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.