SUPPLEMENTARY INFORMATION
Fabrication of heterogeneous nanomaterial array by
programmable heating and chemical supply within
microfluidic platform towards multiplexed gas sensing
application
Daejong Yang1, Kyungnam Kang1, Donghwan Kim2, Zhiyong Li3 & Inkyu Park1,*
1
Mechanical Engineering Department & KI for the NanoCentury, KAIST, Daejeon 305-701, South
Korea
2
Korea Electric Power Research Institute, KEPCO, Daejeon 300-713, South Korea
3
Systems Research Lab, Hewlett Packard Laboratory, Palo Alto, CA, 94304-1126, United States
1. Fabrication process of the sensor chip and microfluidic platform
Figure S1. Schematics and photograph of the sensor chip and microfluidic synthesis
platform; The microheater array platform was fabricated by using conventional
microfabrication process and consists of platinum (Pt) microheaters, SiO2 insulation
layer and gold (Au) interdigitated electrodes; The microchannels were fabricated by
conventional PDMS replication method
2. Fabrication result of single ZnO nanowires
Figure S2. EDS data of the ZnO nanomaterials at the upper and lower regions of
precursor flow stream on (a) first and (b) second electrodes; There are no significant
differences in composition of the nanowires.
3. Synthesis mechanism of the nanomaterials
(a) ZnO nanowires
The synthesis principle of ZnO nanowires in microfluidic platform is the same as that
for the hydrothermal synthesis of ZnO nanowires in bulk-phase. The solubility of Zn2+
plays an important role in the synthesis of ZnO nanowires and is a function of pH,
ammonia concentration and temperature1. Ammonia increases the solubility of ZnO
within an intermediate range of pH2. The solubility of Zn2+ is lower at high temperature
than at low temperature. Therefore, ZnO nanowires are synthesized at high
temperature by the following endothermic reactions3:
(CH2)6N4 + 6H2O → 6HCHO + 4NH3
NH3 + H2O ↔ NH4+ + OH-
2OH- + Zn2+ → ZnO(s) + H2O
(b) CuO nanospikes
The synthesis mechanism of CuO nanospikes is similar to that of ZnO nanowires. The
solubility of Cu2+ ions is lower at high temperature and CuO nanospikes are also
synthesized at high temperature condition by the following endothermic reactions4:
Cu(NO3)2∙3H2O + 2NaOH → Cu(OH)2 + 2NaNO3 + 3H2O
(CH2)6N4 + 6H2O → 6HCHO + 4NH3
NH3 + H2O ↔ NH4+ + OH-
Cu(OH)2 → CuO(s) + H2O
(c) TiO2 nanotubes
For the synthesis of TiO2 nanotubes by liquid phase deposition (LPD) process, template
structures are required. In this work, ZnO nanowires were used as templates. The
synthesis process consists of two major steps: TiO2 deposition and ZnO etching5. TiO2
nanofilm is coated on the surface of ZnO nanowires. The F- ions dissociated from
ammonium hexafluorotitanate are combined with boron ions to accelerate the
hydrolysis reactions of ZnO nanowire templates by the following process6,7:
TiF62- + 2H2O ↔ TiO2 + 6F- + 4H+
H3BO3 + 4H+ + 4F- ↔ BF4- + H3O+ + 2H2O
ZnO + 2H+ → Zn2+ + H2O
4. Fabrication result of heterogeneous nanomaterial arrays
Figure S3. EDS data of the heterogeneous nanomaterial arrays in synthesis sequence
for array of TiO2 nanotubes, CuO nanospikes and ZnO nanowires: (a) ZnO nanowire,
CuO nanospike and ZnO nanowire array after step 1 and (b) TiO2 nanotube, CuO
nanospike and ZnO nanowire array after step 2(I); ZnO nanowires and CuO nanospikes
were synthesized without cross-contamination, and TiO2 nanotubes were also
successfully converted without damage to the neighboring nanomaterials.
Figure S4. EDS data of the heterogeneous nanomaterial arrays in synthesis sequence
for array of ZnO nanowire/CuO nanospike hybrid nanostructures, CuO nanospikes and
ZnO nanowires: (a) ZnO nanowire, CuO nanospike and ZnO nanowire array after step
1 and (b) ZnO nanowire/CuO nanospike hybrid nanostructures, CuO nanospike and ZnO
nanowire array after step 2(II); ZnO nanowires and CuO nanospikes were synthesized
without cross-contamination, and CuO nanospikes were also successfully synthesized
on the pre-synthesized ZnO nanowires without contamination to the neighboring
nanomaterials.
5. Numerical analysis of the precursor solutions
Table S1. Concentration of ions at the center of microheater #1, #2 and #3 during (a)
step 1, (b) step 2(I) and (c) step 2(II)
(a) Step 1
Ions
Ionic concentration at the center (mM)
Microheater #1
Microheater #2
Microheater #3
Zn2+
4.0000218571
0.0000519936
4.0000170410
Cu2+
0.0000011384
3.9999428911
0.0000009858
OH-
16.0041723925
15.9992354879
16.0036988867
(b) Step 2(I)
Ions
Ionic concentration at the center (mM)
Microheater #1
Microheater #2
Microheater #3
Ti+
99.9979529003
0.0652464310
0.0014254546
F-
599.9927243203
0.1405432990
0.0086779298
(c) Step 2(II)
Ions
Ionic concentration at the center (mM)
Microheater #1
Microheater #2
Microheater #3
Cu2+
1.0000005034
0.0000004929
0.0000000000
OH-
3.9797933857
0.0126504278
0.0000000004
Figure S5. Concentration profile of (a) Zn2+ and (b) OH- ions within microfluidic channel
at various flow speeds. The concentration ion profiles of both Zn2+ and OH- get sharper
in high flow speed.
Table S2. Concentration of (a) Zn2+ and (b) OH- ions at the center of microheater #1,
#2 and #3 at various flow speeds
(a) Zn2+
Ionic concentration at the center (mM)
Flow speed (mm/s)
Microheater #1
Microheater #2
Microheater #3
1
0.9965428
0.0005348
0.0000000
5
1.0000005
0.0000004
0.0000000
10
1.0000002
0.0000000
0.0000000
20
1.0000000
0.0000000
0.0000000
50
1.0000000
0.0000000
0.0000000
100
1.0000000
0.0000000
0.0000000
500
1.0000003
0.0000000
0.0000000
1000
1.0000001
0.0000000
0.0000000
(b) OH-
Ionic concentration at the center (mM)
Flow speed (mm/s)
Microheater #1
Microheater #2
Microheater #3
1
3.5699181
0.2575775
0.0045164
5
3.9797922
0.0126504
0.0000000
10
3.9984523
0.0024806
0.0000000
20
4.0000397
0.0002460
0.0000000
50
4.0000061
0.0000022
0.0000000
100
4.0000002
0.0000000
0.0000000
500
3.9999998
0.0000000
0.0000000
1000
4.0000002
0.0000000
0.0000000
Figure S6. Concentration profile of (a) Zn2+ and (b) OH- ions within microfluidic channel
for different ratio of flow speed. The concentration profiles of both Zn2+ and OH- ions
are shifted towards flow stream #1.
Table S3. Concentration of (a) Zn2+ and (b) OH- ions at the center of microheater #1,
#2 and #3 for different ratio of flow speed
(a) Zn2+
Ionic concentration at the center (mM)
Ratio of inlet flow speed
Microheater #1
Microheater #2
Microheater #3
#1 : #2 : #3
1:1:1
1.0000005
0.0000004
0.0000000
1:1.5:1.5
0.9980799
0.0000000
0.0000000
1:2:2
0.9486467
0.0000000
0.0000000
1:3:3
0.4633808
0.0000000
0.0000000
(b) OH-
Ionic concentration at the center (mM)
Ratio of inlet flow speed
Microheater #1
Microheater #2
Microheater #3
#1 : #2 : #3
1:1:1
3.9797922
0.0126504
0.0000000
1:1.5:1.5
3.7428182
0.0004376
0.0000000
1:2:2
3.1787465
0.0000417
0.0000000
1:3:3
1.9502301
0.0000020
0.0000000
6. Effect of flow speed on the temperature distribution and material synthesis
Figure S7. (a) Two dimensional temperature distribution in the precursor solution and
(b) temperature curve from the center of microheater to ceiling of microchannel at
various flow speeds (1 mm/s - 1000 mm/s); The heating area and maximum
temperature were decreased with increasing flow speed.
Figure S8. (a) Top-view and side-view SEM images and of ZnO nanowire bundles
synthesized at different flow speeds. (b) Average nanowire length and area vs. flow
speed curve. Both length and area of ZnO nanowires were decreased with increasing
flow speed.
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