ISTM-CNR
CVD SYNTHESIS AND
PHOTOCATALYTIC ACTIVITY
OF ZnO NANOPLATELETS
Padova
University
INSTM
C. Maccato*2, D. Barreca1, A. P. Ferrucci2,
A. Gasparotto2, C. Maragno2, E. Tondello2
1
ISTM-CNR and INSTM - Padova, Italy
2 Department of Chemistry - Padova University and INSTM Padova, Italy
*[email protected]
VI Convegno Nazionale sulla Scienza e Tecnologia dei Materiali – Perugia 12-15 Giugno 2007
Zinc oxide (ZnO)
Wurtzite
(hexagonal lattice)
Zn
O
n-type
semiconductor
EG3.4 eV
Main interests:
 Optoelectronics
 Gas Sensing
 Energetics
 Photocatalysis
PHOTOCATALYSIS
• The aim of semiconductor
photocatalysis is to effectively
decompose organic pollutants.
• Photons are used to create
electron – hole pairs in
the semiconductor.
e- + O2 → O2h+ + OH- → OH
A ˉ
Conduction
Band
Reduction
A
Valence
Band
+
DOxidation
D
Photocatalytic Activity depends on:
•The competition between separation and recombination processes of
charge carriers (e-, h+).
•Surface catalysts-dye charge transfer and
efficient surface adsorption/desorption processes.
AIM:
Synthesis of nanosystems characterized by a high surface/volume
ratio using a bottom-up CVD approach .
Zn(hfa)2TMEDA
(Hhfa=1,1,1,5,5,5-hexafluoro-2,4pentanedionate;
TMEDA=N,N,N’,N’tetramethyilethylendiamine)
Zn(NO)3·xH2O
+
1,1,1,5,5,5-hexafluoro-2,4-pentanedione
+
N,N,N’,N’-tetramethylethylendiamine
Zn(hfa)2(TMEDA)
C
N
Yield = 63%
H
2nd Generation precursor
O
Zn
• high volatility
and thermal stability
•one-pot synthesis
F
Tobin J. Marks et al., J. Am. Chem. Soc., (2005) 127, 5613.
ZnO NANOSYSTEMS
ZnO
F(O2+H2O) = 40 sccm
F(N2) = 40 sccm
CVD
10 mbar, 60’
Si(100)
T[Zn(hfa)2(TMEDA)] = 60°C
Tsub. = 250-500°C
Zn(hfa)2(TMEDA)
Thickness [Φ(O2+H2O)]
Tsub.
256
209
128 64
250 300 350
400
450 500 °C
111 128
nm
FE-SEM
Water effect on morphology
No H2O
Thin film
Tsub= 400°C
100 nm
100 nm
Nanoplatelets
(NPTs)
With H2O
Tsub= 400°C
100 nm
100 nm
The obtained systems show very different morphologies
Tsub EFFECT
100 nm
100 nm
NPTs mean
thickness ~ 5.5 nm
System porosity is
temperature – dependent.
Different
photocatalytic
activity is expected
No variations
of NPTs morphology
after
thermal treatment
(600°C, 2h)
(a)
250°C
350°C
(d)
350°C
100 nm
100 nm
(e)
250°C
100 nm
100 nm
(c)
(b)
450°C
(f)
450°C
The expected intensity ratio
I002/I101 for ZnO powders is 0.44.
GIXRD
In the synthesized NPTs the
I002/I101 ratio depends on Tsub with
a maximum value of 3.4
at Tsub= 350°C.
500°C
450°C
400°C
350°C
300°C
20
30
(102)
(002)
(101)
(100)
250°C
2 J (degrees)
40
50
O
ZnO(001) surface is polar.
The Lewis acids sites exposed on
the surface are very reactive
toward the chemisorption of both
H2O and OH- groups.
ZnO(001) Surface
Zn
XPS
Auger Parameter - 
Oxygen
Zinc
Silicon
80
KE = hυ - BE
60
 = BE(XPS) +
%
KE (Auger)
Tsub=350°C
40
ZnO
Auger Peak: ZnLMM
XPS peak: Zn2p3/2
ZnO, literature≈ 2010.1 eV
20
0
0
50
100
150
Sputtering time (min)
C and F XPS signals disappear after a
mild sputtering indicating that
they are only surface contaminants.
ZnO ≈ 2010.2 eV
AFM
Photocatalytic activity
ZnO/Si(100)
Orange II solution (2.4*10-6 M , pH ~ 6)
UV irradiation (125 W)
(a)
nm
30
10
200
200
400
400
600
800
600
800
(Cdye/Cdye,0)*100 (%)
100
nm
80
(b)
nm
40
20
60
200
200
400
40
NPTs 400°C
RMSR = 6 nm
400
600
600
800
20
NPTs 350°C
RMSR = 32 nm
ZnO NPTs (350°C)
ZnO NPTs(400°C)
ZnO thin film (400°C)
800
nm
(c)
0
0
100
200
300
Irradiation time (min)
nm
12
6
200
Decomposition process shows
a pseudo-first order kinetics
K350°C (min-1) = 4.9*10-3
200
400
400
600
600
800
800
nm
Film 400°C
RMSR = 2 nm
CONCLUSIONS
Synthesis of ZnO NPTs on Si(100) starting from
Zn(hfa)2·TMEDA.
Tailoring of nanostructure and morphology as a function
of processing conditions.
Higher photocatalytic efficiency of ZnO NPTs
with respect to continuous films.
PERSPECTIVES
Syntesis of ZnO-TiO2 nanocomposites.
Evaluation of their photocatalytic and gas sensing
performances as a function of synthesis parameters.
XPS
Zn
2p3/2
1025
1020
BE (eV)
O1s
Intensity (a.u.)
Intensity (a.u.)
XPS Signals pertaining to ZnO sample deposited at 350°C
Zn-OH
536
Zn-O
532
528
BE (eV)
O
(a)
ZnLL’
O
H
H
H
H
H
H
H
H
O
O
O
O
O
O
Si
Si
Si
Si
Si
Si
Ruolo dei gruppi –OH
nella crescita pseudocolonnare
O
O
OH
ZnLL’
ZnLL’
O
H
H
H
H
H
H
H
O
O
O
O
O
O
Si
Si
Si
Si
Si
Si
O
…
(b)
ZnO
Si(100)
Zn
RISULTATI ANALISI SEM
1.4
1.2
ln v (nm/min)
1.0
0.8
0.6
0.4
0.2
1.3
1.4
1.5
1.6
1.7
-1
1/T (K )
CAMPIONI/ T (°C)
SPESSORE
FILM (nm)
LUNGHEZZA
SCAGLIE (nm)
SPESSORE MEDIO
SCAGLIE (nm)
ZnO19/250
111 ± 2
55
5.5
ZnO18/300
128 ± 3
60
5.5
ZnO17/350
256 ± 5
66
5.5
ZnO14/400
209 ± 5
76
5.5
ZnO15/450
128 ± 3
56
5.5
ZnO16/500
64 ± 2
33
5.5
-3
1.9x10
Alla temperatura del supporto
di 350°C si ha la deposizione
migliore
Zn(hfa)2TMEDA - CARATTERIZZAZIONE
100
m.p.=104-106°C
3.0
e
massa(%)
analisi
1H-
80
13C-NMR
analisi termiche
2.5
2.0
60
1.5
40
ln[velocità vaporizzazione (mmol/min)]
1.0
-4
sperimentale
fit
20
0.5
0
0.0
50
-5
100
150
200
T(°C)
250
-6
► singolo processo di sublimazione
senza decomposizione
-7
► Perdita in peso = 98%
-8
2.6
2.7
2.8
-1
1/T(K )
-3
3.0x10
ln p1 – ln p0 = (H0vap/R)(T0-1 –T1-1)
H°vap = 102  1 kJ/mol
derivata massa (%/°C)
analisi elementare
C=32,29%, H=2,87%, N=4,64%
CVD
Chemical
Vapor Deposition
Centro metallico
Legante
u
y
Gas reattivo
R
v
x
R
w
R
R
R
Substrato
R
z
R
R
CVD - termico
u Trasporto di massa
v Diffusione verso la superficie
w Reazione superficiale
x Desorbimento sottoprodotti
y Eliminazione sottoprodotti
z Nucleazione e crescita
source
J2
J2
detector
J1
J1
XRD
detects only reflections
for planes parallel
to the sample surface
detector
source
GIXRD
enhancement
of surface sensitivity

J2
J1
ORANGE II
Assorbanza (a.u.)
0 min
15 min
45 min
75 min
135 min
195 min
255 min
315 min
300
400
500
 (nm)
600
700