Transcript Micro-wrinkled Pd surface for hydrogen sensing and switched

ICHS 2011
September 13, 2011
The BIOROBOTICS
Institute
Micro-wrinkled Pd surface for
hydrogen sensing and switched
detection of Lower Explosive Limit
Francesco Greco1*, Letizia Ventrelli1,2, Paolo Dario1,2 and Virgilio Mattoli1*
1 Center
for Micro-BioRobotics IIT@SSSA,
Istituto Italiano di Tecnologia, Pontedera, Italy
2 Biorobotics
Institute,
Scuola Superiore Sant’Anna, Pontedera, Italy
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Micro-wrinkled Pd surface for H2 sensing
Summary
 Background
 The Micro-Wrinkled H2 Sensor
 Sensor Fabrication
 Sensor Characterization
 Conclusions
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Micro-wrinkled Pd surface for H2 sensing
Background
Hydrogen sensors have several applications in different fields including industrial
process control, hybrid vehicles and spatial applications.
Due to the increasing interest in the use of hydrogen as a fuel, the development of
reliable hydrogen sensors for the detection of H2 gas leakages and for the continuous
monitoring of its concentration in many different industrial environments is the
object of intensive research.
The use of hydrogen as a possible alternative and “green” resource is indeed limited
by many safety issues related to storage and handling, raised by its flammability and
explosiveness: the Lower Explosive Limit (LEL) of H2 in air is 4%.
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Micro-wrinkled Pd surface for H2 sensing
Background
Different sensing technologies have been proposed and exploited to provide hydrogen
detection; most of these rely on the selective interaction of the hydrogen with
palladium (Pd) or palladium alloys, because of its peculiar permeability to hydrogen
and to the ability to form a stable hydride (PdHx);
Such hydrogen absorption causes large changes in physical properties of Pd, that are
exploited for hydrogen detection in several ways.
The choice of the right technology is essentially related to the application field,
depending on the working ranges considered and the performance required
(in terms of accuracy, reproducibility and response/recovery times).
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Micro-wrinkled Pd surface for H2 sensing
State of the Art
Technologies Based on Palladium / Palladium Alloys
+ Resistive Pd Based Film Sensors
Active elements
+ Pd Coated Optical Fibers
+ Pd Based Schottky Junctions
+ Pd Film-Based Transistors (H-FET / H-MOS)
+ Pd Based SAW Sensors
+ Nanostructured Pd
- Discontinuous ultra-thin films
- Pd nano- or meso-wires
- FIB nanotrenches into Pd microwires
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Reference elements
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Micro-wrinkled Pd surface for H2 sensing
State of the Art
Technologies Based on Palladium / Palladium Alloys
+ Resistive Pd Based Film Sensors
+ Pd Coated Optical Fibers
+ Pd Based Schottky Junctions
+ Pd Film-Based Transistors (H-FET / H-MOS)
+ Pd Based SAW Sensors
+ Nanostructured Pd
- Discontinuous ultra-thin films
- Pd nano- or meso-wires
- FIB nanotrenches into Pd microwires
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Adapted from:
T. Tabib-Azar et al. Sensors and Actuators B 56 (1999) 158-163
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Micro-wrinkled Pd surface for H2 sensing
State of the Art
Technologies Based on Palladium / Palladium Alloys
+ Resistive Pd Based Film Sensors
+ Pd Coated Optical Fibers
+ Pd Based Schottky Junctions
+ Pd Film-Based Transistors (H-FET / H-MOS)
+ Pd Based SAW Sensors
+ Nanostructured Pd
- Discontinuous ultra-thin films
- Pd nano- or meso-wires
- FIB nanotrenches into Pd microwires
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Adapted from:
H.I. Chen et al. Sensors and Actuators B 85 (2002) 10-18
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Micro-wrinkled Pd surface for H2 sensing
State of the Art
Technologies Based on Palladium / Palladium Alloys
+ Resistive Pd Based Film Sensors
+ Pd Coated Optical Fibers
+ Pd Based Schottky Junctions
+ Pd Film-Based Transistors (H-FET / H-MOS)
+ Pd Based SAW Sensors
+ Nanostructured Pd
- Discontinuous ultra-thin films
- Pd nano- or meso-wires
- FIB nanotrenches into Pd microwires
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Adapted from:
Y. Morita et al. Sensors and Actuators B 33 (1996) 96-99
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Micro-wrinkled Pd surface for H2 sensing
State of the Art
Technologies Based on Palladium / Palladium Alloys
+ Resistive Pd Based Film Sensors
+ Pd Coated Optical Fibers
+ Pd Based Schottky Junctions
+ Pd Film-Based Transistors (H-FET / H-MOS)
+ Pd Based SAW Sensors
+ Nanostructured Pd
- Discontinuous ultra-thin films
- Pd nano- or meso-wires
- FIB nanotrenches into Pd microwires
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Adapted from:
W.P. akubik et al. Sensors and Actuators B 82 (2002) 265-271
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Micro-wrinkled Pd surface for H2 sensing
State of the Art
Technologies Based on Palladium / Palladium Alloys
+ Resistive Pd Based Film Sensors
+ Pd Coated Optical Fibers
+ Pd Based Schottky Junctions
+ Pd Film-Based Transistors (H-FET / H-MOS)
+ Pd Based SAW Sensors
+ Nanostructured Pd
- Discontinuous ultra-thin films [1]
- Pd nano- or meso-wires [2]
- FIB nanotrenches into Pd microwires [3]
-…
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From:
F. Yang et al., Small, 6 (2010) 1422–1429
[1] T. Kiefer et al., Nanotechnology, 21 (2010) 505501
[2] F. Favier, et al., Science, 293, No. 5538 (2001) , 2227-2231
[3] T. Kiefer et al., Nanotechnology, 19 (2008) 125502
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Micro-wrinkled Pd surface for H2 sensing
Thin Palladium Film Behavior
Type II
I
Ultra-thin and nanostructured Pd films,
characterized
by some discontinuity
In bulk or continuous
thin films ofand
Pd nanoupon
scale
gaps (nanopores,
nanotrenches,
nanowires,
hydrogen
exposure there
is an increase
in
nanoparticles
assembly).
electron scattering
due to hydrogen atoms acting
These
nano-break
are closed
due to
as defect
sites in junctions
the fcc lattice
of palladium
volume
upon hydrogen absorption
hydride expansion
(PdHx)
Electrical resistance decreases because of
enhanced
electricalupon
charges.
Electrical percolation
resistance of
increases
hydrogen
exposure.
Drawback: typically poor adhesion of Pd on lower
surface energy substrates cause severe stability
problems (delamination, cracking of the sensing
surface, irreversible drift of electrical resistance)
when repeated cycles of absorption/desorption of
hydrogen are imposed
From: F. J. Ibañez et al, Langmuir, 22 (2006) 9789-9796
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Micro-wrinkled Pd surface for H2 sensing
The Idea
Here we present an innovative approach based on the resistive properties of a
microwrinkled Pd surface. By following a method recently proposed for different
materials and applications [1], the reliable formation of microwrinkles on Pd surface
has been accomplished with the use of an extremely easy, fast and low-cost process.
Mask, Pd deposition
(DC sputtering)
Pd thin film
Thermo-retractable PS
Polyshrink
T = 170°C
(PS substrate shrinks to 40% of lateral
dimensions; thickness increase by 800%)
Thermo-retractable
PS Polyshrink®
Micro-Wrinkled
Pd@polyshrink
[1] C. Fu et al., Tunable Nanowrinkles on Shape Memory Polymer Sheets, Advanced Materials, 21, No. 44, 2009, pp. 4472-4476.
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Micro-wrinkled Pd surface for H2 sensing
Sensor Prototypes
Wrinkled Pd Sensor (WPS)
Micro-wrinkled palladium thin films, fabricated as rectangular-shaped
stripes through Pd deposition on polystyrene sheet (Polyshrink™) +
Heating.
Deposition time t varied for obtaining samples with two different Pd
thickness: WPS120 (t = 120s sputtering) and WPS30 (t = 30s sputtering).
Flat Pd Sensor (FPS)
Flat Pd Sensors, fabricated is the same way, except for the final heating
step (no shrinking). Same final size of sensors and active surface.
Samples with different Pd thickness FPS120 and FPS30
(sputtering t = 120s and 30s respectively)
Control Pd Film (CPF)
Control Pd Films, fabricated by sputtering Pd on glass substrate
simultaneously with Wrinkled and Flat Sensors(same conditions)
Samples with different Pd thickness CPF120 and CPF30
(sputtering t = 120s and 30s respectively)
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WPS30
WPS120
FPS30
FPS120
CPF30
CPF120
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Micro-wrinkled Pd surface for H2 sensing
Sensors Prototypes
FPS
FPS
WPS
WPS
FPS
WPS
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Size:
FPS 13.6 x 3.6 x 0.2 mm3
WPS 13.6 x 3.6 x 2.0 mm3
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Active Pd surface
12.0 x 2.0 mm2
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Micro-wrinkled Pd surface for H2 sensing
Sensors Prototypes – the Wrinkled Surface
100 μm
100 μm
100 μm
Uniform isotropic micro and nano-wrinkles over the whole active area of sensors
(24 mm2) with broad wrinkles wavelength distribution.
Improved adhesion and stability of wrinkles respect to flat sensors (no peeling off Pd
film, even scratching the surface), due to integration of Pd wrinkled film with the
supporting PS substrate (heating above glass transition temperature of PS, Tg ≈ 95°C)
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Micro-wrinkled Pd surface for H2 sensing
Sensor Characterization
 Surface Characterization
 Thickness
 Morphology
 Topography
Veeco
Atomic Force Microscope (AFM)
Hirox KH7700
Digital Microscope
 Electrical Characterization
 Resistance
 Sensitivity
Experimental
Parameters
ZEISS
EVO MA15
 Response and recovery time Scanning
Nominal air flow rate:1 l/min;
Electron Microscope (SEM)
Measured H2 concentrations: from 0,45%vol to 4 %vol with 0,5% steps;
Several On-Off cycles (3-Way valve Open/Closed) for each concentration;
Measured signal: R/R0 (%)
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Micro-wrinkled Pd surface for H2 sensing
Sensor Characterization - Thickness
Thickness t of the Pd thin films has been estimated on the CPF samples.
AFM images of Pd films in control samples at different deposition times
CPF30
Thickness (t30) = 30 ± 6 nm
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CPF120
Thickness (t120) = 105 ± 10 nm
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Micro-wrinkled Pd surface for H2 sensing
Sensor Characterization - Topography
Flat Samples
1 KX
10 KX
FPS30
FPS120
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Micro-wrinkled Pd surface for H2 sensing
Sensor Characterization - Topography
Wrinkled Samples
1 KX
10 KX
WPS30
WPS120
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Micro-wrinkled Pd surface for H2 sensing
Sensor Characterization - Hydrogen Sensing (1/2)
FPS120
WPS120
“Activation threshold”
cAct  1,5%
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Micro-wrinkled Pd surface for H2 sensing
Sensor Characterization - Hydrogen Sensing (2/2)
Results can be explained taking into account the peculiar topography of wrinkled sensors
Micro-wrinkles affect electrical properties of the surface:
 improve the active surface area of the sensor with respect to flat ones,
 create new and shorter conductive pathways between points in the surface that would be
very far apart if in a flat surface - wrinkles can come in contact with neighboring wrinkles
 sort of micro-structured “conductive skin” is formed with thickness comparable to
wrinkles amplitude (several micrometers)
These evidences are also confirmed by the value of electrical resistance
FPS120 R = 500 - 600 Ω
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WPS120 R = 20 - 30 Ω
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Micro-wrinkled Pd surface for H2 sensing
Sensor Characterization - Hydrogen Sensing (2/2)
Results can be explained taking into account the peculiar topography of wrinkled sensors
Hydrogen adsorption affects surface topography and Micro-wrinkles affect
electrical properties of the surface:
 Expansion of Pd layer the closing of more and more gaps between adjacent wrinkles
(WPS sensor act as in Type II sensors)
 Further adsorption leads towards a maximum obtainable percolation enhancement
(all nanogaps are closed)  establishment of a “bulk-like”, continuous Pd surface
 Above activation threshold further H2 adsorption causes an increase in resistance of the
sensor, as in continuous Pd films (WPS sensor act as in Type I sensors)
 Restoring the air environment causes a complete recovery of the original resistance.
RR= R0
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Micro-wrinkled Pd surface for H2 sensing
Effect of Pd-Layer Thickness on Sensing Behavior
Recovery
Recovery
Response
Response
FPS: little sensitivity improvement with increasing thickness of Pd layer in the 1% - 2,5% range. Shift in trend slope
change for FPS30 probably due to lower mechanical constraints occurring in thinner Pd films acting against the lattice
expansion induced by  phase transition.
WPS: activation threshold cAct 1,5% for WPS120 while cAct  1,8% for WPS30. Differences in surface topography.
Activation threshold tailoring by fine tuning of micro-wrinkling (Control by Pd Thickness)
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Micro-wrinkled Pd surface for H2 sensing
Sensor Response and Recovery Times
Response Time
Recovery Time
τresp time to reach 90% of the overall resistance
change on H2 exposure
τrec time to reach 90% of the overall resistance
change on air exposure
FPS: τresp and τrec strongly depend on Pd-film thickness.
WPS: complex behavior related to activation threshold. Fast recovery above the activation threshold.
Observed recovery time is suitable for technological application, required performance being τrec = 60s
independent of hydrogen concentration
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Micro-wrinkled Pd surface for H2 sensing
Effects of Hydrogen Exposure on Topography
Before Exposure to H2
After Exposure to H2
FPS30
H2
FPS120
H2
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Micro-wrinkled Pd surface for H2 sensing
Effects of Hydrogen Exposure on Topography
Before Exposure to H2
After Exposure to H2
WPS30
H2
WPS120
H2
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Micro-wrinkled Pd surface for H2 sensing
Conclusions
 A new Hydrogen sensor for detection of H2 in the LEL range has been
presented
 The sensor is based on a easy-to-produce micro-wrinkled Pd surface over
plastic substrate
 It has been characterized in term of morphology, sensitivity and
response/recovery times
 The sensor presents very unique features:
 Switched Response on LEL Range
 Ultra Low Cost
 Fast Recovery
 Easy integration
 Reliability and Resistance
 Highly reproducible in switching behavior with tunable threshold (by thickness)
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Thank you for your attention!
Acknowledgements:
The work is supported by the
Tuscany Region in the framework of
the project “H2 Filiera Idrogeno”
The BIOROBOTICS
Institute
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