Crystalline Thin Films: The Electrochemical Atomic Layer Deposition (ECALD) view MMALEWANE MODIBEDI mmodibedi@csir.co.za
Acknowledgements Outline Context: Crystalline Thin films What is ECALD? Mechanism Instrumental set-up Applications - Electrocatalysis Deposition on various substrates: work done @ EaP - Compound semiconductor Conclusions CSIR 2011 www.csir.co.za
Acknowledgements Dr. Tumaini Mkwizu, PhD work on ECALD Ms. Nikiwe Kunjuzwa, PhD student- ECALD and batteries Prof. Kenneth Ozoemena, Research group leader and electrochemist Dr Mkhulu Mathe, Competence area manager, electrochemist and ECALD expert
Crystalline Thin Films Application Historical: light absorber material CdTe, crystalline Si Modern: light absorber material Solar cells Cu, In, Ga, Se (CIGS)- developed by Vivian Alberts and UJ - low material cost in 2011- potentially low fabrication costs Growth methodologies Molecular beam epitaxy/ chemical vapour deposition Disadvantages: expensive equipment, use toxic precursors, Vacuum and High Temperature Electrochemical methods: co-deposition, precipitation Disadvantages: lack of control during deposition
What is ECALD? Definition: alternated electrodeposition of atomic layers of elements on a substrate, employing under-potential deposition (UPD) in which one element deposits onto another element at a voltage prior to that necessary to deposit the element onto itself. Advantages: ambient temperature, use small concentrations of precursor solutions, optimized solutions and potential separately Offers atomic layer control- fundamental for controlled growth processes
M1-BE Rinse M1-Fill M1-Deposit M2-BE Rinse M2-Fill M2-Deposit M1-BE Rinse M1-Fill M1-Deposit M2-BE Rinse M2-Fill M2-Deposit Steps Mechanism-Sequential deposition Cycle 1 Cycle 2 Repeat Cycles Time
Distribution Valve Block Waste/ Recycling Reference Electrode Electrodeposition flow-cell Pump Counter Electrode Pump Pump Substrate (Working Electrode) Gasket Pump Potentiostat Pump Computer Interface/ DAQ Card Relay Board/ Power Supply Precursor Solution Reservoirs
Potentiostat Flow-cell Peristaltic Pumps Instrumental set-up Pumping system, Potentiostat and Flow-Cell Connectivity
APPLICATIONS Electrocatalysis Noble-Metals studied = Pt, Ru, Au, Pd Substrates = Carbon materials-fuel cell carbon paper, Gold films T.S.Mkwizu, M.K. Mathe, and I. Cukrowski, ECS Transactions, Vol.19, 97-113 (2009) T.S.Mkwizu, M.K. Mathe, and I. Cukrowski, Langmuir, Vol. 26, 570-580 (2010) T.S Mkwizu, M.R. Modibedi, and M. K. Mathe, 219 th ECS Meeting (2011)
Sequential electrodeposition coupled to Surface-limited Redoxreplacement reactions: Synthesis of multilayered bimetallic RuPt electrocatalyst S S S S S S S (1) Clean substrate with blank electrolyte (BE); Inject Cu 2+ solution at E >> E Cu-Cu2+ S S S S S S S Pt Pt Pt Pt Pt S S S S S S S Cu Cu Cu Cu Cu Cu 2+ -2e Cu 2+ Cu2+ S S S S S S S Pt Cu Pt Cu Cu (2) Potentiostatic electrodeposition at E dep > E Cu-Cu2+ (Underpotential Deposition (UPD)) or E dep < E Cu-Cu2+ (small Overpotential Deposition (OPD) - to produce sacrificial Cu adlayer on active sites of the substrate; Rinse with BE (4) Pt nanodeposit on substrate; Rinse with BE and inject Cu 2+ solution at E >> E Cu-Cu2+ Pt 4+ Cu 2+ Pt 4+ (3) Inject H 2 PtCl 6 solution and allow surface-limited redox-replacement (SLRR) of Cu by Pt at open circuit (OC) S S S S S S S Pt Pt Pt Pt Pt Cu Cu Cu Cu Cu Cu 2+ -2e Cu2+ Cu 2+ (5) Potentiostatic electrodeposition at E dep to produce sacrificial Cu adlayer on active sites on Pt adlayers; Rinse with BE S S S S S S S Pt Pt Pt Pt Pt Ru Cu Ru Ru Cu Ru 3+ Ru 3+ Cu 2+ (6) Inject RuCl 3 solution and allow surface-limited redox-replacement (SLRR) of Cu by Ru at OC S S S S S S S Pt Pt Pt Pt Pt Ru Ru Ru Ru Ru S S S S S S S Pt Pt Pt Pt Pt Ru Ru Ru Ru Ru Pt Pt Pt Pt Pt Ru Ru Ru Ru Ru
Tuning Electrocatalysis: Electrochemical Characterisation Electro-activity increases with increasing deposition cycles Impedance of methanol electro-oxidation Pt < PtAu < PtRu Order of activity: Methanol electro-oxidation Chronoamperometry of methanol electro-oxidation T.S.Mkwizu, M.K. Mathe, and I. Cukrowski, ECS Transactions, Vol.19, 97-113 (2009) T.S.Mkwizu, M.K. Mathe, and I. Cukrowski, Langmuir, Vol. 26, 570-580 (2010) (i) Sequentially-deposited with Cu SLRR bimetallic PtRu / GC (ii) Sequentially- codeposited with Cu SLRR bimetallic Pt-Ru/GC (iii) Sequentially-deposited with Cu SLRR monometallic Pt/GC 0.1M HClO 4 + 0.5 M CH 3 OH
Tuning Electrocatalysis on Fuel Cell gas diffusion layer: SEM micrographs and EDX profile Carbon paper TGPH090 Pt/carbon paper TGPH090 C Carbon paper TGPH090 Pt/carbon paper TGPH090 Pt/carbon paper TGPH090 O Pt Cu Pt 0 1 2 3 4 5 6 Energy (kev)
Current Density (ma/cm 2 ) Current Density (ma/cm 2 ) 0.30 0.25 0.20 0.15 0.10 0.05 0.00-0.05-0.10 Tuning Electrocatalysis on Fuel Cell gas diffusion layer: Electrochemical Characterisation Methanol oxidation -0.15 0.2 0.4 0.6 0.8 1.0 1.2 Potential (V) CO adsorption-oxidation 1.0 (ii) 0.1 M HClO4 + CO 0.8 0.6 0.4 0.2 0.0-0.2-0.4-0.6 Cyclic voltammograms at 50 mv/s in (i) 0.1 M HClO4 + 0.1 M Methanol Pt/carbon paper TGPH090 Carbon paper TGPH090-0.2 0.3 0.8 1.3 Potential (V)
Compound Semiconductors Group II - VI, Group III - V, Group IV - VI: Optoelectronic materials CdTe, CdSe, GaAs, HgSe - photovoltaics, photon sensors, lasers etc. Substrates = Gold, carbon material Mkhulu K. Mathe et al. J. Electrochem. Soc., Volume 152, Issue 11, C751-C755 (2005) Mkhulu K. Mathe et al. J. Crystal Growth Volume 271, Issues 1-2, 55-64 (2004) Energy storage Physical form (capacitors) and Electrochemical form (batteries) Obstacle to increasing thin film battery storage capacity: limited diffusion path length of ions and electrons Solution: area enhancement- 3D thin films - Increases total amount of active material while maintaining short diffusion path - Results- high power and energy density
Conclusions ECALD: controlled growth of thin film deposits atomic layer control is key to reducing the amount of PEMFC catalysts possibility of a 3D battery stack
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