micro-wrinkled pd surface for hydrogen sensing and switched detection of lower explosive limit...

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

Sept 13, 2011 http://mbr.iit.it [email protected] - [email protected]

The BIOROBOTICSInstitute

ICHS 2011September 13, 2011


Background

The Micro-Wrinkled H2 Sensor

Sensor Fabrication

Sensor Characterization

Conclusions

Micro-wrinkled Pd surface for H2 sensingSummary


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%.

Micro-wrinkled Pd surface for H2 sensingBackground

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.


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).


+ 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

Micro-wrinkled Pd surface for H2 sensingState of the Art

Technologies Based on Palladium / Palladium Alloys

Reference elements

Active elements










Adapted from: T. Tabib-Azar et al. Sensors and Actuators B 56 (1999) 158-163







+ Nanostructured Pd - Discontinuous ultra-thin films- Pd nano- or meso-wires- FIB nanotrenches into Pd microwires



Adapted from: H.I. Chen et al. Sensors and Actuators B 85 (2002) 10-18










Adapted from:Y. Morita et al. Sensors and Actuators B 33 (1996) 96-99










Adapted from:W.P. akubik et al. Sensors and Actuators B 82 (2002) 265-271







+ Nanostructured Pd- Discontinuous ultra-thin films [1]- Pd nano- or meso-wires [2]- FIB nanotrenches into Pd microwires [3]- …



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


From: F. J. Ibañez et al, Langmuir, 22 (2006) 9789-9796

Micro-wrinkled Pd surface for H2 sensingThin Palladium Film Behavior

Type I In bulk or continuous thin films of Pd upon hydrogen exposure there is an increase in electron scattering due to hydrogen atoms acting as defect sites in the fcc lattice of palladium hydride (PdHx)

Electrical resistance increases upon hydrogen exposure.

Type II Ultra-thin and nanostructured Pd films, characterized by some discontinuity and nano-scale gaps (nanopores, nanotrenches, nanowires, nanoparticles assembly). These nano-break junctions are closed due to volume expansion upon hydrogen absorption

Electrical resistance decreases because of enhanced percolation of electrical charges.

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


Micro-wrinkled Pd surface for H2 sensingThe 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.

[1] C. Fu et al., Tunable Nanowrinkles on Shape Memory Polymer Sheets, Advanced Materials, 21, No. 44, 2009, pp. 4472-4476.

Thermo-retractable PS Polyshrink

Pd thin film

Mask, Pd deposition(DC sputtering)

Thermo-retractable PS Polyshrink®

Micro-WrinkledPd@polyshrink

T = 170°C (PS substrate shrinks to 40% of lateral dimensions; thickness increase by 800%)


Micro-wrinkled Pd surface for H2 sensingSensor 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).

WPS30

WPS120

FPS30

FPS120

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)

CPF30

CPF120

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)


Micro-wrinkled Pd surface for H2 sensingSensors Prototypes

Active Pd surface12.0 x 2.0 mm2

FPS 13.6 x 3.6 x 0.2 mm3

WPS 13.6 x 3.6 x 2.0 mm3

Size:

FPS

WPS

FPS

WPS

FPS

WPS


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)

Micro-wrinkled Pd surface for H2 sensing Sensors Prototypes – the Wrinkled Surface

100 μm 100 μm 100 μm


VeecoAtomic Force Microscope (AFM)

Hirox KH7700Digital Microscope

ZEISS EVO MA15Scanning Electron Microscope (SEM)

Micro-wrinkled Pd surface for H2 sensingSensor Characterization

Surface Characterization Thickness Morphology Topography

Electrical Characterization Resistance Sensitivity Response and recovery time

Experimental ParametersNominal air flow rate:1 l/min;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 (%)


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 CPF120

Thickness (t30) = 30 ± 6 nm Thickness (t120) = 105 ± 10 nm


Micro-wrinkled Pd surface for H2 sensing Sensor Characterization - Topography

1 KX 10 KX

FPS30

FPS120

Flat Samples


Micro-wrinkled Pd surface for H2 sensing Sensor Characterization - Topography

1 KX 10 KX

WPS30

WPS120

Wrinkled Samples


Micro-wrinkled Pd surface for H2 sensing Sensor Characterization - Hydrogen Sensing (1/2)

FPS120 WPS120

“Activation threshold” cAct 1,5%



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

Results can be explained taking into account the peculiar topography of wrinkled sensors

FPS120 R = 500 - 600 Ω WPS120 R = 20 - 30 Ω



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.

Results can be explained taking into account the peculiar topography of wrinkled sensors

R R R = R0


Micro-wrinkled Pd surface for H2 sensing Effect of Pd-Layer Thickness on Sensing Behavior

Response

Recovery

Response

Recovery

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)


Micro-wrinkled Pd surface for H2 sensing Sensor Response and Recovery Times

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

Response Time

τresp time to reach 90% of the overall resistance change on H2 exposure

Recovery Time

τrec time to reach 90% of the overall resistance change on air exposure


Micro-wrinkled Pd surface for H2 sensing Effects of Hydrogen Exposure on Topography

Before Exposure to H2 After Exposure to H2

FPS30

FPS120

H2

H2


Micro-wrinkled Pd surface for H2 sensing Effects of Hydrogen Exposure on Topography

Before Exposure to H2 After Exposure to H2

WPS30

WPS120

H2

H2


Micro-wrinkled Pd surface for H2 sensingConclusions

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)


Thank you for your attention!

Acknowledgements:

The work is supported by the Tuscany Region in the framework of the project “H2 Filiera Idrogeno”

The BIOROBOTICSInstitute

micro-wrinkled pd surface for hydrogen sensing and switched detection of lower explosive limit...

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