Voltammetry of Metal Oxide Nanoparticle Films

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Voltammetry of Metal Oxide Nanoparticle Films Surface processes at Mono- to Multi-Layer Deposits Frank Marken Department of Chemistry, University of Bath, BA2 7AY, UK 16 th September 2008, Liverpool, Electrochem 2008

Acknowledgements: Students & visitors: Dr. Susan J. Stott Dr. Elizabeth V. Milsom Dr. Katy J. McKenzie Dr. Liza Rassaei Michael Bonné Charles Y. Cummings Xiaohang Zhang Collaborators: Dr. Karen J. Edler Prof. Roger J. Mortimer, Prof. Stephen Fletcher Funding: EPSRC, RSC, Tekes, Unilever, & the Royal Society

Overview: Aims & Introduction I. Thin film voltammetry with nano-particle mono-layers: Fe 2 O 3 TiO 2 II. Multi-layer structures: TiO 2 III. Underpotential reduction: CeO 2 IV. Surface stabilisation effects: Fe 2 O 3 Summary & Outlook

Electrochemical Characterisation of Nanoparticle Thin Films: Aims To explore the surface (electro-)chemistry of insulator materials in nano-dimensions To exploit the use of the film thickness in the study of surface processes. To investigate triple phase boundary processes at contact points. To develop strategies for the assembly of novel thin film materials for electrocatalysis and electroanalysis.

Nanoparticle Thin Film Voltammetry I.: Monolayers on ITO and on BDD Hydrous Iron Oxide Adsorbed onto BDD and Immersed in Aqueous Media: Cyclic voltammograms (scan rate 0.02 V s -1 ) for the oxidation of 1 mm hydrogen peroxide in the presence of (i) 0.1 mm, (ii) 0.4 mm, (iii) 1.0 mm, and (iv) 2.0 mm KOH in aqueous 1.0 M KNO 3 at a boron-doped diamond electrode (0.25 cm 2 ) modified with hydrous ferric oxide. Catalytic Species?

Nanoparticle Thin Film Voltammetry I.: Monolayers on ITO and on BDD Hydrous Iron Oxide (Fe 2 O 3 ) Monolayer Reactivity in Aqueous Phosphate Media: Cyclic voltammograms (two cycles, scan rate 0.1 V s -1 ) obtained for the reduction of Fe 2 O 3 nanoparticles adsorbed (by immersion into a Fe 2 O 3 solution 2 mm in Fe) onto a 3 x 4 mm 2 tin-doped indium oxide electrode and immersed (A) in aqueous 0.1 M phosphate buffer at ph 8 and (B) in aqueous 0.1 M phosphate buffer at ph 8 containing 10 mm EDTA.

Nanoparticle Thin Film Voltammetry I.: Monolayers on ITO and on BDD Titanium (IV) Oxide Reactivity in Aqueous Phosphate Media:

Nanoparticle Thin Film Voltammetry I.: Monolayers on ITO and on BDD Titanium (IV) Oxide Reactivity in Aqueous Phosphate Media:

Nanoparticle Thin Film Voltammetry II.: Multi-layer Assembly Multi-layer Film Preparation: two-component dipcoating of: (i) TiO 2 nanoparticle sol (anatase sol, TKS-202, typically 6 nm diameter, 30-37% acidified with HNO 3 ) (ii) 40 mm sodium phytate in H 2 O, acidified with HClO 4 to ph 3.

Nanoparticle Thin Film Voltammetry II.: Multi-layer Assembly Robotic Dip-Coating Unit:

Building Blocks Substrate Glass Carbon Diamond ITO Au Pt Metal Oxide Fe 2 O 3 TiO 2 ZrO 2 CeO 2 Ru 2 O 3 MnO 2 SnO 2 SiO 2 Binder phytic acid C 6 H 6 (CO 2 H) 6. H 2 O hexametaphosphate [Mo 7 O 24 ] 6-

Nanoparticle Thin Film Voltammetry II.: Multi-layer Assembly QCM monitoring of the layer-by-layer growth of a TiO 2 cyclohexane hexacarboxylic acid film

Nanoparticle Thin Film Voltammetry II.: Multi-layer Assembly SEM images of the TiO 2 phytate nanofilm on ITO (A) one layer TiO 2 nanoparticles some agglomerates (B) 10 layers TiO 2 phytate (C) 30 layers TiO 2 phytate Prior to imaging, surface scratched & gold coated

Nanoparticle Thin Film Voltammetry I.: Monolayers on ITO and on BDD Titanium Oxide (TiO 2 ) Reactivity in Aqueous Phosphate Media: Reversible charging processes in aqueous media at a potential negative of 0.5 V vs. SCE. No reaction in phosphate buffer media.

Nanoparticle Thin Film Voltammetry I.: Monolayers on ITO and on BDD Titanium (IV) Oxide Reactivity in Aqueous Phosphate Media: Effect of scan rate: Effect of ph:

Nanoparticle Thin Film Voltammetry II.: Multi-layer Assembly Electrochemical Characterisation of TiO 2 Films: Thickness Effects Reversible one electron process Ru(NH 3 ) 6 3+ (aq) + e - Ru(NH 3 ) 6 2+ ( aq ) New voltammetric response at E p red = -0.33 V vs. SCE Adsorption of Ru(NH 3 ) 6 3+ into TiO 2 phytate film

Nanoparticle Thin Film Voltammetry II.: Multi-layer Assembly Electrochemical Characterisation of 500 o C Calcined TiO 2 Films Reversible one e - process Ru(NH 3 ) 6 3+ (aq) + e - Ru(NH 3 ) 6 2+ ( aq ) (A) with phytate (B) no phytate/with CHHCA (C) in phosphate buffer at ph 7 (D) effect of Ru(NH 3 ) 6 3+ concentration

Nanoparticle Thin Film Voltammetry II.: Multi-layer Assembly Processes within Nafion-TiO 2 -Nafion-PDDA- Au-PDDA Films:

Nanoparticle Thin Film Voltammetry II.: Multi-layer Assembly Processes within Calcined TiO 2 -Au Films: The determination of NO (nitric oxide) is possible at TiO 2 -Au film electrodes:

Nanoparticle Thin Film Voltammetry III.: Reactivity of CeO 2 Assemblies CeO 2 can be assembled with phytate or with cyclohexanehexacarboxylate QCM SAXS

Nanoparticle Thin Film Voltammetry III.: Reactivity of CeO 2 Assemblies Cerium (IV) Oxide Reactivity in Aqueous Phosphate Media: (i) Background ITO only in 0.1 M PBS at ph 7 (ii) one layer of cerium oxide in 0.1 PBS at ph 7 (iii) one layer of cerium oxide in 0.1 M KCl

Nanoparticle Thin Film Voltammetry III.: Reactivity of CeO 2 Assemblies Electrochemical Characterisation of CeO 2 Films Continued potential cycling causes PI to diminish and PII to appear Strong phosphate concentration effect Rate of CePO 4 formation is different, presumably due to more extensive phosphate adsorption and better electron transport

Nanoparticle Thin Film Voltammetry III.: Reactivity of CeO 2 Assemblies Electrochemical Characterisation of CeO 2 Films Process I: 2 CeO 2 (s) +H + +e - CeO 2 H(s) Process II: Ce 3 (PO 4 ) 4 (s) + 3 H + (aq) + 3 e - 3 CePO 4 (s) + H 3 PO 4 (aq) More layers more current

Nanoparticle Thin Film Voltammetry III.: Reactivity of CeO 2 Assemblies Electrochemical Characterisation of CeO 2 Films: Underpotential Reduction

Nanoparticle Thin Film Voltammetry IV.: Reactivity of Fe 2 O 3 Assemblies Fe 2 O 3 assemblies prepared from nanoparticles. SAXS shows particle properties before and after calcination. UV/Vis evidence for film growth.

Nanoparticle Thin Film Voltammetry IV.: Reactivity of Fe 2 O 3 Assemblies Fe 2 O 3 assemblies show two distinct types of surface processes: Fe(III/II) Fe(VI/III) SURFACE STABILISATION

Nanoparticle Thin Film Voltammetry IV.: Reactivity of Fe 2 O 3 Assemblies Fe(VI) Electrocatalytic Detection of Glucose: Fe(VI) at the nanoparticle surface is a strong oxidant e.g. reacting with glucose in solution. However, the anion controls reactivity.

Summary & Outlook: I. Thin film voltammetry with nanoparticle mono-layers: Fe 2 O 3 TiO 2 II. Multi-layer structures: TiO 2 III. Underpotential reduction: CeO 2 IV. Surface stabilisation effects: Fe 2 O 3 V. Voltammetry is a tool for studying oxide surfaces Nanoparticle thin film voltammetry offers a powerful diagnostic tool for both (i) bulk conducting and (ii) surface conducting materials. THANK YOU FOR YOUR ATTENTION!

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