Synthesis and investigation on cobalt layered hydroxide compounds and their related nanostructured materials

Chapter 5 Mechanistic Investigation of Self-Redox Decomposition of Cobalt-Hydroxide-Nitrate Compounds with Different Nitrate Anion Configurations in Interlayer Space 80 Chapter 6 Invest

I would sincerely like to thank Dr Xu Zhiping and Dr Gowry Sampanthar for many useful discussions and their assistance in carrying out research work in many aspects

For technical support, I am especially grateful to Dr Li Sheng and Mdm Sam Fam Hwee Koong for XPS, Mr Chia Phai Ann and Mr Mao Ning for TEM Many thanks go to Ms Lee Chai Keng, Mdm Khoh Leng Khim and Ms Tay Choon Yen for their support manytimes in running other instruments

Special thanks must go to my husband, my parents and Ms Zhang Yingsu and her daughter Mao Bangyuan for their unfailing support, encouragement and understanding during the last three years

(TGA-FTIR) 48

Chapter 4 A Comparative X-ray Photoelectron Spectroscopic

Investigation on Cobalt-Containing Layered Hydroxides 52

Chapter 5 Mechanistic Investigation of Self-Redox

Decomposition of Cobalt-Hydroxide-Nitrate Compounds with

Different Nitrate Anion Configurations in Interlayer Space 80

Chapter 6 Investigation on the Effect of Molecular Symmetry

on Decomposition Processes of PA/TA-Intercalated Layered

Chapter 7 Mechanistic Investigation on Salt-Mediated

Formation of Free-Standing Co3O4 Nanocubes at 95 °C from

Chapter 8 Preparation of Nanorods of Cobalt Hydroxide

Carbonate and the Derived Co3O4 One-Dimensional

8.2 Material Synthesis and Characterization 165

Summary

The research work carried out in this thesis is related to cobalt-containing layered hydroxide compounds and their derived nanostructured materials, due to the growing interest in new applications of such materials as catalysts, organic-inorganic hybrid nanocomposites and nanostructured materials The element cobalt was chosen

as it is an important transition metal having common oxidation states of +2 and +3, which gives interesting properties to the derived compounds, in the form of hydroxide and oxide The layered materials studied in this thesis include hydrotalcite-like compounds and hydroxide salts The basic structure of such compounds consists of metal hydroxide layers and anions intercalated or pillared into the layer structures

Chapter 2 gives a literature review about such materials regarding their physicochemical properties, synthesis methods and applications In addition, some introduction specifically on cobalt-containing layered hydroxide materials is provided

In this chapter, the nanostructured materials are also briefly introduced and the synthesis of cobalt-layered hydroxides and the related nanostructured materials is summarized Chapter 3 is an introduction to the experimental methods involved in this work It presents the methods for materials preparation and characterization techniques

Chapter 4 is concerned with surface chemical states of cobalt-containing layered hydroxides, as such information is rare in open literature despite that the bulk information

of these materials have been well documented The spectra obtained with XPS for various elements indicate the composition and bonding environment in the surface region The effects of metals, population and valency of interlayer anions are observed and discussed

In Chapters 5 and 6, investigation is carried out on decomposition processes of cobalt-layered compounds intercalated with anions Chapter 5 studies in detail about the mechanism of the self-redox decomposition of two nitrate-containing cobalt layered hydroxide compounds, with variable redox reagent contents and different configurations of nitrate anions in the interlayer space It has been elucidated that divalent cations in resulting oxides, rather than in hydroxyl octahedral, are the active reductant for the redox reactions Chapter 6 presents the effect of molecular symmetry

of the intercalated organic bi-carboxylate anions on the decomposition process In this

intercalated into CoAl- and MgAl-hydrotalcite-like compounds It has been found that within the same confined space and chemical environment provided by the hydroxide

charge distribution

described in Chapter 7 The formation of nanosized and faceted nanocubes is found to

diffusion boundary in the solution with high ionic strength In Chapter 8, the effort has

transformation from rod-like cobalt hydroxide carbonate precursors

Finally, Chapter 9 gives conclusions about the proceeding chapters and some suggestions for the future work

Nomenclature

c′ inter-brucite-like-sheet-distance

c 3c′ or 2c′ (unit cell parameter in c-direction)

dhkl distance between reflection planes (hkl)

ED electron diffraction

Figs figures

List of Figures

Figure 2.1

XPS spectra of C 1s for all the samples listed in Table 4.1

XPS spectra of O 1s for all the samples listed in Table 4.1

XPS spectra of Al 2p for all the aluminium-containing samples listed in Table 4.1

cobalt-containing samples listed in Table 4.1

A proposed surface structure for anion grafting (Peak 2, Table 4.6) on external surface of brucite-like layers (Peak 1, Table 4.6); the light-gray areas indicate the brucite-like or

hydrotalcite-like layered structures while the darkened areas represent for anion-grafted brucite-like layers or for cobaltous salts (nitrate and carbonate) in which anion oxygen is directly linked to cobalt

XRD patterns of two cobalt-hydroxide-nitrate compounds

CoII,III-0.2 and CoII-0.5 Reflections of polycrystalline silicon (internal standard) are indicated with asterisk symbol Insert indicates an alternate arrangement of oxidant and reductant in the two compounds and inter-brucite-like-sheet-distance assignment under two different crystal symmetries

FTIR spectra of as-prepared cobalt-hydroxide-nitrate

Nujol-mull techniques

Nitrate anion configurations in interlayer space for a hydrotalcite-like compound with 1/3 of total cobalt cations in trivalent oxidation state In model I, every oxygen atom has two hydroxyl groups respectively from two adjacent brucite-like layers (above and below, marked with 1, 2 and 3) In model II, oxygen atoms are not connected directly to hydroxyl groups (marked with 1, 2 and 3) from the two adjacent brucite-like layers in the normal HT stacking sequence, but they may

Co-be connected to hydroxyl groups directly when a turbostratic stacking occurs in the upper layer Arrows indicate the vibrational directions of atoms in asymmetric stretch mode

I), which leads to a modification of hydroxyl sublattice On the other hand, a nitrate anion could directly attach to the same

compounds heated in nitrogen atmosphere; gas flowrate 100

versus heating temperature in the combined TGA-FTIR

compounds The two samples were heated from room

compounds as well as their respective samples heated over the

compounds as well as their respective samples heated over the

XRD patterns of the as-prepared TA or PA containing hydrotalcite-like samples CoTA, MgTA, CoPA and MgPA

Two possible ways of molecular arrangement of PA in the interlayer space of hydrotalcite-like materials

FTIR spectra of the as-prepared TA or PA containing hydrotalcite-like samples CoTA, CoPA, CoTA(PA), MgTA, MgPA and MgTA(PA)

Decomposition of the as-prepared CoTA, MgTA, CoPA and MgPA samples in air: (a) TGA curves and (b) DrTGA curves

Decomposition of the as-prepared CoTA, MgTA, CoPA and

MgPA samples in nitrogen: (a) TGA curves and (b) DrTGA

measurements for samples CoTA, MgTA, CoPA and MgPA

FTIR spectra of calcined CoTA in nitrogen at 400, 450 and

TEM pictures of calcined samples in nitrogen for 2 h: (a)

TEM pictures of calcined samples in nitrogen for 2 h: (g)

XRD curves of calcined samples in nitrogen for 2 h

XRD pattern evolution for the A series samples after 1.5-36 h

XRD pattern evolution for the B series samples after 1.5-24 h

FTIR spectra for the A series samples after 1.5-36 h (i.e.,

FTIR spectra for the B series samples after 1.5-36 h (i.e.,

oC

DrTGA curves for the A series samples (samples A1.5 to A36)

DrTGA curves for the B series samples (samples B1.5 to B24)

Comparison of SEM images of samples formed after reactions

TEM confirmation of cubic morphology with different tilted

TEM images with different magnifications (a and b) or the

A reaction intermediate mixture of

indicated with arrows

XRD patterns of as-prepared cobalt hydroxide carbonate compounds, A1-A5, by heterogeneous precipitation method

XRD patterns of as-prepared cobalt hydroxide carbonate compounds, B1-B4, by homogeneous precipitation method using urea

FTIR spectra of some representative as-prepared samples A5, B2 and B4

TGA and DrTGA curves of samples B1-B4 (prepared by homogeneous precipitation method using urea), heating rate:

TEM pictures for samples A1-A5 (prepared by heterogeneous

pattern for sample A5 (insert)

TEM pictures for samples B1-B4 (prepared by homogeneous precipitation method using urea) and the free-standing rod for each sample (insert)

SAED pattern of a rod selected from sample B4 (prepared by

XRD patterns of calcined samples in static air, B1-300

TEM and HRTEM pictures of calcined samples at respective

3h; (b) SAED pattern of (a); (d) SAED pattern of (c)

List of Tables

Table 2.1

The ionic radii of some divalent and trivalent cations

HTlcs obtained with different cations

The values of c′ for some HTlcs

Various formulas to determine the maximum value of m

Organic anions incorporated to HTlcs using anion exchange method

Chemical formulas of the samples investigated with XPS method

Relative surface atomic compositions of C, O, Al and Co in the solid compounds

Binding energies (eV) of C 1s of different surface species and their relative percentage atomic ratios (indicated in

parenthesis)

Binding energies (eV) of O 1s of different chemical species and their relative contents (indicated in parenthesis) with respect to Co (as unity)

Binding energies (eV) of Al 2p of different components and their relative contents (indicated in parenthesis) to Co (as unity)

components

Results of compositional analysis and structural analysis for

Results of N 1s and Co 2p spectrum analyses (see Figures 5.9 and 5.10)

The initial solution composition and the estimated brucite-like-sheet distances and the average crystallite sizes

inter-The measured chemical composition and molecular formulas

Samples nomenclature and experimental conditions

TGA and elemental analysis results for as-prepared cobalt hydroxide carbonates samples

140

166

176

Publications Related to Thesis

Mater., 13, 297-303 2001

2 Xu, Z P., Xu, R and Zeng, H C Sulfate-functionalized carbon/metal-oxide

nanocomposites from hydrotalcite-like compounds, Nano Lett., 1, 703-706 2001

4 Xu, R and Zeng, H C Mechanistic investigation on self-redox decompositions

of cobalt-hydroxide-nitrate compounds with different nitrate anion configurations

in interlayer space, Chem Mater., in press

decomposition processes of PA/TA-intercalated clays, in preparation

Chapter 1 Scope of the Thesis

This thesis is concerning the cobalt layered hydroxides and their derived nanostructured materials, mainly cobalt spinel oxides Cobalt layered hydroxide is used as a broad name for several types of compounds in this work Besides the

other two types of cobalt layered hydroxides incorporated with anions, namely, hydrotalcite-like compounds and hydroxide salts or basic salts In general, layered hydroxide consists of two-dimensional hydroxide sheets stacking one on top of each other The sheet is formed by edge-sharing octahedra In each octahedra, metal cation

resulted in the hydroxyl sheet This positive charge is balanced by exchangeable interlayer anions Such type of compounds is named as hydrotalcite-like materials

positive charges are balanced by the interlayer anions which directly coordinate to the metal group Depending on the nature of cations and anions, they may or may not occupy the interlayer space and such compounds in general is called hydroxide salt In the current research, four types of work have been involved:

1) Synthesis of cobalt layered hydroxide materials mainly by coprecipitation method;

2) Characterization of the above materials by XRD/FTIR/TGA/CHN/ICP/XPS, etc.;

3) Detailed investigation on their thermal decomposition behavior by in situ FTIR experiment coupled with other characterization methods listed in 2);

TGA-4) Synthesis of cobalt spinel oxide nanomaterials from cobalt layered hydroxide precursor compounds under oxidative conditions

First of all, some effort was made in Chapter 2 to give an overall introduction

on layered hydroxide materials regarding their general physicochemical properties, synthesis methods and applications, as well as nanostructured materials and in particular nanomaterials of cobalt layered hydroxides and their related compounds, mainly cobalt spinel oxide

Chapter 3 intends to highlight the general experimental synthesis techniques and the principles of all the instrumental methods used in the present thesis work

The work carried out in Chapter 4 presents a detailed investigation on surface chemical states of Co-containing layered hydroxides by X-ray photoelectron spectroscopy (XPS) Over the past decades, extensive chemical and physical characterization work has been conducted for obtaining the bulk information of layered hydroxides, such as crystallographic structure, chemical bonding, thermal behavior and chemical composition However, the surface chemistry of such materials has not received much attention The samples investigated which were readily synthesized in

with carbonate and/or nitrate anions Other commercial available compounds,

complementary information XPS analysis in the surface region was conducted for C 1s, O 1s, Co 2p and Al 2p The chemical species and bonding information were deduced based on the binding energies from the deconvoluted peaks for all the elements The relative atomic compositions on the surface were estimated by quantitative treatment of the XPS spectra obtained The results demonstrated that XPS

can be used as a very sensitive technique for the elucidation of surface information of Co-containing layered hydroxides in great detail

After being familiarized with XPS technique and obtaining a sound understanding about the surface chemistry, the author carried out further research on thermal decomposition behavior of cobalt-containing layered hydroxides, which is presented in Chapters 5 and 6 The chemical reactions under thermal conditions involve both brucite-like sheets and the intercalated anions The former decompose into catalytically active metal oxides, which then accelerate the reactions of the latter Such reactions are worth studying as the reactants (the metal oxides and the interlayer anions) are stacked in alternate layers with molecular level mixing In other words, the surface contact between different species is maximized and the distance that the reactants need to diffuse is minimized Interesting results can be obtained especially when the reactions involve reactants of different redox reactivity and anions of different configurations and symmetries in the interlayer space

In Chapter 5, such experimental findings are presented by the studies on redox decompositions of two nitrate containing cobalt layered hydroxide compounds,

prepared with variable redox reagent contents and different configurations of nitrate anions in the interlayer space Based on the detailed investigation using TGA-FTIR and XPS methods, the decomposition temperatures, sequence of reactions and gas-evolving patterns of the two samples were investigated for a mechanistic understanding of the self-redox decompositions of cobalt-containing layered hydroxides The final decomposition products of both compounds are all in nanophase

Chapter 6 extends the thermal studies to organic anions-intercalated CoAl- and MgAl-layered hydroxides Such organic-inorganic composite materials have potential applications in newly emerging fields, such as organic-inorganic nanocomposites, biomolecular-inorganic nanohybrids and precursors for nanostructured materials To achieve a better design of the precursor compounds, the chemical reactivity between the intercalated organic anions and the metal cations needs to be addressed in detailed

In this chapter, terephthlate and 1,2-phenylenediacetic anions have been intercalated into CoAl-HTlcs and MgAl-HTlcs by the coprecipitation method Due to the difference in the molecular symmetries and coordination patterns of carbonate anions

on the benzene ring for these two anions, the physicochemical properties of the precursors, such as the anion orientations, the particle sizes and anion intercalation selectivity varied All these properties in turn influenced the chemical reactivities of metal groups and the organic anions, which were investigated by TGA-FTIR/FTIR/CHN analysis During the thermal reactions, the heating rate was varied with the aim to control the morphological properties of the decomposed products

The last part of the thesis work was focused on synthesis of cobalt spinel oxide,

ambient condition in aqueous system (Chapter 7) and thermal condition (Chapter 8)

field of interest, although extensive synthesis work has been carried out for such materials over the past decades

As presented in Chapter 7, cobalt layered hydroxides were first prepared with precipitation method by adding cobalt nitrate aqueous solution into sodium hydroxide

hydroxide solution During the continued aging in the mother liquor with air purging, the precursor cobalt layered hydroxides gradually transformed to cobalt spinel oxide The effect of adding an electrolyte is to reduce the thickness of the counter-ion

“atmosphere” surrounding the particles, which then affect the coagulation or

nanocubes As it was our objective to understand the formation mechanism of such particles, the intermediate compounds were also fully characterized to “capture” the transformation process by XRD/FTIR/TGA/TEM methods The same experiment in the absence of salt mediation was also conducted for comparison

made and the results are reported in Chapter 8 The precursor materials, cobalt hydroxide carbonate compounds, were synthesized by precipitation methods, either in heterogeneous or homogeneous way The addition of the solution containing NaOH

the form of monodispersed nanorods At the same time, the homogeneous precipitation of cobalt salts in the presence of urea hydrolysis also led to the formation

of nanorods with larger size of the similar compounds Combined TGA and CHN analysis determined the molecular formula of such compounds The as-formed hydroxide carbonate compounds were then calcined under oxidative condition in order

retained with interconnected sub-particles of cobalt spinel oxides after calcination

rod-shape, obtained by relatively simple synthetic way compared to those conventional self-assembly techniques The microstructures of the pristine cobalt hydroxide

disclose the crystalline orientation and the formation mechanism of rod-like morphology via this simple precipitation method

Finally, Chapter 9 briefly summarizes the major experimental results of the current research work on cobalt layered hydroxides and the related nanostructural materials An attempt to suggest some future work has also been made at the end of this chapter

Chapter 2 Literature Review

2.1 Overview

Since the report of the novel synthesis method for carbon nanotubes in early

development nanostructured materials which can be used in the areas of semiconductor, biomedical and catalysis, etc There are various newly emerged methods in the literature for the preparations of the nanosized particles, such as carbon

nanostructured materials with desired properties Compared to these methods, conventional ambient-temperature precipitation methods are less explored The main reason is that such simple methods normally produce relatively large particles with varied grain sizes in the scale of microns and above However, these are usually the simpler and less costly way of performing synthesis In order to achieve particle size reduction and morphology control, some novelty in terms of alternative pathway or modified parameters may need to be imposed into the conventional synthesis method

In the present PhD studies, research activities have been focused on the synthesis of Co-layered hydroxide compounds as precursors for related nanostructured materials The synthesis methods involved are mainly precipitation at ambient conditions

The literature review in this chapter covers three parts: i) a detailed review of layered hydroxide materials, mainly hydrotalcite-like compounds; ii) a particular discussion about cobalt-layered hydroxides; and iii) a brief introduction of nanostructured materials

2.2 Layered Hydroxide Compounds

2.2.1 Introduction

Layered hydroxide is a broad name, which represents a large family of crystallites with basic constituents of metal hydroxide layers The compositions of such compounds can be varied in a rather flexible manner because the structure can accommodate mixed hydroxides with wide range of interlayer species The most intensively studied layered hydroxide compounds are clays, which can be divided into

nature and always contain impurities, such as quartz, calcite, etc They consists of negatively charged metal (alumino in most case)-silicate layers and the interlayer cations The basic building blocks of cationic clays are the tetrahedral Si(O,OH) and

two-dimensional tetrahedral and octahedral sheet, respectively The combinations of these two types of sheets give rise to electroneutral structures In nature, isomorphous

sheet commonly occurs When the original cations are substituted by atoms with lower valence, the hydroxide layers carry negative charges, which are balanced by

investigation work has been carried out on cationic clays, mainly due to the difficulty and complication in synthesis of the pure form Unlike cationic clays, although less diffused in nature, anionic clays are relatively simple and inexpensive to synthesize Many reports can be found in open literature, regarding the synthesis methods, their properties and applications The review in this section is intended to focus on one family of anionic clays, hydrotalcite-like compounds (HTlcs), because of their

attractive properties as anionic exchangers,21-25 catalyst precursors26-29 and

2.2.2 Structural properties

Hydrotalcite-like compounds (HTlcs) are mixed hydroxides with homogeneous distribution of cations of different type and/or valence in hydroxide

octahedra share edges to form two-dimensional infinite sheets, which are stacked one

on top of each other and held together by weak interactions due to hydrogen bonding

charge is resulted in the hydroxyl sheet This positive charge is balanced by

There are two main stacking sequences of the hydroxyl sheets, rhombohedral

Figure 2.1 Schematic diagrams for (a) brucite lattice; (b)hydrotalcite lattice; and (c) atom composition

-parameters of the unit cell are a and c = 3c′, where c′ is the sum of the thickness of a brucite-like sheet and the interlayer distance The interlayer distance is also referred to

as gallery height The polytype form of hydrotalcite, manasseite, crystallizes with a hexagonal 2H stacking sequence and the parameters of the unit cell are a and c = 2c′

of 1H has been discovered for the most hydrated variety of layered hydroxide compounds with interlayer sulfate anions Its symmetry is still uncertain between

It has been well known that wide range of cations and interlayer anions can be accommodated in the hydrotalcite-like structure Most of these compounds can be

interlayer anion) The formula describes the compounds formed by with divalent and

cations The structure of this type of materials can be described on the basis of the

substitution of MIII by MIV have also been reported.37-39 For example, Taylor has

of HTlcs are much less common than the normal HTlcs of M(II)-M(III) type The understanding of these materials is still subjected to further studies of their detailed physicochemical properties

properties and applications of HTlcs and will be discussed separately in the following subsections

configuration of OH groups in the brucite-like layer Table 2.1 lists the ionic radii of some divalent and trivalent cations It can be seen that many cations have reasonably

too small or too big and thus not suitable for octahedral They usually form other types

Table 2.1 The ionic radii of some divalent and trivalent cations

ion, it usually crystallize to malachite-like phase This is due to the so-called Teller effect, which leads to the energetically favorable formation of distorted

brucite-like sheet are separated from one another and they individually form undistorted

ions situated in nearby octahedra prefer to form distorted octahedral structure, which results in the malachite-like side phase Table 2.2 gives a representative list of HTlcs containing various combinations of metal ions reported by different authors

Table 2.2 HTlcs obtained with different cations

50 51-54

55

56 52,57 58,59

65

66 67,68 69,70

78

38

2.2.2.2 The values of x

parameters differ considerably Therefore, it is understandable that the stoichiometric range of HTlcs does not extend to these two distinctive end members The x value

have shown that it is possible to obtain pure HTlcs only for 0.2≤x≤0.33 When x falls

from each other A large value of x will gives rise to a high number of neighboring

value of x leads a high density of Mg octahedra in the brucite-like sheet, acting as

well crystallized and are not detectable by X-ray diffraction The alternative techniques to be used are thermogravimetric, electron microscopy or solid-state NMR

analyses.81,82 It has also been found that when x is too small, interlayer anion content

is insufficient to separate all brucite-like layers, resulting in interstratification of

also proposed that molecular-scale inhomogeneities might be present for all stoichiometries, although solid-solution behavior can be observed on a macroscopic scale

2.2.2.3 Nature of anions

The general purpose of interlayer anions is to balance the positive charges in brucite-like layers The only restriction to the nature of anions is that they do not form strong complexes with the cations in brucite-like layers There is practically no other limitations regarding their type, valence, size and orientation, which although may deeply affect the properties of the formed HTlcs, such as the interlayer distance, the crystallite size, surface acid/base properties, etc One of the main noteworthy properties of HTlcs is their ability to exchange interlayer anions, thus presenting an enormous potentiality for preparing materials with a wide range of chemical compositions A very versatile range of anions are reported in the literature:

(iii) Complex ions: oxometalate and polyoxometalate anions,59,107-112

The distribution and packing of anions in the interlayer region are closely related to the electrostatic forces between the brucite-like sheets and the anions Some representative values of c′ are reported in Table 2.3 for typical HTlcs with common inorganic anions

Table 2.3 The values of c′ for some HTlcs

In the relatively simple case of HT compound, the oxygen atoms of carbonate and water in the interlayer are at sites located around the symmetry axes, which pass through the hydroxyl ions of adjacent brucite layers Due to the close proximity of OH groups in brucite sheets, only one such site of an octahedral is occupied The carbonate group is situated flat in the interlayer with its three oxygen atoms sitting at three adjacent sets of sites The hydroxyls interact with carbonate directly or via

The interlayer arrangement of other anions, such as hydroxide, nitrate and chloride is similar to that of carbonate

Finally, it should be mentioned that it is very difficult to prepare pure HTlcs

present in the aqueous solution is always unavoidable

2.2.2.4 The values of m

Water molecules are always incorporated into the interlayer space during the synthesis of HTlcs in aqueous medium A tetrahedral configuration can be accepted for water molecule which forms two hydrogen bonds with other oxygen atoms in near

of a close packed configuration of oxygen atoms, it is possible to calculate the

interlayer, and subtracting the sites occupied by the anions Researchers have

interlayer such as the formulas listed in Table 2.4

Table 2.4 Various formulas to determine the maximum value of m

a) 1−Nx/n

b) 1−3x/2+d

c) 0.81−x

N = number of sites occupied

by the anions; n = charge of

d = 0.125

122

123

80 Using formula a), the maximum amount of water in HT is calculated to be 0.625 The actual value of m in natural HT is 0.5 and the synthetic HT compounds usually give m lower than 0.5

The value of m is also affected by the interlayer distance In certain cases with large interlayer distance, more water can accumulate in the interlayer region by

interlayer space influences the interlayer distance Water molecules can associate with the anions, thus causing an expansion of the interlayer distance Such situations have

conditions of relative humidity higher than 50%, the values of c' are higher (10.8 and

the values of c' are 8.9 and 9.2 Å, respectively Interestingly, there were no

The amount of water is usually determined with thermogravimetric analysis of weight loss However, it is commonly agreed that the value of m is difficult to evaluate accurately due to the several reasons:

2) The surface of HTlcs usually absorbs substantial amount of moisture water, especially when the crystallinity is poor;

3) The dehydration and dehydroxylation often partially overlap when the crystallinity of the HTlcs is poor

2.2.3 Synthesis methods

The synthesis of HTlcs can be carried out by various methods, such as coprecipitation (or salt-base) method, anion exchange method, hydrothermal synthesis, sol-gel method, oxidation method, electrochemical synthesis, rapid crystallization method assisted by microwave, etc The choice of synthesis method generally affects the properties of the final products, such as crystallinity, particle size, porosity and

preparing HTlcs with anions other than carbonate In order to prepare such HTlcs in a

pure form, methods other than coprecipitation needs to be used Special care is to be

form other compounds A short discussion of various synthesis methods is provided in the following subsections

2.2.3.1 Coprecipitation method

It has been reported that HTlcs are thermodynamically favored over the separate hydroxides under the normal synthesis conditions This is actually the drive

precipitations in conditions of supersaturation This is usually achieved by keeping the

pH of the solution higher than or equal to the one at which the more soluble hydroxide

required to form the HTlcs In general, a pH value in the range of 8-10 is practically used to form HTlcs There are three different methods for coprecipitation synthesis:

base solutions are slowly added to the container;

very quickly to the base solution

Research groups have their own preferences in choosing which method to use

In general, it is easier to perform under conditions of variable pH values The first synthetic HTlc was prepared by titration of very dilute solutions of Mg and Al with a

materials with more interesting properties, from the technological applications point of

view, are obtained at constant pH Higher crystallinity, smaller particle size, higher specific surface area and higher average pore diameter were found for materials obtained by coprecipitation at constant pH, when compared to the materials obtained at

supersaturation synthesis usually gives rise to precipitates which are more crystalline with respect to those obtained at the high supersaturation condition Under the latter circumstance, the rate of nucleation is higher than that of crystal growth and hence, a large number of smaller particles are obtained The most commonly used basic media

a constant level in the case of low supersaturation, besides adding dilute streams of reagents at the very low speed, buffer solutions can be filled into the container at the beginning of the synthesis so that the pH of the medium can be effectively maintained around the pKa value of the buffering acid used For example, a buffer solution

After the precipitation, aging is usually applied to improve the crystallinity of

hours

2.2.3.2 Anion exchange method

This technique is a direct implementation of the anion exchange property of

In general, the anion exchange reaction is performed by simply dispersing the precursor HTlcs in an aqueous solution containing an excess of the anions that is to be intercalated To carry out such reactions, the anions, especially organic anions of interest, and the respective brucite-like layers must be stable at the pH of exchange, which is usually kept in a weak acidic range

It is worth mentioning that Bish and Brindley first synthesized NiAlA-HTlcs

et al carried out the systematic investigations and derived the sequence of exchange

-The above sequence shows that the carbonate anion is most difficult to be exchanged out by other anions, due to its high affinity to the brucite-like layers On the other hand, HTlcs containing nitrate anions are, therefore, the most suitable precursors for the preparation of new HTlcs with bulky and complex anions such as carboxylic acids, polyoxometalate anions and other macromolecules, since nitrate anions are relatively easier to be displaced from the interlayer

Table 2.5 lists some of the organic anions exchanged with the original anions in the precursor HTlcs By incorporating big or supra molecules such as oxometalate

out that sometimes an intermediate HTlcs containing relatively simple organic anions (e.g terephthalate anion) are used for exchange of more complicated anions (e.g

Table 2.5 Organic anions incorporated to HTlcs using anion exchange method

ordered in crystalline structure compared to those obtained by direct exchange method This modification can also be applied in the rehydration method, which will be discussed shortly Crepaldi et al claimed another type of anion exchange method for a surfactant-intercalated HTlcs The HTlc precursor contains an anionic surfactant in the interlayer region The surfactant anion is removed from the interlayer by the formation

of the salt with cationic surfactant, which then migrates to the organic phase This process effectively provides the chance for the anions of interest to be incorporated into the interlayer space Since the surfactant salt is insoluble (or only sparingly soluble) in the aqueous phase, its migration to the organic medium enhances the efficiency of the anion exchange process by displacing the equilibrium to the

The preparation, properties and applications of organo-HTlcs have been extensively investigated and some review articles can be referred for detailed

2.2.3.3 Rehydration method

This method was first reported by Miyata who found that after calcination of

and reformed back to a HTlc in the presence of anions This unique “memory effect”

of HTlcs provides an effective synthetic route to obtain HTlcs with desired inorganic and organic anions, and to avoid the incorporation of competing inorganic counter

In the presence of glycerol or other glycols as swelling agent, Dimotakis and Pinnavaia showed the formation of exceptionally well-ordered HTlcs containing carboxylate and

either vapor phase or in the glycerol solution It is found that the temperature of glycerol treatment is important Hansen and Taylor reported the difference when using

treatment with glycerol vapor gives an interlayer spacing of 14.4 Å (an increase in interlayer spacing of 6.7 Å) Considering the size of glycerol molecule, these data

2.2.3.4 Hydrothermal synthesis

The intercalation of large inorganic or organic anions into HTlcs materials is of considerable interest in the search for advanced materials However, these large anions usually have low selectivity, which is determined by their charges and sizes Synthetic limitation occurs to intercalate these anions if using the above-mentioned three methods To overcome the selectivity problem, an alternative reaction route has