Ultrasonic spray pyrolysis and electroless deposition for the synthesis of nanostructured metal / carbon microspheres

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LEABHARLANN CHOLAISTE NA TRIONOIDE, BAILE ATHA CLIATH TRINITY COLLEGE LIBRARY DUBLIN

OUscoil Atha Cliath

The University of Dublin

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Ultrasonic spray pyrolysis and electroless

deposition for the synthesis of nanostructured

metal/carbon microspheres

A thesis submitted to the University of Dublin for the degree of Doctor of

Philosophy

by

Paul Duffy B. A. (Mod.)

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T R !N rrY C O LL E G E

5 JAN 2015

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Declaration

I d e c la re th a t this thesis has not been s u b m itte d as an exercise fo r a d e g re e a t this or any other university and it is entirely m y own work. I agree to deposit this thesis in the

U n iv e rs ity ’ s open access institutional repository or a llo w the library to do so on my

behalf, subject to Irish C opyright Legislation and T rin ity College L ib ra ry conditions o f

use and acknowledgement.

I declare that this report details entirely m y own work. Due acknowledgements and

references are given to the w ork o f others where appropriate.

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Acknowledgements

I w o u ld like to th a n k D r P au la C o l a v it a fo r h e r su p p o rt an d p atien ce o v e r the c o u rs e o f

m y P h D studies. S h e has b een a c o n s ta n t g u id e in the im p le m e n ta tio n o f this study and

w a s alw ays av a ilab le for d isc u ssio n s a n d insight.

I w o u ld a lso like to tha nk D r L a u r a Soldi an d D r D ilu sh a n Ja y a s a n d ra th e post d o c s in

m y group. Both g av e im m e n s e h elp in the im p l e m e n ta tio n o f m y stu dies, p rac tica lly and

via ideas a n d sug g e stio n s. I w o u ld also like to a c k n o w le d g e all the su p p o rt given to m e

by the o th e r m e m b e rs o f o u r re se arch group. I w o u ld also like to a c k n o w le d g e all the

p e o p le at the c h e m is try d e p a rtm en t, w ith o u t training on in stru m e n ta tio n and b a c k g ro u n d

support and se rvic e, none o f this w o u ld h a v e b ee n possible. In particular, I w o u ld like to

tha nk D r M a n u e l R u e th e r for m u c h help o v e r the years. A lso D r K arsten R o d e fo r help

in data inte rpretatio n.

I w o u ld a lso like to a c k n o w le d g e D r K ev in M e tz , o u r co ffee lovin A m e ric a n

co llaborator. M a n y go o d talks w ere to be h a d on the w ay to S E M sessions.

Lastly, I w o u ld like to thank fam ily a n d frie n d s for su p p o rtin g me th r o u g h o u t m y P h D

studies.

I grate fully a c k n o w le d g e su p p o rt f ro m the E n v iro n m e n t a l P rotection A g e n c y , Ireland

(E P A , Ireland) fo r funding. T rinity C o lle g e D u b lin for its start up fu n d s an d the S chool

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Abstract

T h e o b je c tiv e o f this w o rk is to synthesis various n a n o -m e ta llic carbon m icro s p h e re

c o m p o s ite m a te ria ls fo r e n v iro n m e n ta l an d ca ta ly tic ap p licatio n s. U ltraso nic spray

pyrolysis and electro less d ep ositio n te c h n iq u e s w e re used to achieve this. The

co m p o sites m a teria ls w e re ch arac teris ed and th e ir su b se q u en t a c tiv ity w as

d e m o n s tra te d .

C arbon m icro s p h e re p article o f various m o rp h o lo g y and su rface area have been

synthesised using ultraso n ic spray pyrolysis. T hese particles w e re ch arac teris ed using a

c o m b in a tio n o f Ram an spectroscopy, scanning e le c tro n m icroscopy (SEM ), BET

a d s o rp tio n , FTIR and zeta p o te n tia l m e a s u re m e n ts . SEM im ag e ry d e te rm in e d th e

sh ap e an d m o rp h o lo g y o f th e various carb on m a te ria ls . It w as d e m o n s tra te d via

Ram an sp ectro scop y th a t carb on m icro p a rticles w e re g ra p h itic in n a tu re , h o w e v e r,

FTIR and Zeta p o te n tia l m e a s u re m e n ts d e m o n s tra te d th e p resen ce o f c a rb o x ylate

groups on th e ca rb o n m icro s p h e re surface.

It w as d e m o n s tra te d th a t carbon m icrosp heres synthesised using ultraso n ic spray

pyrolysis had c o n tro lla b le size by leveragin g p rec u rso r c o n c e n tra tio n s . This was

c o n firm e d via a c o m b in a tio n o f SEM im ages and d y n a m ic light sc atte rin g e x p e rim e n ts .

FTIR and zeta p o te n tia l e x p e rim e n ts d e m o n s tra te d th a t C M surfaces display ch em istry

w h ich a llo w e d th e g ra ftin g o f d iffe rin g ch em ical m o ie tie s via d ia zo n iu m ch em istry.

Using electro less d ep o s itio n te c h n iq u es , n a n o p a rtic le s o f th r e e m e ta l m a teria ls w e re

n u c le a te d and g ro w n on th e C M surfaces. In p articu lar. P allad iu m and silver m e tal

w e r e n u c le ate d on th e CM surface fro m m e ta l salts using c o ffe e as a g ree n reducing

a g e n t, a t ro o m te m p e ra tu r e . M e ta l co m p o site m a te ria ls w e r e ch arac teris ed using a

c o m b in a tio n o f SEM , x-ray d iffractio n (XRD), e n erg y dispersive x-ray spectroscopy

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p a r tic le s o b s e rv e d v ia S E M . U s in g T G A a n d EDS in c o m b in a tio n , m ass lo a d in g s w h e r e

e s tim a te d w it h A g /C a n d P d /C m ass ra tio s o f ( 1 3 .5 ± 1 .5 )% a n d ( 7 .4 ± 1 .0 )% w / w ,

re s p e c tiv e ly .

S u zu k i c o u p lin g r e a c tio n c o n f ir m e d t h e v ia b ility o f t h e P d /C c o m p o s ite m a t e r ia l f o r

c a ta ly tic a p p lic a tio n s . A g /C c o m p o s ite re d u c e d 4 - n it r o p h e n o l, a c h ie v in g re a c tio n ra te s

w h ic h a r e c o m p a r ib le t o o t h e r s u p p o r t s ilv e r n a n o p a r tic le s in t h e lit e r a t u r e .

Iro n a n d iro n o x id e n a n o p a r tic le s w e r e s y n th e s is a t t h e c a rb o n m ic r o s p h e r e s u rfa c e

u s in g e le c tr o le s s d e p o s itio n . T h e r e p o r te d e le c tr o le s s d e p o s itio n a p p ro a c h p r o d u c e d a

c o m p o s ite F e /F e O x /c a r b o n m ic r o s p h e r e w it h n a n o p a rtic le s o f a n a r r o w ly d is p e rs e d

size . A c o m b in a tio n o f X - r a y p o w d e r d if f r a c tio n (X R D ) a n d X -ra y a b s o rp tio n

s p e c tro s c o p ie s (EXAFS a n d X A N E S ) w a s u s e d in o r d e r t o d e t e r m in e t h e s tr u c tu r e a n d

c o m p o s itio n o f t h e F e /F e O x /c a r b o n m ic r o s p h e re s . M ic r o s p h e r e s w e r e f o u n d to d is p la y

( 1 4 ± 1 )% iro n c o n t e n t ( w /w ) , w h e r e b y (1 2 ± 3 )% o f Iro n a to m s w e r e p re s e n t as

m e t a llic iro n a n d t h e r e m a in in g as m a g h e m it e (F e2 0 3>. F in a lly , w e s h o w t h a t t h e r e m o v a l c a p a c ity o f F e /F e O x /c a r b o n m ic r o s p h e re s f o r C r (V I) is ( 2 0 ± 2 ) m g g ' l a n d t h a t

t h e m a x im u m s u r fa c e d e n s ity f o r C r a d s o rb a te s Is ( 6 0 ± 6 ) m g m ‘^, th u s s u g g e s tin g t h a t

t h e s e a r e p ro m is in g m a te r ia ls f o r t h e r e m o v a l o f w a t e r p o llu ta n ts f r o m a q u e o u s

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3 .1 .1 USP m e th o d s fo r carb on m icro s p h e re synthesis... 4 7 3 .1 .2 M o d ific a tio n o f surface ch e m is try and c h a rg e ... 4 8 3 .2 E x p e rim e n ta l... 4 9 3 .2 .1 M a te ria ls ... 4 9 3 .2 .2 Synthesis o f porous ca rb o n m ic ro s p h e re s ... 50

3 .2 .3 F u n ctio n a lisa tio n o f ca rb o n m icro s p h e res... 51

3 .2 .4 C h arac terizatio n te c h n iq u e s ... 52

3 .3 Results and Discussion... 53

3 .3 .1 C h aracterisatio n o f C M s... 53

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3.3.3 S u rfac e c h a r g e a n d c h e m i s t r y ... 62

3 .3.4 Discussion a n d s u m m a r y ... 68

3.3.4.1 C arb o n m i c r o s p h e r e c h a r a c t e r i s a t i o n ... 68

3.3.4.2 Size co n tro l o f c a r b o n m i c r o s p h e r e s ... 71

3.3.4.3 S u rfac e c h e m is try a n d f u n c ti o n a lis a ti o n o f c a r b o n m i c r o s p h e r e s ... 78

3.4 C o n c lu sio n s... 79

R e f e r e n c e s ... 81

4 Sy n th esis o f M e t a l / C a r b o n c o m p o s i t e s via g r e e n e l e c t ro l e s s d e p o s i t i o n 83 4 .1 I n t r o d u c t i o n ... 84

4 .1.1 C arb o n s u p p o r t m a t e r i a l ... 84

4.1.2 Reduction of m e ta l ions using c o f f e e ... 85

4.2 E x p e r im e n ta l... 86

4.2.1 M a teria ls a n d r e a g e n t s ... 86

4 .2.2 Synthesis of c a r b o n a n d m e t a l / c a r b o n m i c r o s p h e r e s ... 86

4.2.3 Reactivity s t u d i e s ... 87

4 .2.3 .1 Ag/CM rea ctivity... 87

4 .2.3 .2 Pd/CM rea ctivity... 87

4.2.4. C h a ra c te riz a tio n t e c h n i q u e s ... 88

4.3. R e su lts... 89

4.3.1. S ynthesis a n d c h a r a c t e r is a tio n of m e ta l /C M c o m p o s i t e s ... 8 9 4.3.1.1 S ynthesis a n d c h a r a c t e r i s a t i o n o f a c t i v a t e d C M ... 89

4.3.2 Catalytic activity of m e t a l / c a r b o n c o m p o s i t e m a t e r i a l s ... 101

4.3.2.1. R e d u ctio n o f 4 - n it r o p h e n o l using Ag/CM c o m p o s i t e s 102 4.3.2.2 Suzuki co u p lin g using Pd/C M c o m p o s i t e s ... 104

4.3.3 Discussion a n d s u m m a r y ... 106

4.3.3.1 Effect of S ensitisation s t e p o n c o m p o s i t e s ... 107

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4 .4 Conclusions... 113

References... 114

5 Synthesis of Fe/FexOy nanoparticles on porous carbon microspheres: Structure and surface reactivity... 118

5.1 Introduction... 119

5.1.1 Electroless deposition of Iro n ... 119

5.2 Experim ental... 121

5.2.1 M aterials and reagents... 121

5.2.2 Composite m aterial synthesis... 122

5.2.3 Characterization... 123

5.3 Results and discussion... 124

5.3.1 Characterisation of activation steps... 125

5.3.2 Iron deposition utilising m ethod A... 127

5.3.2.1 Synthesis of Fe/FexOy CMs using M etho d A ... 127

5.3.3 Iron deposition utilising m ethod B... 133

5.3.3.1 Synthesis and characterisation of Fe/FexOy CM using method B... 133

5.3.3.2 Removal o f C r(V I)... 140

5.3.4 Discussion and sum m ary... 142

5.3.4.1 M etho d A... 142

5.3.4.2 M etho d B... 145

5.4 Conclusions... 151

References... 153

6 Conclusions and fu tu re w/ork... 157

6.1 Conclusions... 158

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List of figures

Figure 1.1: Two polym er families used as metal NP supports fo r catalysis

Figure 1.2: Nanoparticles encapsulated in PAMAM or PPI dendrimers: complexation o f a

metal cation, then reduction to metal(O) by NaBH4, and aggregation giving the NPs inside the

dendrim er. Specifically, the preparation o f dendrimer-encapsulated bim etallic NPs is show/n

Figure 1.3: (a) Schematic illustration o f the steps involved in the functionalization o f carbon

nanofibers and subsequent proceudure fo r electroless deposition, (b) Chemical

transform ations involved in the nanofiber m odification

Figure 1.4: Core shell model o f an iron nanoparticle

Figure 1.5: TEM showing FeNP aggregates; Scale bar 500 nm

Figure 1.6: Model o f Surface charges and potentials. All potentials are defined w ith respect to

the potential in the bulk solution.

Figure 1.7: Electroless deposition processes: (a) Autocatalytic: The reduced noble metal

serves as the catalyst fo r fu rth e r reduction of the metal salt by the external reducing agent,

(b) Substrate catalyzed: The substrate surface catalyzes the reduction o f the metal salt by the

reducing agent (c) Galvanic displacement: The surface serves as the reducing agent and

electron source fo r reduction o f the metal salt

Figure 1.8: Schematic showing electroless deposition utilising catalytic seed particles

Figure 2.1: Schematic o f the USP system used in our studies.

Figure 2.2: Optical setup fo r DLS measurements

Figure 2.3: Example o f an experim ental correlation function

Figure 3.1: Schematic dem onstrating an example of a diazonium grafting reactions.

Figure 3.2: Photo o f the USP setup used fo r the synthesis o f CMs in our laboratory.

Figure 3.3: Raman spectrum o f microspheres (exc. 457 nm); the profile displays the D and G

bands th a t are characteristic o f amorphous carbons.

Figure 3.4: a) Shows SEM image o f a CM synthesized using LiDCA as a precursor solution;

Scale bar = 200 nm, b) Size distribution obtained fro m SEM images from 100+ particles fo r 1.5

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Fig u re 3 .5 : a) S h o w s SEM i m a g e o f a CM s y n t h e s i z e d using NaDCA a s a p r e c u r s o r s o l u t io n ; Sc ale b a r = 2 0 0 n m , b) Size d i s t r i b u t i o n o b t a i n e d f r o m SEM i m a g e s f r o m 10 0 + p a r ti c l e s fo r 1.5 M NaDCA p r e c u r s o r so l u t io n .

F ig u re 3 .6 : a) S h o w s SEM i m a g e o f a CM s y n t h e s i z e d u sin g KDCA a s a p r e c u r s o r s o l u t i o n ; Scale b a r = 2 0 0 n m , b) Size d i s t r i b u t i o n o b t a i n e d f r o m SEM i m a g e s f r o m 1 0 0 + p a r ti c l e s f o r 1.5 M KDCA p r e c u r s o r s o l u t io n .

F ig u re 3 .7 : Typical DLS size d i s t r i b u t i o n o f c a r b o n p a r ti c l e s p r o d u c e d by USP o f 1.5 M s o l u t i o n s o f a) LiDCA a n d b) NaDCA.

F ig u re 3 .8 : DLS size d i s t r i b u t i o n s f o r 0 . 1 2 5 , 1 . 0 0 0 a n d 1 . 5 0 0 M c o n c e n t r a t i o n LiDCA p r e c u r s o r s o lu tio n .

F ig u re 3. 9: Size d i s t r i b u t i o n o b t a i n e d f r o m SEM i m a g e s f r o m 1 0 0 + p a r ti c l e s f o r 1.0 M LiDCA p r e c u r s o r s o l u t io n .

F ig u re 3 .1 0 : DLS size d i s t r i b u t i o n s o b t a i n e d u s in g 2 . 5 4 MHz Piezo e l e c t r i c disk f o r 0 . 1 2 5 , 0 . 5 0 0 a n d 1 . 0 0 0 M LiDCA.

F ig u re 3 . 1 1 : (a) I n f r a r e d t r a n s m i s s i o n s p e c t r u m o f c a r b o n m i c r o s p h e r e s ; (b) 0 - p o t e n t i a l of c a r b o n m i c r o s p h e r e s in a q u e o u s s u s p e n s i o n s a s a f u n c t i o n o f pH.

F ig u r e 3 .1 2 : FTIR s p e c t r u m s o f p r i s t in e c a r b o n (Red) vs CM f u n c t i o n a l i s e d (Blue) w i t h a) p -c a r b o x y b e n z e n e d i a z o n i u m b) p - s u l p h o n a t e d i a z o n i u m a n d -c) n , n - d i e t h y l a n i l i n e d i a z o n i u m . Fig u re 3 .1 3 : z - p o t e n t i a l m e a s u r e m e n t s o f p r i s t in e C M s s h o w n in c o m p a r i s o n t o z - p o t e n t i a l o f CMs f u n c t i o n a l i s e d w i t h a) p - c a r b o x y b e n z e n e d i a z o n i u m b) p - s u l p h o n a t e d i a z o n i u m a n d c) n ,n - d i e t h y l a n i l in e d i a z o n i u m .

Fig u re 3 .1 4 : Plot o f t h e p a r ti c l e d i a m e t e r c u b e d , d e t e r m i n e d via DLS vs c o n c e n t r a t i o n o f LiDCA s o l u t io n

F ig u r e 3 .1 5 : SEM i m a g e s o f C Ms s y n t h e s i s e d f r o m t h e 2 . 5 4 MHz p i e z o u sin g a) 0 . 1 2 5 M a n d b) 0 . 8 5 M LiDCA; Sc a le b a r = 2 0 0 n m

F ig u re 3 .1 6 : Size d i s t r i b u t i o n via SEM o f 6 0 C M s s y n t h e s i s e d u s in g 1.0 M NaDCA

F ig u re 3 .1 7 : a) Typical DLS d i s t r i b u t i o n s o b t a i n e d u sin g NaDCA C M s b) Plot o f t h e p a r ti c l e d i a m e t e r c u b e d vs c o n c e n t r a t i o n o f t h e NaDCA s o l u t io n .

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Figure 4.2: Typical SEM images o f th e Ag (a) and Pd (b) carbon m icro sphe res o b ta in e d by using co ffe e as a re d u c ta n t, 3 m m o l so lu tio n A g lN H jjj (a) and (b) 5 m m ol so lu tio n PdClz;

Scalebar = 200 nm.

Figure 4.3; C o n tro l e xp e rim e n ts excluding Sn se n sitisa tio n step fo r A g/C M (a) and Pd/CM (b) co m p o site synthesis; scalebar = 500 nm.

Figure 4.4:(a-c) Surface m o d ific a tio n caused by sen sitisa tion and a c tiv a tio n using 8 m m o l (a), 50 m m o l (b) and lO O m m ol (c) in th e sen sitisa tion step. Figure 4 (d -f) Resulting electroless

de p o sitio n s fro m activ a tio n s observed in fig u re 4.4a-c, resp ective ly. Scalebar= 100 nm , except

b = 50 nm

Figure 4.5: A g/C M synthesized using 0.3 m m o l (a) and 30 m m o l (b) Ag(NH3)2*;scalebar (a) = 100 nm Scalebar (b) = 200nm

Figure 4.6: XRD p a tte rn s o b ta in e d fro m th e A g/C M (a) and Pd/CM (b) samples. Figure 4.7: TGA curves o b ta in e d in air fo r p ris tin e CMs, A g/C M and Pd/CM com posites. Figure 4.8: E volution o f th e UVeVis a b so rp tio n spectra o f 4 -n itro p h e n o l in th e presence o f 1.0 X 10'^ M NaBH4 and A g/C M particles as a fu n c tio n o f re a ctio n tim e ; all spectra w e re corre cte d fo r s c a tte rin g [40]. The firs t sp e ctru m was take n im m e d ia te ly a fte rin je c tio n o f 4 -n itro p h e n o l,

w h ere as th e last sp e ctru m was take n a fte r 27 m in.

Figure 4.9: L o ga rithm ic p lo t o f th e no rm alized absorbance change as a fu n c tio n o f tim e . The lin e a r f it ne ar tim e zero was used to calculate th e ra te c o e ffic ie n t fo r th e re d u c tio n reaction.

Figure 4.10: Suzuki re a ctio n b e tw e e n 4 -b ro m o to lu e n e and p h e n y lb o ro n ic acid, run fo r 18 h at ro o m te m p e ra tu re . Yields o f 40 ± 10% w e re achieved, even up on th e th ir d use o f th e Pd/CM

catalysts.

Figure 4.11: TGA curves fo r washed and unw ashed Ag/C, D iffe re n ce can be a ttrib u te d to u n s u p p o rte d particles.

Figure 4.12: EDX m apping placing highest c o n c e n tra tio n s o f Ag to locatio ns w h e re particles are observed a t th e CM surface.

Figure 5.1: SEM im ages o f th e surface o f carbon m icrospheres as p re pa red (a), a fte r se n sitiza tio n in a Sn^* s o lu tio n (b), and a fte r a ctiv a tio n in a Pd^* s o lu tio n (c); scale bar =200

Figure 5.2: XP sp e ctru m o f carbon m icro sphe res a fte r un d e rg o in g th e tw o step s e n s itiz a tio n /a c tiv a tio n process th a t nucleates Pd° n a n o p a rtic le s at th e carbon surface (a SEM

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Figure 5.3: SEM image showing CM samples after deposition in hypophosphite and Fe^* showing a) the heterogeneous reaction occurring on the CM surface and b) colloidal particles

present which represent the bulk o f the sample; Scale bar = 200 nm.

Figure 5.4: SEM images showing CM surface a fte r utilising m ethod A w ith 0.0125 M Fe^* in solution. Scale bar = 200 nm.

Figure 5.5: SEM images showing CM surface after utilising m ethod A w ith 0.025 M sodium hypophosphite concentration in solution. Scale bar = 200 nm

Figure 5.6: SEM images showing CM surface a fte r utilising m ethod A at room tem perature. Scale bar = 200 nm

Figure 5.7: SEM images showing CM surface a fte r utilising m ethod A at pH value 6. Scale bar = 300 nm

Figure 5.8: Typical SEM images o f CMs obtained after deposition in DMAB/Fe^* solution after 0.5 h (a) and a fte r 1.5 h (b); scale bar = 200 nm. The size of primary particles increases w ith

deposition tim e, (c) Size distrib u tio n o f iron clusters obtained a fte r 1.5 h o f deposition

Figure 5.9: Size d istribution o f iron particles obtained after 1.5 hr deposition.

Figure 5.10: XRD pattern obtained a fte r sensitization/activation and deposition in DMAB/Fe^* solutions fo r 1.5 h on activated CM powders

Figure 5.11: Fe K-edge absorption threshold obtained from (a) a-Fe, (b) hem atite, (c)magnetite and (d) maghem ite standards compared to th a t o f (e) Fe/CM. The linear com bination (LC) fit

w ith the parameters reported in Table 1 and the residual are drawn in the b o tto m traces.

Figure 5.12: k3-weighted EXAFS data o f Fe/CM compared to the fittin g curve in k-space (a) and R-space (b). The model did not include distances larger than 0.35 nm.

Figure 5.13: Cr(VI) removal fro m solution as a function o f tim e fo r pristine CM and Fe/CM particles

Figure 5.14: Shows the resulting decomposition o f Bath A absent glycine; Scale bar = 4 nm Figure 5.15: TGA showing th e residual masses le ft for CMs a fte r undergoing deposition fo r various ED times

Figure 5.16: SEM showing NPs at the CM surface a fte r 2.5 hr deposition utilising m ethod B Figure 5.17: XRD o f m ethod B perform ed on a flat carbon surface

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[image:16.521.43.511.40.762.2]

List of Tables

Table 3.1: Surface area and pore size results from BET analys

Table 3.2: Comparison of 2.54 and 1.67 M HZ using 1.000 and 0.125 M LiDCA

Table 3.3: Average Zeta potential difference after surface functionalisation of CMs

Table 4.1: Summary of the effect of metal ion concentration on particle growth at the CM

surface

Table 5.1: Demonstrating the components of Methods A and B

Table 5.2: Results of a linear combination fit on XANES data of Fe/CM sample (see Figure 6).

Error bar is reported in brackets

Table 5.3: Summary of scattering paths obtained from a best fit of the EXAFS spectrum of

Fe/CM powders

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Chapter 1

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1.1 Background

C o n trary to p o p u la r b elie f, n a n o te ch n o lo g y is n o t exclusive to m o d e rn science.

C olloidal gold n a n o p a rtic le s (NPs) w as disco vered o v e r 2 5 0 0 years ago [1]. Colloidal

gold w/as used to m a k e ruby glass and to im b u e ceram ics w ith co lou r, and th ese

ap p licatio n s a re still co n tin u in g n ow . This w as d u e to th e in ten se colours th a t colloidal

n o b le m e ta l NPs m a ke . Also, o f in te re s t w as th e use o f co llo idal gold NPs in th e

diagnosis o f syphilis in th e m id d le age. A m e th o d w h ich re m a in e d ac tiv e and in use

u n til th e 20'*'’ c e n tu ry , a m e th o d w hich also h appens to be c o m p le te ly u n re lia b le [2].

H o w e v e r, ev en th o u g h NPs had b een aro u n d f o r this long, it w a s n 't until F araday

sh o w ed th a t th e in te n s e co lo u r is d ue to m e ta llic gold in co llo idal fo rm th a t

u n d erstan d in g o f th e p o te n tia l ap p licatio n s o f NPs w as slo w ly realised [3 ,4 ]. In 19 0 8 ,

M ie e x p la in ed th e p h e n o m e n a o f co lo u re d co llo idal m e ta l particles by solving

M a x w e ll's eq u a tio n s fo r th e ab so rp tio n and sc atte rin g o f e le c tro m a g n e tic ra d ia tio n by

spherical m e ta llic particles [5]. In te res tin g ly , th e s e tw o results in d icate d th a t m e ta l

nan o m a te ria ls had d iffe re n t o ptical p ro p e rtie s th a n th e ir b ulk c o u n te rp a rts , k n o w as

q u a n tu m size e ffe c ts [6 ,7 ]. These effects a re n ot a resu lt o f a scaling fa c to r b u t are

d ire c tly d ue to th e size and shape o f th e NPs. In fa c t, m a n y o f th e p ro p e rtie s o f bulk

m a te ria ls change w h e n th e m a te ria l is p ut in to th e n an o scale. O p tical, physical,

ch em ical and e le ctrica l p ro p e rties have all been o b s erv ed to be size d e p e n d e n t fo r

various m a te ria ls [8 ,9 ]. Effects such as this d ue to sm all p a rtic le size led to g re a t

curiosity a b o u t th e p ro p e rtie s o f nano-sized m a te ria ls in th e scientific c o m m u n ity

w h ich has c o n tin u e d to this day. In fa c t, in th e last te n years alo n e , th e n u m b e r o f

research papers on n a n o m a te ria ls has g ro w n e x p o n e n tia lly in n u m b e rs, ind icatin g th a t

research into n a n o m a te ria ls is still a re la tiv e ly n e w and u n e x p lo re d field [1 0 ]. This

research is d riven by a desire to d iscover m a teria ls w ith n e w p ro p e rtie s an d to

u n d e rs ta n d th e science b eh in d q u a n tu m size effe cts fo r d iffe rin g m a teria ls in th e hop e

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All o f this research has led to m a n y p o te n tia l ap p licatio n s o f n a n o m a te ria ls fro m biological, optics, e n v iro n m e n ta l an d catalysis. H o w e v e r, in this re p o rt w e w ill be focusing on th e n a n o m a te ria ls w hich have p o te n tia l c a talytic and e n v iro n m e n ta l applications.

Synthesis o f n a n o m a te ria ls and p article is g e n e ra lly achieved in tw o w ays, to p d o w n ap p ro ach and b o tto m up ap p ro ac h . T op d o w n ap p ro ac h g en era l involved th e use o f p h o to c h e m is try o r e le c tro n b eam lith o g ra p h y fo r etchin g surfaces to c re a te n a n o m e tre sized structures and fe a tu re s [1 1 ,1 2 ], This ro u te fo r synthesis o f n a n o m a te ria ls w ill n ot be discussed o r re p o rte d on h ere in .

T he b o tto m up ap p ro ac h g e n e ra lly involves th e n u c le a tio n o f particles fo llo w e d by su b se q u en t g ro w th o f th e p article. T he easiest m e th o d fo r o b ta in in g NPs using this ap p ro ach is utilising w e t ch em istry. This ty p e o f synthesis starts o ff w ith m o le c u la r p recurso r solutions. Typically, th e y include a m e ta l salt, a reducing ag en t and a s u p p o rt/s ta b ilis in g a g e n t. G e n e ra lly th e m e ta l ion in so lutio n is re du ced to its m e ta llic s ta te , fo rm in g a p article. This process Is g o v ern e d by th e th e o ry o f n u c le atio n and g ro w th [6 ]. O th e r m e th o d s fo r b o tto m up synthesis h ave b een ac h ie ved . Specifically, m e th o d s involving sp ray pyrolysis [1 3 ,1 4 ], e le c tro -d e p o s itio n [1 5 ,1 6 ], and sol gel. Synthesis o f n an o s tru ctu res using e le c tro -d e p o s itio n is discussed In m o re d e ta il In section 1 .5 .3 .

1.2 Introduction to supported nanoparticle systems

1.2.1 Stabilising agents

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crucial fo r m a n y p o te n tia l ap p lications. In s tab ility in so lutio n com es fro m a ttra c tiv e

forces b e tw e e n NPs in so lutions such as van d e r vaals fo rce s and m a g n e tic in te rac tio n s

[1 8 ]. If b o tto m up m e th o d s a re being used th e re m u st be a m ech an ism fo r restricting

g ro w th o f th e NP to keep it in th e desired size range. This is g e n e ra lly achieved using

stabilising ag en ts and s u p p o rt m aterials.

By in tro d u cin g stabilising s u p p o rt m a teria ls , it is possible to m a n ip u la te th e size o f NPs

d u rin g synthesis and NP s ta b ility post synthesis. The sta b ilis atio n o f NPs is g e n e ra lly o f

th e fo rm e le c tro s ta tic , steric, ele c tro s te ric and use o f ligands. Stabilisers w o rk by

crea tin g an e n e rg e tic b a rrie r to c o u n te ra c t th e van d e r W a a ls an d m a g n e tic (m a g n etic

m a te ria ls ) a ttra c tio n s b e tw e e n n an op articles. This can be used to in h ib it g ro w th in th e

n u c le atio n processes o f NP fo rm a tio n or to stabilise dispersions o f NPs in so lutio n [6].

1 .2 .1 .1 Polym ers

An e x a m p le o f a steric stabilising ag en t is polym ers. Figure 1 .1 shows p o ly (N -v in y l-2 -

p yrro lid o n e) (PVP) and p o ly (2 ,5 -d im e th y lp h e n y le n e o xid e ) (PPO ) w h ich are used in th e

synthesis o f NPs b ecause th e y b ehaves as steric and ligand stabiliser.

P V P

poly( vi ny IpyiTol idone)

PPO

poly(2.3-cliniethylphenylene oxide)

[image:20.521.46.510.23.821.2]

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Using th is p o ly m e r, Pt, Pd and Rh NPs have been synthesised w hich catalyse o le fin and

b e n ze n e h y d ro g e n a tio n reactions [1 9 ]. H o w e v e r th ro u g h o u t th e lite ra tu re a vast

n u m b e r o f p o lym ers have b een used in NP synthesis, including, polyacrylic acid [20 ],

oligosaccharides, p o ly e th y le n e glycol [2 1 ], chitosan [22] and p o ly e le c tro ly te film s [23 ].

1 .2 .1 .2 D e n d rim e rs

D e n d rim e rs h ave also seen use fo r s u p p o rtin g and stabilising m e ta l NPs. M e ta ls NPs

such as Cu, Au, Pt, Pd, Fe, Ag and Ru have all b een synthesis using d e n d rim e rs [2 4 -2 6 ].

D e n d rim e rs b e h a v e sim ilarly to p o lym ers in th a t th e y en cap su late th e NP, p articu larly

if h e te ro ato m s o f t h e m e ta l a re lo c a te d in th e d e n d rim e r in te rio r. T he branches o f th e

d e n d rim e r th a n c o n tro l access to th e n u c le atin g NP, w hich lim its g ro w th and stabilise

th e NP in so lutio n. Figure 1.2 show s th e stra te g y p io n e e re d by croo k e t al fo r th e

synthesis o f NPs via en ca p su la tio n in a d e n d rim e r [24].

P A M A M -O H

Complexatlon

Product Reactant

Pd/Rh bim etallic cat. Reduction

[image:21.521.10.510.41.789.2]

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dendrim er. Specifically, the preparation of dendrim er-encapsulated bimetallic NPs is shown

[27],

O th e r fo rm s o f stabilisers used include Ligands [1 ], m icro -e m u lsio n s, re ve rse m icelles

[9] an d su rfa cta n ts [10 ]. These m e th o d s ach ieve sta b ilis atio n and c o n tro lle d p article

g ro w th by using th e sam e principles and co n cepts as ex am p les previously o u tlin e d

ab o v e . H e n c e, th e y w ill n ot be discussed fu r th e r in this re p o rt. In g en era l, use o f som e

fo rm o f stabilising agents is essential in o rd e r to o b ta in nan o-sized m a te ria ls using a

b o tto m up s o lu tio n based ap p ro ach.

1 .2 .1 .3 Solid su p p o rt system s

In re c e n t lite ra tu re , th e re is an Increasing n u m b e r o f re p o rts on NPs a re being placed

on va rio u s solid su p po rts a re being published. T h e b e n e fit o f an ch o ring NPs a t a solid

su rfa ce is in c rea tin g synergies b e tw e e n th e d e s ira b le p ro p e rties o f m e ta l

n a n o p a rtic le s and th o s e o f th e solid m a trix. For e x a m p le , ease o f re m o va l and h an dling

o f NP system fro m a re ac tio n m e d iu m and m in im is a tio n o f p article a g g reg a tio n are

sig nifican t b en efits. A n o th e r ap p lica tio n fo r solid su p po rts is in use as a te m p la te

m a te ria l in o rd e r to achieve stru c tu red n a n o m a te ria ls . For e x a m p le , Kim e t a l[2 8 ],

n u c le a te d a Pd film by h e a t tre a tin g silica spheres w hich had p a lla d iu m

a c e ty la c e to n a te (P d (aca c)2 ) adsorbed a t th e silica su rface. S u bseq uent etchin g o f th e

silica s p h e re w ith HF le ft a h o llo w Pd m e ta l s p h e re n a n o s tru c tu re . M e n o n e t al u tilised

a p o ly c a rb o n a te m e m b ra n e as a te m p la te fo r synthesis Au n a n o e le c tro d e s in th e 10

nm size ra n g e [2 9 ]. In th e lite ra tu re , m o st solid su p po rts ta k e th e fo rm o f m e ta l oxides.

For e x a m p le . Si [3 0 ], Al [3 1 ], and Tl [32 ] oxides have all b een utilised as solid s u p p o rt

m a te ria ls fo r NPs. O xide su pports are n o t exclusive to th e s e m etals and w ill n o t be

discussed fu r th e r in this re p o rt.

M o re re c e n tly , carb o n m a teria ls have seen use as solid s u p p o rt m a te ria ls fo r NPs. T h e

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properties fo r a variety o f applications. Carbon provides m any advantages over

polym eric o r inorganic supports due to its resistance to corrosion and its relatively

good b iocom patibility. F urtherm ore, th e surface chem istry o f carbon can be fin e tuned

to display d iffe re n t chemical groups th a t can fo r instance im p art charge, regulate

basicity/acidity, co ntrol w e ttin g behaviour o r p ro m p t biological recognition. Finally,

carbon supports can be designed to display a large specific surface area w hich can be

leveraged fo r the delivery o f large loads o f nanoparticles o r small molecules. For this

reason, th ere have recently been increased e ffo rts aimed at developing new

m ethodologies fo r th e c o ntrolle d d ep osition /e m b e dd in g o f m etal nanoparticles at

carbon scaffolds [33-36].

Examples o f carbon m aterials fo r nanom aterials support Include: M etz et al. have

synthesized nano-structured com posite Au/C and Pt/C electrodes fo r energy storage

and conversion applications, by using vertically aligned carbon nano-fibres (VACNF) as

a scaffold [36-38].

Photochemical Electroless

VACNF functionallzation

b)

CH3 D

'C=0

1) hv, 254nm

2) deprotection 1) 2)

OH OH C=0 CO

H H H

±

Au* Au

Au Au N ^ Sn^* ✓ V

o

'CO po

VACNF VACNF VACNF VACNF

Figure 1.3: (a) Schematic illustration of the steps involved in the functionalization of carbon

nanofibers and subsequent proceudure for electroless deposition, (b) Chemical

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Figure 1.3 shows h o w IVletz and co w o rk e rs used solid ca rb o n fibres as a s u p p o rt fo r g ro w in g gold nano - ele ctro d es. T he ca rb o n fibres act as a te m p la te in o rd e r to shape th e g ro w th o f th e m e ta l n an o m a te ria l. Lipshuts e t al [40 ] use o f Nickel on Charcoal N i/C as an Inexp en sive H e te ro g e n e o u s C atalyst fo r Cross-Couplings b e tw e e n Aryl C hlorides and O rg an o m eta llics is a n o th e r e x a m p le o f a carb on su p p o rte d n a n o m a te ria l.

1.3 Applications of nanoparticles

In this section th e use o f n an o m a te ria ls w ith reg ard to specific areas w ill be discussed. T h e firs t p art w ill consist o f n a n o m a te ria ls fo r catalysis. A sh o rt re v ie w w ill be given, fo llo w e d by ap p lica tio n in catalysis fo r th e tw o m e tals utilised in th is re p o rt fo r catalysis. Part tw o w ill consist o f n a n o m a te ria ls fo r e n v iro n m e n ta l ap p licatio n s. H e re in , a re v ie w w ill be given o f c u rre n t te c h n o lo g ie s in e n v iro n m e n ta l re m e d ia tio n , th e p o te n tia l o f n a n o m a te ria ls and th e p ro b lem s w ith im p le m e n ta tio n o f n a n o m a te ria ls in this field .

1.3.1 Nanomaterials for catalysis

[image:24.521.15.511.52.597.2]

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b o n d s , m ak in g it po ssib le t o f u n c tio n alis e olefins a n d a r o m a t i c s in a facile m a n n e r . M o r e o v e r, significant n u m b e r s o f d iv e rse r e a c tio n s c a n b e c a ta ly s e d by v ario u s n oble m e ta l NPs, w h ic h a r e b e y o n d t h e s c o p e o f th is r e p o r t [10]. For th is r e p o r t , w e will utilise Pd a n d Ag m e ta l s in o r d e r t o achieving t h e aim s s t a t e d p reviously (s ee se c tio n 1.6.2).

1.3.1.1 Palladium n a n o m a t e r i a l s in cata lytic ap p lic a tio n

Pd n a n o p a r t i c l e s fin d i m p o r t a n t a p p lic a tio n s as c a ta ly s ts in h y d r o g e n a t i o n a n d C-C b o n d f o rm i n g r e a c tio n s like Suzuki, Heck a n d S o n o g a s h ir a co u p lin g [45,46]. H o w ev er, p r o b l e m s w ith ca ta ly s t re c o v e ry m e a n t t h e p r e s e n c e of u n d e s i r e d m e t a l c o n t a m i n a n t s in t h e e n d p r o d u c t s in r e a c tio n s f o r w hich it w a s utilised. Im m o b ilizatio n of catalytically ac tiv e Pd n a n o p a r t i c l e s a t solid s u p p o r t s fac ilita tes c a ta ly s t r e m o v a l an d rea c tio n w o rk up w h e n c o m p a r e d t o h o m o g e n e o u s cata ly sts. T h e r e f o r e , m u c h e ffo rt has r e c e n tly b e e n d e v o t e d t o d e v e lo p in g a n c h o r i n g p r o to c o l s f o r Pd n a n o p a r tic l e s , t h u s f o rm i n g a c o m p o s i t e m a te r ia l t h a t p r e s e r v e s t h e original ca taly tic p r o p e r t i e s w h ile im p roving h a n d lin g a n d r e a c tio n w o rk up. Mei e t al u s e d p o ly e le c tr o ly te b r u s h e s a n d c o r e shell m icro gels in o r d e r t o e n c a p s u l a t e t h e PdNP [26]. O t h e r g r o u p s h a v e u s e d c a r b o n n a n o t u b e s [47,48], silica [49,50], p o ly e le c tr o ly te films [51], a n d g r a p h i t e [52]. Im p o rt a n tl y t h e resu lts o b t a i n e d by m a n y of t h e s e r e s e a r c h e r s in d ic a t e t h a t t h e n a t u r e o f t h e s u p p o r t u s e d c an h a v e an i m p a c t o n t h e r a t e o f catalysis. This is n o t u n e x p e c t e d as catalysis is a h e t e r o g e n e o u s p ro c e s s . T h e r e f o r e , a n y m a te r ia l t h a t blocks a c c e s s t o t h e NP s u r f a c e h a s t h e p o te n t i a l t o r e d u c e c a ta ly tic p e r f o r m a n c e .

1.3.1.2 Silver n a n o m a t e r i a l s in ca taly tic ap p lic a tio n

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[2 5 ,5 9 ], In o rd e r to achieve this goal, several research groups h ave fo cu sed on

d e v e lo p in g s u p p o rte d Ag n an o p artic le s, e.g. A g /p o lysa cc h a rid e [2 2 ,5 8 ], A g /g ra p h e n e

[6 0 ] o r A g / p o ly m e r [61 ] n a n o c a rrie r com p o sites, im p ro vin g tra n s p o rt and m o d u la tin g

ag g reg a tio n o f Ag n a n o p a rtic le active cen tres. Sim ilar to Pd p articles, th e su p p o rt

system chosen can in flu en ce th e ra te o f catalysis fo r AgNPs. This m eans selectio n o f an

a p p ro p ria te s u p p o rt m a te ria l can be essential fo r utilising NPs in c a talytic ap p lications

in an e ffic ie n t m a n n e r (See section 1.4).

1.3.2 Nanomaterials for environmental applications

1 .3 .2 .1 R eview o f th e issues

D u e to g ro w in g aw a ren es s a b o u t th e effe cts o f subsurface co n ta m in a n ts in soil and

g ro u n d w a te r in th e e n v iro n m e n t, increased research and fu n d in g is being allo c a te d to

th e a re a o f re m e d ia tio n tec h n o lo g ie s and c o n ta m in a n t m a n a g e m e n t. R e m e d ia tio n o f

th e s e c o n ta m in a te d sites is a vast and c o m p lex a re a . T he vast scale o f th e p ro b le m is

e x a s p e ra te d by th e s h e e r n u m b e r o f c o n ta m in a te d sites. In th e US th e re a re o v e r 1 5 0 0

o f w h a t are kn o w n as su p erfu n d sites and on av erag e a su p erfu n d site costs

a p p ro x im a te ly $ 2 5 m illio n to re m e d ia te . R ecently, it has been e s tim a te d th a t Irela n d

has close to 2 0 0 0 c o n ta m in a te d sites d u e old ind u stries, u n m a n a g e d spills and o th e r

in a d e q u a te w as te m a n a g e m e n t p ro ced u res [6 2 -6 4 ]. T h e c o m p le x ity o f th e p ro b le m is

in th e v a rie ty o f c o n ta m in a n ts a t th e sites, v a rie ty o f c o n ta m in a n t source ty p e and

lo catio n s and also th e v a rie ty o f soils and porous m e d ia w h ich co n tain th e

c o n ta m in a n ts . This m eans d iffe re n t m e th o d s fo r clean up h ave to be id e n tifie d in a

case by case basis. T ra d itio n a l ap p ro aches to tre a tin g soil an d g ro u n d w a te r

c o n ta m in a n ts have used p u m p an d t r e a t o r c o n ta in m e n t m e th o d s via p e rm e a b le

re a c tiv e b arriers (PRB). These m e th o d s h ave serious disad vantages. T he biggest

d is a d v a n ta g e is th e cost associated w ith th e m . Installing, m anagin g and re m o vin g

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c o n ta m in a n t sources in d e e p aq u ife rs and u n d e r buildings m ay n ot be accessible to

th ese tec h n o lo g ie s [6 5 -6 7 ].

In re c e n t tim e s th e use o f n a n o te c h n o lo g y fo r e n v iro n m e n ta l re m e d ia tio n has been investig ated [1 8 ]. N a n o p article s have a h ig h er p ro p o rtio n o f a to m s n e a r o r a t th e

surface. This leads to a h ig h er p ro p o rtio n o f a to m s having d angling bonds and h ig h er surface en erg y . This m eans th a t th e s e a to m s h ave a m u ch g re a te r cap acity to

p ro m o te a d s o rp tio n and to in te ra c t w ith o th e r m o lecules in o rd e r to c o m p en s ate fo r

th e excess surface en ergy. A lre a d y , n a n o m a te ria ls ex h ib itin g prom ising re a c tiv e and

ad so rp tive p ro p e rtie s have b een successfully used in w a te r p u rific a tio n and

e n v iro n m e n ta l re m e d ia tio n . N a n o p article s also have th e a d v a n ta g e o f being ab le to p e n e tra te in tra p a rtic le pores o f soils. This en ab les th e ir use in slurry reactors fo r th e

re m e d ia tio n o f c o n ta m in a te d soils and s e d im en ts. C u rre n tly , th e m o st prom ising

n a n o m a te ria ls fo r e n v iro n m e n ta l a p p lica tio n s, co n sidering re m e d ia tio n p o te n tia l and cost a re m e ta llic iron an d iron oxides.

1 .3 .2 .2 M e ta llic iron as re m e d ia tio n m a te ria l

In re c e n t lite ra tu re , m e ta llic iron Fe° has b een re p o rte d as a successful re m e d ia tin g ag e n t fo r e n v iro n m e n ta l co n ta m in a n ts . Fe° is an e x c e lle n t e le c tro n d o n o r in w a te r ,

resulting in its use as a re d u ctiv e ag e n t th a t can e ffe c tiv e ly re m e d ia te c o n ta m in a n ts

such as ino rg an ic anions [6 8 -7 3 ], heavy m e tals [7 4 -7 6 ], and o rg an o h alid e s [7 7 -7 9 ]. In

fa c t, m e ta llic iron has b een so ve rs a tile in re m e d ia tin g m a n y d iffe re n t v a rie tie s o f

co n ta m in a n ts th a t an explosion o f w o rk has o ccu rred in this research fie ld in th e last fifte e n years.

Fe° particles can be d escribed using a c o re shell m o d e l as show n in Figure 1.4 . T he

co re m ain ly consists o f Fe w h e re a s th e o u te r shell consists o f a m ix tu re o f Fe^'^ and

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H,0

Figure 1.4: Core shell model of an iron nanoparticle[18]

Figure 1 .4 also shows tw o o f th e m ain pro po sed m ech an ism s by w hich re m e d ia tio n

occurs on p a rtic le surfaces in th e case o f a c h lo rin a te d o rg an ic c o m p o u n d . Several

c o n ta m in a n ts ad so rb on th e oxide phase layer, o x id a tio n o f th e Fe° in th e p article

pro vides e le ctro n s fo r th e re d u ctio n o f tric h lo ro e th y le n e . T he Fe° core shrinks and is

used up in th e re ac tio n . T he particles a re no lon g er ac tiv e as a redu cin g a g e n t once all

th e Fe° is oxidised.

T h e re d u ctio n re a c tio n ind u ced by Fe° is a h e te ro g e n e o u s process. This m eans th a t

th e to ta l n u m b e r o f ac tiv e surface sites w hich can in te ra c t w ith co n ta m in a n ts

d e p e n d s only on th e to ta l surface area . H ence, by increasing th e surface to v o lu m e

ra tio {i.e d ecrea se d p article size), th e efficie n c y an d p e rfo rm a n c e o f th e re d u ctiv e

processes can be im p ro v e d . This led to th e in te re s t in n ano-sized Fe° across research

g roups fo r e n v iro n m e n ta l re m e d ia tio n . In te re s tin g ly , Fe°NPs have displayed

p ro p e rtie s u n iq u e to nan o-sized p article ind icatin g th e r e a re so m e q u a n tu m size

effe c ts . For e x a m p le , nano iron can d e c h lo rin a te p o ly c h lo rin a te d bip h en yl com p o un d s

(PCBs) a t ro o m te m p e ra tu r e and pressure w h e re a s g ra n u la r iron does n ot [8 0 -8 2 ].

H o w e v e r, m a n y re d u ctio n d u e to iron d o n 't have an y q u a n tu m e ffe cts . In fa c t w h e n

n o rm a lis e d fo r su rface a re a , re actio n ra te co nstants a re s im ilar fo r m a n y re d u ctiv e

re ac tio n s [6 5 ,8 0 ,8 3 ].

For successful use o f n an o m a te ria ls a t a large scale, cost e ffe c tiv e and robust m eth o d s

[image:28.521.13.509.33.720.2]

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q u a n titie s o f re m e d ia tio n agents a t lo w costs. Fe°NPs have b een synth esised using a

w id e ra n g e o f m e th o d s: d e c o m p o s itio n o f iron p e n ta c a rb o n y l in o rg an ic solvents

[8 4 ,8 5 ], e le c tro d e p o s itio n o f fe rro u s salts , vacu u m s p u tte rin g [8 6], re d u c tio n o f

g o e th ite an d h e m a tite p articles w ith h yd ro g e n gas a t high te m p e r a tu r e [8 7 ] and

fin a lly , re d u ctio n o f F e[lll] and F e[ll] salts using sodium b o ro h y d rid e o r o th e r redu cin g

agents. [8 8]. This last m e th o d uses aq u eo u s based c h e m is try an d is fa c ile to

im p le m e n t. Fe° is p re p a re d according to th e fo llo w in g re ac tio n :

4Fe^^ + 3BH4 + 9H2O ^ 4 F e ° 4 . + 3H 2 B 03 ' + 12H^ +6H2 t (e q n 1 .1 )

W h e n th e re a c tio n is ca rrie d o u t u n d e r co n s ta n t stirrin g an d u n d e r in e rt a tm o s p h e re

it leads to th e fo rm a tio n o f p olyd isp erse p olycrystallin e o r a m o rp h o u s n a n o p a rtic le s ,

w ith d ia m e te rs ty p ic a lly in th e rang e 2 0 -1 0 0 nm . This e q u a tio n show s th e basic

re ac tio n th a t occurs. It is im p o rta n t to n o te th a t m a n y side re ac tio n s can o ccu r h ere .

For e x a m p le , th e re a c tio n sh o w n in eq n 1.2 b e lo w occurs

B H 4 + 2 H 2 O - > B O 2 ■ + 4 H 2 (eq n 1.2)

This re a c tio n causes th e fo rm a tio n o f th e basic M e ta b o r a te ion. This can th e n lead to

th e fo rm a tio n o f m e ta llic Boron in o u r m ix tu re via th e h a lf re a c tio n

BO2 +2H2 0 + 3 e ^ B + 4 0 H ' (eq n 1 .3 )

These reactio n s re su lt in a F e/B alloy. T h e Boron c o n c e n tra tio n o f th is allo y is sensitive

to p a ra m e te rs such as pH, te m p e r a tu r e , c o n c e n tra tio n o f salts an d t h e ra tio o f th e

fe rro u s salt to th e B o ro h yd rid e salt. T h e se have b een in v e s tig a te d in d e ta il in th e

p revious lite ra tu re [8 9 -9 1 ].

T h e increased p e rfo rm a n c e o f Fe° o ffers m any ad va n ta g es; (a) im p ro v e d p e rfo rm a n c e

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h a r m f u l b y - p r o d u c t in t e r m e d ia t e s . F in a lly , n a n o - m a t e r ia ls can b e d e liv e r e d d ir e c tly

in to t h e c o n t a m in a n t s o u rc e v ia in je c tio n s o f a q u e o u s s lu rrie s c o n ta in in g

n a n o p a r tic le s . T h is in je c tio n c a p a b ility e n a b le s s ite s s u c h as c o n t a m in a t e d d e e p

a q u ife r s t o b e s e le c tiv e ly t a r g e t e d [ 9 2 - 9 6 ] , th u s re p la c in g c o s tly e x c a v a tio n s .

H o w e v e r , a n u m b e r o f c h a lle n g e s m u s t b e o v e r c o m e in o r d e r t o im p le m e n t n a n o F e°

in t h e fie ld ; k e y issues t h a t r e p r e s e n t a n o b s ta c le a r e o u t lin e d in t h e f o llo w in g s e c tio n .

1 .3 .2 .3 C h a lle n g e s in t h e im p le m e n t a t io n o f iro n n a n o p a r tic le s

T h e f ir s t p r o b le m w ith t h e im p le m e n t a t io n o f F e ° is t h e p re s e n c e o f c o m p e t in g

o x id a n ts . In a c o n t a m in a n t p lu m e F e° n o t o n ly re a c ts w it h c o n t a m in a n ts b u t a ls o w ith

d is s o lv e d o x y g e n a n d w a t e r a c c o rd in g to t h e f o llo w in g r e a c tio n s :

2Fe°(s) + 02(g) + 4H ^ 2Fe^^(aq| + 2 H 2 0 (i) (e q n 1 .4 )

2Fe°,s) + 2 H 2 0 (|) 2Fe'",aq) + H2,g) + 20H -,aq ) (e q n 1 .5 )

Fe^'^ c a n b e f u r t h e r o x id is e d t o Fe^'^ a c c o rd in g to :

4 F e % ) + 02(g) + 4 H " 4Fe'"(aq> + 2 H 2 0 (,) (e q n 1 .6 )

T h e s e s id e re a c tio n s can a f f e c t t h e s e rv ic e lif e t im e o f F e ° NPs b a s e d PRB's, s in c e

c o n t a m in a n t p lu m e s c a n t a k e s e v e ra l y e a rs to c o m p le t e ly pass a p o in t in t h e

s u b s u r fa c e . This p r o b le m has t h e r e f o r e r e s tric te d t h e u s e o f F e° as a r e m e d ia t io n

a g e n t t o g r o u n d w a t e r , w h ic h is a r e la t iv e ly a n o x ic m e d iu m . H o w e v e r , t h e b ig g e s t

p r o b le m w it h f u r t h e r a p p lic a tio n o f F eN P s is in its m o b ilit y in p o ro u s m e d ia [9 7 ].

R e c e n t s tu d ie s o n b a r e u n s u p p o r te d F e °N P s h a v e r e p o r t e d lim it e d m o b ilit y in

s a tu r a t e d p o ro u s m e d ia [9 7 ] . T r a n s p o r t d is ta n c e s o f a f e w c e n t im e t r e s o r in c h e s h a v e

b e e n r e p o r t e d . T h is p o o r m o b ilit y is c a u s e d v ia t w o m e c h a n is m s , s u c c e s s fu l f ilt r a t io n

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an o p tim u m size range fo r particles in o rd er to minimise th e ir filtra tio n by porous media. In general, sm aller particles below 100-200nm are susceptible to adsorptive processes on soil grains. Electrostatic interactions, London van der vaals forces and Brownian m otio n all influence these absorptive processes; consequently, soil pH, ionic strength and w a te r com position can all affect adsorption. Larger particles, above 1-2 nm in size, are susceptible to filtra tio n via sedim entation instead. Therefore, depending on soil typ e and w a te r conditions, the optim um range fo r particles to m inim ise filtra tio n is in th e o rd er o f 100 nm to 2000 nm. This means th a t Fe°NPs are vulnerable to filtra tio n by adsorptive process due to th e ir small size [98,99].

The colloidal chem istry and fe rro m a gn etic behaviour o f Fe particles results in severe aggregation. Aggregation prevents particle flo w through porous media such as soils a fte r aggregates reach a fe w microns in diam eter. Lowry et al have rep orte d DLS m easurem ents taken at d iffe re n t tim es a fte r sonication o f Fe° NPs dispersions, and observed th a t average p article size increased w ith in 15 min fro m lOOnm to SOOOnm in a concentrated solution. Even fo llo w in g attem pts to prepare extrem ely dilute dispersions it was not possible to reduce aggregation.

500nm

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Even w ith an im p ro v e m e n t in th e result and m e a s u re m e n t q u a lity , th e ag g reg atio n

ap p ea rs to be still occurring. Figure 1.5 shows a T E M o f FeNPs synthesised in o ur

la b o ra to ry using th e b o ro h y d rid e m e th o d o u tlin e d ab o ve. Large m ic ro m e te r sizes

chain like ag g reg ates can be o bserved . A g g regation such as this, also have th e e ffe c t

o f lo w e rin g re a c tiv ity d u e to su b se q u en t lo w e rin g o f specific surface area av a ila b le fo r

ad s o rp tio n an d re a c tio n [1 0 0 ,1 0 1 ].

In re c e n t tim e s , m a n y groups h ave trie d to o ve rc o m e th e s e d ifficu ltie s by use o f

su rfa ce m o d ific a tio n and su pports fo r th e NPs. S urface m o d ifica tio n s h ave involved

th e use o f su rfa cta n ts [9 9 ,1 0 2 ], ca rb o h y d ra te s [1 0 3 ,1 0 4 ], p o lye le ctro lyte s [1 0 5 ] and

tri-b lo c k co p o lym ers [1 0 6 ], These app ro aches aim at p re v e n tin g ag g reg a tio n via

e le c tro s ta tic repulsion a n d /o r steric stabilisatio n. In lite ra tu re th e s e m eth o d s have

b e e n sh o w n to im p ro v e tra n s p o rt in m o d el soils and to m in im is e ag g lo m e ra tio n ,

increasing sta b ility . H o w e v e r, this o fte n occurs at a loss in re a c tiv ity since th ese

su rfa c e m o d ifica tio n s have th e e ffe c t o f sim u ltan eo u s ly blocking and inh ib itin g

re a c tiv e sites on th e particle.

T h e re fo re , th e re is a need fo r n e w m e th o d s and te c h n o lo g ie s th a t can im p ro v e

tra n s p o rt an d m o b ility in soils fo r th e n an o p articles. These n e w te c h n o lo g ie s should

a im to im p ro v e tra n s p o rt and m o b ility w h ils t m a in ta in in g th e ad va n ta g e o f increased

re a c tiv ity g ained fro m using FeNP.

1 .3 .2 .4 Iro n o xid e in e n v iro n m e n ta l ap p lications

H ig h e r su rfa ce en ergies o f m e ta l o xide n an o p artic le s d u e to surface to v o lu m e ratio's

m a k e iron oxide n a n o p a rtic le m o re p ro n e to ad so rp tio n effects. A large body o f w o rk

has been d o n e on ad so rp tio n m echanism s o f ions o n to m e ta l o xid e surfaces [1 0 7 ,1 0 8 ].

S u rfac e h yd roxides in m e ta l oxides have a d o u b le p air o f ele ctro n s w ith a dissociable H

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[1 8 ,1 0 9 ]. Iron o xide NPs have also been used in e n v iro n m e n ta l ap p licatio n s. Iro n oxide NPs have been sh o w n to be e ffe c tiv e in th e ad so rp tio n o f C r(V I) [1 1 0 ], a rs e n ite and

ars e n a te [1 1 1 ].

1.4 Support material for Ag, Pd and Fe/FCxOy particles

Porous carb on m icro s p h e res (C M s) a re p a rtic u la rly a d va n ta g eo u s fo r n a n o p a rtic le s u p p o rt since th e y can lev era g e all o f th e advan tag es o f ca rb o n m a te ria ls w h ile displaying a high specific surface and good tra n s p o rt/d e liv e r y p ro p e rtie s , as re c e n tly sh o w n by w o rk in o u r g ro u p [1 1 2 ]; h o w e v e r, t h e ir ap p licatio n s as n a n o p a rtic le s u p p o rt have re m a in e d re la tiv e ly u n e xp lo re d .

C arb on m a te ria ls displayed c h em istry w h ich fa c ilita te s th e fu n c tio n a lis a tio n o f th e su rfa ce [1 1 3 -1 1 5 ]. This fa c ilita te s a m ech an ism fo r changing su rfa ce ch arg e and ch e m is try in C M s. Surface ch arge m o d ific a tio n can be m e a s u re d using th e zeta p o te n tia l fo r susp end ed particles.

1.4.1 Zeta potential and surface charge

M o s t p articles in a colloidal suspension have a surface charge associated w ith th e m . This surface ch arge a ttra c ts an excess o f a co u n te rio n species in so lutio ns. This leads to th e fo rm a tio n o f tw o layers o f o p p o site charge. These a re th e fix e d surface ch arg e and th e d iffu se charge fro m ions in so lu tio n . This is kn o w n as th e ele c tric a l d o u b le la y e r (E D L )[1 1 6 ].

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th e o u te r H e lm h o ltz lay er (OHL) to distinguish b e tw e e n m o ieties th a t have a chem ical a ffin ity fo r th e surface as w e ll as co u lo m b ic in te rac tio n s. T an g e n tial flu id flo w along a ch arged su rface can be caused by ap p lyin g an ele c tric fie ld . It has b een sh o w n th a t a th in la y e r o f flu id sticks to th e surface. This is kn o w n as th e h yd ro d y n a m ica ily stag n an t layer. This la y e r goes fro m th e surface to so m e d istance d®'' w h e re a slip plane is assum ed to exist and flu id flo w begins again. This assu m p tion m eans th a t th e v o lu m e c o n ta in e d w ith in th e slip p la n e Is c o n stan t and h en ce th e space charge fo r e v e ry th in g w ith in th e slip plane is co n stan t as e v e ry th in g is assum ed to be static w ith in th e slip p lane. T he p o te n tia l a t this p o in t is d e fin e d as th e zeta p o te n tia l. T h e slip p lane w ill be a g re a te r th a n o r eq u al to d is tan ce fro m th e su rfa ce as th e OHL.

a° a' 0^ a®*'

IH P

O H P

Slip p la n e

d d®'^

Distance

Figure 1.6: M odel o f Surface charges and potentials. All potentials are defined with respect to th e potential in the bulk solution.

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The zeta potential is not directly measurable but its value can be obtained via electrokinetic m easurem ents coupled to modelling of the electrophoretic mobility. In our studies w e have used this approach to investigate the effects of surface functionalisation on CM surface charge using methods outlined in section 2.2.2.

1.4.2 Carbon microsphere w ith controllable size

Also, CMs w/ere synthesised utilising ultrasonic spray pyrolysis. Using this m ethod CMs with varying porosity have been synthesised[117]. This m ethod also has potential to control CM size (See section 2.1.1). Tufenkji-Elim elech model predicts th a t filtratio n effects are minimized fo r particles b etw een 0.1 nm and 1 nm in d iam eter [118]; th erefore, CM size is in th e optim al range fo r applications requiring delivery of nanoparticles through porous matrices. These CM o ffe r a vast range of selectivity and tuneable param eters fo r potential applications as a nanomaterials support. If control of CMs used fo r support can be gained, th ey would provide an excellent support fo r Ag, Pd and Fe/FexOy NP's in all th ere potential applications.