VI Convegno Nazionale sulla Scienza e Tecnologia dei Materiali – Perugia 12-15 Giugno 2007VI Convegno Nazionale sulla Scienza e Tecnologia dei Materiali – Perugia 12-15 Giugno 2007

CVD SYNTHESIS AND CVD SYNTHESIS AND

PHOTOCATALYTIC ACTIVITY PHOTOCATALYTIC ACTIVITY

OF ZnO NANOPLATELETSOF ZnO NANOPLATELETS

C. Maccato*C. Maccato*22, , D. BarrecaD. Barreca11, A. P. Ferrucci, A. P. Ferrucci22, , A. GasparottoA. Gasparotto22, C. Maragno, C. Maragno22, E. Tondello, E. Tondello22

11 ISTM-CNR and INSTM - Padova, Italy ISTM-CNR and INSTM - Padova, Italy22 Department of Chemistry - Padova University and INSTM - Department of Chemistry - Padova University and INSTM -

Padova, ItalyPadova, Italy

*chiara.maccato@unipd.itchiara.maccato@unipd.it

Padova University

ISTM-CNR

INSTM

Zinc oxide (ZnO)Zinc oxide (ZnO)

WurtziteWurtzite(hexagonal lattice)(hexagonal lattice)

nn-type-typesemiconductorsemiconductor

EEGG3.4 eV3.4 eV

Main interests:Main interests:

Optoelectronics Optoelectronics Gas Gas SensingSensing

Energetics Energetics PhotocatalysisPhotocatalysis

OZn

PHOTOCATALYSIS PHOTOCATALYSIS

AA ˉ̄

Valence Band

++

Conduction Bandhh > E

> Egg

ReductionReduction

AA

OxidationOxidation

DD--

DD

• The aim of semiconductor The aim of semiconductor photocatalysis is to effectivelyphotocatalysis is to effectivelydecompose organic pollutants.decompose organic pollutants.

• Photons are used to createPhotons are used to createelectron – hole pairs in electron – hole pairs in

the semiconductor.the semiconductor.

ee- - + O+ O22 → O→ O22--

hh++ + OH + OH- - → OH→ OH

•The competition between separation and recombination processes ofThe competition between separation and recombination processes ofcharge carriers (echarge carriers (e--, h, h++)).

•Surface catalysts-dye charge transfer and Surface catalysts-dye charge transfer and efficient surface adsorption/desorption processes.efficient surface adsorption/desorption processes.

Photocatalytic Activity depends on:Photocatalytic Activity depends on:

AIM:AIM:Synthesis of nanosystems characterized by a Synthesis of nanosystems characterized by a highhigh surface/volume surface/volume

ratio using a bottom-up ratio using a bottom-up CVDCVD approach approach .

Zn(hfa)Zn(hfa)22TMEDATMEDA

(Hhfa=1,1,1,5,5,5-hexafluoro-2,4-pentanedionate;

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)Zn(hfa)22(TMEDA)(TMEDA)

Yield = 63%Yield = 63%

Tobin J. Marks et al., J. Am. Chem. Soc., (2005) 127, 5613.

Zn

O

H

F

N C

22ndnd Generation precursor Generation precursor

• high volatility high volatility and thermal stabilityand thermal stability

•one-pot synthesisone-pot synthesis

ZnO NANOSYSTEMSZnO NANOSYSTEMS

Zn(hfa)Zn(hfa)22(TMEDA) (TMEDA)

CVD

Si(100)

ZnO(O(O22+H+H22O) = O) = 40 sccm40 sccm

(N(N22) = 40 sccm) = 40 sccm

10 mbar, 60’10 mbar, 60’

TT[[Zn(hfa)Zn(hfa)22(TMEDA)(TMEDA)]] = 60°C = 60°C

TTsub.sub. = 250-500°C = 250-500°C

Thickness [Thickness [ΦΦ(O(O22+H+H22O)O)]]

TTsubsub.. 250250 300300 350350 400400 450450

111111 128128 256256 209209 1281286464 nmnm

500 °C500 °C

100 nm 100 nm

Water effect on morphologyWater effect on morphology

Tsub= 400°C Thin filmThin film

With HWith H22OOTTsubsub= 400°C= 400°C

100 nm100 nm

The obtained systems show very different morphologies

NanoplateletsNanoplatelets(NPTs)(NPTs)

FE-SEMFE-SEM

TTsub sub EFFECT EFFECT

System porosity is System porosity is temperature – dependenttemperature – dependent.

Different Different photocatalyticphotocatalytic

activity is expectedactivity is expected

NPTs mean NPTs mean thickness thickness ~ 5.5 nm~ 5.5 nm

No variationsNo variationsof NPTs morphology of NPTs morphology

afterafterthermal treatment thermal treatment

(600°C, 2h)(600°C, 2h)

100 nm

100 nm100 nm

250°C 250°C

350°C350°C

450°C 450°C

(a)

(c)

(e) (f)

(d)

(b)

100 nm 100 nm

100 nm 100 nm100 nm

100 nm100 nm100 nm100 nm

250°C 250°C

350°C350°C

450°C 450°C

100 nm 100 nm

100 nm100 nm

GIXRD GIXRD The expected intensity ratio The expected intensity ratio

II002002/I/I101101 for ZnO powders is 0.44. for ZnO powders is 0.44.

In the synthesized NPTs the In the synthesized NPTs the

II002002/I/I101 101 ratio depends on Tratio depends on Tsubsub with a with a

maximum value of 3.4 maximum value of 3.4

at Tat Tsubsub= 350°C.= 350°C.

ZnO(001) SurfaceZnO(001) Surface

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.

O Zn504030202 (degrees)

500°C

400°C

450°C

350°C

300°C

250°C

(100

)

(002

)(1

01)

(102

)

XPSXPS

C and F XPS signals disappear after amild sputtering indicating that

they are only surface contaminants.

Auger Parameter Auger Parameter - -

ZnOZnO

Auger Peak: ZnLMMXPS peak: Zn2p3/2

ZnO, literature≈ 2010.1 eV

ZnOZnO ≈ 2010.2 eV≈ 2010.2 eV

BE(XPS) + KE (Auger)BE(XPS) + KE (Auger)

KE = hKE = hυυ - BE - BE

80

60

40

20

0

150100500Sputtering timeSputtering time (min)(min)

Oxygen Zinc

Silicon

%%TTsubsub=350°C=350°C

200400

600800

200400

600

800

nm

4020

nm

(b)

400600

800

126

nm

nm

200

800

200400

600

(c)

200400

600800

200400

600800

nm

30

10

nm(a)

200400

600800

200400

600

800

nm

4020

nm

(b)

200400

600800

200400

600

800

nm

4020

nm

200400

600800

200400

600

800

nm

4020

nm

400600

800

126

nm

nm

200

800

200400

600

(c)

400600

800

126

nm

nm

200

800

200400

600

200400

600800

200400

600800

nm

30

10

nm(a)

200400

600800

200400

600800

nm

30

10

nm

200400

600800

200400

600800

nm

30

10

nm

200400

600800

200400

600800

nm

30

10

nm

AFMAFM

NPTs 350°CNPTs 350°CRMSR = 32 nmRMSR = 32 nm

NPTs 400°CNPTs 400°CRMSR = 6 nmRMSR = 6 nm

Film 400°CFilm 400°CRMSR = 2 nmRMSR = 2 nm

Photocatalytic activityPhotocatalytic activity

ZnO/Si(100)Orange II solution (2.4*10-6 M , pH ~ 6)

UV irradiation (125 W)

Decomposition process shows a pseudo-first order kinetics

K350°C (min-1) = 4.9*10-3

Irradiation time (min)

100

80

60

40

20

0

(Cd

ye/C

dye

,0)*1

00 (

%)

3002001000

ZnO NPTs (350°C) ZnO NPTs(400°C) ZnO thin film (400°C)

CONCLUSIONSCONCLUSIONS

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.

PERSPECTIVESPERSPECTIVES

Syntesis of ZnO-TiO2 nanocomposites.

Evaluation of their photocatalytic and gas sensing

performances as a function of synthesis parameters.

Inte

nsi

ty (

a.u

.)536 532 528

BE (eV)

O1s

Zn-OZn-OH

BE (eV)

Inte

nsi

ty (

a.u

.)

1025 1020

Zn 2p3/2

XPSXPS

XPS Signals pertaining to ZnO sample deposited at 350°CXPS Signals pertaining to ZnO sample deposited at 350°C

ZnO

Si

O

Si

OHHH

Si

O

Si

OHHH

ZnLL’

O

O

Si

O

Si

OHHH

ZnLL’

O

O

Si

O

Si

OHH

ZnLL’OH

O

…

(a)

Si

OH

Si

OH

Si

OH

Si

OH

(b)

Si(100)

ZnO

Si

O

Si

OHHH

Si

O

Si

OHHH

ZnLL’

O

O

Si

O

Si

OHHH

ZnLL’

O

O

Si

O

Si

OHH

ZnLL’OH

O

…

(a)

Si

OH

Si

OH

Si

OH

Si

OH

Si

O

Si

OH

Si

OH

Si

OHHHHHH

Si

O

Si

OH

Si

OH

Si

OHHHHHH

ZnLL’

O

O

Si

O

Si

OH

Si

OHHHHH

ZnLL’

O

O

Si

O

Si

OH

Si

OH

Si

OHHHH

ZnLL’OH

O

…

(a)

Si

OH

Si

OH

Si

OHH

Si

OH

Si

OH

Si

OHH

Si

OH

Si

OH

Si

OHH

Si

OH

Si

OH

Si

OHH

(b)

Si(100)

ZnO

(b)(b)

Si(100)

ZnO

Ruolo dei gruppi –OH nella crescita pseudo-

colonnare

RISULTATI ANALISI SEMRISULTATI ANALISI SEM

Alla temperatura del supporto Alla temperatura del supporto di 350°C si ha la deposizione di 350°C si ha la deposizione

miglioremigliore

1.4

1.2

1.0

0.8

0.6

0.4

0.2

ln v

(n

m/m

in)

1.9x10-3

1.71.61.51.41.31/T (K

-1)

CAMPIONI/ T (°C)CAMPIONI/ T (°C) SPESSORE SPESSORE FILM (nm)FILM (nm)

LUNGHEZZA LUNGHEZZA SCAGLIE (nm)SCAGLIE (nm)

SPESSORE MEDIO SPESSORE MEDIO SCAGLIE (nm)SCAGLIE (nm)

ZnO19/250ZnO19/250 111 111 ± 2± 2 5555 5.55.5

ZnO18/300ZnO18/300 128 128 ± 3± 3 6060 5.55.5

ZnO17/350ZnO17/350 256 256 ± 5± 5 6666 5.55.5

ZnO14/400ZnO14/400 209 209 ± 5± 5 7676 5.55.5

ZnO15/450ZnO15/450 128 128 ± 3± 3 5656 5.55.5

ZnO16/500ZnO16/500 64 64 ± 2± 2 3333 5.55.5

Zn(hfa)Zn(hfa)22TMEDA - CARATTERIZZAZIONETMEDA - CARATTERIZZAZIONE

m.p.=104-106°C

analisi elementareC=32,29%, H=2,87%, N=4,64%

analisi 1H- e 13C-NMR

analisi termiche

ln p1 – ln p0 = (H0vap/R)(T0

-1 –T1-1)

H°vap = 102 1 kJ/mol

► singolo processo di sublimazione senza decomposizione

► Perdita in peso = 98%

-8

-7

-6

-5

-4

ln[v

eloc

ità

vap

oriz

zazi

one

(mm

ol/m

in)]

3.0x10-3

2.82.72.6

1/T(K-1

)

sperimentale fit

100

80

60

40

20

0

mas

sa(%

)25020015010050

T(°C)

3.0

2.5

2.0

1.5

1.0

0.5

0.0

derivata massa (%

/°C)

Trasporto di massaTrasporto di massa

Diffusione verso la superficieDiffusione verso la superficie

Reazione superficiale

Desorbimento sottoprodotti

Eliminazione sottoprodottiliminazione sottoprodotti

Nucleazione e crescita

CVD - termico

Centro metallico

R

RRR

R

R

R

Legante

Substrato

R

CVDChemical

Vapor DepositionGas reattivo

1 1

2 2

sourcedetector

GIXRDGIXRD

detector2

1

source

enhancementof surface sensitivity

XRDXRD

detects only reflectionsfor planes parallel

to the sample surface

Ass

orba

nza

(a.u

.)

700600500400300(nm)

0 min15 min45 min75 min

135 min195 min255 min315 min

ORANGE IIORANGE II