1. Micro-wrinkled Pd surface for hydrogen sensing and switched detection of Lower Explosive Limit Francesco Greco 1*, Letizia Ventrelli 1,2, Paolo Dario 1,2 and Virgilio Mattoli 1* 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 virgilio.mattoli@iit.it - francesco.greco@iit.it The BIOROBOTICS Institute ICHS 2011 September 13, 2011

2. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it  Background  The Micro-Wrinkled H 2 Sensor  Sensor Fabrication  Sensor Characterization  Conclusions Micro-wrinkled Pd surface for H 2 sensing Summary

3. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it 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 H 2 in air is 4%. Micro-wrinkled Pd surface for H 2 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 H 2 gas leakages and for the continuous monitoring of its concentration in many different industrial environments is the object of intensive research.

4. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 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 (PdH x ); 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).

5. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it + 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 H 2 sensing State of the Art Technologies Based on Palladium / Palladium Alloys Reference elements Active elements

6. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it + 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 H 2 sensing State of the Art Technologies Based on Palladium / Palladium Alloys Adapted from: T. Tabib-Azar et al. Sensors and Actuators B 56 (1999) 158-163

7. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it + 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 H 2 sensing State of the Art Technologies Based on Palladium / Palladium Alloys Adapted from: H.I. Chen et al. Sensors and Actuators B 85 (2002) 10-18

8. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it + 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 H 2 sensing State of the Art Technologies Based on Palladium / Palladium Alloys Adapted from: Y. Morita et al. Sensors and Actuators B 33 (1996) 96-99

9. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it + 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 H 2 sensing State of the Art Technologies Based on Palladium / Palladium Alloys Adapted from: W.P. akubik et al. Sensors and Actuators B 82 (2002) 265-271

10. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it + 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] - … Micro-wrinkled Pd surface for H 2 sensing State of the Art Technologies Based on Palladium / Palladium Alloys 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

11. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it From: F. J. Ibañez et al, Langmuir, 22 (2006) 9789-9796 Micro-wrinkled Pd surface for H 2 sensing Thin 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 (PdH x ) 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

12. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 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. [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-Wrinkled Pd@polyshrink T = 170°C (PS substrate shrinks to 40% of lateral dimensions; thickness increase by 800%)

13. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 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: WPS 120 (t = 120s sputtering) and WPS 30 (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 FPS 120 and FPS 30 (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 CPF 120 and CPF 30 (sputtering t = 120s and 30s respectively)

14. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 sensing Sensors Prototypes Active Pd surface 12.0 x 2.0 mm 2 FPS 13.6 x 3.6 x 0.2 mm 3 WPS 13.6 x 3.6 x 2.0 mm 3 Size: FPS WPS FPS WPS FPS WPS

15. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Uniform isotropic micro and nano-wrinkles over the whole active area of sensors (24 mm 2 ) 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, T g ≈ 95°C) Micro-wrinkled Pd surface for H 2 sensing Sensors Prototypes – the Wrinkled Surface 100 μm

16. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Veeco Atomic Force Microscope (AFM) Hirox KH7700 Digital Microscope ZEISS EVO MA15 Scanning Electron Microscope (SEM) Micro-wrinkled Pd surface for H 2 sensing Sensor Characterization  Surface Characterization  Thickness  Morphology  Topography  Electrical Characterization  Resistance  Sensitivity  Response and recovery time Experimental Parameters Nominal air flow rate:1 l/min; Measured H 2 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/R 0 (%)

17. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 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 CPF 30 CPF 120 Thickness (t 30 ) = 30 ± 6 nmThickness (t 120 ) = 105 ± 10 nm

18. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 sensing Sensor Characterization - Topography 1 KX10 KX FPS 30 FPS 120 Flat Samples

19. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 sensing Sensor Characterization - Topography 1 KX10 KX WPS 30 WPS 120 Wrinkled Samples

20. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 sensing Sensor Characterization - Hydrogen Sensing (1/2) FPS 120 WPS 120 “Activation threshold” c Act  1,5%

21. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 sensing Sensor Characterization - Hydrogen Sensing (2/2) 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 FPS 120 R = 500 - 600 ΩWPS 120 R = 20 - 30 Ω

22. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 sensing Sensor Characterization - Hydrogen Sensing (2/2) 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 H 2 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 RR R = R 0

23. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 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 FPS 30 probably due to lower mechanical constraints occurring in thinner Pd films acting against the lattice expansion induced by  phase transition. WPS: activation threshold c Act  1,5% for WPS120 while c Act  1,8% for WPS30. Differences in surface topography. Activation threshold tailoring by fine tuning of micro-wrinkling (Control by Pd Thickness)

24. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 sensing Sensor Response and Recovery Times FPS: τ resp and τ rec 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 τ rec = 60s independent of hydrogen concentration Response Time τ resp time to reach 90% of the overall resistance change on H 2 exposure Recovery Time τ rec time to reach 90% of the overall resistance change on air exposure

25. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 sensing Effects of Hydrogen Exposure on Topography Before Exposure to H 2 After Exposure to H 2 FPS 30 FPS 120 H2H2 H2H2

26. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 sensing Effects of Hydrogen Exposure on Topography Before Exposure to H 2 After Exposure to H 2 WPS 30 WPS 120 H2H2 H2H2

27. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it Micro-wrinkled Pd surface for H 2 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)

28. Sept 13, 2011 http://mbr.iit.it virgilio.mattoli@iit.it - francesco.greco@iit.it 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