CVD SYNTHESIS AND PHOTOCATALYTIC ACTIVITY OF ZnO NANOPLATELETS

1. Padova University ISTM-CNR 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 *chiara.maccato@unipd.it CVD SYNTHESIS AND PHOTOCATALYTIC ACTIVITY OF ZnO NANOPLATELETS VI Convegno Nazionale sulla Scienza e Tecnologia dei Materiali – Perugia 12-15 Giugno 2007

2. O Zn Zinc oxide (ZnO) Wurtzite (hexagonal lattice) n-type semiconductor EG3.4 eV Main interests:  Optoelectronics  Gas Sensing Energetics Photocatalysis

3. A ˉ Reduction Conduction Band h > Eg A D- Valence Band + Oxidation D 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→ O2- • h+ + OH- → OH

4. 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.

5. C N H O Zn F Zn(NO)3·xH2O + 1,1,1,5,5,5-hexafluoro-2,4-pentanedione + N,N,N’,N’-tetramethylethylendiamine Zn(hfa)2TMEDA(Hhfa=1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA=N,N,N’,N’-tetramethyilethylendiamine) Zn(hfa)2(TMEDA) Yield = 63% • 2nd Generation precursor • high volatility • and thermal stability • one-pot synthesis Tobin J. Marks et al., J. Am. Chem. Soc., (2005) 127, 5613.

6. CVD F(O2+H2O) = 40 sccm F(N2) = 40 sccm 10 mbar, 60’ T[Zn(hfa)2(TMEDA)] = 60°C Tsub. = 250-500°C Thickness [Φ(O2+H2O)] 256 209 128 64nm 111 128 Tsub. 300 350 400 450 250 500 °C ZnO NANOSYSTEMS ZnO Si(100) Zn(hfa)2(TMEDA)

7. Nanoplatelets (NPTs) With H2O Tsub= 400°C 100 nm 100 nm The obtained systems show very different morphologies FE-SEM Water effect on morphology No H2O Tsub= 400°C Thin film 100 nm 100 nm

8. Tsub EFFECT 100 100 nm nm 100 100 nm nm • NPTs mean • thickness ~ 5.5 nm • System porosity is • temperature – dependent. (a) 250 250 ° ° C C (b) 250 250 ° ° C C 100 100 100 nm nm nm 100 100 100 nm nm nm Different photocatalytic activity is expected (c) 350 350 ° ° C C (d) 350 350 ° ° C C 100 100 100 nm nm nm 100 100 100 nm nm nm No variations of NPTs morphology after thermal treatment (600°C, 2h) (e) 450 450 ° ° C C (f) 450 450 ° ° C C

9. 500°C 450°C 400°C 350°C Zn 300°C 250°C (102) (100) (002) (101) 20 30 40 50 O J 2 (degrees) GIXRD • The expected intensity ratio I002/I101 for ZnO powders is 0.44. • In the synthesized NPTs the I002/I101 ratio depends on Tsub with a maximum value of 3.4 • at Tsub= 350°C. ZnO(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.

10. Oxygen 80 Zinc Silicon KE = hυ - BE 60  = BE(XPS) + KE (Auger) % 40 20 0 0 50 100 150 Sputtering time(min) XPS Auger Parameter -  Tsub=350°C ZnO Auger Peak: ZnLMM XPS peak: Zn2p3/2 ZnO, literature≈ 2010.1 eV • ZnO≈ 2010.2 eV C and F XPS signals disappear after a mild sputtering indicating that they are only surface contaminants.

11. AFM Photocatalytic activity ZnO/Si(100) Orange II solution (2.4*10-6 M , pH ~ 6)UV irradiation (125 W) NPTs 350°C RMSR = 32 nm (a) (a) nm nm nm nm nm 30 30 30 30 30 10 10 10 10 10 200 200 200 200 200 200 200 200 200 200 400 400 400 400 400 400 400 400 400 400 600 600 600 600 600 600 600 600 600 600 800 800 800 800 800 800 800 800 800 800 100 nm nm nm nm nm (b) (b) 80 NPTs 400°C RMSR = 6 nm nm nm nm nm )*100 (%) 40 40 40 40 20 20 20 20 60 dye,0 200 200 200 200 200 200 200 200 400 400 400 400 /C 400 400 400 400 600 600 600 600 40 600 600 600 600 dye 800 800 800 800 (C 800 800 800 800 nm nm nm nm ZnO NPTs (350°C) 20 ZnO NPTs(400°C) (c) (c) ZnO thin film (400°C) Film 400°C RMSR = 2 nm nm nm nm 0 12 12 12 6 6 6 Irradiation time (min) 0 100 200 300 200 200 200 200 200 200 Decomposition process shows a pseudo-first order kinetics K350°C (min-1) = 4.9*10-3 400 400 400 400 400 400 600 600 600 600 600 600 800 800 800 800 800 800 nm nm nm

12. 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.

14. O1s Zn 2p3/2 Zn-O Zn-OH Intensity (a.u.) Intensity (a.u.) 536 532 528 1025 1020 BE (eV) BE (eV) XPS XPS Signals pertaining to ZnO sample deposited at 350°C

15. Zn O Ruolo dei gruppi –OH nella crescita pseudo-colonnare

16. RISULTATI ANALISI SEM • Alla temperatura del supporto di 350°C si ha la deposizione migliore

17. Zn(hfa)2TMEDA - CARATTERIZZAZIONE m.p.=104-106°C analisi elementare C=32,29%, H=2,87%, N=4,64% analisi 1H- e 13C-NMR analisi termiche ►singolo processo di sublimazione senza decomposizione ►Perdita in peso = 98% ln p1 – ln p0 = (H0vap/R)(T0-1 –T1-1) H°vap = 102  1 kJ/mol

18. CVD y x v z w u CVD - termico R R R R R R R R Centro metallico Legante Chemical Vapor Deposition Gasreattivo Substrato uTrasporto di massa vDiffusione verso la superficie wReazione superficiale xDesorbimento sottoprodotti yEliminazione sottoprodotti zNucleazione e crescita

19. 2 2 source 1 1 XRD detects only reflections for planes parallel to the sample surface detector detector 2 source 1 GIXRD  enhancement of surface sensitivity

20. ORANGE II