Electronic and Mechanical Antagonist Effects in Resistive Hydrogen

Article pubs.acs.org/JPCC

Electronic and Mechanical Antagonist Effects in Resistive Hydrogen Sensors Based on Pd@Au Core−Shell Nanoparticle Assemblies Prepared by Langmuir−Blodgett Khalil Rajoua,* Linda Baklouti, and Frèdéric Favier* Institut Charles Gerhardt Montpellier UMR 5253 CNRS, Université Montpellier 2, cc1502, Montpellier 34095 Cedex 05, France S Supporting Information *

ABSTRACT: Pd@Au core−shell nanoparticles, synthesized with a good control of the shell thickness, can be assembled by a simple Langmuir−Blodgett method as 2D assembles and transferred onto glass chips to fabricate H2 resistive sensors. Thanks to the specific reactivity of palladium toward hydrogen leading to the reversible conversion of palladium as palladium hydride, these core−shell nanoparticle layers can be used to detect hydrogen in extended H2 concentration ranges. Fabricated sensors show attractive sensing performances including high signal amplitudes, good specificity toward hydrogen, and short response and recovery times. Depending on the Pd shell thickness and H2 concentration, distinct response types are observed, either resistive or conductive. These responses, in terms of amplitude and sign, strongly depend on the balanced contribution of two antagonist mechanical and electronic effects, promoted by the palladium hydride formation under H2 atmosphere. By using the percolation theory and simple data modeling, these Pd thickness-dependent contributions are decorrelated, and the sensing mechanisms are described.

■

limited lifetime and high cost.6,7 The work-function-based sensors present both high sensitivity and selectivity. They are however suffering from signal saturation at low concentration and baseline drift.8 Optical-based sensors mainly use optical fibers partially or totally coated with Pd and/or WO3. These sensors have a very high sensitivity but are subject to aging effects and interferences from ambient light, both promoting signal drifts.9,10 The mechanical-based sensors are usually based on cantilevers. These micromachinable devices have small size but slow response time and are highly sensitive to poisoning.11 Among present day hydrogen gas sensors, resistive hydrogen sensors are probably those able to simultaneously show high sensitivity, short response time, low power consumption, etc.1,3,12 Typically, these sensors are prepared by deposition of a film, a drop of (nano)particles, or an array of fibers or wires in between microelectrodes at the surface of an insulating substrate.1,13−15 The sensing materials can be based on Pd, Pt, Ag, Au... metals or alloys but can also be semiconductors such as SnO2, WO3, etc.16,17 Noble metals are more suited than semiconductors since the latter cannot, usually, operate at room temperature and show lower selectivity. The choice of the sensing material can also depend on the operating conditions. For example, depending on the nature of the atmosphere, the use of Pt-based sensing material is limited to measurements in

INTRODUCTION Hydrogen is anticipated as an efficient and clean(er) energy carrier to replace fossil fuels for generalized industrial, home, domestic, mobile, and macro- and micro- uses. It however has a severe drawback as an explosive gas at concentrations from 4% to 75% in air. Moreover, this odorless, colorless, and tasteless gas is undetectable by the human senses, hence the need for hydrogen sensors. There are currently numerous kinds of hydrogen sensors such as catalytic, thermal conductivity, electrochemical, work function, mechanical, optical, acoustic, and resistance-based devices.1−4 For each, several design and fabrication strategies have been explored during the last 20 years as evidenced by the impressive literature on the subject with more than 1000 published scientific papers (ref: web of knowledge (http:// apps.webofknowledge.com/), “hydrogen gas sensor” topic) and 200 000 patent applications filled (ref: google patent (https:// www.google.com/?tbm=pts&=ssl)). Only a few of them show the performance required for industrialization and commercialization in terms of concentration range, sensitivity, specificity (cross sensitivity), response and recovery times, robustness, lifetime, cost, large-scale fabrication process, size, power consumption, setup, etc. For example, catalytic and thermal conductivity sensors need to be heated at given temperature ranges to detect specific gases.1,4,5 As a consequence, they have high power consumption and lack in selectivity. In contrast, electrochemical-based sensors have high sensitivity and low power consumption, but their main disadvantages are their © 2015 American Chemical Society

Received: February 17, 2015 Revised: April 7, 2015 Published: April 13, 2015 10130

DOI: 10.1021/acs.jpcc.5b01636 J. Phys. Chem. C 2015, 119, 10130−10139

Article

The Journal of Physical Chemistry C

Figure 1. Pathway for the synthesis of Pd@Au core−shell nanoparticles and fabrication of the sensitive layer. From aqueous gold colloid (a), adsorbed ascorbic acid (b) acts as a Pd2+ reductive agent for the surface decoration of Au nanoparticles by a Pd shell (c). After addition of hexane (d), a Langmuir−Blodgett film is formed by the transfer of a Pd@Au core−shell nanoparticle at the water−hexane interface promoted by EtOH addition to the aqueous phase (e). The resulting monolayer is transferred by dip-coating at the surface of a glass substrate supporting interdigitated electrodes (SIE) (f).

the Langmuir−Blodgett method, to get an organized monolayer of close-packed particles28−33 eventually made of palladium and from which nanogap-based sensing could be anticipated. Unfortunately, Pd colloids are a real challenge to prepare. In contrast, aqueous gold colloids are easily prepared by reduction of a Au(III) salt with an organic mild reductive agent.34,35 These colloids show an enhanced stability thanks to electrostatic repulsion provided by negatively charged absorbed species. The gold surface is available for functionalization, by either grafting or localized precipitation. As such, metals, alloys, and metal oxides have been used for the decoration of gold colloids.36,37 Depending on the nature and thickness of the surface deposit, colloids of the resulting core−shell particles can be fairly stable. This work demonstrates the use of monolayers of Pd@Au core−shell nanoparticles obtained by the Langmuir−Blodgett technique as highly sensitive hydrogen gas sensors. The sensing performances are discussed on the basis of the shell thickness. The observed sensing discrepancies are assigned to the transduction mechanism, controlled by two antagonistic electronic and mechanical contributions.

O2-based atmospheres since the sensing mechanism involves the adsorption of O2.18,19 In such a case, Pd-based sensors will be preferred. In such sensors, after H2 dissociation at the Pd catalytic surface, the diffusion of hydrogen increases the scattering of conducting electrons and thus the material resistivity (a factor of 1.8 from Pd to PdH 0.7).20 In discontinuous Pd nanostructures, another effect counterbalances the resistivity increase in the presence of H2 as Pd undergoes a reversible volume expansion during hydride formation. This material swelling can cause the opening/ closing of nanoscopic gaps or breaks in the sensing layer, leading to a responsive system with signal changes up to several orders of magnitude. Since the first sensors based on nanowire arrays,13,15,21 various approaches toward sensing layers built on discontinuous Pd nano-objects have been explored, such as nanodot patterns,21,22 nanoparticulate thin films,23−25 nanoparticulate flexible films,26,27 etc. Besides, the conservation of the attractive performances including fast response, roomtemperature operation, low power consumption, and low crosssensitivity toward other gases, sensor design, and fabrication routes have been developed for cost-effective and mass production.1,3,4,12 Self-assembling is a very attractive bottom-up technique to easily get organized arrays of complex nano-objects by largescale compatible and high-throughput processes.28,29 The bottom line for these approaches is the need for the nanoobjects to be stabilized as individual/unaggregated objects in a solvent before being assembled and for the control of the interaction forces during the whole assembling process. Colloids are perfectly suited, especially when assembled by

■

EXPERIMENTAL METHODS Figure 1 depicts the general pathway for the synthesis of Pd@ Au core−shell nanoparticles, the formation of Langmuir− Blodgett films, and dip-coating transfer for the fabrication of the sensitive layer. Gold Colloid Preparation. All glassware used in the following procedures was cleaned using freshly prepared “aqua regia” solution (3:1 HCl:HNO3) and rinsed thoroughly with 10131


Article


operating at 120 kV. Sampling was performed by depositing a drop containing the assembled particles on carbon on copper grids (Agar Scientific). Scanning electron microscopy (SEM) imaging was carried out with a Hitachi S4800 microscope equipped with a detector of secondary and backscattered electrons. The acceleration voltage ranges from 0.1 to 30 kV. Samples were coated with a platinum layer using a SC7620 mini Sputter Coater (Sputter current at 30 mA during 30 s) from Quorum Technologies before imaging. The chemical composition of the sensing layers was measured with a scanning electron microscope with energy-dispersive X-ray spectrometer (SEM-EDX) Quanta FEG 200 from FEI.

H2O prior to use. All solutions were prepared with distilled water (Elga H2O purification system, σ ≥ 18.2 MΩ·cm−1). Gold nanoparticles were prepared by the Turkevich-Frens’s method.34,35 Briefly, 5 mL of a sodium citrate tribasic dihydrate, C6H5Na3O7·2H2O (Sigma-Aldrich), 38.8 mmol L−1 aqueous solution, was added to a 50 mL boiling aqueous 1.0 mmol L−1 solution of sodium tetrachloroaurate(III) dihydrate, NaAuCl4· 2H2O (99.99%, Alfa Aesar), under vigorous stirring. During the 20 min stirring, the color of the solution gradually turned from yellow to colorless, dark blue, and finally, to red. Then, the solution was gradually cooled to room temperature, and the stirring was extended by 20 min. Synthesis of Pd@Au Core−Shell Nanoparticles. The almost identical synthetic method has been described elsewhere.38 Briefly, 5 mL of the as-prepared gold colloid was stirred at 60 °C for 20 min. Then, 1.5 mL of a freshly prepared 0.1 mol·L−1 aqueous solution of L-ascorbic acid, C6H8O6 (99%, Sigma-Aldrich), was added to the hot gold colloidal solution. An amount of 10 mL of a palladium(II) chloride, PdCl2 (99.999%, Sigma-Aldrich), aqueous solution at 3.5, 5.0, or 7.0 mmol L−1 was added to the mixture under vigorous stirring. The mixture changed color from red to brown, and the stirring was kept for 20 min. Fabrication of Sensitive Layers by Interfacial Assembly Using the Langmuir−Blodgett Method. A simple 50 mL glass beaker was used as a Langmuir−Blodgett throw for the nanoparticle assembly. An amount of 10 mL of hexane (p.a., Carlo Erba) was gently added to 20 mL of the Pd@Au colloid in the beaker. Then, 3 to 10 mL of ethanol (p.a. 99.8%, Normapur VWR) was added very carefully in the aqueous phase by using a syringe with a needle. EtOH addition caused the migration of the Pd@Au nanoparticles at the hexane/water interface, and a 2D close-packed nanoparticle layer was gradually formed at the interface. Finally, the excess of hexane was removed with a syringe. Glass slides with interdigitated electrodes (SIEs: see Supporting Information for fabrication details, Figure SI1) were immersed into the aqueous phase at a certain inclined angle, and the nanoparticle array was transferred onto the electrode side of the chip by slowly pulling it vertically. Sensor Testing. The fabricated sensors were tested using a specially designed test line allowing the generation of controlled flows of humidified hydrogenated gas mixtures in the concentration range from 0.1% to 100% of H2 in N2 or synthetic air used as carrying gases. Sensing measurements were performed in a Plexiglas flow cell equipped with gas inlet and outlet and copper cable connectors for electrical measurements. Electrical measurements were carried out using a potentiostatgalvanostat PGSTAT100 from Metrohm Autolab B.V. controlled by the general purpose electrochemical system (GPES) from Eco Chemie B.V. software. A 0.5 V potential bias was applied between the interdigitated electrodes, and the corresponding current passing through the nanoparticle sensing layer was measured over time by chronoamperometry under various hydrogenated atmospheres. For a constant relative humidity level, gas mixtures were bubbled in a water tank during sensing measurements. Measurements were done at room temperature. Characterization Techniques. UV−visible spectroscopy measurements were done on a JASCO V-670 UV−vis−NIR equipment using 10 mm quartz cells. Transmission electron microscopy (TEM) images were obtained with a transmission JEOL 1200 EXII microscope using a field emission gun

■

RESULTS AND DISCUSSIONS The strong absorption band at 520 nm in the UV−visible spectrum (solid triangle) shown in Figure 2 is characteristic of

Figure 2. UV−visible spectra of a 5 mM PdCl2 solution (solid line) and gold (solid triangle) and Pd@Au colloids from 3.5 mM (solid circle), 5 mM (open circle), and 7 mM (open square) PdCl2 solutions.

the surface plasmon resonance band of the aqueous gold colloid prepared by reduction of a Au3+ solution by sodium citrate (Figure 1a).39 After adsorption of ascorbic acid (Figure 1b), the spectrum remains; however, by addition of palladium chloride (Figure 1c) from solutions of various Pd2+ concentrations, this plasmonic band disappears, and a sloppy absorption background is observed (Figure 2, solid and open circles and open square for suspensions from 3.5, 5, and 7 mM PdCl2 solutions, respectively). The disappearance of the plasmonic band is characteristic of drastic nanoparticle morphological and chemical changes in the colloid. For reference, the spectrum of a 5 mM PdCl2 solution is also shown (Figure 2, solid line). The band at 426 nm, characteristic of the Pd2+ d−d transition, fully disappears after reaction with the ascorbic acid absorbed at the Au colloid surface as a proof of the complete reduction of the Pd2+ species. As depicted in Figure 3, the reduction of Pd2+ cations by ascorbic acid adsorbed at the gold particle surface leads to Pd@ Au core−shell colloids. TEM imaging confirms the absence of parasitic Pd particle growth from solution. In contrast, palladium forms a dense shell at the gold surface, and the resulting Pd@Au nanoparticles have a roughly spherical morphology. As shown in histograms in Figures 3b−3d, the particle size distributions, measured over 200 individual units per micrograph, broaden with the palladium salt concentration. The nanoparticle diameter however increases with the palladium chloride concentration as it averages at 18.0 (4) nm, 21.7 (3) nm, and 24.9 (2) nm for [PdCl2] = 3.5, 5, and 7 mM, respectively. The diameter of the pristine Au nanoparticles 10132


Article


Figure 3. TEM micrographs and particle size distributions of gold core colloids (a), Pd@Au core−shell nanoparticles with [PdCl2] = 3.5 mM (b), [PdCl2] = 5 mM (c), and [PdCl2] = 7 mM (d). Thickness of the shell is calculated from the size difference with raw Au particles (12 nm).

from TEM pictures on the dependence of the shell thickness with the PdCl2 concentration as thicker shells obviously lead to higher Pd/Au ratio. However, a simple comparison (Figure SI2, Supporting Information) of the relative volumes (VPd/VAu) from TEM dimension measurements and from EDX (ρAu/ρPd × mPd/mAu) shows some discrepancies that can be assigned to cumulative errors from either the EDX analysis or broad particle size distribution. Despite this discrepancy, several assessments can be done to suggest a growth mechanism of Pd at the Au surface leading to the observed Pd@Au core−shell particles. First, the absence of raw Pd particles suggests that the reduction of PdCl2 by ascorbic acid is preferentially promoted (either thermodynamically or kinetically) at the surface of gold particles. Second, the Pd shell thickness dependence on PdCl2 concentration suggests the reduction of Pd2+ to involve a dynamic adsorption of the ascorbic acid (in excess in solution) at the growing Pd surface as it is consumed during the reduction. Third, the smooth morphology of the shell suggests the Pd growth step to be favored toward the nucleation step.

remains at 12 nm after decoration by Pd, and the average shell thickness can be calculated by diameter differences at 3, 4.9, and 6.4 nm when using, respectively, a 3.5, 5, and 7 mM palladium chloride solution. The shell thickness is actually shown to vary almost linearly with the palladium precursor concentration (Figure SI2, Supporting Information). In contrast, increasing the volume of ascorbic acid 0.1 M solution from 0.5 mL to 1.5 and 6 mL while keeping the same 10 mL of 7 mM PdCl2 solution did not induce any change in the resulting shell thickness which remained close to 6.4 nm. This observation confirms that, in the presence of an excess of ascorbic acid in the solution, the shell size only depends on the Pd precursor concentration. The chemical composition (in atomic and weight percentages as measured by EDX) of the prepared core−shell Pd@Au nanoparticles is given in Table SI1 (Supporting Information) as a function of the PdCl2 concentration used for the synthesis. The higher the PdCl2 concentration, the greater the palladium content. These observations are consistent with those made 10133


Article

The Journal of Physical Chemistry C The relatively broad size distributions of the prepared Pd@Au particles are indicative of a fast growth process probably promoted by an efficient adsorption of the ascorbic acid at the Pd surface. The prepared Pd@Au colloids are rather stable and can be used for the preparation of nanoparticle films by the Langmuir−Blodgett method. To do so, ethanol was introduced in the aqueous phase to promote the migration and selfassembly of the core−shell nanoparticles at the water−hexane interface (Figure 1e). The volume of introduced EtOH is critical for the film formation and its characteristics. If the volume of ethanol was below 2.5 mL, the formed film density was too low for the nanoparticles to close-pack. In the present case, the most suited EtOH volume to get the most ordered monolayer of Pd@Au nanoparticles ranged from 2.5 to 5 mL (Figures SI3a and SI3b, Supporting Information). In contrast, if the volume of ethanol was larger than 5 mL, the film formation resulted in the production of multilayer aggregates and islands (Figures SI3c, Supporting Information). For the three suspensions of synthesized Pd@Au core−shell particles, after addition of 5 mL of EtOH, the film formed was easily transferred by dip-coating onto a glass slide with interdigitated electrodes (SIEs) allowing electrical measurement. The current responses of the fabricated Pd@Au nanoparticle films under hydrogenated atmospheres are depicted in Figure 4. Measurements were done in the concentration range from 0.1% to 100% hydrogen in N2. Each H2/N2 mixture exposure lasted for 100 s and was followed by a 200 s cell flushing by H2-free N2. The current responses are quite complex since exposures to hydrogenated gas mixtures led to an increase or a decrease of the measured current, depending on the shell thickness and/or H2 in N2 concentration. Such a size-dependent behavior has already been observed in devices based on ultrasmall palladium nanowires.40 More precisely, the current decreased (i.e., the resistance of the sensitive layer made of Pd@Au nanoparticles increased) under hydrogenated gas mixtures in the case of (i) the thinner shells at 3 and 4.5 nm (Figures 4a and 4b) for H2 concentrations above 1 or 2% in N2 and (ii) the thicker shell at 6.4 nm (Figure 4c) for H2 concentrations below 1% in N2. In these cases, sensor responses are called “resistive”. The other way around, “conductive” responses were observed in the case of (i) the thinner shells at 3 and 4.5 nm (Figures 4a and 4b) for H2 concentrations below 1 or 2% in N2 and (ii) the thicker shell at 6.4 nm (Figure 4c) for H2 concentrations above 1% in N2. As a critical performance marker, the response and recovery times of the fabricated sensors have been estimated. As an example, Figure SI4 (Supporting Information) shows the signal profile of a sensitive film made of Pd@Au nanoparticles with a 6.4 nm shell thickness under [H2] = 20% in N2. A t90 response time of 15 s was measured for this sensor. It corresponds to the time needed to reach 90% of the full signal response. A short recovery time, trec, of 18 s was also measured where trec is defined as the time needed for the signal to decrease by 90% of the full signal response. Furthermore, when measured in the same conditions, response and recovery times were found independent of the Pd shell thickness as they were typically measured at 15 ± 5 s for all the prepared sensors. Figure 5 shows that the response signal amplitudes of the fabricated sensor under various hydrogenated atmospheres are, for both conductive and resistive responses, proportional to the H2 concentration to a large extent. The drastic change in the curves as the H2 concentration rises from 0 to 5% in N2

Figure 4. Current response (solid line) versus hydrogen concentration in N2 (gray histograms) for Pd@Au nanoparticle assemblies prepared by the Langmuir−Blodgett method with Pd shells at 3 (a), 4.5 (b), and 6.4 nm (c). Voltage bias at 0.5 V.

demonstrates the high sensitivity of these sensors toward H2 level in this concentration range and their potential as safety devices. As depicted in Figure 5a, the signal amplitudes relative to the baseline current (ΔI/I0) are larger for sensors made of Pd@Au nanoparticles with thick Pd shells (at 6.4 nm) than those for sensors from thinner Pd shells (at 3 and 4.9 nm). For the former, they change by up to 28% from pure N2 to pure H2, while signal amplitudes are limited at most to 4% change for the latter in the same concentration range. This point emphasizes the fact that conductive sensor responses are more ample than resistive responses. Together with the “sign” of signal change from conductive to resistive, this amplitude discrepancy strongly suggests distinct sensing mechanisms for thick and thin Pd shell sensors. Regarding the sensor responses relative to the maximum amplitude, as depicted in Figure 5b as ΔI/ΔIMax as a function of the H2 in N2 concentration, both resistive and conductive sensors behave similarly, in absolute value. Between 1 and 2% H2/N2, they switch from resistive, at lower concentrations, to conductive, at higher concentrations for the sensors made of Pd@Au nanoparticles with 6.4 nm thick Pd shells, and the other way around when Pd shell 10134


Article


Figure 5. Amplitudes of response as current variation relative to the current baseline ΔI/I0 (a) and to the largest sensing signal ΔI/ΔIMax (b) versus hydrogen concentration for sensing layers made from core−shell nanoparticles of various shell thicknesses.

gaps in discontinuous Pd-based arrays, caused by the swelling of Pd undergoing hydrogen absorption.13 In the absence of hydrogen gas, closed nanogaps reverted to open circuits, and the sensor signal recovers the baseline current. In such cases, the sensors exhibit a sigmoid shape of the current response versus hydrogen concentration (similar to those observed in Figure 5). These sigmoids could be fitted using Boltzmann functions confirming the probabilistic nature of the conduction process across the layer.50,51 At low hydrogen concentration, an α-palladium hydride phase is formed without any change on the crystal cell characteristics (fcc with aPd = 3.887 Å).43,49 Increasing H2 concentration leads to the formation of a β-palladium hydride phase which remains fcc but shows a larger cell parameter at aβ = 4.043 Å.43 This conversion from α to β palladium hydride corresponds to a 12% cell volume increase (Figure SI5c, Supporting Information). Moreover, this maximum volume change corresponds to the fully hydrogenated PdH0.7 phase, but the β-phase is also formed at lower H/Pd ratio, for lower volume changes, by exposure at intermediate H2 concentrations. These volume changes account for the material swelling and gap closing at the origin of the mechanical effect. In a network formed of Pd@Au nanoparticles, the swelling of palladium shells induces an increase of the number of conduction paths in the array thanks to electron tunneling and/or direct ohmic contacts and consequently a decrease of the sensing layer resistance (Figure SI6, Supporting Information).13 Under H2 exposure, both electronic and mechanical effects obviously happen simultaneously: the metallic palladium is converted as semiconductive palladium hydride (resistive contribution) and the swelling of the shell closes the eventual interparticle gaps creating conduction paths through the 2D array (conductive contribution). For any hydrogen concentration, the response of these sensors could be conductive or resistive according to the prevalence of these antagonist electronic and mechanical effects, but in any cases, the overall

thicknesses are less than 4.9 nm. This behavior inversion is assigned to two antagonistic electrical contributions from distinct physical origins as the Pd shell is exposed to hydrogenated gases and converted to palladium hydride. One is related to changes in the electronic characteristic20 and the other one to induced structural changes in Pd metal-based materials.41−43 In the following sections, the sensor responses are discussed on the basis of the relative contribution of both of these electronic and mechanical effects. Pd crystallizes in a face centered cubic (fcc) cell. After adsorption, the dissociation of hydrogen molecules, H2, to hydrogen atoms, H, occurs at the surface of the palladium shells, preferentially at the first atomic (111) layer on adsorption sites built on three octahedral vacancy sites (Figure SI5a, Supporting Information). Afterward, dissociated hydrogen interstitially diffuses into the bulk to form a palladium hydride, PdHx (Figure SI5b, Supporting Information). The hydride composition (x) strongly depends on the H2 concentration or relative pressure.41,44−48 From Pd to PdHx, the conversion as hydride induces a drastic change in the electronic structure moving from metallic palladium to semimetallic palladium hydride because of the insertion of H in the octahedral site of the fcc structure. As a direct consequence, the resistance of the material increases. It has been demonstrated that this increase is quasi-linearly proportional to the hydrogen content for 0 < x < 0.7. At x = 0.7 the palladium hydride resistance is maximum and corresponds to a 1.8 increase toward that of metallic palladium. Consequently, the resistive behavior of the sensors is assigned to this electronic effect; i.e., the resistance of the sensing layer increases when palladium is converted as palladium hydride.20,41,42,49 On the other hand, the mechanical effect accounts for the conductive behavior of the sensors. This behavior has been described elsewhere for nanogap-based hydrogen sensing devices and originates from the reversible closing of nanoscopic 10135


Article


obviously decrease as the shell thickness increases. In a 2D percolation system, there is not any empirical relationship between the electrical conductivity and the percolation threshold. However, Granqvist et al.51 have introduced an analytical expression of the electrical conductance of the network in the vicinity of the percolation threshold (Scheme SI1, Supporting Information). To evaluate the preponderance of both mechanical and electronic effect, it is necessary to introduce the concept of apparent percolation threshold as it is a critical parameter for the percolation phenomenon and so for the contribution of the mechanical effect to the sensor response. As a matter of fact, Stauffer et al. and, later, Masihi et al. have defined the apparent percolation threshold for various patterns and coordination numbers of the percolation site network.22,52−54 These empirical relations were obtained by Monte Carlo simulations.

electric behavior of the sensing layer is balanced by both effects. As demonstrated by the sensor signal amplitudes depicted in Figures 4 and 5, several parameters impact the sensor response under hydrogenated atmospheres: the hydride rate (x), the thickness of the palladium shell (ePd), the particle packing in the 2D layer, and the percolation threshold induced by the mechanical effect. As demonstrated by TEM and SEM imaging (Figure 3 and Figure SI3, Supporting Information) the sensing layers prepared by the Langmuir−Blodgett method are made of Pd@Au core−shell nanoparticles organized in a disordered hexagonal 2D packing. In such assemblies, each particle and interparticle contact point can be considered as electrically equivalent and can be modelized as an electrical resistance which depends on the particle dimensions and composition.50 Linear (quasi 1D), square, and hexagonal 2D networks of resistances are shown in Figure SI7 (Supporting Information). Whatever the shell thickness, 2D Pd@Au nanoparticle arrays can be modelized as hexagonal resistor networks (Z = 6) of different sizes (more precisely as a network of hexagonal networks). Two extra parameters are used to characterize the resistor network, nx and ny, which correspond to the maximum number of resistors in the directions parallel and perpendicular to the microelectrodes, respectively. For the microelectrode length and span used in this study, respectively, at 3000 and 100 μm, nx and ny only depend on the Pd@Au nanoparticle size and, more precisely, on the Pd shell thickness (Table 1). Both

̃ = (1 − 0, 51/ nx )1/ ny Pc1D

Pc̃2D = pc∞ + c(w1/ υ − 1)ny−1/ υ

nx ny

3.5 mM, 3 nm

5 mM, 4.9 nm 7 mM, 6.4 nm

1D

166667 5556 0.9978

138249 4608 0.9974

120968 4032 0.9970

2D

0.6078

0.6101

0.6120

Pc̃ Pc̃

(z = 4−6)

(1) (2)

With: - nx = the number of electrical resistance in the direction parallel to the microelectrodes - ny = the number of electrical resistance in the direction perpendicular to the microelectrodes ∞ - p∞ c = percolation threshold in an infinite system, where Pc 2D 1D ∞ = 1 for Pc̃ and Pc = 0.59275 for Pc̃ - c = constant ∼0.82 according to Monte Carlo simulations - w = aspect ratio of the system: w = (ny/nx) (∼30 for the presently studied arrays) and w = 1 in the isotropic case - υ = correlation length exponent ∼4/3 (for a 2D network). The apparent percolation thresholds computed for the prepared sensors are listed in Table 1. For comparison purposes, they were calculated for the three palladium shell thicknesses at 3, 4.9, and 6.3 nm and for two kinds of ideal or perfect resistor networks with z = 2 for parallel linear chains (quasi-1D) and with z = 4 or 6 for the square or hexagonal lattice (2D). For the parallel linear chains, the apparent

Table 1. Value of the Apparent Percolation Threshold Calculated from Equations 1 and 2 for Various Sensors Depending on the Pd Shell Thickness [PdCl2], Pd shell thickness

(z = 2)

Figure 6. Balance between the electronic and mechanical contributions in the H2 sensing responses of prepared Pd@Au nanoparticle assemblies with thin (a) and thick (b) Pd shells. The mechanical contribution (open circle), the electronic contribution (open square), and convoluted data (open diamond) are depicted. 10136


Article


low concentration percolation threshold and by the electronic effect for sensors made of Pd@Au particles with thicker Pd shells (ePd = 6.4 nm). In range (II), from 0.8 to 5 H2/N2 %, contributions are inverted from mechanical to electronic and vice and versa, and response types switched from resistive to conductive or the other way around. In range (III), for concentrations above 5 H2/N2 %, contributions are at their maximum, and the overall sensor signal depends on the greatest contribution, either mechanical or electronic.

percolation threshold decreases as the palladium shell thickness is increased. This means that the percolation, as an evidence of the mechanical effect preponderance, is reached at lower hydrogen concentrations for sensors based on thick palladium shells than for those on thin palladium shells. Conversely, for the hexagonal lattice of nanoparticles, the apparent percolation threshold increases by increasing the palladium shell thickness. As such, the mechanical effect contribution is greater at lower hydrogen concentrations for sensors based on thin palladium shells than for those on thick palladium shells. On the basis of the experimental observations and modeling results discussed above, the electronic and mechanical contributions to the sensing responses can be individually addressed as a function of the Pd shell thickness and the H2 concentration. Figure 6 shows electronic and mechanical contributions and corresponding convolution for both types of sensors made of Pd@Au nanoparticles with either thin (Figure 6a) or thick (Figure 6b) Pd shells. Below 2% of H2 in N2, the electronic effect (open square) results in a quasi-linear increase of the resistivity of the material for both thick and thin shells.20 At greater concentrations, the resistivity reaches a plateau. By considering a 1.8 increase in the resistance from pure Pd to PdH0.7, this plateau corresponds to a decrease of ΔI/I0 (current change relative to the baseline current) by 44% (from R = 1.8R0 and ΔI/I0 = (1 − 1.8)R0/1.8R0 = −0.44). From this modeled electronic contribution (open square), the contribution of the mechanical effect (open circle) was “calculated” for the overall response to fit the experimental data (open diamond) corresponding to the convolution of both electronic and mechanical contributions. As expected, from the percolation theory, the curves associated with the mechanical effect are sigmoids whose shape and features strongly depend on the percolation threshold and intrinsically on the thickness of the palladium shells. For sensors made of Pd@Au nanoparticles assemblies with thin Pd shells, the percolation threshold was shown to be the lowest in the sensor series, inducing the sigmoid to be shifted to low H2 concentration. Moreover, because of the thin Pd shell, the swelling induced by the hydride formation generates a limited number of percolation paths through the sensing layer. As a result, the amplitude of the mechanical effect is the lowest. On the contrary, for sensors from Pd@Au with thicker shells, the higher percolation threshold slightly shifts the sigmoid to higher H2 concentrations. The signal amplitude is also greater. Overall, the balancing of these antagonist electronic and mechanical contributions accounts for the observed sensing results as the sum of the corresponding curves matches the experimental data (open diamond) pretty well. Two specific H2 concentrations are characteristic of the observed sensing behaviors: the contribution switch which corresponds to the concentration at which the mechanical effect takes over the electronic one, or the other way around, and the response switch from resistive to conductive sensing responses. In the series, the calculated values of the percolation thresholds are close for the various sensors. This observation points out why, whatever the thickness of palladium shells, both contribution and response switches were experimentally found to take place at hydrogen concentration in the same range, between 0.5 and 1 and 1 and 2 H2/N2 %, respectively. Depending on the Pd shell thickness, three H2 concentration ranges can be defined: In range (I), from 0 to ∼0.8 H2/N2 %, the sensing behavior is controlled by the mechanical effect for sensors made of Pd@Au particles with thin Pd shells (ePd = 3 and 4.9 nm) because of the

■

CONCLUSIONS Pd@Au core−shell nanoparticles were prepared with a good control of the shell thickness. 2D assemblies of the prepared core−shell particles were obtained by a simple Langmuir− Blodgett method and transferred onto glass chips for resistive sensing measurement under hydrogen gas. These core−shell nanoparticle monolayers detected hydrogen in extended H2 concentration ranges and presented attractive performances. They showed high amplitude responses and a good specificity toward hydrogen. Their response time and recovery time were shown to be short. The comprehensive study of the sensing mechanism demonstrated that the sensor responses depend on the specific reactivity of palladium toward hydrogen. Depending on the Pd shell thickness, two distinct response types were observed, resistive or conductive, depending on the balanced contribution of two antagonist effects, either mechanical or electronic, promoted by the reversible conversion of palladium as palladium hydride. By using the percolation theory, the sensor signal was shown to depend on the Pd thickness.

■

ASSOCIATED CONTENT

S Supporting Information *

Chemical composition of Pd@Au nanoparticle layers obtained by SEM-EDX. SEM and TEM micrographs of Pd@Au monolayers for the different volumes of ethanol introduced during the Langmuir−Blodgett process. Response time (t90) and recovery time (trec) measurement. Scheme of the swelling process of the Pd to the PdHx during the hydrogen exposition. Scheme of the various 2D resistor networks and expression of the electrical conductivity of the network close to the percolation threshold. Competition between the mechanical and the electronic effect. Fabrication process of the substrates with interdigitated electrodes (SIEs) by photolithography. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b01636.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The Centre National de la Recherche Scientifique is gratefully acknowledged for funding.

■

REFERENCES

(1) Aroutiounian, V. Hydrogen Detectors. Int. Sci. J. Altern. Energy Ecol. 2005, 23, 21−31.

10137


Article

The Journal of Physical Chemistry C (2) Züttel, A.; Remhof, A.; Borgschulte, A.; Friedrichs, O. Hydrogen: the Future Energy Carrier. Philos. Trans R. Soc., A 2010, 368, 3329− 3342. (3) Buttner, W. J.; Post, M. B.; Burgess, R.; Rivkin, C. An Overview of Hydrogen Safety Sensors and Requirements. Int. J. Hydrogen Energy 2011, 36, 2462−2470. (4) Hübert, T.; Boon-Brett, L.; Black, G.; Banach, U. Hydrogen Sensors − a Review. Sens. Actuators, B 2011, 157, 329−352. (5) Han, C. H.; Hong, D. W.; Kim, I. J.; Gwak, J.; Han, S.-D.; Singh, K. C. Synthesis of Pd or Pt/Titanate Nanotube and its Application to Catalytic Type Hydrogen Gas Sensor. Sens. Actuators, B 2007, 128, 320−325. (6) Janata, J.; Josowicz, M.; Vanýsek, P.; DeVaney, D. Chemical Sensors. Anal. Chem. 1998, 70, 179−208. (7) Korotcenkov, G.; Han, S. D.; Stetter, J. R. Review of Electrochemical Hydrogen Sensors. Chem. Rev. 2009, 109, 1402− 1433. (8) Potje-Kamloth, K. Semiconductor Junction Gas Sensors. Chem. Rev. 2008, 108, 367−399. (9) Bearzotti, A.; Caliendo, C.; Verona, E.; D’Amico, A. Integrated Optic Sensor for the Detection of H2 Concentrations. Sens. Actuators, B 1992, 7, 685−688. (10) Chadwick, B.; Gal, M. Enhanced Optical Detection of Hydrogen Using the Excitation of Surface Plasmons in Palladium. Appl. Surf. Sci. 1993, 68, 135−138. (11) Baselt, D. R.; Fruhberger, B.; Klaassen, E.; Cemalovic, S.; Britton, C. L., Jr; Patel, S. V.; Mlsna, T. E.; McCorkle, D.; Warmack, B. Design and Performance of a Microcantilever-Based Hydrogen Sensor. Sens. Actuators, B 2003, 88, 120−131. (12) Boon-Brett, L.; Bousek, J.; Black, G.; Moretto, P.; Castello, P.; Hübert, T.; Banach, U. Identifying Performance Gaps in Hydrogen Safety Sensor Technology for Automotive and Stationary Applications. Int. J. Hydrogen Energy 2010, 35, 373−384. (13) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Hydrogen Sensors and Switches from Electrodeposited Palladium Mesowire Arrays. Science 2001, 293, 2227−2231. (14) Walter, E.; Favier, F.; Penner, R. Palladium Mesowire Arrays for Fast Hydrogen Sensors and Hydrogen-Actuated Switches. Anal. Chem. 2002, 74, 1546−1553. (15) Fournier, C.; Rajoua, K.; Doublet, M.-L.; Favier, F. Palladium− Silver Mesowires for the Extended Detection of H2. ACS Appl. Mater. Interfaces 2013, 5, 310−318. (16) Hyodo, T.; Baba, Y.; Wada, K.; Shimizu, Y.; Egashira, M. Hydrogen Sensing Properties of SnO2 Varistors Loaded with SiO2 by Surface Chemical Modification with Diethoxydimethylsilane. Sens. Actuators, B 2000, 64, 175−181. (17) Ippolito, S. J.; Kandasamy, S.; Kalantar-zadeh, K.; Wlodarski, W. Hydrogen Sensing Characteristics of WO3 Thin Film Conductometric Sensors Activated by Pt and Au Catalysts. Sens. Actuators, B 2005, 108, 154−158. (18) Yang, F.; Donavan, K. C.; Kung, S.-C.; Penner, R. M. The Surface Scattering-based Detection of Hydrogen in Air Using a Platinum Nanowire. Nano Lett. 2012, 12, 2924−2930. (19) Patel, S. V.; Gland, J. L.; Schwank, J. W. Film Structure and Conductometric Hydrogen-Gas-Sensing Characteristics of Ultrathin Platinum Films. Langmuir 1999, 15, 3307−3311. (20) Baranowski, B. High Pressure Research on Palladium-Hydrogen Systems. Platinum Met. Rev. 1972, 16, 10−15. (21) Villanueva, L. G.; Fargier, F.; Kiefer, T.; Ramonda, M.; Brugger, J.; Favier, F. Highly Ordered Palladium Nanodot Patterns for Full Concentration Range Hydrogen Sensing. Nanoscale 2012, 4, 1964− 1967. (22) Kiefer, T.; Villanueva, G.; Brugger, J. Conduction in Rectangular Quasi-One-Dimensional and Two-Dimensional Random Resistor Networks Away From the Percolation Threshold. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2009, 80, 021104. (23) Kiefer, T.; Villanueva, L. G.; Fargier, F.; Favier, F.; Brugger, J. The Transition in Hydrogen Sensing Behavior in Noncontinuous Palladium Films. Appl. Phys. Lett. 2010, 97, 121911−121913.

(24) Xu, T.; Zach, M.; Xiao, Z.; Rosenmann, D.; Welp, U.; Kwok, W.; Crabtree, G. Self-Assembled Monolayer-Enhanced Hydrogen Sensing with Ultrathin Palladium Films. Appl. Phys. Lett. 2005, 86, 203104. (25) Ibañez, F. J.; Zamborini, F. P. Ozone-and Thermally Activated Films of Palladium Monolayer-Protected Clusters for Chemiresistive Hydrogen Sensing. Langmuir 2006, 22, 9789−9796. (26) Su, P.-G.; Chuang, Y.-S. Flexible H2 Sensors Fabricated by Layer-by-Layer Self-Assembly Thin Film of Multi-walled Carbon Nanotubes and Modified in Situ with Pd Nanoparticles. Sens. Actuators, B 2010, 145, 521−526. (27) Wang, L.; Luo, J.; Yin, J.; Zhang, H.; Wu, J.; Shi, X.; Crew, E.; Xu, Z.; Rendeng, Q.; Lu, S. Flexible Chemiresistor Sensors: Thin Film Assemblies of Nanoparticles on a Polyethylene Terephthalate Substrate. J. Mater. Chem. 2010, 20, 907−915. (28) Zasadzinski, J.; Viswanathan, R.; Madsen, L.; Garnaes, J.; Schwartz, D. Langmuir-Blodgett Films. Science 1994, 263, 1726−1738. (29) Chen, S. Two-Dimensional Crosslinked Nanoparticle Networks. Adv. Mater. 2000, 12, 186−189. (30) Greene, I. A.; Wu, F.; Zhang, J. Z.; Chen, S. Electronic Conductivity of Semiconductor Nanoparticle Monolayers at the Air/ Water Interface. J. Phys. Chem. B 2003, 107, 5733−5739. (31) Huang, S.; Minami, K.; Sakaue, H.; Shingubara, S.; Takahagi, T. Effects of the Surface Pressure on the Formation of Langmuir-Blodgett Monolayer of Nanoparticles. Langmuir 2004, 20, 2274−2276. (32) Pang, S.; Tetsuya, O.; Tomoyuki, W.; Kondo, T.; Kawai, T. Studies on 2D Hybrid Films of Half Surfactant-Covered Au Nanoparticles at the Air/Water Interface. J. Colloid Interface Sci. 2005, 285, 634−639. (33) Park, Y.-K.; Yoo, S.-H.; Park, S. Assembly of Highly Ordered Nanoparticle Monolayers at a Water/Hexane Interface. Langmuir 2007, 23, 10505−10510. (34) Turkevich, J.; Stevenson, P. C.; Hillier, J. A Study of the Nucleation and Growth Processes in the Synthesis of Colloidal Gold. Discuss. Faraday Soc. 1951, 11, 55−75. (35) Frens, G. Controlled nucleation for the regulation of the particle size in monodisperse gold suspensions. Nature 1973, 241, 20−22. (36) Daniel, M.-C.; Astruc, D. Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related Properties, and Applications Toward Biology, Catalysis, and Nanotechnology. Chem. Rev. 2004, 104, 293−346. (37) Gong, J. Structure and Surface Chemistry of Gold-Based Model Catalysts. Chem. Rev. 2011, 112, 2987−3054. (38) Hu, J.-W.; Zhang, Y.; Li, J.-F.; Liu, Z.; Ren, B.; Sun, S.-G.; Tian, Z.-Q.; Lian, T. Synthesis of Au@Pd Core−Shell Nanoparticles with Controllable Size and Their Application in Surface-Enhanced Raman Spectroscopy. Chem. Phys. Lett. 2005, 408, 354−359. (39) Ghosh, S. K.; Pal, T. Interparticle Coupling Effect on the Surface Plasmon Resonance of Gold Nanoparticles: from Theory to Applications. Chem. Rev. 2007, 107, 4797−4862. (40) Zeng, X. Q.; Latimer, M. L.; Xiao, Z. L.; Panuganti, S.; Welp, U.; Kwok, W. K.; Xu, T. Hydrogen Gas Sensing with Networks of Ultrathin Palladium Nanowires Formed on Filtration Membranes. Nano Lett. 2011, 11, 262−268. (41) Lewis, F. A. Palladium hydrogen system. Platinum Met. Rev. 1982, 26, 20−27. (42) Grashoff, G.; Pilkington, C.; Corti, C. The Purification of Hydrogen. Platinum Met. Rev. 1983, 27, 157−169. (43) Manchester, F.; San-Martin, A. Pitre, The H-Pd (HydrogenPalladium) System. J. Phase Equilib. 1994, 15, 62−83. (44) Mitsui, T.; Rose, M.; Fomin, E.; Ogletree, D.; Salmeron, M. Dissociative Hydrogen Adsorption on Palladium Requires Aggregates of Three or More Vacancies. Nature 2003, 422, 705−707. (45) Lopez, N.; Łodziana, Z.; Illas, F.; Salmeron, M. When Langmuir is too Simple: H2 Dissociation on Pd(111) at High Coverage. Phys. Rev. Lett. 2004, 93, 146103. (46) Sykes, E. C. H.; Fernández-Torres, L. C.; Nanayakkara, S. U.; Mantooth, B. A.; Nevin, R. M.; Weiss, P. S. Observation and Manipulation of Subsurface Hydride in Pd(111) and its Effect on 10138


Article

The Journal of Physical Chemistry C Surface Chemical, Physical, and Electronic Properties. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 17907−17911. (47) Johansson, M.; Skulason, E.; Nielsen, G.; Murphy, S.; Nielsen, R. M.; Chorkendorff, I. Hydrogen Adsorption on Palladium and Palladium Hydride at 1 bar. Surf. Sci. 2010, 604, 718−729. (48) Lee, E.; Lee, J. M.; Koo, J. H.; Lee, W.; Lee, T. Hysteresis Behavior of Electrical Resistance in Pd Thin Films During the Process of Absorption and Desorption of Hydrogen Gas. Int. J. Hydrogen Energy 2010, 35, 6984−6991. (49) Lewis, F. The Hydrides of Palladium and Palladium Alloys. Platinum Met. Rev. 1960, 4, 132−137. (50) Kirkpatrick, S. Percolation and Conduction. Rev. Mod. Phys. 1973, 45, 574. (51) Buhrman, R.; Granqvist, C. Log-normal Size Distributions from Magnetization Measurements on Small Superconducting Al Particles. J. Appl. Phys. 1976, 47, 2220−2222. (52) Stauffer, D. Scaling Theory of Percolation Clusters. Phys. Rep. 1979, 54, 1−74. (53) Masihi, M.; King, P. R.; Nurafza, P. Effect of Anisotropy on Finite-Size Scaling in Percolation Theory. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2006, 74, 042102. (54) Masihi, M.; King, P. R. Percolation Approach in Underground Reservoir Modeling. Water Ressources Management and Modeling; InTech Open Access Publisher, 2012; pp 263−287.

10139


Electronic and Mechanical Antagonist Effects in Resistive Hydrogen

Recommend Documents