hexagonal symmetry of honeycomb lattice.[9] The
diameter dt of a carbon nanotube is given as π
nm + m2 + n2 a = π ch = dt , where the lattice constant a=2.49 Å, ch is the length of chiral vector and
chiral angle is described as
n + 2m n 3 =
θ tan 1- . The translational vector T
is parallel to nanotube axis and perpendicular to chiral vector Ch, can be
given in terms of basis vector a1 and a2
T = q1a1 + q2a2, with dR n + m 2 = q1 , dR m + n 2 = q2
where dR is the greatest common devisor of (2m+n) and (2n+m), which
can be related to the greatest common devisor d for n and m as follows dR = d if n-m is not multiple of 3d and dR = 3d when n-m is multiple of 3d.
The length of the translational vector T is given by T = 3L/dRwhere L is
the circumferential length of the nanotube. The number of hexagon per unit cell N can be accomplished as
4
Figure 1.1: (a) By joining the O-A and B-Bˊ nanotube can be formed. The chiral vector Ch and
translational vector T are defined by OA and OB, respectively. R is the symmetry vector. (b)
possible vectors chosen by defining a pair of integers (n,m) appropriately for general carbon nanotubes, including zigzag, armchair and chiral nanotubes.[10]
a2 a1 Ch × T × = N
5
The chirality and diameter define wether a nanotube will be metallic or semiconducting. [11] For instance (1,1), (2,2), (3,3) nanotubes are metallic,
while (4,2), (4,3) and (5,3) nanotubes are semiconducting.
Carbon fibers are graphite-like materials, which are very similar to MWCNTs in their structures and described as one-dimensional filamentous carbon exhibiting a high aspect ratio. Like graphite, carbon fibers are composed of atomic sheets of carbon atoms positioned in a regular hexagonal fashion. The way these sheets are wrapped creates different structures.
Graphene consists of only one sheet of carbon atoms arranged in a hexagonal honeycomb lattice structure. Its synthesis through micromechanical cleavage [12] was first reported in 2004 by the Nobel
laureates A. K. Geim and K. S. Novoselov. Since then graphene has received much attention due to its unique properties and wide range of applications. Graphene exhibits a high mobility of charge carriers as well as extraordinary thermal, optical and electrical properties. [13-15] Various ways to synthesize
graphene have been developed over the past decade, the most popular ones being chemical exfoliation of graphite [16], sublimation of Si at the
surface of SiC[17] and chemical vapour deposition.[18] Important for this
dissertation is also the oxidized form of graphene, graphene oxide, obtainable by exfoliation of graphite oxide. Graphite oxide was first reported in 1859[19], but only nearly a century later, in 1962, Boehm et al.
found that the chemical reduction of dispersions of graphite oxide yielded lamellar carbon and concluded “that the thinnest of the lamellae really consisted of single carbon layers”. [20]
1.2
Carbon based hybrid materials
The word “hybrid” in materials science was coined to refer to the combination of two or more materials at a low dimensional scale, resulting in superior properties due to synergetic effects. Mostly, one of these constituents is inorganic and other one organic. To combine the properties of organic-inorganic components is an age-old challenge. Based on the interaction and stability among joint constituents, hybrid composite
6
materials can be divided into category I and II materials. [21-23] In category I
hybrids the constituents are bound by van der Waals, weak electrostatic interactions or hydrogen bonding. In the case of category II hybrids, firm chemical interactions exist between the constituents, as for example for the building blocks of organic-inorganic hybrids that are covalently bonded to each other. In general, two types of schemes are adopted for the preparation of organic-inorganic hybrids. The first type covers cases where the hybrid nanosystems are formed in-situ via a single- or multi-step process, including aggregation, thermal, chemical or electrochemical reduction and interconnection of nanostructures. The second type comprises the cases where individual nanostructures are fabricated prior to synthesis of hybrid system.
Among all the hybrid systems, carbon-based ones are a unique category of materials with potential for diverse applications. Isolated carbon nanostructures such as CNFs, CNTs and graphene have also been realized into many devices. [24-27] Further, the unique electrical, mechanical, optical
and thermal properties of carbon nanostructures have been object to significant research focussing on combining them with inorganic materials. The leading step for CNT-inorganic hybrids was introduced by filling of CNTs with yttrium carbide (Y3C), titanium carbide (TiC), lead oxide (PbO), metallic
nickel (Ni) and ruthenium chloride (RuCl3).[28-31] Since then carbon-based
organic-inorganic hybrid nanostructures have been investigated for their application in photovoltaics, capacitors, batteries, flexible electronics, catalysis and sensors as well as for biomedical and electrochemical technologies.[32-43] In recent years, the focus has broadened to bring metal
oxides systems in the fold. In particular, considerable research efforts have been devoted to probe the underlying finer aspects of the metal oxide-graphitic interfaces, which in turn greatly affect the architectures, interaction and properties of the hybrid structure.[44-47] Furthermore,
fabrication procedures have been adopted with an aim to design application-oriented interfaces.[48-50] Despite the significant progress, the
control over size, morphology, locations, understanding of the interfacial processes and development of an applications specific interface of organic-inorganic hybrids are still challenging.
7 1.3
Motivation of the thesis
The structural versatility and properties of carbon nanostructures allows to include them in devices and hybrid functional materials that hold promise for emerging technological developments. The carbon nanostructures such as SWCNTs synthesized by chemical vapour deposition (CVD) comprise both semiconducting and metallic nanotubes. [51-53] The electron transport and
optical properties are strongly affected by helicity, diameter and organization of these nanotubes into devices. The aligned and unaligned bundled networks of SWCNTs have been realized into integrated circuits and functional electronic arrangements. [54-56] However, the optical response
and charge transfer mechanism of chiral nanotubes (unaligned, vertically and horizontally aligned tubes: bundled or isolated tubes) is still not fully explored. Therefore, a better understanding of the optical properties and intertube interactions responsible for charge carrier dynamics is necessary for increasing the efficiency of carbon nanostructure-based devices. Two chapters of this dissertation will be dedicated to this theme.
Carbon-based hybrid nanostructures have already presented remarkable potential for applications in various fields including nano-electronics, photovoltaics, catalysis, and nano-sensors. [57 58] However, in fabrication of
carbon-based hybrid nanostructures, typical issues persist in the control over morphology and interfacial behaviour, chemical composition, and the structure of different nanoscale building blocks, as well as the accurate induction of these blocks in the final assembly design. The size, shape, geometry and phases of nanostructure building blocks are also the prime parameters related to the functioning of the hybrid systems. For instance, theoretical studies have revealed that the use of metallic CNT interconnects may yield energy efficient and fast integrated circuits. [59] To realize such a
scenario, control over the precise positioning of interconnections and integration of CNFs/CNTs with network nanowires (NWs) to form the hybrid nanostructures is necessary. It is also important to determine the stability, compatibility and effectiveness of these interconnections. Therefore, it is highly desirable to develop a viable process to connect individual, suspended or aligned CNFs to form joined or bridged robust architectures.
8
As CNTs/CNFs connecting the nanowires are free of substrate surface or intramolecular interactions, they could be suitable for integration in materials to achieve enhanced mechanical, electromechanical and electrical properties. These types of building blocks are not easy to realize, particularly over a large area. One promising approach for the integration of CNTs/CNFs in real devices is the synthesis of self-assembled and controlled CNF/NW hybrid structures, which we shall describe in one chapter of this dissertation.
In a similar way, the graphene - transition metal oxide hybrid nanostructures such as those combining graphene with zinc oxide (ZnO) and titanium dioxide (TiO2), have received remarkable attention due to their
high efficiency in energy storage and conversion devices. [40 60 61]
Nevertheless, the chemistry and interface design of individual building blocks are also a critical issue. The hybrid boundary and the associated interfacial interactions are of particular interest because they substantially effect on the properties and functions of the hybrid systems. One chapter of this thesis will be dedicated to this subject in the case of graphene-based zinc oxide core shell hybrid nanostructures.
Recently, assemblies of perovskite solar cells (PSC) and metal oxide semiconductors (TiO2, ZnO) have been used as an electron transport layer
(ETL). [46 62-64] The ETL plays an important part in the device architecture to
assure the efficient transport of electrons from the light absorber (perovskite) to the electrode. TiO2 as charge transport layer has been widely
explored in PSC devices due to its non-toxicity, chemically stability, cost effectiveness and easy availability, and because it has a large band gap. [65]
However, the electron-hole pair recombination is very high at TiO2/perovskite interfaces and that is a serious issue [63 65-67] because it
reduces the energy conversion efficiency in solar cells. In mesostructured cells, a number of studies have been done to facilitate charge transport by employing modified TiO2. [68-73]
In this context graphene-TiO2 hybrid composites have received much
9
insoluble, current efforts have headed to solution-processable graphene oxides (GO) from exfoliation of graphite powders with strong oxidizing reagents. Furthermore, the availability of reactive carboxylic acid groups at the edge and epoxy/hydroxyl groups on the basal plane of GO sheets facilitate functionalization of graphene, allowing to tune the optoelectronic properties while preserving a good solubility in polar solvents. [74 75]
Moreover, GO can be produced and processed in solution at large scale with low cost, which is particularly attractive for industrial applications. GO materials have been used in almost every part of polymer solar cell devices, including as electrode and charge extraction layers. [76-79] The last
experimental chapter of this thesis will be devoted to GO-TiO2 hybrids. 1.4
Thesis outline
The research work presented in this dissertation is divided into chapters as follows:
Chapter 2 introduces the experimental spectroscopic and microscopic tools employed for the characterization of carbon-based hybrid nanostructures and SWCNT bundles.
Chapter 3 presents the growth of CNFs on vertically aligned indium arsenide nanowires (InAs NWs) by the CVD method. Scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), were used to characterize the hybrid structures discussed in the chapter. Chapter 4 is dedicated to the synthesis of vertically aligned and unaligned SWCNTs bundles using the CVD technique. Transient reflectivity experiments were carried out to study the photoinduced charge transfer mechanism in aligned and unaligned carbon nanotubes.
Chapter 5 reports on colour transient reflectivity measurements conducted on bundled aligned SWCNTs and the comparison of the findings with already published results on unaligned nanotubes. Results showed that it is the nanotubes’ bending, which induces the free charge carriers rather than structural defects or the arrangement of nanotubes in form of bundles.
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Chapter 6 presents the controlled synthesis and subsequent characterization of reduced graphene oxide-based zinc oxide core shell hybrid nanostructures. The interfacial properties of these hybrids were studied as a function of temperature by employing ambient pressure XPS. In Chapter 7 reports on the synthesis of GO-TiO2 nanocomposites to serve
as an electron transport layer in perovskite solar cells. The preparation of thin films on an ITO substrate, their phase and composition characterization and their electrical properties are discussed.
At the end, results are summarized and an outlook is given on future research perspectives in this field. A summary in English and Dutch, curriculum, list of publications and acknowledgements conclude the thesis.
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