AIChE646d
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1) A model was developed to understand the sharp transition between copper oxide phases observed during CVD. The model considers oxygen transport and the single degree of freedom in the Cu2O-O2-CuO system. 2) 1D and 2D models were used to predict spatial oxygen profiles and examine the effects of temperature, oxygen flow, and surface/gas phase reaction rates on the transition location. 3) Comparisons to experimental film measurements suggest surface reactions play a primary role in the transition, though gas phase reactions may also contribute. Further work aims to estimate adsorption rates and refine surface/gas phase reaction mechanisms.
![Introduction Methodology Results Conclusion
1D oxygen partial pressure profile
L1 L2
O2
substrate exhaust
Ar
CuI boat
dCO2
JO2 ,z = −DO2 ,Ar + vCO2
dz
JO2 ,z constant within each
segment, [0, L1 ) and
(L1 , L1 + L2 ];
Flux discontinuity at the feed
point L1 .
max
PO2 exp (v (z − L1 − δshift )/D) z ≤ L1
PO2 = max
PO2 z > L1](https://image.slidesharecdn.com/dac646dbcopy-130104164212-phpapp01/85/AIChE646d-11-320.jpg)
![Introduction Methodology Results Conclusion
Computed O2 pressure profile
O2 pressure profile estimated by solving the 2D governing equation with
a parabolic velocity profile v (r ). The units in the color bar are [Pa].](https://image.slidesharecdn.com/dac646dbcopy-130104164212-phpapp01/85/AIChE646d-16-320.jpg)
AIChE646d
- 1. Introduction Methodology Results Conclusion Modeling Spatial Variations In Thin-Film Processing of Copper Oxide Films for Solar Energy Applications and Hydrogen Production David Arana-Chavez and Raymond A. Adomaitis Department of Chemical & Biomolecular Engineering Institute for Systems Research University of Maryland January 5, 2012 Support: NSF CBET-0828410
- 2. Introduction Methodology Results Conclusion 1 Introduction Film growth by chemical vapor deposition The sharp transition 2 Methodology Estimating the cuprous oxide to cupric oxide ratio Design of experiments - temperature and oxygen flow 3 Results Oxygen partial pressure profiles Limiting cases: gas phase reaction vs surface deposition Comparison with experiments 4 Conclusion Summary
- 3. Introduction Methodology Results Conclusion Solar photoelectrochemical H2 production from water Hydrogen O2 H2 Tremendous interest and hv research focused on counter electrode (anode) transparent conductor 2e- + 2H2O 2OH- + H2 e- h+ hydrogen production at p-type semicond photo-cathode this meeting (ICE 2011) OH- Substantial discussion of 2OH- H2O + 1/2 O2 + 2e- mass production, manufacturability issues e- PEC Goal: direct solar conversion of water to H2 PEC system components: a semiconductor (metal oxide) in contact with an electrolyte inside an electrochemical cell
- 4. 0.5 Introduction Methodology Results Conclusion dark light Copper oxide current (mA/cm2) 2 0 PEC (p-type) semiconductors we investigate: Cu2 O, CuO Precursors available for a −0.5 −1 −0.5 0 0.5 1 relatively benign CVD process bias (V) Measured band gap: 2.06 eV 11 NaOH 0.5 for Cu2 O, 1.8 eV for CuO dark light CVD films are stable in high pH current (mA/cm2) 2 electrolytes, physically robust 0 Photoactivity has been established for both CVD films by collaborators Cu2 O is a promising catalyst for −0.5 −1 −0.5 0 0.5 1 various reactions as well as for bias (V) arsenic removal from Cu2 O sample contaminated water
- 5. Introduction Methodology Results Conclusion Copper oxide CVD reactions/reactor 2CuI(g) + 1/2 O2 (g) → Cu2 O(s) + I2 (g) O2 Ar 2CuI(g) + O2 (g) CuI Substrate → 2CuO(s) + I2 (g) Tubular CVD reactor elements: gas flow controllers (left), the process tube and furnace (center), and gas scrubbing system (right).
- 6. Introduction Methodology Results Conclusion Initial experimental results Experimental conditions: 0.1g CuI; total P = 1atm; T = 775o C ; Ar flow O2 flow; dep time = 5min; quartz substrate. (Red) Cu2 O at substrate leading edge and O2 feed tube; decrease in (black) CuO film thickness in the direction of gas flow
- 7. Introduction Methodology Results Conclusion Dependence on total O2 flow Modeling motivation: Understand the unique CVD behavior exhibited (e.g., unusually sharp and stable composition transitions); Control the deposition behavior, including film thickness, composition, and morphology; Assess potential effects of substrate and previous deposition layers.
- 8. Introduction Methodology Results Conclusion Why is the Cu2 O/CuO transition so sharp? Consider a system of O2 (g) and D.o.F . = c − π + 2 − r crystalline CuO(s) and Cu2 O(s): = 3−3+2−1=1 1 Both phases can only coexist at Cu2 O(s) + O2 (g ) ↔ CuO(s) 2 eq 2 PO2 = Po /Keq = 24Pa at 750o C 24 Pa PO2 < < PO2 The white bar corresponds to 1 mm
- 9. Introduction Methodology Results Conclusion Cuprous to cupric oxide ratio from substrate images Photos were converted to RGB images in MATLAB. Two major color modes detected with separation around gray scale 70. This representation allows for calculation of the percentage of the area of the films covered by Cu2 O.
- 10. Introduction Methodology Results Conclusion Effects of temperature and pressure Design of experiments Response surface model Cu2 O Area m = -1,0,1 → qO2 = 4.3, y= Total Area × 100 6.4, 8.5 sccm t = -1,0,1 → T = 725, 750, y = 52.4 − 18.6 · m + 8.3 · t 775 o C
- 11. Introduction Methodology Results Conclusion 1D oxygen partial pressure profile L1 L2 O2 substrate exhaust Ar CuI boat dCO2 JO2 ,z = −DO2 ,Ar + vCO2 dz JO2 ,z constant within each segment, [0, L1 ) and (L1 , L1 + L2 ]; Flux discontinuity at the feed point L1 . max PO2 exp (v (z − L1 − δshift )/D) z ≤ L1 PO2 = max PO2 z > L1
- 12. Introduction Methodology Results Conclusion Comparison with DoE Initially the 1D profile shows c PO2 off the substrate. An estimated displacement due to radial diffusion added (δshift ). After the spatial shift, 1D calculations agree with response surface model.
- 13. Introduction Methodology Results Conclusion 2D oxygen partial pressure profile model Axial and radial O2 diffusion; Initially investigate limiting case of no O2 consumption by rxn: 1 ∂ ∂CO2 ∂ 2 CO2 v ∂CO2 fO2 r + 2 − = r ∂r ∂r ∂z DO2 ,Ar ∂z DO2 ,Ar ∂CO2 with BCs JO2 = −DO2 ,Ar ∂z + vCO2 = 0 @ z =0 ∂CO2 −DO2 ,Ar ∂z =0 z =L ∂CO2 JO2 = −DO2 ,Ar ∂r =0 r = Ro ∂CO2 ∂r =0 r =0 δ (r ) where fO2 = qO2 δ (r , z − L1 ) = qO2 δ (z − L1 ) 2πr
- 14. Introduction Methodology Results Conclusion 2D oxygen partial pressure profile model solution m n CO2 = αi,j ψi (r ) θj (z) i j Solved through the projection of the concentration on a set of eigenfunctions. Eigenfunctions ψi (r ) and θj (z) based on 1D Sturm-Liouville similar problems with eigenvalues λi and ωj , respectively. where v ψi (r ) = J0 (λi r /Ro ) , φj (z) = Dωj sin (ωj z) + cos (ωj z) Oxygen feed modeled as a point source at feed tube exit location: m n fO2 = βi,j ψi (r ) θj (z) i j
- 15. Introduction Methodology Results Conclusion 2D model solution: eigenfunctions r -direction z-direction Eigenfunction equation Eigenfunction equation 1d dψ d 2θ r = λ2 ψ = ω2θ r dr dr dz 2 BCs BCs dψ(r ) dr = 0 at r =0 −D dθ(z) + v θ(z) = 0 dz z =0 dψ(r ) dθ(z) dr =0 r = Ro dz = 0 z = L1 + L2 Eigenvalue equation Eigenvalue equation J1 (λj ) = 0 (D/vL)ωi = tan (ωi L)
- 16. Introduction Methodology Results Conclusion Computed O2 pressure profile O2 pressure profile estimated by solving the 2D governing equation with a parabolic velocity profile v (r ). The units in the color bar are [Pa].
- 17. Introduction Methodology Results Conclusion O2 Convergence test Mass balance equation Ro /L qO2 − 0 (v (r )CO2 (m, n, r , z = L)2πr ) dr MB(m, n) = × 100 qO2
- 18. Introduction Methodology Results Conclusion O2 Profile - COMSOL Model Newtonian incompressible fluid Axial symmetry Velocity profile results parabolic and is develop soon after the oxygen feed point.
- 19. Introduction Methodology Results Conclusion O2 Comparison with COMSOL Model Comparison of the oxygen partial pressure profile at the wall of the reactor Model with 100 eigenfunctions in both the r and z directions
- 20. Introduction Methodology Results Conclusion Limiting Case - O2 consumption in the gas phase 1 ∂ ∂CO2 ∂ 2 CO2 v ∂CO2 kG fO2 r + 2 − − CO2 = r ∂r ∂r ∂z DO2 ,Ar ∂z DO2 ,Ar DO2 ,Ar Oxygen is only consumed by reactions in the gas phase; 1st-order reaction constant kG for the consumption of oxygen.
- 21. Introduction Methodology Results Conclusion Limiting Case - O2 consumption by deposition B.C. at r = Ro ∂CO2 kpO2 · PO2 = kadsO2 CO2 = −DO2 ,Ar ∂r The eigenvalues λ are obtained from the following equation DO2 ,Ar λ kads J0 (λRo /L) = J1 (λRo /L) L Oxygen is consumed only by deposition on the walls and the substrate. The governing equation does not change, only the boundary condition at the walls.
- 22. Introduction Methodology Results Conclusion Limiting Cases Comparison Representative experimental film profiles: Da = Damk¨hler o number DaG = kG L/vavg
- 23. Introduction Methodology Results Conclusion Limiting cases comparison Representative experimental film profiles: DaS = kadsO2 Ro 2 /DO2 ,Ar DaS (750o C) = 464 max kads related to kPO2 : max RT kadsO (T ) = 2 2πMwO2 RT
- 24. Introduction Methodology Results Conclusion Conclusions We account for a spatially sharp film composition transition by the single degree of freedom in the Cu2 O-O2 -CuO system. Transition location can be estimated from the O2 partial pressure profile in the reactor. Transport models in 1 and 2 dimensions were created to predict the O2 spatial profiles to probe the effect of T and O2 flow on transition position. Transition location positions as a function of surface and homogeneous gas phase O2 consumption rates were examined over a range of rate constants. Qualitative comparisons to observed film measurements tend to support the likelihood of the primary role of surface reactions, although gas-phase reactions cannot be ruled out. Current work focuses on estimating dissociative O2 adsorption rates and more detailed surface- and gas-phase reaction mechanisms.