Transparent Pd Wire Network based Areal Hydrogen Sensor with ...

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Transparent Pd Wire Network based Areal Hydrogen Sensor with Inherent Joule Heater Sunil Walia, Ritu Gupta, K. D. Mallikharjuna Rao, and Giridhar U. Kulkarni ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08275 • Publication Date (Web): 17 Aug 2016 Downloaded from http://pubs.acs.org on August 18, 2016

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ACS Applied Materials & Interfaces

Transparent Pd Wire Network based Areal Hydrogen Sensor with Inherent Joule Heater† Sunil Walia,a, c Ritu Gupta,b K. D. M.Rao,a Giridhar U. Kulkarnic *‡ a

Chemistry & Physics of Materials Unit and Thematic Unit of Excellence in Nanochemistry, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bengaluru, 560064, India.

b

Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur, Rajasthan 342011, India.

c

Centre for Nano and Soft Matter Sciences (CeNS), Jalahalli, Bengaluru, 560013. India.

‡On lien from JNCASR, Bengaluru.

KEYWORDS: Transparent Heater, Flexible, Gas Sensor, Hydrogen, Joule Heating ABSTRACT: High degree of transparency in devices is considered highly desirable for futuristic technology. This demands that both the active material and the electrodes are made of transparent materials. In this work, a transparent Pd wire network (~ 1 cm2), fabricated using crackle lithography technique with sheet resistance and transmittance of ~ 200 Ω □-1 and ~ 80% respectively, serves multiple roles; besides being an electrode, it acts as an active material for H2 sensing as well as an in-built electrothermal heater. The sensor works over a wide range of hydrogen (H2) concentration down to 0.02% with response time ~ 41 s which could be improved to ~ 13 s by in-situ Joule heating to ~ 75 °C. Importantly, the device has the potential of scale-up to window size transparent panel and be flexible when desired.

Introduction In optoelectronics devices such as flat panel displays, light emitters and solar cells, one of the electrodes is kept transparent suiting the device functionality while the device itself is not required to be transparent. In nonoptoelectronic devices by definition, transparency does not play a significant role as the device functioning does not involve any form of light. However, futuristic technology demands are unconventional; a current trend is to make device as a whole transparent. Electronics being transparent1 adds new dimension to our lives creating that extra, even if little, crucial visual space. It is but natural that the focus is on optoelectronic devices and in this direction, there have been efforts in the literature to make solar cells,2 photodiodes3,4 etc. visually transparent. Smart windows with control over color and transparency are the finest examples. Interestingly, other devices such as capacitor,5 battery,6 transistor,7 defroster,8,9 heater10,11 have also been fabricated to be transparent. This trend is encompassing the field of sensors as well. Instead of small area opaque sensors, the technology of sensing is gradually drifting towards transparent sensors.

For example, Jong et al.12 have fabricated a graphene based transparent strain sensor. Zhenan et al.13 have used transparent carbon nanotube (CNT) film for pressure and strain sensor applications. Similarly, micromolded Pd nanowire based semitransparent conducting film has been employed for a mechanical strain sensor.14 CdS nanowires have been used to make a transparent multicolor photodetector array.3 In this direction, even gas sensors have also been explored for transparency in applications such as automobiles where spatial measurement is important for fast and sensitive detection. Jang et al.15 demonstrated NO2 sensing using self-activated graphene with transparency of 80%. Choi etal.16 have reported graphene based NO2 gas sensor with self-integrated heater and transparency of as high as 90%. Moon et al.17 fabricated thin film of self-activated WO3 on indium tin oxide (ITO) as large area, chemoresistive, transparent (~ 90%) gas sensor. Au/SWCNTs (singlewalled carbon nanotubes) and SWCNTs films were fabricated as transparent-flexible ammonia gas sensor.18,19 A transparent Pd coated TiO2 nanotube array on glass also has been tried for H2 sensing.20 Given the growing importance of H2 as a fuel,21 its usage in realistic

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applications such as automobiles is of utmost safety concern.21 In this work, we have explored the fabrication of large area transparent H2 sensors. In the literature, many gas sensors have been reported, a few being transparent and few being flexible but very few being transparent and flexible simultaneously. The present work deals with the fabrication of a transparent and flexible H2 sensor with a built-in microheater. Transparent electronics requires all components to be transparent. It is relatively straightforward to design transparent electrodes based on current practices; conventionally, ITO has been used as transparent conductor in such electronic prototypes. For example, Mitsubayashi et al.22 fabricated transparent glucose sensors using ITO while Makhija et al.,23 used ITO for ethanol and ammonia gas sensing. However, ITO is not preferred due to high cost and its brittle nature particularly when the application is over large areas.24 Alternate materials such as CNT networks,25,26 graphene,27,28 metal nanowire networks29, metal grids with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)30,31,32 are also being employed as substitute for ITO, including metal network based transparent conductors.5, 33 However, conceiving active materials that are transparent is not something trivial. Often, devices are rendered semi-transparent due to limited transparency or color of the active material.34 What is desirable is that both electrode and active materials permit high transparency and in this context, we have developed a H2 sensor by patterning Pd using crackle lithography,33 a method pioneered in this laboratory with which we could conceive, the electrodes and the active material into a single transparent component. The Pd wire network was fabricated on a flexible and flat substrate using previously developed crackle lithography33 as illustrated in Figure 1a. Briefly, the crackle precursor is spin coated on the substrate resulting in a spontaneously formed crackle network. Pd metal (~ 50 nm, Figure 1b) is thermally evaporated over the substrate with crackle layer acting as a mask for metal deposition. The lift-off process of crackle precursor leaves behind interconnected cracks filled with Pd metal in the form highly connected wire network. Scanning electron microscopy (SEM) image (Figure 1c and Figure S-1a) reveals the presence of densely packed nanoparticles forming wires in the network;

Figure 1 (a) Schematic demonstrating the process steps for fabricating transparent Pd wire network with overlaid optical microscope images. (b) Atomic force microscope (AFM) height profile. Inset is the Pd wire showing line profile in AFM image. (c) SEM image and (d) Optical transmittance spectrum of Pd/glass. Inset is a photograph of the Pd wire 2 network device with Ag contact pads (~ 1 cm ).

energy dispersive X-ray spectroscopy (EDS) data (Figure S-1b) clearly shows Pd signal from the wire network. The optical transmittance spectra of the Pd wire network on glass was flat in the visible as well as near IR regions (Figure 1d), with average transmittance of ~80% over the visible range (400-800 nm) with a standard deviation of ~0.9%. The density of Pd wire network is calculated to be ~ 490/mm2 (Figure 1c) using image analysis.35 A device with an active area of ~ 1 cm2 with Ag contacts pads showed a sheet resistance of 200 Ω□-1. Figure 2a shows the photograph of H2 sensing set up. The resistive signal is measured using the electrical connectors across a sealed chamber with gas inlet and outlet. For sensing action, H2 is premixed with N2 (carrier gas) using a mass flow controller before passing into the test chamber. The concentration of H2was varied and the total flow of gas was kept constant at 1,000 sccm. The relative resistance change is shown in Figure 2b for Pd wire network when exposed to varying H2 concentrations (0.02% - 5 vol %). We note that the pristine Pd exhibited

Table 1: Different Pd wire networks with varying device characteristics.

1

glass

Transmittance (%) (at 650 nm) 73

450

44

15.9

35-45

2

glass

77

365

63

16.6

30-40

Device no.

Substrate

Resistance (Ω)

Response time (s)

Fill factor (%)

Thickness (nm)

3

glass

81

~1000

reverse signal

16.1

25-35

4

glass

79

118

53

17.1

42-50

5

glass

73

220

66

17.7

40-50

6

PET

75

~1000

14

18.7

12-21

7

PET

60

275

18

19.6

28-42


2

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Figure 2 H2 gas sensing characteristics of Pd wire network (a) Photograph of a Pd wire network based H2 sensor in a test chamber. (b) Response and recovery curves for various H2 concentrations. Inset showing the response behavior for 1% H2 before and after electrical activation of Pd wire network. Here, ∆R is Rf- Ri, and Rf is maximum resistance after 2 minutes of H2 purging while Riis the base resistance before purging. (c) Response time with respect to H2 concentration for pristine and electrically activated Pd wire network (d) Optical profiler images of Pd wire network, (i) after introducing a stream of H2 gas (ii) and after purging with N2 (iii). The color bar is shown alongside relates to height variations.

a sluggish response and recovery at all concentrations (Figure S-2a, b) probably due to some surface contamination from the residual crackle precursor. However, a small electrical activation by applying a voltage pulse of 10 V for 10 s, improved the nature of the Pd wire network as shown in the inset of Figure 2b and in the Figure S-3a, b. A slight reduction in sensitivity seen is probably due to increased particle coupling induced by Joule heating (see response curve in Figure S-4). However, there is a prominent improvement in response time from 58 to ~44 s (Figure 2c) as well as recovery time from 145 to~101 s for 1% H2 concentration (Inset Figure 2b). Here, the response and recovery times correspond to the periods required for 90% of the total change in the signal. For concentrations above 2%, a sluggish response was obtained due to the retarding effect of phase transition at this concentration regime (2%-4%) which is known in the literature.37 The response and recovery curves for low H2 concentrations (0.02%-0.2%) show distinguishable changes in resistance which is noteworthy. The nonlinearity in ∆R/R (Figure S-4) is understandable as the resistance change is not saturated at lower concentrations (0.02% - 0.2%) within the exposure time of 2 min. The

response time, sensitivity and reproducibility of the Pd wire network based sensor can be seen from the repeated response curves for 1%, 2%, 3% and 4% of H2 concentration (see Figure 2b). There is no significant change in the response curves for 1% to 3%, which shows repeatability and reliability of the sensor. However, resistance change saturates for H2 concentration greater than 3% which agrees well with the literature.37-38 Usually, in case of Pd, the change in resistance is attributed to the formation of Pd-hydride (PdHx), which has relatively higher electrical resistance than Pd metal.38 PdHx has two phases, ‘α’ at lower H2 concentration while ‘β’ at higher concentration where the latter shows no change in resistance with increase the concentration of H2. In the present device, the response may saturate due to the formation of the ‘β’ phase and hence ∆R/R saturates at higher concentration. In all, seven devices were made for the sake of comparison over glass and PET(polyethylene terephthalate) substrates, as listed in Table 1, which, however, exhibit varying response. However, no systematic trend is observed with the wire width due to several other influencing factors

3



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Figure 3 Heating and sensing (a)Thermal images (scale bar 5 mm) with sample surrounding marked in dotted rectangle and (b) Temperature plot of Pd wire network with power per unit area (c) Response for various concentration of H2 at ~ 47 °C under applied voltage of 6 V (d) H2 sensing action at 1% H2 at different temperatures. The kink feature is marked with an arrow (e) Response and recovery time with respect to temperature of the substrate.

such as surface area, particle coupling, surface adhesion etc. Indeed, two devices with similar resistance (~ 1000 Ω) and transmission characteristics but on two different substrates (glass and PET) exhibited widely differing behaviour. Importantly, devices 4 and 5 which have similar mesh characteristics on a given substrate show similar sensing behaviour (Table 1). We also note that the Pd based H2 sensor with thinner wire (> 30 nm) possesses high resistance (~ kΩ) and concomitantly shows reversal in signal (decrease in resistance upon hydrogenation, see Table 1) due to enhanced particle coupling in the wire upon hydrogenation. This kind of behavior is known in literature for ultrathin Pd films.36 We have examined in-situ changes in the morphology of Pd wire network during H2 sensing by using optical profilometry as shown in Figure 2d. Optical profilometry is indeed useful to monitor minute changes in thickness.39 Figure 2d(i) shows the image of Pd wire network after electrical activation. The average thickness of Pd wire is estimated to be ~ 48 ± 5 nm. When Pd wire network was exposed to H2, Pd wires exhibit an increase in height due to lattice expansion while converting to PdHx as can be seen from occurrence of red spots in optical profiler (OP) image shown in Figure 2d(ii). Upon recovery in air atmosphere (Figure 2d(iii)), the Pd wire network regained its pristine nature. These observations confirm that the Pd wire network absorbs and releases H2 reversibly thus acting like an H2 sensor. For rapid response and recovery, an electrothermal heater is sometimes integrated below the active gas-sensing material. However, in the device, the active material Pd being metallic, the Pd wire network itself can be Joule heated, thus serving as a heater. The heating response of Pd wire network at different voltages (1, 5 and 9 V) can be

seen from the thermal images in Figure 3a. As can be seen from the images, heating is uniform which indicates that the sensor has uniform sheet resistance and thickness all over the electrodes. The rise in temperature of Pd wire network is linear with respect to applied power (Figure 3b), as expected with thermal resistance of ~100 cm2K/W. As seen by the plot in Figure 3b, 6 V (~ 0.2 W/cm2) is enough to raise the temperature of the Pd wire network to~ 47 °C. Figure 3c is the H2 sensing action at ~ 47 °C for different H2 concentrations (1%-4%). The response is observed to be linear even for higher concentration (~ 4%) upon heating. However, the sensing is not reliable for lower concentrations (