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
Results from Batch Reactor Case Study
Intensity [a.u.]
Microreactor
Deposition has similar morphology for both reactors
Vial Reactor
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.
Raman shows equal quality nanowires from microreactor
Microreactor’s faster growth
rate did not alter E2 peak
position
Indicates no excess stress in
the deposition
Differences in E2 peak
height due to void fraction
differences
Nanowires
Seeds (x10)
(100)
30
31
(101)
(002)
32
33
34
2θ [deg.]
35
36
37
Vial Reactor
Hexagonal
Dense, well-aligned from
seeding
80-100 nm diameter
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
30 min
4h
2h
Bulk
Reaction
Adsorption
onto surface
Deposition
Traditional reactors have low surface-to-volume ratios
Bulk reaction dominates → massive precipitation → low yield
ZnO Deposition Rate [(µg/mm )/h]
Surface
Reaction
0.5
0.25
0
0
Time [h] 0.5
1
Microreactor
Vial Reactor
0
2
Precipitation
[µg/mm2]
Mass of ZnO Deposition [µg/mm2]
Diffusion to
surface
Speciation
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
0.75
2
1
0.75
Vial
Reactor
200
300
Glass
Substrate
400
500
Raman Shift [cm-1]
600
700
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.
Fundamentals of Solution Deposition
(Metal Salts/Complexing Agents)
Microreactor
Continuous Flow Microreactor
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
Aqueous Precursors
E2(high): 438
4h
2h
Microreactor
15 min
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
Results from Batch Reactor Case Study (cont.)
Intensity [a.u.]
Background & Motivation
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
Microreactor
0.5
Vial Reactor
ZnO nanowire length vs. position downstream
0.25
00
1
2
3
4
Time [h]
5
6
Microreactor for Solution Deposition
Microreactor provides high yield deposition
Contact heater
Plexiglass
ZnO[moles]deposited
%Yield =
× 100
Zn[moles]initial
Substrate
1mm
ZnO
deposition
Deposition
channel
Microreactor gives order of
magnitude better yield than
vial reactor
Slower heating rates result in
higher yield
Sub-millimeter reaction channel
Contact-heated substrate forms one wall of reaction channel
50
Microreactor (8 °C/min)
Microreactor (34 °C/min)
Vial Reactor
40
% Yield
Gasket
Aluminum
Clamp
30
20
10
Batch Reactor Case Study
00
1
2
3
Time [h]
4
5
6
We compared ZnO nanowires deposited using a traditional vial
reactor and microreactor by investigating:
Flow rate affects deposition uniformity
Morphology of deposition
ZnO deposition mass
2+
Bath [Znaq
]
Crystal quality
Slow flow → graded
deposition
Fast flow → more uniform
deposition
*
C6H12N4 + 6 H2O −
)
− 6 HCHO + 4 NH3
–
+
*
NH3 + H2O −
NH
+
OH
)
−
4
*
2 OH – + Zn 2+ −
)
− ZnO(s) + H2O
Nanowire Length (normalized)
What species control the ZnO deposition rate?
4 h growth at various flow rates
1
2.88 mL/h
0.75
0.5 0.72 mL/h
1.44 mL/h
0.25
pH range during deposition
100
Substrate
Seeding
Method
Aqueous
Precursors
Bath Volume
Temperature
Vial Reactor
Microreactor
Soda lime glass
Dip coat in Zinc Acetate, Ethanol,
MEA then anneal
0.025 M Zn(NO3)2 & 0.025 M
Methenamine (HMT)
27 mL
0.8 mL
90 ◦C
90 ◦C at interface
+
0.1
ZnOH
Zn(OH)3-
0.01
0.001
0.0001
Zn(OH)2(aq)
2
3
4
5
6
7
pH
8
−
9
[OH ] initially limiting
2+
Initial excess Zn
Slow thermohydrolysis
of HMT →
− OH –
2+
Becomes [Zn ] limited
pH gradually rises
throughout reaction
2+
Zn is depleted
10
11
12
13
1.5
ln(ZnO Deposition Rate)
Experimental conditions for the batch reactors
Zn is the predominant
zinc species at deposition
conditions. We examined
the deposition rate as a
2+
function of [Zn ] in the
bath.
1
ZnO Deposition Rate [(µg/mm2)/h]
ZnO Properties
ZnO Applications
II-VI semiconductor
Transparent conducting
oxides
Wide band gap (3.37 eV)
Nanostructured photovoltaics
Large exciton binding energy
(60 meV)
Gas sensors
UV Lasers
2+
Zn(OH)42-
10
% Zn
Solution deposition can be used to deposit oxides & chalcogenides
(CdS, CdSe, etc.), we chose ZnO because:
ZnO(s)
Zn2+
1.25
1
0.75
Microreactor
Vial Reactor
0
-1
-2
[Zn2+]
limited
-3
-4
0
0.005 0.01 0.015 0.02
Zn2+(aq) [mol/L]
2+
8
12
16
20
24
Distance from Inlet [mm]
28
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
[OH ]
limited
References
0.25
0
4
-
[Zn ]
limited
0.5
[OH-]
limited
00
0
0.005
0.01
2+
0.015
Zn (aq) [mol/L]
0.02
0.025
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.