Thoracic Trauma Christopher McPeak, MD Department of Emergency Medicine Albany Medical Center.
Kevin M. McPeak and Jason B. Baxter
1
Microreactor for High Yield Solution Deposition of ZnO Nanowires Kevin M. McPeak and Jason B. Baxter Department of Chemical and Biological Engineering, Drexel University, Philadelphia PA Background & Motivation Solution deposition is a simple deposition method requiring only that a substrate be placed in a vessel containing a heated aqueous solution of common precursors. Advantages of solution deposition include low cost, operation at low temperature and atmospheric pressure, and scalability to large area substrates. One of the major drawbacks of solution deposition is inefficient utilization of reactants and significant waste solvent generation. Deposition on the substrate is limited by competing precipitation in solution and deposition on the reactor walls. Careful design of the reactor geometry is critical to efficient utilization of reactants and minimization of waste solvent. Microreactors offer the following advantages over traditional reactors: High surface-to-volume ratio suppresses homogeneous reactions Low thermal mass → precise temperature control Scale up without re-engineering the reactor We report on the use of a microreactor to deposit ZnO nanowires and films from a sub-millimeter reaction channel with the following results: Order of magnitude increase in yield 60% increase in deposition rate Improved bulk crystal quality Fundamentals of Solution Deposition Aqueous Precursors (Metal Salts/Complexing Agents) Speciation Diffusion to surface Adsorption onto surface Surface Reaction Deposition Precipitation Bulk Reaction Traditional reactors have low surface-to-volume ratios Bulk reaction dominates → massive precipitation → low yield Microreactor for Solution Deposition Plexiglass Aluminum Deposition channel Substrate Contact heater 1mm Gasket ZnO deposition Clamp Sub-millimeter reaction channel Contact-heated substrate forms one wall of reaction channel Batch Reactor Case Study We compared ZnO nanowires deposited using a traditional vial reactor and microreactor by investigating: Morphology of deposition ZnO deposition mass Bath [Zn 2+ aq ] Crystal quality Solution deposition can be used to deposit oxides & chalcogenides (CdS, CdSe, etc.), we chose ZnO because: ZnO Properties II-VI semiconductor Wide band gap (3.37 eV) Large exciton binding energy (60 meV) ZnO Applications Transparent conducting oxides Nanostructured photovoltaics Gas sensors UV Lasers Experimental conditions for the batch reactors Vial Reactor Microreactor Substrate Soda lime glass Seeding Method Dip coat in Zinc Acetate, Ethanol, MEA then anneal Aqueous Precursors 0.025 M Zn(NO 3 ) 2 & 0.025 M Methenamine (HMT) Bath Volume 27 mL 0.8 mL Temperature 90 ◦ C 90 ◦ C at interface Results from Batch Reactor Case Study Deposition has similar morphology for both reactors Microreactor Vial Reactor 15 min 2 h 4 h 30 min 2 h 4 h Microreactor Vial Reactor 30 31 32 33 34 35 36 37 Intensity [a.u.] 2θ [deg.] (100) (002) (101) Nanowires Seeds (x10) Hexagonal Dense, well-aligned from seeding 80-100 nm diameter Measuring deposition volume from SEM images results in errors due to non-uniform morphology Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analysis of acid digested ZnO deposition gives precise ZnO deposition mass 0 1 2 Mass of ZnO Deposition [μg/mm 2 ] Microreactor Vial Reactor 0 0.25 0.5 0.75 1 0 1 2 3 4 5 6 ZnO Deposition Rate [(μg/mm 2 )/h] Time [h] Microreactor Vial Reactor 0 0.25 0.5 0.75 0 0.5 [μg/mm 2 ] Time [h] Microreactor’s fast heating increases growth rate Microreactor reduces induction time Microreactor results in 60% higher deposition rate Vial reactor’s large volume deposits more Microreactor provides high yield deposition %Yield = ZnO [moles ] deposited Zn [moles ] initial × 100 Microreactor gives order of magnitude better yield than vial reactor Slower heating rates result in higher yield 0 10 20 30 40 50 0 1 2 3 4 5 6 % Yield Time [h] Microreactor (8 °C/min) Microreactor (34 °C/min) Vial Reactor What species control the ZnO deposition rate? C 6 H 12 N 4 + 6H 2 O - * ) - 6 HCHO + 4 NH 3 NH 3 + H 2 O - * ) - NH + 4 + OH – 2 OH – + Zn 2+ - * ) - ZnO (s) + H 2 O 0.0001 0.001 0.01 0.1 1 10 100 2 3 4 5 6 7 8 9 10 11 12 13 % Zn pH Zn 2+ ZnOH + Zn(OH) 2(aq) Zn(OH) 3 - Zn(OH) 4 2- ZnO (s) pH range during deposition Zn 2+ is the predominant zinc species at deposition conditions. We examined the deposition rate as a function of [Zn 2+ ] in the bath. [OH - ] initially limiting Initial excess Zn 2+ Slow thermohydrolysis of HMT - → OH – Becomes [Zn 2+ ] limited pH gradually rises throughout reaction Zn 2+ is depleted 0 0.25 0.5 0.75 1 1.25 1.5 0 0.005 0.01 0.015 0.02 0.025 ZnO Deposition Rate [(μg/mm 2 )/h] Zn 2+ (aq) [mol/L] [OH - ] limited [Zn 2+ ] limited [OH - ] limited [Zn 2+ ] limited Microreactor Vial Reactor -4 -3 -2 -1 0 0 0.005 0.01 0.015 0.02 ln(ZnO Deposition Rate) Zn 2+ (aq) [mol/L] Results from Batch Reactor Case Study (cont.) Raman shows equal quality nanowires from microreactor Microreactor’s faster growth rate did not alter E 2 peak position Indicates no excess stress in the deposition Differences in E 2 peak height due to void fraction differences 200 300 400 500 600 700 Intensity [a.u.] Raman Shift [cm -1 ] E 2 (high): 438 Microreactor Vial Reactor Glass Substrate Continuous Flow Microreactor The batch microreactor’s small bath volume limits the deposition thickness. To overcome this limitation we have developed a continuous flow microreactor for solution deposition. Removes deposition thickness limitation Bath concentration constant at fixed position i.e. plug flow reactor Allows for further investigation of growth mechanisms Preliminary Results from Continuous Microreactor Interference fringes highlight changes in deposition thickness as reactants are depleted ZnO nanowire length vs. position downstream Flow rate affects deposition uniformity Slow flow → graded deposition Fast flow → more uniform deposition 0 0.25 0.5 0.75 1 0 4 8 12 16 20 24 28 Nanowire Length (normalized) Distance from Inlet [mm] 4 h growth at various flow rates 0.72 mL/h 1.44 mL/h 2.88 mL/h Conclusions Nanowire morphology equivalent in both reactors Microreactor increased deposition yield by order of magnitude Deposition rate increased by 60% in microreactor Raman shows equal crystal quality in microreactor Batch microreactor’s small bath volume limits deposition thickness, continuous microreactor lifts this limitation Flow rate in continuous microreactor affects deposition uniformity References 1. McPeak, K.M.; Baxter, J.B., Microreactor for High-Yield Solution Deposition of Semiconductor Nanowires and Films. I&EC Research, Accepted. 2. McPeak, K.M.; Baxter, J.B., Drexel University (2008) Microreactor for Solution Deposition and Method of Use, Prov. U.S. Pat. 61,054,911.
Transcript of Kevin M. McPeak and Jason B. Baxter
Microreactor for High Yield Solution Deposition of ZnO NanowiresKevin M. McPeak and Jason B. Baxter
Department of Chemical and Biological Engineering, Drexel University, Philadelphia PA
Background & Motivation
Solution deposition is a simple deposition method requiring onlythat a substrate be placed in a vessel containing a heated aqueoussolution of common precursors. Advantages of solution depositioninclude low cost, operation at low temperature and atmosphericpressure, and scalability to large area substrates. One of the majordrawbacks of solution deposition is inefficient utilization ofreactants and significant waste solvent generation. Deposition onthe substrate is limited by competing precipitation in solution anddeposition on the reactor walls. Careful design of the reactorgeometry is critical to efficient utilization of reactants andminimization of waste solvent.
Microreactors offer the following advantages over traditionalreactors:
High surface-to-volume ratio suppresses homogeneous reactionsLow thermal mass→ precise temperature controlScale up without re-engineering the reactor
We report on the use of a microreactor to deposit ZnO nanowiresand films from a sub-millimeter reaction channel with thefollowing results:
Order of magnitude increase in yield60% increase in deposition rateImproved bulk crystal quality
Fundamentals of Solution Deposition
Aqueous Precursors(Metal Salts/Complexing Agents)
Speciation Diffusion tosurface
Adsorptiononto surface
SurfaceReaction
Deposition
Precipitation
BulkReaction
Traditional reactors have low surface-to-volume ratiosBulk reaction dominates→ massive precipitation→ low yield
Microreactor for Solution Deposition
Plexiglass
Aluminum
Depositionchannel
Substrate
Contact heater
1mm
Gasket ZnOdeposition
Clamp
Sub-millimeter reaction channelContact-heated substrate forms one wall of reaction channel
Batch Reactor Case Study
We compared ZnO nanowires deposited using a traditional vialreactor and microreactor by investigating:
Morphology of depositionZnO deposition massBath [Zn 2+
aq ]Crystal quality
Solution deposition can be used to deposit oxides & chalcogenides(CdS, CdSe, etc.), we chose ZnO because:
ZnO PropertiesII-VI semiconductorWide band gap (3.37 eV)Large exciton binding energy(60 meV)
ZnO ApplicationsTransparent conductingoxidesNanostructured photovoltaicsGas sensorsUV Lasers
Experimental conditions for the batch reactors
Vial Reactor MicroreactorSubstrate Soda lime glassSeedingMethod
Dip coat in Zinc Acetate, Ethanol,MEA then anneal
AqueousPrecursors
0.025 M Zn(NO3)2 & 0.025 MMethenamine (HMT)
Bath Volume 27 mL 0.8 mLTemperature 90 ◦C 90 ◦C at interface
Results from Batch Reactor Case Study
Deposition has similar morphology for both reactors
Micro
re
acto
rV
ia
l R
ea
cto
r
15 min 2 h 4 h
30 min 2 h 4 h
Micro
re
acto
rV
ia
l R
ea
cto
r
30 31 32 33 34 35 36 37
Inte
nsity
[a.u
.]
2θ [deg.]
(100)(002)
(101)
Nanowires
Seeds (x10)
HexagonalDense, well-aligned fromseeding80-100 nm diameter
Measuring deposition volume from SEM images results in errorsdue to non-uniform morphologyInductively Coupled Plasma Mass Spectrometry (ICP-MS)analysis of acid digested ZnO deposition gives precise ZnOdeposition mass
0
1
2
Mas
s of
ZnO
Dep
ositi
on [µ
g/m
m2 ]
MicroreactorVial Reactor
0
0.25
0.5
0.75
1
0 1 2 3 4 5 6
ZnO
Dep
ositi
on R
ate
[(µg
/mm
2 )/h]
Time [h]
Microreactor
Vial Reactor
0
0.25
0.5
0.75
0 0.5
[µg/
mm
2 ]
Time [h]
Microreactor’s fast heatingincreases growth rate
Microreactor reducesinduction timeMicroreactor results in 60%higher deposition rateVial reactor’s large volumedeposits more
Microreactor provides high yield deposition
%Yield =ZnO[moles]deposited
Zn[moles]initial× 100
Microreactor gives order ofmagnitude better yield thanvial reactorSlower heating rates result inhigher yield
0
10
20
30
40
50
0 1 2 3 4 5 6
% Y
ield
Time [h]
Microreactor (8 °C/min)Microreactor (34 °C/min)
Vial Reactor
What species control the ZnO deposition rate?
C6H12N4 + 6 H2O −⇀↽− 6 HCHO + 4 NH3
NH3 + H2O −⇀↽− NH +4 + OH –
2 OH – + Zn 2+ −⇀↽− ZnO(s) + H2O
0.0001
0.001
0.01
0.1
1
10
100
2 3 4 5 6 7 8 9 10 11 12 13
% Z
n
pH
Zn2+
ZnOH+
Zn(OH)2(aq)
Zn(OH)3-
Zn(OH)42-
ZnO(s)
pH range during deposition
Zn 2+ is the predominantzinc species at depositionconditions. We examinedthe deposition rate as afunction of [Zn 2+] in thebath.
[OH−] initially limitingInitial excess Zn 2+
Slow thermohydrolysisof HMT −→ OH –
Becomes [Zn2+] limitedpH gradually risesthroughout reactionZn 2+ is depleted
0
0.25
0.5
0.75
1
1.25
1.5
0 0.005 0.01 0.015 0.02 0.025ZnO
Dep
ositi
on R
ate
[(µg
/mm
2 )/h]
Zn2+
(aq) [mol/L]
[OH-]limited
[Zn2+]limited
[OH-]limited
[Zn2+]limited
MicroreactorVial Reactor
-4
-3
-2
-1
0
0 0.005 0.01 0.015 0.02ln(Z
nO D
epos
ition
Rat
e)
Zn2+(aq) [mol/L]
Results from Batch Reactor Case Study (cont.)
Raman shows equal quality nanowires from microreactor
Microreactor’s faster growthrate did not alter E2 peakpositionIndicates no excess stress inthe depositionDifferences in E2 peakheight due to void fractiondifferences
200 300 400 500 600 700
Inte
nsity
[a.u
.]
Raman Shift [cm-1]
E2(high): 438
Microreactor
VialReactor
GlassSubstrate
Continuous Flow Microreactor
The batch microreactor’s small bathvolume limits the deposition thickness.To overcome this limitation we havedeveloped a continuous flowmicroreactor for solution deposition.
Removes deposition thicknesslimitationBath concentration constant at fixedposition i.e. plug flow reactorAllows for further investigation ofgrowth mechanisms
Preliminary Results from Continuous Microreactor
Interference fringes highlight changes in deposition thicknessas reactants are depleted
ZnO nanowire length vs. position downstream
Flow rate affects deposition uniformity
Slow flow→ gradeddepositionFast flow→ more uniformdeposition
0
0.25
0.5
0.75
1
0 4 8 12 16 20 24 28
Nan
owire
Len
gth
(nor
mal
ized
)
Distance from Inlet [mm]
4 h growth at various flow rates
0.72 mL/h 1.44 mL/h
2.88 mL/h
Conclusions
Nanowire morphology equivalent in both reactorsMicroreactor increased deposition yield by order of magnitudeDeposition rate increased by 60% in microreactorRaman shows equal crystal quality in microreactorBatch microreactor’s small bath volume limits depositionthickness, continuous microreactor lifts this limitationFlow rate in continuous microreactor affects depositionuniformity
References
1. McPeak, K.M.; Baxter, J.B., Microreactor for High-YieldSolution Deposition of Semiconductor Nanowires and Films.I&EC Research, Accepted.
2. McPeak, K.M.; Baxter, J.B., Drexel University (2008)Microreactor for Solution Deposition and Method of Use, Prov.U.S. Pat. 61,054,911.
