Energy transfer mechanisms from plasmonic metal nanoparticles to core-shell black-tio 2.

Transcription

1 Photocatalysis for energy [PHOTO4E] Lyon,15-17 October 2014 Energy transfer mechanisms from plasmonic metal nanoparticles to core-shell black-tio 2. A. Naldoni 1, F. Fabbri 2, M. Altomare 3, M. Marelli 1, R. Psaro 1, E. Selli 3, G. Salviati 2, V. Dal Santo 1. 1 CNR Istituto di Scienze e Tecnologie Molecolari and ERIC laboratory Materials for Hydrogen Production Via C. Golgi 19, Milano 20133, Italy 2 IMEM-CNR Institute, Parco Area delle Scienze 37/A, Parma, Italy 3 Dipartimento di Chimica, Università degli Studi di Milano, Via Golgi 19, Milano 20133, Italy presenting author v.dalsanto@istm.cnr.it

2 Solar energy exploitation roadmap Energy Environ. Sci., 2013, 6, 1711.

3 Photocatalytic water splitting Artificial photosynthesis Water splitting using TiO Absorption, bandgap 1.23 ev < Eg < 2.5 ev Charge carriers transport Band energy position Surface features for maximixing interfacial charge transfer Stability, cost, non-toxicity Chen et. al., Chem. Rev. 2010, 110, ;

4 TiO 2 + & - and solutions TiO2 N-doped TiO2 large availability cheapness stability non-toxicity JACS 2006, 128, drawback is absorption only in UV region Hydrogenated TiO2 Chen et. al., Chem. Rev. 2010, 110,

5 LSPR Localized Surface Plasmon Resonance high concentration of energetic e - Collective oscillation of surface free electrons d NM << λ inc intense, localized electromagnetic field PLASMON RESONANCE ADVANTAGES Coupling of optical & electronic propertiers Catching all the solar spectrum-tunable absorption through nanostructures control Bumajdad A. et al., Phys. Chem. Chem. Phys. 16 (2014) 7146; Clavero C.,. 1 Nature Photon. 8 (2014) 95; Govorov A. et al., Nano Today 9 (2014) 85.

6 LSPR - Applications 1. Metal oxide - Metal nanoparticles coupling Metal oxide photoactivity increased (LEMF) Naldoni A. et al., Appl. Catal. B: Environm (2013) Plasmonic-induced photoactivity Plasmon resonance to excite the metal oxide Lee J. et al., Nato Lett. 12 (2012) 5014.

7 TiO 2 supported-au nanoparticles 1 hot e - E F h + ϕ SB metal E g CB VB semiconductor Hot e - -transfer Band alignment Schottky barrier height (φ SB ) Au/TiO 2 φ SB ~ 0.9 ev Δε ~ 1 ev 2 - CB E F E g + VB metal semiconductor Energy Transfer (PRET) Intra band-gap defect states V O s, slightly below CB Non-metal doping, slightly above VB 3

8 Aim of the work Use CL spectroscopy better discerne the 2 mechanisms P25 TiO 2 : no intra band-gap states Black-TiO 2-x : defect states below the CB Au nanoparticles loading: 1 wt.% to evaluate charge transfer and induced electric field variations Excitation of Au plasmon band ( >400nm) (TiO 2 band gap ~ ev)

9 Photocatalysts synthesis P25-TiO 2 commercial TiO 2 powder, 500 C in O 2 black-tio 2 Nanoactive commercial TiO 2 powder, 500 C in H 2 Au, Pt NPs deposition (1 wt.%): HPtCl 6 /HAuCl 4 + NaBH 4 Photocatalysts characterization Plasmon-photoactivity X-ray Diffraction Uv-vis TEM Catholuminescence spectroscopy H 2 O H O 2 CH 3 OH + H 2 O 3H 2 + CO 2 H 2 O photosplitting / methanol photoreforming

10 Black TiO 2 1 P25-TiO 2, ~ 80 wt.% A + ~ 20 wt.% R b-tio 2, ~ 81 wt.% A + ~ 19 wt.% R Nanoactive - TiO 2, 100 wt.% amorphous 1. Red shift of absorption E g 2. Continuous absorption nm b-tio 2 P25; Nanoactive TiO 2 Naldoni A. et al., J. Am. Chem. Soc. 134 (2012)

11 P25 XPS Black TiO 2 2 EPR black TiO2 Ti 4+ Ti 3+, no O2 - VO in the core EPR and XPS analysis indicate that Ti 3+ is present only in the core of black TiO2 NPs

12 Black TiO 2 vs. P25 Black TiO 2 : crystalline core / amorphous shell P25: fully crystalline nanocrystal

13 Black TiO 2 summary Crystalline core/disordered shell morphology V O s in the bulk anatase crystalline phase - disordered NP surface stoichiometric. Bandgap narrowing synergisty V O s / surface disorder.

14 Plasmon-photoactivity 1 H 2 O H O 2 Water photosplitting CH 3 OH + H 2 O 3H 2 + CO 2 methanol photoreforming CO(g)

15 Photoreforming device (A) Photoreactor cross section (B) photocatalyst bed (C) Pyrex glass window (GC) gas chromatograph (E) six ways sampling valve (D) bellow pump (F) thermostated bubbler (FC) gas flow meter (TI) temperature indicator (PI) pressure indicator G.L. Chiarello et al., Photochem. Photobiol. Sci., 10 (2011) 355. TiO 2 powders (14 mg) on quartz beads Photocatalytic Plexiglas cell + closed recirculation apparatus N 2 flow 40 ml min -1 Vapour feed from 20 vol.% MeOH/H 2 O solution (x MeOH,l = 0.1) 300 W Xe arc lamp (solar simulator, cut off filter >400 nm) H 2 + MeOH oxidation products monitored by on-line GC analysis T = 30 ± 2 C and P i = 1.2 bar

16 Photoreforming activity Un-filtered solar simulator light Au/P25 Pt/P25 >> P25 M/P25 > M/b-TiO 2 Au/b-TiO 2 > Pt/b-TiO 2 >> b-tio 2

17 Photoreforming selectivity Real water photosplitting Less photoreforming Pt more selective than Au

18 Pure visible light ( >400nm) 0,012 r H 2 / mmol h -1 gcat -1 0,01 0,008 0,006 0,004 0,002 No H 2 for black TiO 2 Au vs. Pt 360% enhancement ( 33% increase under unfiltered solar light) 0 black TiO2 black TiO2/Au black TiO2/Pt

19 CL System CL spectroscopy system: Wavelength ranges: Visible nm Ge/InGaAs photodiodes NIR detector LT transfer EBIC ampl. Ge NIR nm Extended InGaAs Short wavelength IR nm CL EL ampl. Temperature: 6 300K PhotoMultiplier Tube Visible Detector SE Microscope: Accelerating Voltage: 250 ev 40 kev Beam Current: 100 pa 10 A

20 E C B A Exciton N D h ea DAP D G E A E A A E V P INTRINSIC TRANSITIONS E D E D EXTRINSIC TRANSITIONS DONORS ACCEPTORS Two types of transitions can be distinguished: - Intrinsic emissions which are due to recombination of electrons and holes across the fundamental energy gap, by interband transitions from the bottom of the conduction band to the top of the valence band. In the spectral region close to the energy gap it is possible to detect free excitons or bound excitons, with one of the carriers localized at an impurity centre. - Extrinsic luminescence which is due to radiative transitions involving states in the band-gap, shallow or deep, mainly due to native defects and impurities complexes acting as donor or acceptor centers. Different processes of recombination between free carriers and trapped carriers can take place, basically indicated as freeto-bound (e.g. donor-to-free-hole D h, free-electron-to-acceptor ea ) and donor-toacceptor pair (DAP) transitions.

21 P25 vs black TiO 2 BLACK TiO P ev = self-trapped exciton 2.62 ev = oxygen vacancies (V O s) 2.36 ev = oxygen vacancies (V O s) MUCH HIGHER NUMBER OF DEFECT IN BLACK TiO 2

22 Au, Pt P25, black-tio 2 P25 Black Pt luminescence quenching (better e - /h + TiO separation) 2 Au luminescence enhancement (plasmonic interactions = energy transfer)

23 More on Au The disordered black-tio 2 nanoparticle outer layer induces a reduced charge mobility Less hot e - transfer. The signal at 2.36 ev is doubled PRET SPR(Au) V O s TiO 2

24 Conclusions 1. Higher photoactivity of Au/TiO 2 vs Pt/TiO 2 2. Role of SPR and Plasmonic interactions 3. By Catholuminescence spectroscopy: Hot e - transfer: dominant process for non-defective titania Plasmon Resonance Energy Transfer: reduced band-gap black titania Role of intra band-gap oxygen vacancies defect states (CB)

25 Acknowledgements Thanks for your kind attention! Coworkers of Thanks to: Financial support from Italian MIUR (FIRB OXIDES_RBAP115AYN, FIRB PEC_RBFR13XLJ9 projects) Collaborators Prof. S. Santangelo Dr. M. Altomare Prof. E. Selli Dr. Salviati Dr. Fabbri