Polymorphism control in nanocrystalline binary metal oxides

8/11/2019 Polymorphism control in nanocrystalline binary metal oxides

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Polymorphism Control in Nanostructured Metal Oxides

A Dissertation Presented

by

Shantanu Sood

to

The Graduate School

in Partial Fulfillment of the

Requirementsfor the Degree of

Doctor of Philosophy

in

Materials Science and Engineering

Stony Brook Uniersity

August 20!


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!o"yright byShantanu Sood

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Stony "roo# $ni%ersity

The Graduate School

Shantanu Sood

#e$ the dissertation committee for the aboe candidate for the

Doctor of Philoso"hy degree$ hereby recommend

acce"tance of this dissertation%

Prof& Pelagia '& (ouma ) Dissertation Ad%isorProfessor* Materials Science and Engineering

Dr& Dilip (ersappe + Chairperson of Defense

Professor* Materials Science and Engineering

Dr& Milutin Stanace%ic

Associate Professor* Electrical Engineering

Dr& Ming,hao -iu

Assistant Materials Scientist* "roo#ha%en National -a.oratory

This dissertation is acce"ted by the Graduate School

!harles TaberDean of the Graduate School

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Abstract of the Dissertation

Polymorphism control in nanostructured metal oxides

by

Shantanu Sood

Doctor of Philosophy

in

Materials Science and Engineering

Stony Brook Uniersity

20!

Polymor"hic "hase transformations are common to all nanocrystalline binary metal

o&ides% The "olymor"hic nature of such metal o&ides makes aailable a large number of "hases

'ith differing crystal structures$ each stable under certain conditions of tem"erature$ "ressure$

and(or "article si)e% These different crystal structures translate to unique "hysical and chemical

"ro"erties for each structural class of "olymor"hs% Thus "redicting 'hen "olymor"hic "hase

transitions are likely to occur becomes im"ortant to the synthesis of stable functional materials

'ith desired "ro"erties% Theoretical calculations using a heuristic a""roach hae resulted in an

accurate estimation of the critical "article si)e "redicting metastable to stable "hase transitions%

This formula is a""lied to different case studies* for anatase to rutile titania+ ,-Alumina to .-

Alumina+ and tetragonal to monoclinic )irconia% The theoretical alues calculated hae been seen

to be ery close to the e&"erimental results from the literature%

/anifestation of the effect of "hase transitions in nanostructured metal o&ides 'as

"roided in the study of metastable to stable "hase transitions in #01% 2ano'ires of tungsten

trio&ide hae been synthesi)ed in-situ inside an electron microsco"e% Such structure of tungsten

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trio&ide result due to a metastable to stable "hase transformation$ from the cubic to the

monoclinic "hase% The transformation is massie and com"lete% The structures formed are unique

one-dimensional nano'ires% Such a method can be scaled inside any equi"ment equi""ed 'ith an

electron gun$ for e&am"le lithogra"hy systems either using ST3/ or 3-beam lithogra"hy%

Another study on nano'ire formation in binary metal o&ides inoled the synthesis of

stable orthorhombic /o01by means of blend electros"inning% Both a traditional single 4et

electros"inning set u" and a noel high-through"ut "rocess to get high as"ect ratio nano'ires%

The latter is a 4et-controlled and flo' controlled electros"inning%

The mechanism of the formation of nano'ires of both tungsten trio&ide and molybdenum

trio&ide are discussed in relation to the "olymor"hic nature of the o&ides%

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Dedicated to

/y highly motiating and su""ortie family%

And

3s"ecially to my mother$ Dr% /adhoolika Sood%

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/a.le of Contents

List of Tables..........................................................................................................................ix

Preface...................................................................................................................................xi

Acknowledgments.................................................................................................................xiii

Vita.......................................................................................................................................xv

Publications (Journal and Proceedings)...............................................................................xvi

Chapter 1 !ntroduction.......................................................................................................2

&& Polymorphs of O1...............................................................................................................3

&2& Polymorphs of MoO1.............................................................................................................4

&1& Polymorphs of other .inary metal oxides..............................................................................5

&!& Metasta.ility in nanocrystalline .inary metal oxides...........................................................6

&& Phase /ransformations due to nanoscale particle si,e........................................................10

&3& Polymorphic nano4ires of O1and MoO1.........................................................................12

&5& Applications of nanocrystalline .inary metal oxides...........................................................14

&6& Polymorphic nano4ires as sensing materials......................................................................22

&7& Statement of the pro.lem.....................................................................................................24

References............................................................................................................................................................25

Chapter " #$perimental %etails........................................................................................35

2&& Synthesis Methods...............................................................................................................35

2&&& Sol+(el synthesis..............................................................................................................35

2&&2& Electrospinning................................................................................................................35

2&&1& 8igh throughput electrospinning.....................................................................................37

2&2& Characteri,ation Methods...................................................................................................39

2&2&& 9+ray diffraction..............................................................................................................39

2&2&2& :aman Spectroscopy........................................................................................................39

2&2&1& /ransmission electron microscopy...................................................................................40

2&2&!& 8igh resolution /ransmission electron microscopy.........................................................40

2&2&& Scanning Electron Microscopy........................................................................................40

References............................................................................................................................................................41

Chapter & 'euristic model of polmorphic transitions......................................................42

1&&& /heory..............................................................................................................................43

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1&&2& Entropic contri.ution......................................................................................................44

1&&1& Surface ;ree Energy........................................................................................................45

1&&!& Surface Stress...................................................................................................................46

1&&& ;ree Energy due to particle shape...................................................................................46

1&&3& Critical particle si,e.........................................................................................................47

1&2& Case Studies.........................................................................................................................48

References............................................................................................................................................................48

Chapter Tungsten Trio$ide nanowire in*situ T#+ snthesis..........................................53

!&& Precursor phase...................................................................................................................53

!&2& 'n+situ nano4ire synthesis...................................................................................................56

!&1& /hermodynamics of nano4ire formation............................................................................61

References............................................................................................................................................................65

Chapter , +olbdenum Trio$ide nanowires b electrospinning........................................67

&& MoO1Electrospun nano4ire formation..............................................................................67

&2& Single P and CA fi.ers....................................................................................73

&1&2& Process Parameters..........................................................................................................75

&1&1& Nanofi.er Synthesis.........................................................................................................77

References............................................................................................................................................................80

Chapter - %iscussion and future work..............................................................................84

3&& Polymorphic transitions and critical particle si,e...............................................................84

3&2& Phase transitions and O1nano4ire synthesis...................................................................89

3&1& Polytypism in nano4ires......................................................................................................92

3&!& ;uture 4or#.........................................................................................................................95

References............................................................................................................................................................99

Appendi$ A Comparison between bulk and nano phases................................................105

Appendi$ . Crstalline /tructures..................................................................................107

Appendi$ C !nternal pressure calculations.....................................................................108

Appendi$ % 0ree energ due to particle shape calculations............................................109

Appendi$ # Per mole conersion of surface energ2tension and surface stress terms.....110

Appendi$ 0 +echanism behind the 'igh throughput electrospinning............................114

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-ist of ;igure

;igure ?Traditional electros"inning set u"%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%15;igure 2?@a/odified flo' controlled and 4et controlled high through"ut electros"inning set u"% @.Source disk$ the metallic "late 'here one terminal of the high oltage source attaches is isible$ 56equidistant 7mm diameter holes drilled in to the inner base of the disk 'ith one hole "lugged for attachingthe oltage terminal+ @cSchematic of the setu"$ dimensions and other details are labeled%%%%%%%%%%%%%%%%%%%%%%%16;igure 1?The source disk is a critical com"onent of the set u"+ @aSchematic sho'ing the side ie' ofthe source disk% @.Schemtic sho'ing the bottom ie' of the source disk% @cThe bottom ie' of thesource disk% @dThe to" ie' of the source disk%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%180igure 3(a)Low magnification SEM image of sol-gel synthesied nanoparticles of tungsten trio!ide

annealed at "#$%, (b)shows the higher magnification SEM image with uniformly distri&uted particle

sie' (c)Shows the Raman spectra of the nanoparticles, the &ands o&sered are mared' (d)Shows the

*R+ of the nanoparticles'%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%87;igure ? @-eftT3/ microgra"h of the cubic nano"articles of Tungsten trio&ide$ @:ight SAD "attern$brighter rings can be inde&ed to 97::;+ 977:;+ 957:; "lanes of the cubic "hase%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%850igure -3 (Left)EM of nanowires grown on a Silicon itride support film, (4ight)EM of nanowires

grown on formar support film on a %opper mesh grid'%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%860igure 53(Left)ypical Electron diffraction pattern taen from a region with high concentration of

nanowires' (4ight)Schematic of crystal structure of the monoclinic phase, with the preferred growth

direction, the distortions hae &een e!aggerated to sere the purpose of illustration'%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%88;igure 6?@-eft gro'th direction along the long a&is+@:ight!orres"onding diffraction "attern%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%8?0igure 63.igh resolution EM of the structure of the nanowires/ (!nset)S0+ pattern from indiidual

nanowire'%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%8@;igure 0?Gra"h "lotting the free energy for cubic and monoclinic "hase due to "article si)e$ and oerlayis the difference in the free energies for cubic nano"articles and monoclinic nano'ires as a function of theas"ect ratio of the nano'ires kee"ing the radius of nano'ires constant at the obsered alues 5:nm%%%%%%?:;igure @aS3/ microgra"h of metal o&ide enca"sulated electros"un nanofibers% @./olybdenumtrio&ide enca"sulated inside ele&tros"un fibers$ @'nset3DS "attern of the enca"sulated "articles%%%%%%%%%%%??;igure 2* @aS3/ microgra"h of heat treated nanofibers at 8::!+ @.S3/ microgra"h of high as"ectratio nano'ires%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%??0igure 1&3 (a)anowire growth after heat treatment at 1#$% and (inset) its corresponding S0+ pattern/

(b)anowire growth after heat treatment at #$$% and (inset) its corresponding S0+ pattern/ (c)

.REM image of the nanowire and (inset) its corresponding S0+ pattern'%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%?@;igure !? @a


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;igure 7* @a!rystal structure of orthorhombic /o01 along 'ith )one a&is as marked%@. -Raydiffraction of /o01 orthorhombic nano'ires synthesi)ed by both traditional and high through"utnano'ires$ the "lanes marked are from C!PDS :8-:8:E%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%@8;igure 20?Schematic describing the gro'th of monoclinic tungsten trio&ide nano'ires fromnano"articles of cubic "hase in a ste"'ise fashion% Ste" 7 sho's the starting nano"articles 'hich hae aradius of bet'een 1: to 6:nm% As an electron beam is sho'n onto these nano"articles the "articles start togro' into nano'ires as sho'n in ste" 5% As the nano"articles get consumed they start to coagulate andgro' into nano'ires% Ste" 1 sho's ho' these nano"articles gro' from "referred direction to formrandomly oriented nano'ires before being totally consumed and forming highly aligned nano'ires oftungsten trio&ide as sho'n in ste" 6% The 'hole "rocess may take a fe' micro seconds to com"lete oncethe nano"articles receie the required amount of energy%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%E1;igure 2* FFT images of the nano'ires+ 9eft; Usually obsered nano'ire+ 9right; "eriodic nano'irestructure%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%E6;igure 22* Analysis of electron diffractions of differently oriented nano'ires$ "ossibly sho'ing thereason for the streaking "resent in the diffraction "atterns%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%E@0igure "&3Rotating source dis design'%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%:Figure 56* 9a; Structure 7* Rutile-ty"e Tetragonal+ 9b;Structure 5* Peroskite-ty"e+ 9c;Structure 1*

!orundum-ty"e Rhombohedral+ 9d;Structure 6* !a!l5-ty"e 0rthorhombic+ 9e; Structure8* Rh501%%%%%%%%%8;igure 2?A liquid dro"let 'ith radius HrI$ a hy"othetical cylindrical dro"let 'ith radius HrI and heightHhI%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%?;igure 23 @aDro"let formation at the surface of the metallic disk+ @.As the dro"let gro's aboe thecritical si)e$ it forms into a conical 4et+ @cThis 4et gets solidified as nanofibers at the cylindrical collector%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%E;igure 25?/ulti"le 4et formation on the disk surface%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%E;igure 26?The electric field distribution$ 9a; bet'een the outer collector cylinder and source disk+ 9b; onthe surface of the source disk%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%;igure 27?Geometric inter"retation of the electros"inning "rocess$ 9Jnset; sho'ing the double fiberformation in the 4et oerla" region%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%7::

;igure 10?Plot of theoretical yield 9g(h; s number of holes and loss of yield due to K oerla" s numberof holes%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%7:7

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-ist of /a.les/a.le * /etastable "hases of /etal 0&ides%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%@/a.le 2?!om"arison bet'een bulk and nano "hase at ambient tem"erature and "ressure%%%%%%%%%%%%%%%%%%%%%%%%77/a.le 1?Standard entro"y of formation a fe' Polymor"hs%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%6:/a.le !* Alumina$ titania$ )irconia critical "article si)e calculations%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%66/a.le * !ontributions from the three main factors influencing the nano'ire gro'th%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%8E/a.le 3* Data accumulated from the literature$ and the free energy calculations%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%8E/a.le 5?Preious attem"ts to increase "roduction rates of electros"un fibers%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%?E/a.le 6?!om"arison bet'een traditional needle electros"inning and high through"ut electros"inning forPP%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%% @:/a.le 7* /odels for "hase transformation behaior de"endence on "article si)e%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%E:/a.le 0?!om"arison bet'een bulk and nano for same "hase and different conditions for tem"eratureand "ressure%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%1

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Preface

The 'ork done here focuses on understanding and controlling "hase transitions in

nanostructured binary metal o&ides$ 'ith em"hasis on the synthesis of nano'ires of the stable

"hases of molybdenum trio&ide and tungsten trio&ide$ res"ectiely%

!ha"ter 7 introduces the conce"t of o&ide "olymor"hism and discusses bulk "hase

"olymor"hs of arious metal o&ides like$ #01$ /o01$ Ti05$ Sn05$ !r05$ Lr05$ Jn501$ Fe501$ and

Al501% Further the nanocrystalline "hases of these metal o&ides hae been tabulated% The

"olymor"hs that result from "hase transformations at the nanoscale are also discussed% Parts ofthis 'ork hae been "ublished inanomaterials and Energy295;$ 7 95:71;%

!ha"ter 5 focuses on the e&"erimental details and both synthesis techniques and

characteri)ation techniques used for tungsten trio&ide and molybdenum trio&ide nanomaterials%

!ha"ter 1 discusses a heuristic a""roach used to arrie u"on the critical "article si)e for

the "hase transformation bet'een a metastable and stable "hase of a metal o&ide% !ase studies

hae also been used to ascertain the accuracy of the e&"ression% This 'ork has a""eared in

"ublication$2ournal of 0merican %eramic Society7395;$ 178 95:71;%

!ha"ter 6 discusses the in-situ synthesis of #01based on a "olymor"hic "hase reaction%

The formation of nano'ires by such a non-inasie route by the irradiation by an electron beam

has im"ortant consequences for deelo"ment of a non-chemical route for nano'ire synthesis%

The im"artation of energy using an electron beam can "otential be achieed by any electron

beam based system%

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!ha"ter 8 adds to the earlier 'ork done on the synthesis of single crystal nano'ires of

/o01 by ada"ting a multi4et system% Such a system has been designed to achiee high

through"ut synthesis of nanfibers% Jt is a 4et-controlled and flo'-controlled system% Part of this

'ork has a""eared in "ublications$2ournal of anoenegineering and anomanufacturing$ !97;$

195:76; andMRS eproceeding$ 37$ 5:76%

!ha"ter ? discusses the im"ortant consequences of the 'orks described in this thesis% The

im"ortance of a""lication of critical "article si)e "henomenon$ in-situ nano'ire synthesis of

#01and unique as"ects of the high through"ut /o01nanofiber synthesis are discussed% The

future research directions as eoled from this 'ork are also outlined

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Ac#no4ledgments

J 'ould like to e&tend the foremost gratitude to my adisor$ Dr% P% Gouma% She has been a

source of great su""ort all throughout my stay at the uniersity% She has been a there through all

the u"s and do'ns and it is only due to her encouragement that J hae been able to achiee

anything% She is a kno'ledgeable mentor and the most "atient adisor%


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ei i$ Cing Lhang$ Ruiyao !ui$ Ciahao huang$ ynne iao$ Selda To"cu hae all contributed to

making my e&"erience in lab a "leasant one%

And finally J 'ould like to thank the most im"ortant "eo"le in my life$ my family$ 'ho J

kno' 'ould al'ays be there no matter 'hat% /y father$ mother$ brother and both sisters and

brother-in-la'%

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>ita

Shantanu Sood

3orn4 Canuary 7@$ 7E$ Rhotak$ Jndia%

3ducation*

"&E&9Culy 5::? /ay 5:7:;$ /etallurgical 3ngineering$ Pun4ab 3ngineering !ollege Uniersity of

Technology$ !handigarh$ Jndia%

M&S&9Canuary 5:77 /ay 5:71;$ State Uniersity of 2e' Oork$ Stony Brook$ 2O$ USA%

3&"erience*

:esearch Assistant90ctober 5:77 "resent;$ De"artment of materials science and engineering$ State

Uniersity of 2e' Oork$ Stony Brook$ 2O$ USA%

/eaching Assistant9Se"tember 5:75 /ay 5:71;$ De"artment of materials science and engineering$

State Uniersity of 2e' Oork$ Stony Brook$ 2O$ USA%

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Pu.lications @


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Although$ "olymor"hism 'as discoered by Nla"roth in 7@E Q7$ 'hen he sho'ed that the

chemical com"osition of minerals calcite and aragonite is same$ !a!01$ fe' gae any

im"ortance to it at the time% Jt 'as the result of the 'ork of a young chemist and amateur

mineralogist$ 3ilhardt /itscherlich Q5 in 7E5:s$ that "olymor"hism 'as uniersally acce"ted%

/itscherlich "ublished "a"ers on arsenates Q1$ "hos"hates Q1 and sulfur Q6$ /itscherlichs la'

of isomor"hism is sim"ly stated* Substances that crystalli)e in isomor"hous forms 9i%e%$ hae

identical crystalline forms and form mi&ed crystals; hae similar chemical com"ositions% Bulk

state o&ide "olymor"hism has been 'idely studied% Jn modern times 'ith the adent of

nanoscience$ "olymor"hism has made aailable a s"ectrum of materials 'hich 'ere hitherto not

aailable to science% S"ecifically nanocrystalline metal o&ides are of increasing interest Q8$ ?

and it 'as sho'n that certain metal o&ides deiate significantly from their bulk state "hase

transformation "ro"erties 'hen they are sub4ect to similar conditions at the nanoscale% Follo'ing

'e discuss certain binary metal o&ides 'hich crystalli)e in metastable forms as they are sub4ect

to differing conditions of "ressure and tem"erature Q@% These metastable states become aailable

at ambient conditions due to the nanometer si)e of the "articles or grains% Table S7 in A""endi&

A tabulates the bulk "hase transitions in the binary o&ides%

Polymor"hic nanocrystalline metal o&ides hae been "roen time and again to hae better

"ro"erties than there bulk counter"arts% 2anocrystalline !r05a half metallic ferromagnetic QE

o&ide has seen a""lications in magnetoelectronics Q8 and there magnetotrans"ort "ro"erties haebeen inestigated Q8% 0rthorhombic "hase of Sn05 is ery im"ortant in terms of gas sensing

a""lications Q% 2anocrystalline Tungsten trio&ide$ is another im"ortant sensor material and its

sensing "ro"erties hae been 'ell studied Q7:% /echanism for sensing behaior of metal o&ides

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can be described by the surface "henomenon$ 'hen there are surface acancies on o&ide surface$

the o&idising gas can get adsorbed$ and for e&am"le 20 gas is adsorbed at the monoclinic

#01 surface$ on the contrary 'hen there is an access of electrons on the surface a reducing gas

can get adsorbed% Polymor"hism can also be an im"ortant "henomenon 'hen considering

electrochemical reactions in fuel cells$ different "olymor"hs of a metal o&ide can hae different

ion conductiity$ for e&am"le$ in the case of .-Bi501$ -Bi501$ ,-Bi501$ V-Bi501 the ion

conductiities 'ere found to be 17:-6$ 57:-1$ 87:-1$ 7 res"ectiely Q77% Polymor"hism is

also im"ortant in "hotocatalytic behaior% Tungsten Trio&ide is a metal o&ide that e&ihibits

"hotocatalytic behaior% Titania is by far the most 'idely used "hotocatalytic material$ but due toits 'ide bandga" it has some limitations$ 'hereas tungsten trio&ide 'hich has much smaller

bandga" of bet'een 5%6 to 5%Ee Q75 and has better "hotocatalytic "ro"erties% Polymor"hism

also "lays an im"ortant role in electrochemical cells and batteries% The lattice of orthorhombic

molybdenum trio&ide due to its layered structure is "articularly suitable for iW insertion reaction

Q71% Authors of ref% Q7? re"orted ithium ca"acity u" to 7%8 i(/o and discharge ca"acity oer

1:: mAh gX7% The lattice of


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of #01is close to the Re01structure% For bulk state$ "hase transformations in #01 take the

follo'ing route Q7@$ 7E$ the monoclinic Y-#019'ith s"ace grou"Pc; 'ith lattice constants$ a Z

8%5@@ [$ b Z 8%788 [$ c Z @%??1 [$ transforms to triclinic V-#0 1 9P6; at 51:N$ 'hich then

transforms to monoclinic ,-#01 9P768n; 'ith lattice contants$ a Z @%5@ [$ b Z @%81 [$ c Z

@%?EE [ at 1::N% The room tem"erature monoclinic "hase is the stable form of #01 under

normal conditions and transforms to orthorhombic -#01 9Pnma; 'ith lattice constants a Z

@%1E6[$ b Z @%875 [$ c Z 1%E6? [ at ?51N% -#0 1tranforms to tetragonal .-#01 9P18ncc; at

::N 'hich is the high tem"erature "hase%

7%5%Polymor"hs of /o01

/o01e&hibits seeral "olymor"hs* the .-"hase 9s"ace grou" Pbmn; that is stable under

ambient conditions+ the -"hase 9P57(c;$ a metastable "olymor"h+ /o01-JJ 9P57(m; a metastable

high-"ressure "hase+ and the he&agonal "hase$ h-/o01% Both bulk .- and -/o01hae

/o0? octahedra a the basic structural unit and their structure is formed by the linkage of

distorted /o0? octahedra Q7% .-/o01crystalli)es 'ith lattice constants a Z 71%E88 [$ b Z

1%?? [ and c Z 1%?1 [ 9S"ace Grou" Pbnm;% -/o01is similar to #01 and is related to the

three-dimensional Re01structure$ 'hich consists of corner-connected octahedra net'ork% Jt

crystalli)es 'ith lattice constants a Z @%755 [$ b Z 8%1?? [$ and c Z 8%8?? [ 9s"ace grou" P57(c;%

Phase transition bet'een "olymor"hs in the bulk state has been obsered to be much different

than at the nanoscale% /c!arron 97E?; Q5: first obsered the -"hase to .-"hase transformation

in the bulk state and concluded that /o01transforms from amor"hous to -"hase at 881N and

then to .-"hase aboe ?@1N% Similar results 'ere later confirmed by Leng 97E; Q57 and it 'as

concluded that the -"hase to the .-"hase reaction is both e&othermic and "hotochromic$ 'ith a

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yello'-colored -/o01conerting to the 'hite-colored . "hase aboe ?6EN at moderate heating

rates%

7%1%Polymor"hs of other binary metal o&ides

/iO2naturally occurs as crystalline "hases Q55$ 51* rutile tetragonal 9a Z 6%81@ [$ c Z

5%8E@ [; 9P65(mnm;$ anatase tetragonal 9a Z 1%@E68 [$ c Z %8761 [; 9J65(amd; and brookite

orthorhombic 9Pcab; 9a Z 8%688E [$ b Z %7E7 [$ c Z 8%765 [;% Rutile is thermodynamically

stable at room tem"erature$ and anatase is kinetically stable and transforms to rutile at higher

tem"eratures Q56%

SnO2 crystalli)es in the rutile-ty"e tetragonal structure at ambient conditions and it

transforms to !a!l5-ty"e$ then to 9-Pb05ty"e$ and finally a "yrite 9modified fluorite;-ty"e

structure under high-"ressure Q58% Rutile-ty"e tetragonal "hase and !a!l5-ty"e orthorhombic

"hase are the t'o most common and im"ortant "olymor"hs of Sn05% 0ther high "ressure

"olymor"hs e&ist 9the reader is directed to follo'ing references; Q5?$ 5@%

CrO2is one of the sim"lest kno'n half-metals$ a metastable chromium o&ide and can easily

decom"ose to chromia 9!r501;$ it crystalli)es into the rutile structure Q5E 9S"ace grou"

P65(mnm$ LZ5; at ambient conditions$ a structure commonly found in many metal dio&ides

9/05+ /ZTi$ !r$ /n$ Sn$ Ge$ Pb$ etc%;% The synthesis of nanocrystalline !r05is difficult since$

as it is a metastable state$ most attem"ts hae resulted in transformation into the more stable

!r501o&ide of chromium% Jn atmos"heric "ressure !r05decom"oses to !r501at as lo' as 6@1N

Q5%

rO2 can ado"t three different crystalline structures$ that is$ cubic$ tetragonal$ and

monoclinic "olymor"hs% The monoclinic "hase$ stable at room tem"erature$ is transformed to

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materials result in ariable structural configurations at arying tem"eratures and ho' e&actly the

"hases do transform% Table J lists the o&ide "olymor"hs that hae been synthesi)ed at the

nanoscale$ along 'ith the synthesis techniques$ "rocessing conditions$ and relatie "article si)e

of the transforming material%

Table 14 Metasta&le phases of Metal 5!ides

PhasePhase

transition

Conditions of

P and /Particle Si,e Synthesis /echniues 4eference

Ti05

Rutile Aboe 7751N Belo' 77nm SolXgel method :16; < :17;

Anatase RT 77 to 18nm SolXgel method :16; < :17;

Brookite -- m 3lectron-beam lithogra"hyand Reactie ion etching *iao et al', :1#

Sn05!a!l5-ty"e

"hase

-- 1:-6:nm diaThermal ea"oration of Sn0"o'der

=hou et al':1?;

-- 8:nm3a"oration(!ondensationTechniques

0r&iol et al':1

E6EN Grain si)e 5:nm/echanical milling of tinmono&ide "o'ders

Lamelas:1A;

PhasePhase

transition

Conditions of

P and /Particle Si,e Synthesis /echniues 4eference

Lr05 TetragonalPhase

7661N 7:nm --Barie and

Boss:1C;

?::N-7:@1N 1%7nm 2onhydrolytic solXgelmethod

2oo et al':#$;

22


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/onoclinicPhase

Aboe 75@1N %8nm to 5:nmiquid Thermal S"raysynthesis

%hrasa et al':#6;

/o01

.-/o01

Aboe @51N ?:nm Jon-beam de"osition :#7;, :#";, :#1

@@1N

7:-8:nm dia%

2ano'ires$ length7-5m 3lectros"inning

Bouma et al'

:##;

-/o01 Belo' @51N 2ano"articles Sol-gel method0' D' Prasad,:#?;,:#1;

h- /o01

RT As"ect ratio of ?:o'-Tem"erature !hemicalmethod

:#@;,:#A;

Phase changeto -/o01aboe @::N

7D structures of5:: nm dia length of E:: nm

!hemical "reci"itationtechnique

%hitham&arar

and 3ose,:#C;

661N to?@11N

8:nm diameter 7m length

#et !hemical route +hage et al',:?

#01

h-#01

8@1N to @51N 8:-7::nmThermal annealing ofammonium tungsten bron)e

:?6;

?::N 1:-8:nm thick Soft !hemical route3alasi et al'

:?7, ?";/:?1;

,-#01 ?@1N 2ano"articles Sol-gel method :?#;

Y-#01 RT 5:nm dia Flame S"ray PyrolysisBouma et al'

:??;

-#01 ?6EN to @@1N E:nm dia2ano'ires 3lectros"inning :?1;,:?@;

.-#01 u"to ?6EN 2ano'ires 3lectros"inning :?1;,:?@;

PhasePhase

transition

Conditions of

P and / Particle Si,e Synthesis /echniues 4eference

!r501 !orundum"hase

7:@1N 2anobelts and2anorods


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7:-1::N

Single and"olycrystallinenanorods$ diameter8-nm$ length 7:-78nm

/icro'ae "lasma 'ithchromiumhe&acarbonyl "recursor

+' Follath et a

:?C;

--


26/124

transition P and /

Al501 ,-Al501Ambient!onditions

S"ecific surfacearea of alumina islarger than 758m5(g$ or "article

si)e less than about71 nm

--Mchale et al'

:A6;

6@1N 1%5nm% 2onaqueous Sol-Gel route :A7;

7%8%Phase Transformations due to nanoscale "article si)e

/etastability is key in "hase reactions inoling nanocrystals% Polymor"hic "hases often

e&ist in metastable states for long "eriods of time$ 'hich makes these states ery useful in

functional a""lications like gas sensing$ catalysis etc% At the nanoscale surface "ro"erties of

"olymor"hic materials differ and in most cases are a""reciably enhanced% For e&am"le the

catalytic "ro"erties of nanoscale !o0 are im"roed com"ared to the bulk state at lo'

tem"eraturesQE1% Thus it is im"ortant to study indiidual metal o&ides and come u" 'ith the

nanoscale characteristics of each$ such characteristics can find e&tensie uses in "hotocatalysis$

gas sensing$ electrochemical$ electrochromic deices to name a fe'%

Such differing "hase transformations due to nano si)e are not 4ust seen in "olymor"hic

metal o&ides$ but in general in all metal o&ides$ "olymor"hic or not$ for e&am"le the Room

tem"erature bulk "hase of !u conerts to cubic !u50 by o&idation at ery high tem"eratures of

greater than 75::N QE6$ if the tem"eratures are still increased the monoclinic !u0 forms%

!ontrary it 'as obsered QE8 that !u50 nano"hase starts to form aboe 851N$ and !u0 starts to

form beyond ?51N$ the "article si)e at 851N and ?51N 'ere 7? and 1:nm res"ectiely% Jt is

obsered that cubic !u50 is more stable at the nanodimentions also QE?$ E@ than the monoclinic

!u0 due to the increase ionic nature caused by reduction in the "article si)e% !u50 nanocrystals

25


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at lo'er than bulk tem"eratures hae been "re"ared by ion irradiation QEE$ s"uttering QE$

reduction route Q:$ etc%

Thus$ e&"erimental obserations confirm that for nanocrystalline metal o&ides "hase

transformation occurs ia different mechanism at the nano scale$ such as reersal in "hase

stability at small "article si)e is obsered in some systems$ melting tem"eratures are generally

lo'er in nano"articles$ transition tem"eratures to high tem"erature "hases are often lo'er$ 'hile

transition "ressures to high "ressure "hases can be higher or lo'er% 0ne of the reasons that

nano"articles sho' such differing "ro"erties is because of the significantly higher surface areas

at nano scale 'hich makes the contribution from surface energies much more significant Q7%

Thermodynamic as"ects of the "hase transformations at nanoscale hae been elaborated in the

ne&t sections% Table 5 gies a com"arison bet'een bulk and nano "hase at ambient tem"erature

and "ressure% Similarly table in a""endi& A gies com"arison bet'een bulk and nano for same

"hase and different conditions for tem"erature and "ressure%

Table "3%omparison &etween &ul and nano phase at am&ient temperature and pressure

Metal

Oxide at

am.ient

conditio

ns

"ul# phase Nano Phase

4ef

Phase

Conditions

of

temperatur

e and

pressure

Phase

Critical

particle

si,e

/emperatu

re

Sn05 Rutile-ty"e

tetragonal "hase

RT and 7

atm

!a!l5-ty"e

orthorhombic"hase

Belo'

8:nm

RT and 7

atm

C7, C",

1@, 1A

n501 !ubic bi&byite-ty"e structure

RT and 7atm

!ubic bi&byite-ty"e structure W!orundum-ty"ehe&agonalstructure

Belo'E:nm

RT and 7atm

@A, @C,

A$

26


28/124

Al501 !orundum ty"etrigonal .-"hase

RT and 7atm

!ubic , "hase Belo'7@nm andbelo'6@1N

RT and 7atm

C1, A6,

A7

Ti05 Rutile RT and 7

atm

Anatase Belo'

77nm

RT and 7

atm

16, 17

/o01 0rthorhombic ."hase

RT and 7atm

0rthorhombic ."hase W


29/124

Synthesis method Possi.le analyte sensed 4ef

/o01nano'ires 3lectros"inning Ammonia 6$1, 6$##01nano'ires 3lectros"inning 2itrogen Dio&ide CA!u0 nano'ires Tem"late assisted gro'th -- 6$"1D !u0 nanogrids 3lectros"inning and

thermal o&idation

3thanol 616


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/o01 nano'ires$ in "articular hae also been synthesi)ed by other researchers% Pure

orthorhombic structured nano'ire arrays of molybdenum trio&ide 'as synthesi)ed by Lhou et al%

Q7:E$ these 'ere gro'n on a silicon substrate% Due to the instability of trio&ide form of

molybdenum o&ides$ many 'orks hae managed to synthesi)e molybdenum dio&ide in

con4unction 'ith molybdenum trio&ide% Such structures hae been reie'ed e&tensiely by /ai

et al% Q7: and the readers are directed to the source for more information%

7%@% A""lications of nanocrystalline binary metal o&ides

As has been elaborated aboe nanocrystalline metal o&ides hae "ro"erties much different

from bulk metal o&ides% For e&am"le$ orthorhombic "hase of Sn05is ery im"ortant in terms of

gas sensing a""lications Q77:% This high tem"erature and high "ressure bulk "hase has been

re"orted at nano scale as single "hase Q77:$ 777 and as coe&istence of orthorhombic Sn0 5and

rutile-ty"e Sn05 "hase nano'ire Q775% 2anocrystalline #01$ is another im"ortant sensor

material and its sensing "ro"erties hae been 'ell studied Q771%

Sensingbehaior of nanocrystalline metal o&ides has been outlined by Gouma 95::?;

Q776$ gas selectiity can be achieed by achieing a s"ecific crystallogra"hic "hase of a "ure

metal o&ide% Surface interactions on metal o&ides are "rimarily due to gas adsor"tion$ hydrogen

e&traction$ or o&ygen addition% Due to structural differences the orientation of the surface atoms

of metal o&ides differ 'hich intern causes differing catalytic behaiors for eery indiidual "hase

of different metal o&ides% /echanism for sensing behaior of metal o&ides can be described by

the surface "henomenon$ 'hen there are surface acancies on o&ide surface$ the o&idi)ing gas

can get adsorbed$ and e%g% 20gas is adsorbed at the monoclinic #01surface$ on the contrary

'hen there is an access of electrons on the surface a reducing gas can get adsorbed% /ost of the

aboe mentioned metal o&ides sho' sensitiity or selectiity to different gases% The monoclinic

29


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32/124

ability of the catalyst to create electronhole "airs$ 'hich generate free radicals like hydro&yl

grou" 'hich are able to undergo secondary reactions% Ability of titania for 'ater catalysis 'as

the discoery by Fu4ushima and


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7%E% Polymor"hic nano'ires as sensing materials

2ano'ires nanostructures are solid$ one dimensional materials 'ith diameters in the 7:-E:

nm range$ and are metallic or semiconducting in nature$ made from metals or metal o&ides most

often% #ires made from molecular entities and diameter less than 8nm are often called /olecular

2ano'ires$ and these are different from 2ano'ires% !arbon nanotube research and nano'ires

research are often "arallel% Though carbon nanotube materials are different$ the sensing

mechanisms and underlying behaior are usually ery similar% 2ano'ire hae large surface area

to olume ratio "romising high sensitiity and the si)e of the nanostructures is similar to the si)e

of s"ecies being sensed$ thus nano'ires make good candidate transducers for "roducing the

signals that are then read and recorded by conentional instruments% Since they 'ere discoered$

silicone nano'ires hae "rimarily been inestigated for sensing a""lications in biological

systems Q75% Biomedical research 'ould greatly benefit from the adent of nano'ires as

sensors and they hae been ie'ed as one of most "romising areas Q71:% Techniques for

generating arious ty"es of im"ortant nano'ires$ nanorods$ nanobelts$ and nanotubes$ synthetic

strategies$ design of sensing deices$ and sensing mechanism has been reie'ed Q717%

2ano'ire nanostructure sensors of metal o&ides hae enhanced "ro"erties as o""osed to

nano"article nanostructures$ this is "articularly true for sensing sensitiity% For e&am"le$

sensitiity of nano'ire structures of orthorhombic "hase of /o01to 7:: ""m ammonia gas 'as

5:%8 in com"arison to nano"articles 'here the sensitiity 'as ?%E for same concentration Q715$

711% Similarly in the case of #01the sensor sensitiity analysis to'ards 205sho'ed that the

sensitiity im"roed significantly 'hen nano'ires 'ere used as o""osed to nano"articles Q716%

Single crystals are of "articular im"ortance for sensing of analytes$ as com"ared to the

multigranular crystals% Jnstabilities may form at the grain .oundaries$ 'hich cause lo'ering of


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35/124

7%% Statement of the "roblem

Polymor"hic reactions in binary metal o&ides are abundant% Due to the numerous number of

crystalline configurations aailable$ the range of "olymor"hs of any binary metal o&ide offers a

ariety of materials 'ith constant com"osition but distinct "ro"erties suitable to an array of

engineering a""lications%

/ost of these aailable "olymor"hs occur in high tem"erature and high "ressure range in the

bulk form% Although due to the inherent energies inoled 'hen the "article si)e is lo'ered to

sufficiently lo' alues$ many of the high "ressure and high tem"erature "olymor"hs could be

aailable at ambient conditions at the nanoscale% This fact makes it im"eratie to study the

"article si)e effect in the control of "olymor"hic reactions and the determination of "hase

stability fields for nanostructured o&ides%

2anostructures of metal o&ides occur in many different sha"es% S"herical nano"articles are

the most common structures% 2ano'ire structures hae higher surface area and therefore are of

greater use in engineering a""lications% This thesis also e&"loits "hase 9meta; stability for the

synthesis of one-dimensional nanostructures%

References

7% /%


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% Cordi Arbiol$ 3lisabetta !omini$ Guido Faglia$ Giorgio Sbereglieri$ Coan Ramon /orante%

Cournal of !rystal Gro'th 17:97;$ 58195::E;7:% Geng #ang$ Miang #ang$ #u u$ and Cinghong i% C% Phy% !hem% B$ 77:961;$ ""

55:595::?;%77% 2% Randrianantoandro$ A% /% /ercier$ /%


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18% 0)den 0)demir and Subir N% Baner4ee% Geo"hysics research letters 7791;$ 7?797E1;%1?% Cung-Fu in$ 0lga Degtyarea$ !harles T% Pre'itt$ Pr)emysla' Dera$ 2agayoshi Sata$

3ugene Gregoryan)$


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8% Angamuthura4 !hithambarara4 and Arumugam !handra Bose% Beilstein C% 2anotechnol% 5$

8E895:77;?:% Dhage S% R%$


39/124

@% Dabin Ou$ Shu-


40/124

% % #ang$ P% Gouma 95:75;% Integrated Microanalytical Systems$ cha"ter 8 9eds% /% A%

!ar"enter et al%;$ 7?@-7EE%7::% R%S% #agner$ #%!% 3llis 97?6;% a"or-liquid-solid mechanism of single crystal gro'th%

0ppl' Phys' Lett'6$ E%7:7% G% % Frey$ Rothschild$ C% Sloan$ Rosentseig$ R% Po"oit) Biro and R% Tenne 95::7;%

Jnestigations of nonstoichiometric tungsten trio&ide nano"articles%2' Solid State %hem' 7?5$

1::%7:5% O%F% Lhang$ O%


41/124


42/124

71% N% Sa'icka$ /% Naradge and P% J% Gouma$ 0&idation synthesi)ed !u0 nano'ires for gas

sensing a""lications$ /icrosc% /icroanal% 7: 9Su""l 5;$ ""% 1?:-1?7$ 5::676:% P% J% Gouma and /% C% /ills$ _Anatase to Rutile Transformation in Titania Po'ders`% C%

Am% !eram% Soc%$ E6 Q1$ ?7-?5595::7;%767% C% ee and P%J% Gouma$ Tailored 1D !u0 2anogrid Formation$ C% 2anomaterials$

5:77 95:77;$ Article JD E?1?17%765% Cun iu$ Lai"ing Guo$ Nai&ing Lhu$ #en4un #ang$ !haofeng Lhang and iaolong !hen%

C% /ater% !hem%$ 5:77$ 57$ 77675761% P%J Gouma and N% Nalyanasundaram% A""l% Phys% ett% 1$ 5667:5$ 5::E%

Chapter 2& Experimental Details

5%7% Synthesis /ethods

5%7%7% Sol-Gel synthesis

Jn a ty"ical colloidal sol-gel "rocess the "recursor material 9either an inorganic salt or

metal alko&ide solution; is chemically "rocessed to form hydrous metal o&ides or hydro&ides

9hydrolysate;% !olloidal dis"ersions 9sols; of the hydrolysate are "re"ared by "e"ti)ation 9the

"rocess res"onsible for the formation of stable dis"ersion of colloidal "articles in 'ater;$ a gel is


43/124

formed by dehydration or "< control$ and the resulting body calcined to form the stable o&ide

"roduct% A unique as"ect of sol-gel "rocessing is the ability to "roceed from molecular "recursor

to final ceramic "roduct$ enabling intimate control oer all stages of "rocessing and the synthesis

of _tailor-made` materials%

Both #01and /o01colloidal "articles 'ere "roduced using sol-gel synthesis% Tungsten

Jso"ro&ide 98K '( in Jso"ro"anol$ Alfa Aesar; 'as used hydroly)ed 'ith deioni)ed 'ater to

synthesi)e the sol% The mi&ture 'as ultrasonicated and aged for 56 hours$ u"on 'hich 'hite

"reci"itates 'ere obsered$ indicating the com"letion of hydrolysis% The synthesi)ed

nano"articles 'ere subsequently dried in air and the finally the "o'der 'as heat-treated at 18:!

in a tube furnace for E hours% Sol-gel synthesis 'as carried out on the "recursor /olybdenum

Jso"ro"o&ide 98K '( in Jso"ro"anol$ Alfa Aersar;% Precursor 'as hydroly)ed using deioni)ed

'ater to get :%7m/ solution% This solution 'as allo'ed to age for 56 hours%

5%7%5% 3lectros"inning

3lectros"un nanofiber mats "ossess high surface area and ariable "orosity Q7% The

traditional 4et-electros"inning "rocess requires a "olymer solution 'hich is loaded onto a

syringe$ a flo' meter to control the flo' rate of the solution$ a needle 'ith a diameter in the

millimeter range$ a collector "late 'hich is grounded and a high oltage "o'er su""ly% Jn the

traditional set u"$ a dro"let forms at the ti" from the source$ 'hich is the syringe and needle$ and

there is a buildu" of electrostatic charge at the surface of the dro"let 'hich induces the formation

of a 4et% 3en though the single 4et technique makes a aluable research tool to e&"eriment on

getting the best "recursors and "rocess "arameters for nanofiber mat formation of the desired

configuration$ it has largely "roduced small amounts of nanomaterials that hae satisfied the

engineering curiosity$ rather than offering a "rocessing route for nanofiber manufacturing%


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0igure 13raditional electrospinning set up

/o01 nanofibers 'ere synthesi)ed using this method% The sol-gel synthesi)ed /o01

colloidal "articles 'ere mi&ed 'ith :%7 molar PP solutions in ratios of 7*6 and 7*8% The

enca"sulation ma&imi)ed at 7*8 ratio% The solution 'as electros"un 97EN oltage+ :%1:m(min

flo' rate+ 7:cm collector to needle distance; to get metal o&ide enca"sulated nanofibers%

/olybdenum trio&ide nano'ires 'ere then formed after the nanofibers 'ere heat treated in a

tube furnace% Jt 'as obsered that electros"un nano'ires al'ays gre' into the orthorhombic .-

/o01 structure no matter the heat treatment tem"erature%

5%7%1%


45/124

and$ outer diameter 7: cm 'as used% The de"th of the source disk 'as 5mm$ 'hile the inner

diameter : mm% Fluid solutions 'ere "um"ed into the hollo' source disk 'ith the hel" of a

"rogrammable syringe "um" 9Nd scientific;% The flo' rate 'as ke"t at 7: to 75 times higher than

the traditional needle electros"inning 9T23;% Testing to "roe the design conce"t 'as carried out

using 51 equidistant holes drilled at the inner base of the disk% The 'orking distance 'as ke"t at

78cm$ 'hich 'as the difference bet'een the outer collector cylinder and the source disk% The

'hole set-u" 'as made out of aluminum% The charge distribution 'as such that the ma&imum

charge accumulation occurred at the ring at the leel of the holes% Jn order to achiee the required

leel of electrostatic charging at the dro"let surface$ the source disk bottom 'as machined to aminimum thickness for bottom surface% The "roblem of refilling the disc 'as oercome Q5 by

using a re"lenishing source to constantly fill the disc%

0igure "3(a)Modified flow controlled and et controlled high throughput electrospinning set up'

(b)Source dis, the metallic plate where one terminal of the high oltage source attaches is

isi&le, 71 eJuidistant 6mm diameter holes drilled in to the inner &ase of the dis with one hole

plugged for attaching the oltage terminal/ (c)Schematic of the setup, dimensions and other

details are la&eled'


46/124


47/124

of 6:k and 1:mA% -ray 'aelength is of !u N radiation 'hich is 7%867E6 [% The d alues are

then com"ared 'ith standard C!PDS "o'der diffraction data for the material under test and

"ossible "hases are identified%

5%5%5% Raman S"ectrosco"y

Raman studies 'ere "erformed on a #itec Al"ha combination near field o"tical and

raman microsco"e% Jn combination 'ith -ray diffraction Raman data 'as used to ascertain the

"hase of the material% Jt 'as also used to eliminate the "ossibility of #-! and #-0< bond

"resence in the colloidal solution%

5%5%1% Transmission electron microsco"y

Transmission electron microsco"e$ in a C30 76:: Soft-bio 'as carried to determine the

"hase and to obsere the grains of ceramic nano"articles synthesi)ed by sol-gel method%

Additionally$ nano'ires 'ere obsered under the microsco"e and the as"ect ratio 'as

ascertained%

The C30 76:: T3/ has a aB? cathode electron source% Jt o"erates at a oltage of

75:Ne and has a resolution of 5nm% Jt is es"ecially suited for soft and bio materials% The

electron diffraction ca"abilities of the microsco"e 'as e&tensiely used to determine the crystal

structures of the materials%

5%5%6%


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5 angstrom% This makes it es"ecially suited for determining the lattice structure of nano'ires%

Additionally$ it is also 'ell suited to obsere the defects in the crystals of ceramic nano"articles%

5%5%8% Scanning 3lectron /icrosco"yS3/ imaging 'as "erformed on Schottky Field 3mission Scanning 3lectron /icrosco"e

9S3/; 930 Gemini 788:; and analytical high resolution S3/ 9C30 @?::F;% S3/ analysis

has been used e&tensiely throughout the research% The nano"article si)e distribution analysis is

carried out using S3/% Traditional and high through"ut electros"inning nanofiber formation and

diameter si)e obserations are also carried out using S3/%

References7% A%/% A)ad and P%J% Gouma% 3ncyclo"edia of 2anoscience and 2anotechnology$


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Chapter 1& 8euristic model of polymorphic transitions

As described in the "reious section$ binary metal o&ides go through a "olymor"hic

transition 'hen they are heat treated at high tem"eratures% Polymor"hic transformation

tem"eratures in nanocrystalline binary metal o&ides are obsered to be lo'er than bulk%

3&am"les include ,-Fe501 to .-Fe501Q7$5$ monoclinic to orthorhombic transformation in

/o01Q1$ anatase to rutile transformation in TitaniaQ6$ , to . Alumina transformationQ8$?%

/any models e&ist 'hich deelo" our understanding of the mechanism of melting tem"erature

lo'ering due to reduction in "article si)eQ@$ cohesie energy based models to "redict

thermodynamic "ro"erties of metallic nano"articlesQE$% But only a fe' attem"ts hae been

made to study the thermodynamics of solid-solid "hase transformations of nanocrystalline binary

metal o&ides$ theoretical alues for anatase to rutile transformation tem"erature Q7: 'ere

calculated to be bet'een ?:: and E::N at a critical "article si)e of 76nm% Gouma et al% 97;

Q77 obsered rutile gro'th at as lo' as ?@1N and 'ith a critical "article si)e of Enm$ 'hich 'as

in agreement 'ith the earlier theoretical "redictions% Jn this communication for the first time$ an

attem"t is made to "roide a generali)ed formula$ accounting for entro"ic$ si)e$ and other effects

that control the onset of "olymor"hic "hase reactions in nanocrystalline o&ide systems%


50/124


51/124

Follo'ing 'e demonstrate ho' these factors contribute in "redicting the critical "article si)e

belo' 'hich the behaior of metal o&ides so noticeably changes%

For the conce"tuali)ation of the thermodynamics inoled for nanocrystalline metal

o&ides$ 'e need to find an e&"ression for the gibbs free energy of "hase transformation$

jG\.9T$r;$ from a "hase to . "hase% From 3quation 91%7;+

jG\.9T$r; Z j


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Gamma 85%1: C(N(molQ7?Delta 8:%?5 C(N(molQ7?

Fe501 Al"ha E@%6 C(N(molQ7?Gamma 1 C(N(molQ7@

Ti05 Anatase 6% C(N(molQ7?

Rutile 8:%55 N(molQ7?

Although for many materials data for standard free energy of formation is aailable in

s"ecific tem"erature range Q7?$ and for them the alues can be "lugged into the follo'ing

equation and free energy at s"ecific transformation tem"erature can be directly found$

G9T; Z A W B%T W !%T%lnT W D%T5W 3%T1W F(T 91%6;

But for most binary metal o&ides such data may not be aailable$ in those cases the

a""ro&imation$ jG\.9T$r; Z j


53/124

Thus$ equation 91%8; can be 'ritten as$

jGs\.9T$r; Z 9m%, m.%,.;(r 91%E;

1%7%6% Surface Stress

Jt has been obsered that there e&ists a "ressure contribution due to the e&istence of

surface stress and due to the curature of nano"articles Q15% 3hrman97;Q75 obsered in the

case of silica that the obsered theoretical "redictions of "article si)e 'ere not in agreement 'ith

the obsered e&"erimental results and attributed this discre"ancy u"on the internal "ressure

effects% Similar internal "ressure effects 'ere also obsered in another study Q11 done on

anatase titania% The authors had to a""ly additional "ressure to nanocrystalline anatase to

com"ensate for the internal "ressure due to the nanosi)e%

This e&cess "ressure can be quantified into the Gibbs free energy of "hase

transformationQ7:$

jGs\.9r; Z Pin%m 1%Pin.%m. 91%;

Jnternal "ressure can be e&"ressed in term of fs$ 'hich is the surface stress due to the

e&cess internal "ressureQ76$56$

PinZ 5% fs(r 91%7:;

3quation 91%7:; can be re'ritten as$

jGP\.9r; Z 95%fs%m 5%fs.%m.;(r 91%77;


54/124


55/124

Aboe equation can be 'ritten in terms of density H^I of "hase . or $ by substituting$ mZ /(^$

'here / is the molar mass$

rc\.9T;Z /%9,.(^. ,(^W 5%fs(^-5%fs.(^.; (9j


56/124

5 0)den 0)demir and Subir N% Baner4ee$ _


57/124


58/124

5@ /% R% Ranade$ A% 2arotsky$


59/124

Chapter !& /ungsten /rioxide nano4ire in+situ /EM synthesis

Jn this section 'e describe the gro'th of highly aligned nano'ires of the monoclinic

"olymor"h transformed from metastable "olymor"hs under the influence of an electron beam%

The mor"hological transformation is a ra"id manifestation of a self-catalytic massie-ty"e "hase

reaction occurring in acuum% The reaction a""ears to be self-cataly)ed due to the instability of

the metastable "hase nano"articles under the influence of the beam% Jt is sho'n that the

nano"articles due to the electron energy of the beam become 'ire-like gro'ing into highly

aligned "eriodic crystalline structures% This finding has im"ortant im"lications for achieing

scalable and controlled nano'ire synthesis%


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6%7% Precursor "hase

The #01nano"articles 'ere synthesi)ed using a sol-gel route% Tungsten Jso"ro"o&ide 98K

'( in Jso"ro"anol$ Alfa Aesar; 'as used in equal quantities 'ith$ ethanol and acetic acid 9to

control "


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in figure 89Right; 'here 'e can obsere the "resence of other dimmer rings in addition to the

rings corres"onding to the 97::;+ 977:;+ 957:; "lanes of the metastable cubic "hase of tungsten

trio&ide% The nano"articles of cubic tungsten trio&ide are sho'n in figure 89eft;$ 'ere

characteri)ed under a C30 C3/-76:: analytical T3/%

0igure 3(a)Low magnification SEM image of sol-gel synthesied nanoparticles of tungsten

trio!ide annealed at "#$%, (b)shows the higher magnification SEM image with uniformly

distri&uted particle sie' (c) Shows the Raman spectra of the nanoparticles, the &andso&sered are mared' (d)Shows the *R+ of the nanoparticles'

0igure , 3 (Left)EM micrograph of the cu&ic nanoparticles of ungsten trio!ide, (4ight)

S0+ pattern, &righter rings can &e inde!ed to (6$$)/ (66$)/ (76$) planes of the cu&ic phase'


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6%5% Jn-situ nano'ire synthesis

The "re"ared nano"articles 'ere dro" coated onto a T3/ grid consisting of a Formar

su""orting film 95:: /esh !o""er grids 'ith su""ort films$ T3D P3A Jnc%;% Formar is a

family of "olymers formed from "olyinyl alcohol and formaldehyde as co"olymers 'ith

"olyinyl acetate Q5$ they are non-reactie class of com"ounds Q1 and additionally hae a

2FPA @:6 reactiity rating of :% A 75:k soft(bio materials analytical transmission electron

microsco"e 9C30 C3/-76::; 'as used to gro' the nano'ires in-situ% The formar film 'as

intact after the gro'th and 3DS analysis 9A""endi& G; sho'ed no carbon on the nano'ires 9ery

small atomic "ercentages 9small "eak; is obsered$ 'hich can be attributed to the "resence of

su""orting film in the icinity$ similarly the co""er "eak is due to the co""er mesh of the T3/

grid;% To com"letely rule out the "ossibility of the transformation being affected by the "resence

of carbon on the su""ort film$ the nano'ire gro'th 'as carried on a grid containing Silicon

2itride as su""ort film$ the nano'ires gre' similar to the formar su""ort film%

A 75:ke electron beam 'as used for the nano'ire gro'th% The beam 'as s"read initially

'hile obsering the nano"articles and it 'as then slo'ly conerged on the sam"le$ the cubic

"hase of the nano"articles instantaneously gre' into highly aligned nano'ires 9Figure ?; due to

the energy im"arted by the electron beam% #hereas the stable monoclinic "hase did not

transform$ this is consistent 'ith obseration of certain nano"articles not transforming% Such

energy can be calculated% The C30 C3/76:: T3/ uses a aB ?filament as the electron source

'hich gies a current density of 76A(cm5on the s"ecimen 'hen using a :%Emrad illuminating

a"erture Q6% This current density can be conerted to energy density using the "o'er formula of

current times oltage$ 'hich gies a alue of 7?E: 4oules(cm5for 7 second at 75:k accelerating

oltage% For 8::: magnification$ a focused electron beam of 75:k 'ould "roide @?ke of


63/124


64/124

The

high

0igure 53 (Left) ypical Electron diffraction pattern taen from a region with

concentration of nanowires' (4ight)Schematic of crystal structure of the monoclinic p

with the preferred growth direction, the distortions hae &een e!aggerated to ser

purpose of illustration'


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concentration of nano'ires causes a distinct diffraction "attern at high magnification$ the

diffraction "attern can be made into a ring "attern by reducing the magnification and taking a

larger sam"le area into focus$ figure @9left;% The rings corres"ond to lattice "lanes 9::5;$ 95::;$

955:;$ 9575;$ 9717;$ 9::6;$ 9767;$ 9 15;$ in Figure @9left;$ of the monoclinic ,-#03 19C!PDS 61-

7:18;% A Re01ty"e corner sharing octahedral forms the basic building block of all "hases of

tungsten trio&ide% The crystal lattice symmetry of monoclinic #01is a distorted rhenium o&ide

cubic configuration$ figure @9Right;%

0igure 73(Left).ighly aligned nanowires, showing the K$$6 growth direction along the

long a!is/ (4ight)%orresponding diffraction pattern'The nano'ires gro' in to highly aligned crystals$ figure E9eft; along the =::7> direction$

'ith the corres"onding SAD "attern is sho'n in figure E9Right;% The high resolution T3/ 'as

carried out on C30 57::F Field 3mission orent) T3/% The microgra"h in figure @ along 'ith

the SAD "attern in figure 9Jnset; gies a more detailed "icture of the crystal structure% The

s"acing of :%1E1nm and :%11nm corres"ond to the 9::5; and 975:; "lanes of the monoclinic


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"hase of tungsten trio&ide 9C!PDS 61-7:18;% Gro'th direction of =::7> "er"endicular to the

9::5; "lane is along the long a&is of the nano'ires%

0igure 63.igh resolution EM of the structure of the nanowires/ (!nset)S0+ pattern from

indiidual nanowire'


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6%1% Thermodynamics of nano'ire formation

By e&"loring the theoretical mechanism 'hich goerns the metastable to stable "hase

transitions in nanocrystalline materials QE 'e can further e&"lain the gro'th of stable "hase

nano'ires% The thermodynamic data for a "suedocubic "hase 'as unaailable$ thus that of cubic

is used% The free energy of a nano"articles system 9jG2S9T$r;; or a nano'ire system

9jG2#9T$r;; de"ends on three factors$ surface free energy 9jGS9T$r;;$ surface stress

9jGP9T$r;;$ and free energy due to "article sha"e 9jGG9T$r;;$ as discussed in "reiously

"ublished 'ork by the authors QE%

Thus$

jG2S9T$r; Z jG9T$r; W jGS9T$r; W jGP9T$r; W jGG9T$r;

96%7;

jG2S9T$r; Z Am%, W Pin% W .%


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Therefore$ the free energy for the nano'ire 9jG2#9T$r;; and nano"articles

9jG2S9T$r;;$ systems can be gien by$

jG2S9T$r; Z 96%%r5;%, W 95%fs(r;%96%%r1(1; W .%S


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Table -4 +ata accumulated from the literature, and the free energy calculations

0bsered 2ano'ire radius 5:nm

Predicted 2ano'ire length 8?:nm

Aerage nano"article radius 6:nm

,monoclinic :%?58 C(m5Q

,cubic 7%?@ C(m5for 9::7; "lanes Q

S


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Table @ tabulates all the alues substituted and the results for the free energy% Jt can be

seen that the free energy alues for nano'ires is greater than for nano"articles% Thus for the

transformation to occur an e&ternal source 'ith the energy equialent to the difference in the free

energy of nano'ire and nano"article is necessary for the gro'th to occur% jG2#9T$r; -

jG2S9T$r;%

To ascertain the a""licability of this theory$ a lot of data "oints 'ere "lotted using

/icrosoft 3&cel% The gra"h in figure 7: "lot difference in free energy 9jG2#9T$r; -

jG2S9T$r;; s nano"article radius% Jn the eent that a single nano"article formed a single

nano'ire s"ontaneously 9'ithout any e&ternal "erturbation;$ then a 1:-6:nm aerage "article

si)e 'ould form a nano'ire 'ith radius bet'een E and 7:nm 'ith as"ect ratio 1:$ this ha""ens

'hen jG2#9T$r; - jG2S9T$r; goes to )ero% Although this is not 'hat is obsered in our

e&"eriments$ the reaction is not s"ontaneous 9e&ternal "erturbation is required; and the gro'th is

due to coalesce and gro'th of nano"article therefore the aerage nano'ire radius obsered is

5:nm 'ith lengths of u" to half a micron%


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Figure 10: (Lef)Free energy (in KJ/mole) vs nanopar!le ra"i#s $or "i%eren& ra"ii an" aspe!& rao

nano'ires. (Right)Free energy (in KJ/mole) vs nanopar!le ra"i#s eeping aspe!& rao !ons&an&.

References

7% G% % Frey$ Rothschild$ C% Sloan$ Rosentseig$ R% Po"oit) Biro and R% Tenne 95::7;%

Jnestigations of nonstoichiometric tungsten trio&ide nano"articles%2' Solid State %hem'

7?5$ 1::%5% 3% Daison$ #% !olquhoun 97E8;% Ultrathin formar su""ort films for transmission

electron microsco"y2' Elect' Micros' ech% 2* 18-61%1% /% Stadermann$ S% 0% Nucheye$ C% e'icki$ and S% A% etts95:75;% Radiation tolerance

of ultra-thin Formar films%0ppl' Phys' Lett% 7:7$ :@7:E%6% C% !%


72/124

E% S% Sood$ P% Gouma 95:71;% Polymor"hic "hase transitions in nanocrystalline binary metal

o&ides2' 0m' %eram' Soc',? Q5$ 187186%

% Peter /% 0lier$ Ste"hen !% Parker$ Russell G% 3gdell and Frances


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Chapter & Moly.denum /rioxide nano4ires .y electrospinning

8%7% /o013lectros"un nano'ire formation

Single crystals of ceramic materials in one-dimensional nano'ire mor"hology by a single

ste" non-catalytic "rocess has been inented and demonstrated by P% Gouma and her grou" Q7%

2anofiberous mats hae been "roduced by blend electros"inning 'hich is a bottom u" synthesis

method Q5% These com"osite nanofibrous mats are heat treated and after doing so "ure metal

o&ide nano'ires are formed$ and there e&ist no organic residual material% !ontinuous single

crystals of e&tremely high as"ect ratio 9nm diameter s mm; are "roduced in this 'ay%

Such a high as"ect ratio gro'th needs to be analy)ed further$ and main issues need to be

analy)ed 'ith regards to their mor"hology and structure% The reasons for the gro'th into single

crystals and gro'th into high as"ect ratio structures$ both need to understood in de"th%

/etastable to stable "olymor"hic reactions in metal o&ides hae been demonstrated earlier Q1-6%

Jn such reactions a critical "article si)e nuclei is initially formed and further the nanocrystalline

aggregates gro' into "referentially oriented high as"ect ratio structures 'hile retaining

mor"hology of the original aggregates Q6%

The "olymeric fiber 'all structure is im"ortant for determining the mor"hology of

nanocrystalline aggregates in the sol-gel o&ide in electros"un com"osites 9ref Gouma et al%;% The

sol-gel com"onent in the com"osite gro's undisturbed until it runs out of material forming into

single crystal "olymor"hs% Relatie "hase stability of "olymor"hs determine the heat treatment

conditions$ this is unique for each metal o&ides and yield stable "hase in single crystalline

nano'ire structures% This 'as first sho'n in our grou" for #01 system% A PP-#01 Q5


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com"osite 'as used to electros"un single crystal nano'ires of #01$ these nano'ires 'ere

different from the ones formed in earlier 'ork% Such a "rocess forms ery large as"ect ratio

nano'ires$ the as"ect ratio is much larger than the earlier formed nano'ires although they hae

larger diameters than before% This 'as the foundation of the later 'ork done on /o01QE in our

grou"$ and therefore to assess the uniersality of such a system the 'ork done here is also based

on such a system% Jn such a system a com"osite system similar to PP-#0 1$ a PP-/o01

system is electros"un to form enca"sulated nanofibers$ 'hich are then heat treated to form the

nano'ires$ similar to ref 7C$ but much different from the 'ork described earlier%

/o01is kno'n for gro'ing in anisotro"ic mor"hologies% The 9orthorhombic "hase; consists

of distorted edge-sharing octahedra% 0ctahedral layers share edges along the a-a&is Q7:: and

corners along the c-a&is Q::7% Along the b-a&is$ the layers are bound by an der #aals forces%

Gro'th along the a-a&is is energetically faorable o'ing to the fact that only one /o0 bond

has to be created as o""osed to the c-direction$ in 'hich t'o such bonds 'ill hae to be formed%

arious methods hae been used to crystalli)e different "olymor"hic forms of /o01% /o01

nano"articles hae been synthesi)ed using sol-gel$ hydrolysis of /olybdenum iso"ro"o&ide

leads to colloidal /o01nano"articles 9Figure 6;$ u"on heat treatment these colloidal "articles

crystalli)e in different "olymor"hic structures Q@$ $ metastable -/o01 monoclinic structure

bet'een 6::! to 68:! and stable .-/o01orthorhombic bet'een 68:! and 878!%

0rthorhombic /o01 'as synthesi)ed by ion beam de"osition Q8$ ?$ @ and as electros"un

nano'ires QE for gas sensing a""lications% 2anostructured orthorhombic /o01$ the stable

"olymor"h of /o01has also been synthesi)ed by Ding et al% Q7: using thermal o&idation of

/olybdenum and i et al% Q77 synthesi)ed /o01 nanostructures using an enironmentally

friendly chemical route% The metastable "olymor"hs$ -/o01monoclinic and h-/o01he&agonal


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structures are much more com"licated to synthesi)e as nanostructures% -/o01 monoclinic

nanosheets 'ere de"osited onto silicon substrate using atmos"heric micro"lasma QE%


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0igure 11 (a) SEM micrograph of metal o!ide encapsulated electrospun nanofi&ers' (b)

Moly&denum trio!ide encapsulated inside ele!trospun fi&ers, (!nset) E+S pattern of the

encapsulated particles'

To get molybdenum trio&ide nano'ires$ the nanofibers 'ere heat treated in a tube

furnace% The heat treatment 'as carried out at tem"eratures ranging from 65:! to 8::!% The

yield of nano'ires 'as obsered 9Figure 8a;% Jt 'as found that the yield ma&imi)ed at 8::!%

Although still not ideal$ the nano'ires 'ere ery high as"ect ratio 9Figure 8b; 'hich is useful for

sensing a""lications%

0igure 1"4 (a)SEM micrograph of heat treated nanofi&ers at #$$%/ (b)SEM micrograph of

high aspect ratio nanowires

a) b)

a) b)


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The nano'ires gre' in high as"ect ratio structures after heat treatment% Preious DS!

results Q7E hae sho'n that /olybdenum trio&ide is monoclinic -/o01u"to tem"erature of

6E8! and then transforms to its stable form orthorhombic .-/o01% Although it 'as obsered

that nano'ires al'ays gre' into the orthorhombic .-/o01 structure no matter the heat

treatment tem"erature% At 68:!$ figure ?9a; the nano'ires 'ere orthorhombic 9C!PDS :8-

:8:E;$ similarly at 8::! 9Figure ?9b;;% Figure ?9c; sho's high resolution T3/ image sho'ing

the lattice structure of the nano'ires$ the orthorhombic .-/o01 consists of distorted edge-

sharing /o0?octahedra$ 'hich share edges along the a-a&is Q7:: and corners along the c-a&is

Q::7% The measured d-s"acing for a-a&is 'as 1%@[ and 1%@5[ for c-a&is$ 'hich is in agreementto the unit cell "arameters for the aboe crystals aZ1%?1: [$ bZ71%E8? [$ and cZ1%??? [

corres"onding to the standard C!PDS file :8-:8:E% Jt is kno'n that 9:7:; is the closed "acked

"lane in the .-/o01$ and the gro'th along Q::7 direction has been 'idely documented Q7$ 5:%

0igure 1&3 (a)anowire growth after heat treatment at 1#$% and (inset) its corresponding

S0+ pattern/ (b)anowire growth after heat treatment at #$$% and (inset) its corresponding

S0+ pattern/ (c).REM image of the nanowire and (inset) its corresponding S0+ pattern'

8%1%/ulti-4et

3en though the single 4et technique makes a aluable research tool to e&"eriment on getting

the best "recursors and "rocess "arameters for nanofiber mat formation of the desired


78/124

configuration$ it has largely "roduced small amounts of nanofibers that hae satisfied the

engineering curiosity$ rather than offering a "rocessing route for nanofiber manufacturing% Thus

focus has diersified into high through"ut techniques% Researchers hae "ro"osed arious

techniques and mechanisms$ enlisted in Table E% Although none so far has seen a""ro"riate for

easy ado"tion$ mainly due to shortcomings like$ requirement for high oltages in some cases

Q57$ requirement for s"ecific material iscosity in others Q5?% Jt is our belief that an

electros"inning mechanism closest to traditional electros"inning technique 'hile simultaneously

increasing the yield to a""ro"riate leels$ in addition to being scalable is the requirement% Thus

'e "ro"ose a scalable set u" 'ith an o"erating mechanism similar to the traditional set u" 'hich"roduces fibers of similar mor"hology% All the "rocess "arameters$ oltage$ 'orking distance$

flo' rate$ material concentration$ ariable in traditional set u" are also ariable in the scaled u"

set u"%

Table 53Preious attempts to increase production rates of electrospun fi&ers

Source Collector >oltage Production :ate 4eferences

Flat disk !ylindrical 88N for initiation+ 7?N

'orking

U" to :%?E6gm(hour :76;

Porous cylindricaltube

#ire meshty"e

5:k 8gm(hour :77;

/ulti"le needles Rotatingsolid cylinder

5:k U" to :%71Egm(hour :7";

Graity("ressuredrien nine 4et

no))les

Flat collector 7:k across allnine no))les

55%8 ml(9cm5min;to 55%8 l(9cm5min;"er 7 cm5of thes"inneret "late

:71;

!onical metal 'ire-coil

Flat collector ?:k U" to 5%@8g(h :7#;


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/agnetic fieldinduced

Flat collector U"to 1:k 75-fold increase :7?;

8 no))les 'ith holeson the surface and

conductie fluid

/oing flat'all

7:-6:k -- :7@;

8%1%7% !ase study on PP and !A fibers

Polymeric solutions named$ P7$ and P5 'ere used for electros"inning% Solution P7 'as

:%7m/ "olyinyl"yrrolidone 9PP+ 9!


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S3/ and T3/ microgra"hs% Thus 'ith similar "rocess "arameters this scalable set u" can

"roduce fibers com"arable to T23%

Table 73%omparison &etween traditional needle electrospinning and high throughputelectrospinning for PFP

P7 P5T23 Scaled u" set u" T23 Scaled u" set u"

Production :%55? g(h 2&36 gh :%556 gm(hour 2&52 ghFlo' Rate :%:18-:%:68

ml(min:%6-:%8 ml(min :%:18-:%:68

ml(min:%6-:%8 ml(min

oltage 5:k 58k 55k 58k!oncentration :%7m/ :%7m/ 7:K solent 7:K solentSource tocollectordistance

7::mm 78:mm 7::mm 78:mm

Ag% fiber diameter 7::-1::nm 5::-18:nm :%?-7 micron :%E-7%8 micron

0igure 13 (a).REM of .igh-throughput P6 electrospun fi&ers/ (b)SEM of E processed P6

electrospun fi&ers


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0igure 1,3(a)SEM of .igh-throughput P7 electrospun fi&ers/ (b)SEM of E processed P7

electrospun fi&ers

8%1%5% Process ParametersAscertaining the "rocess "arameter is essential to control the nanofiber mor"hology% To

do so 'e use the "olymer Polyinyl"yrrolidone 9PP; 'ith ethanol as the solent% The ceramic

nanofibers are synthesi)ed using a com"osite of PP and /o01sol-gel% Thus using PP as a

reference system to deelo" these "arameters can gie a fair control oer the nanofiber synthesis%

!a"illary action generated by the needle in traditional needle electros"inning technique is an

essential com"onent to get enca"sulation of /o01sol-gel inside the PP "olymer%

Figure sho's the de"endence of oltage on the nanofiber diameter$ there seems to be a

"arabolic de"endence$ 'ith fiber diameter smallest at 55k and 1:k$ although at lo'er

concentration of :%:Em/ there 'as bead formation and at higher oltage of 1:k there 'as high

distribution of nanofiber diameter as can be seen by larger error bars$ thus 56k 'as chosen for

/o01 nanofiber synthesis% Figure also sho's negligible de"endence of concentration as

com"ared to oltage to'ards nanofiber diameter% The "roduction rate has a similar "arabolic

de"endence to oltage and the error bars sho'n are due to the concentration ariation% 0n

aerage the "roduction rate is 5%8gm(hr%


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@c:%7m/ 56k @d:%7m/ 5?k

0igure 153 (a*d)SEM micrographs for different oltages and concentration for PFP fi&ers'

8%1%1% 2anofiber Synthesis

Jn this section 'e demonstrate an a""lication of this set u" for "roducing high yield of

ceramic nanofibers% A com"osite material of "olymer and ceramic sol is electros"un to form high

as"ect ratio nanofibers of single crystal /o01similar to that accom"alished by Gouma et al%

earlier Q17% The source disk of the set u" 'as modified from the earlier set u" Q5?$ the holes

diameter 'as reduced to mm as sho'n in figure 7: 9a-d;$ and the holes 'ere "laced at the

bottom corners of the disk at an angle of 68 degrees% Figure 5 and 1 sho's the schematic of the

set u" used% The source is constantly re"lenished using a syringe "um" 9Nd scientific; at a

constant flo' rate% The 'orking distance although ariable$ is ke"t constant at 78cm for this


84/124

study%


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@c T3/ of /o01nanofibers synthesi)edusing T23$ 9Jnset; sho's the selectedarea diffraction$ 'ith the "laneslabeled for =:7:> )one a&is%

@d T3/ of /o01 nanofibers synthesi)edusing high through"ut set u"$ 9Jnset;sho's the selected area diffraction$'ith the "lanes labeled for =::7> )one

a&is%

0igure 173Electron microscopy comparison &etween the nanofi&ers synthesied &y traditional

needle setup (a 8 c) and those synthesied &y high througput set up (b 8 d)'


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5% N% Sa'icka and P% J% Gouma$ 3lectros"un com"osite nanofibers for functional

a""lications$ C% 2ano"article Research$ E9?;$ ""% @?-@E7$ 5::?%1% P% J% Gouma$ P% N% Dutta$ and /% C% /ills$ Structural Stability of Titania Thin Films$

2anostructured /aterials$ 779E;$ ""% 7517-751@$ 7%6% P% J% Gouma and /% C% /ills$ Anatase to Rutile Transformation in Titania Po'ders$ C%

Am% !eram% Soc%$ E6 Q1$ ""% ?7-?55$ 5::7%8% Reactiely S"uttered /o01 films for ammonia sensing$ A%N% Prasad$ P%J% Gouma$ D% C%

Nubinksi$ C%

Documents

Polymorphism control in nanocrystalline binary metal oxides