Enhancing Sensitivity of SAW Sensors using Nanostructured Materials Ophir Ortiz1, Shekhar Bhansali1, Venkat Bhethanabotla2 1Department of Electrical Engineering 2Department of Chemical Engineering University of South Florida 4202 East Fowler Ave. Tampa,

1
Enhancing Sensitivity of SAW Sensors
usingNanostructured MaterialsOphir Ortiz1,
Shekhar Bhansali1, Venkat Bhethanabotla21Departme
nt of Electrical Engineering2Department of
Chemical EngineeringUniversity of South
Florida4202 East Fowler Ave.Tampa, FL 33620
2
Outline

• Purpose
• Experimental Setup
• Fabrication of Nanoparticles/Nanowires
• Effects of morphology changes on the
  nanostructures due to variations of
• Deposition time
• Deposition Potential
• WECE ratio
• Conclusion

3
Purpose

• Create palladium nanoparticles and nano wires
  (NWs) by electrodeposition
• These nanostructures will be used as the sensing
  element in hydrogen gas sensors
• Primarily based for Surface Acoustic Wave (SAW)
  sensor applications

4
Surface Acoustic Wave

• Surface Acoustic Wave devices use an acoustic
  (mechanical) wave as the sensing mechanism
• Piezoelectric material generates
  acoustic wave on application of ac potential
  through IDTs (Interdigitated Transducers)
• The IDTs produce the electric field necessary to
  displace the substrate in order to form an
  acoustic wave

http//www.sensorsmag.com/articles/1000/68/main.sh
tml
5
Application of Nanostructure as SAW Sensing
Element

• Surface wave velocity variations that result from
  the faster absorption of a specific gas species
  can be detected as either phase shifts or as
  frequency shifts

6
SAWs Coated with Nanostructured Materials

• Pd Nanostructured Materials can be placed between
  two IDTs as the hydrogen sensing layer

b
7
Advantages of using Palladium NW for SAW Sensor
Applications

• Selectively absorbs hydrogen gas
• Responds to H2 gas in near real time due to
  faster changes in frequency
• Operates at low (room) temperature
• Resistant to poisoning effects from CO, O2, CH4
• Sensitive to low ppm levels of hydrogen

8
Experimental Setup

• A three electrode system was used for
  electroplating
• Counter Electrode (CE) Platinum foil or wire
• Working Electrode (WE) HOPG
• Reference Electrode (RE) Saturated Calomel
  Electrode
• All depositions took place in an electrochemical
  cell
• Electrolytic Solution 2mM PdNO3 with .1M HClO4

9
Electrochemical Setup
10
Working Electrode HOPG Characteristics

• Highly Oriented Pyrolitic Graphite Surface
• High purity carbon serves the purpose of a
  template because it has atomic layers of graphite
  that are highly oriented among each other
• Renewable and smooth surface
• Apply tape to the surface and peel off to create
  cleaved surface
• 20-40 cleavings per block

11
HOPG
http//www.mpip-mainz.mpg.de/jonas/Master_Surf_Ch
em/lecture_IntroSurfChem_1e.pdf
12
Procedure for Nanowire or Nanoparticle Fabrication

• Cleave HOPG
• Electroplate
• Apply pulsing potential of -.2V for 5ms for
  nucleation
• Apply deposition potential for growth

13
Typical Cyclic Voltammogram Readout
14
Searching for Optimal Nanoparticles/Nanowires
15
Effects of Changing Conditions

• Change in morphology of palladium nanostructures
  was observed under different conditions. The
  following variables were investigated
• 1) Deposition time
• 2) Deposition potential
• 3) Counter Electrode (CE) to Working Electrode
  (WE) ratio (Platinum foil vs. wire)

16
Change in Deposition Time

• Effects of varying the deposition time on pd
  nanowire morphology were investigated by changing
  the deposition time with each plating. All other
  conditions stayed constant.
• The samples for this part shared the following
  characteristics
• 0.33v deposition potential
• Platinum foil (CE)
• -0.2v Pulsing Potential
• Temperature of plating bath (room temp)
• The change in time was observed for
• 300s, 400s, 500s, 600s

17
Palladium Nanowire300s deposition time
Width (Y) 74.7 nm
18
Palladium Nanowire300s deposition time
19
Palladium Nanowire400s deposition time
The average width of these NW ranged from 90nm to
100nm
20
Palladium Nanowire500s deposition time
These NW ranged from under 100 nm to over 120 nm
21
Palladium Nanowire600s deposition time
Width(Y) 131.6 nm
22
Figure 6 Graph
23
Change in Deposition Potential

• Change in deposition potential on pd nanowire
  morphology was investigated by increasing the
  deposition potential of each sample. All other
  conditions stayed the same.
• The samples shared the following characteristics
• 400s deposition time
• Platinum wire (CE)
• -0.2v pulsing potential
• Temperature of plating bath (room temp)
• The change in potential was observed for
• .29v, .33v, .37v

24
0.29 v Deposition Potential
Palladium Nanoparticles formed measuring between
33nm and 40 nm
25
0.33v Deposition Potential
NW 1 Width 28.5 nm NW 2 Width 36.2 nm
26
0.37v Deposition Potential
Width Section 1 40 nm Width Section 2 44.9nm
27
Change in CE to WE ratioPt Foil vs Wire Contd

• NWs formed using the Pt wire CE are smoother and
  thinner than the NWs from the Pt foil CE
• The Pt wire allows for a lower deposition current
  due to the CEWE ratio. The Pt wire has a smaller
  cross-sectional area and correspondingly the
  current is lower
• This Pt wire directly affects the WE because the
  cell current Ic flows between the WE CE

28
CE to WE ratioPt Foil vs Wire
Pt foil (CE) Width 90-100nm 400s dep time .33v
dep potential
Pt wire (CE) Width 28.5 -36.2 nm 400s dep
time .33v dep potential
29
Conclusion

• An increase in width with time was observed
• The most favorable potential for continuous NWs
  was found to be 0.33V and for Nanoparticles to be
  0.29V.
• A foil CE yields wider NWs than using a wire CE.
• Wire CE causes the resulting current during
  deposition to be much lower and constant.
• NWs fabricated with the Pt wire CE have less
  mushrooming. NWs are
• Smooth
• Continuous
• Decreased lateral growth
• (mushrooming less likely)