Article pubs.acs.org/JPCC
Effect of Humidity on Electrical Conductivity of Pristine and Nanoparticle-Loaded Hydrogel Nanomembranes Musammir Khan, Swen Schuster, and Michael Zharnikov* Angewandte Physikalische Chemie, Universität Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany ABSTRACT: We studied here the effect of humidity on the electrical conductivity of pristine and nanoparticle-loaded hydrogel nanomembranes. The membranes were fabricated by the thermally activated cross-linking of amine- and epoxy-terminated, starbranched poly(ethylene glycol) (PEG) oligomers. The resistance of the pristine membrane changed by ∼5.5 orders of magnitude upon relative humidity (RH) variation from 0% to 100%, which is an unprecedented response for homogeneous materials. The dependence of the resistance on the moisture uptake into the membrane could be coarsely described by an exponential function. The loading of the membranes with gold and silver nanoparticles (NPs) resulted in a noticeable improvement of their conductance at low RH but in a small improvement or even a negative effect on the conductance at high RH. Both pristine and NP-loaded PEG hydrogel membranes have significant potential as highly sensitive elements in humidity sensors and moistureresponsive nanoelectronic devices.
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INTRODUCTION There is a continuous demand for light, small, and flexible assemblies based on advanced inorganic and organic materials for different applications as sensors and stimuli-responsive systems. A particularly important area is humidity-sensitive systems, which are of significance in various fields, such as medicine, agriculture, industry, goods storage, and environmental monitoring. In this context, different materials including but not limited to metal oxides, ceramics, and polymers have been used, relying on different kinds of transduction techniques, such as capacitive, resistive, hygrometric, and gravimetric ones.1,2 Since recently, nanostructured materials and organic/inorganic hybrid systems are utilized because of their superior performance in comparison to macroscopic onecomponent systems. Among nanostructured materials, monoclinic VO2 nanostructures3 and VS2 ultrathin nanosheets4 can be mentioned, showing particularly high sensitivity at a relative humidity (RH) above 50%.4 As representative organic/ inorganic hybrid systems, tin oxide nanoparticle (NP) loaded cellulose,5 Li-loaded nanoporous polymers,6 multilayer graphene oxide/polyelectrolyte nanocomposite films,7 multiwalled carbon nanotubes dispersed in cross-linked polyelectrolyte,8 and LiCl−polymer composite nanofibers9 can be listed, showing quite promising performance at high humidity5,7 but also in the entire RH range.7−9 Among organic materials used in humidity-responsive hybrid assemblies, hydrogels are in particular attractive because of their intrinsic ability to absorb moisture, affecting their mechanical, optical, and electrical properties.10−12 Some of the recently developed humidity-sensitive systems include superabsorbent poly(acrylamide)−montmorillonite composite hydrogels,13 fluophore-loaded acrylamide,12 hydrogel-actuated nanorod assembly,11 and conductive polymer hydrogel cross-linked by phytic acid in poly(N-isopropylacrylamide) matrix.14 Most of © 2015 American Chemical Society
the above assemblies are, however, complex, requiring in some cases also a quite sophisticated preparation procedure. In addition, some of these systems have only a limited potential for miniaturization, which is indispensible for the fabrication of nanoscale humidity-sensing and responsive devices. In this context, ultrathin hydrogel films can be of potential interest as far as one succeeds to cross-link individual chains sufficiently, providing a stable, elastic network that is able to swell when hydrated. This process can be mediated by different means but mostly involves exposure to UV light15−17 or a predefined chemical coupling,18−21 requiring a custom modification or design of the monomers and/or addition of further chemicals or cross-linking agents. This works efficiently for macroscopic hydrogels and micrometer thin films but is less established for nanometer thin films, where problems of homogeneity, stability, and precise thickness control become crucially significant. In this context, we have suggested recently an alternative, simple, and reliable procedure, relying on thermally driven cross-linking of hydrophilic oligomers terminated by complementary active groups.22,23 Amine- and epoxy-terminated star-branched poly(ethylene glycols) (STARPEGs) were selected as such oligomers. The controlled crosslinking of these oligomers results in a well-defined hydrogel film with a tunable thickness of 4−350 nm.22 An additional advantage of such films is that they can be easily separated from the substrate and exist as ultrathin membranes,24,25 which allows their subsequent placement into a suitable “device” as a humidity-responsive or sensing element. This can be combined, if necessary, with patterning of the membranes, providing a predefined lateral shape of their sensing part.23 Received: April 14, 2015 Revised: May 26, 2015 Published: June 1, 2015 14427
DOI: 10.1021/acs.jpcc.5b03572 J. Phys. Chem. C 2015, 119, 14427−14433
Article
The Journal of Physical Chemistry C
AgNPs were synthesized via the reduction of silver nitrate by a combination of two reducing agents, viz., sodium citrate and tannic acid.31,32 The loading of the PHMs with NPs was performed by their immersion into a NP solution for different intervals of time, viz., 1, 2, and 6 h. The NP-loaded PHMs were subsequently rinsed with water and dried overnight. Morphology. Morphology of the composite PHMs was analyzed by SEM using an LEO 1530 Gemini microscope (Zeiss). The energy of the primary electron beam was set to 5 keV; the pressure was better than 10−5 mbar. UV−Vis Spectroscopy. The UV−vis absorption spectra of the composite PHMs were recorded with a grating spectrometer (HR2000, Ocean Optics) using a deuterium− tungsten halogen lamp (DH-2000-BAL, Micropack GmbH). Ellipsometry under RH Variation. The thickness of the pristine PHM was measured as a function of RH by a spectroscopic ellipsometer (M-44, J. A. Woollam) at a fixed angle of 75°. RH was controlled in a dynamical fashion, by passing nitrogen gas flow through two parallel-adjusted PYREX gas washing bottles filled with pure water and concentrated sulfuric acid (a slightly modified protocol of ref 4). The gas flow between these bottles was precisely controlled by three-way valves (Rotilabo-tubing valves), leading to certain RH values. The RH could be varied reliably from 15% to 80%. It was, however, not possible to maintain stable RH beyond this range because of the turbulent nature of the flowing air. The RH was monitored by a commercially available hygrometer (BL-20 TRH thermo/hygrometer, an accuracy of ±3.5%). The measurements were repeated several times at each RH value; the results were well-reproducible. The experiment was also performed at the ambient conditions, corresponding to 40− 50% RH. Conductivity Measurements. The conductivity of pristine and NP-loaded PHMs was measured using a custom-made, four-probe arrangement connected to SourceMeter Channel A (Keithley 2635A) by triax cables. The current was applied in the outer circuit, while the inner circuit gave the voltage output. A custom-designed program, generated in Test-script builder “TSP” software, was used. The four-probe tool consisted of four gold electrodes with a flat contact area of 1 mm attached to the spring contacts and arranged in a straight line at an equal distance of ∼1.6 mm from each other on an insulating ebonite holder. The probes touched the surface of PHMs, providing electrical contacts. In the case of NP-loaded PHMs, this arrangement was advantageous because of having a direct contact with the layer of the embedded NPs located predominantly in the topmost part of the membranes.22 The spring contact nature of the probes allowed enough flexibility for the membranes during the swelling process. Moreover, the flat end of the probes facilitated a proper contact with the sample surface and prevented the membranes from damage. A current in the range of 1 pA to 100 nA was applied, and the potential drop across the middle two probes was measured. The resistance was determined from the slope of the voltage vs current plot. Generally, four successive scans were performed for each sample, viz., from negative to positive, from positive to negative, from zero to negative, and from zero to positive current. A typical plot of such scans is shown in Figure 2. All these scans exhibited very similar voltage values at a certain current, which underlines the reliability of the method and lack of hysteresis. For consistency, we usually relied on the “negative to positive” scan to get a resistance value for a particular sample and RH setting, repeating the scan at least four times and
In view of these exceptional properties and potential applications as humidity-responsive elements, as described above, we studied here the electrical transport properties of the STAR-PEG hydrogel films (PHFs) transferred as PEG hydrogel membranes (PHMs) on a nonconductive substrate. Along with the pristine membranes, PHMs loaded with gold and silver NPs (denoted below as AuNPs and AgNPs, respectively) were also investigated, in view of the potentially promising properties of NP−hydrogel composites.26−29 These NPs could be imbedded in the PHM matrix using a simple immersion procedure,22 with the density controlled by the immersion time. The composite membranes were analyzed by scanning electron microscopy (SEM) and UV−vis spectroscopy, whereas the conductivity of both pristine and NP-loaded PHMs was investigated by a fourprobe method as a function of RH.
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EXPERIMENTAL SECTION Preparation of PHFs and PHMs. Amine- and epoxyterminated, four-arm STAR-PEGs were purchased from Creative PEGWorks. Both components were separately dissolved in chloroform at a concentration of 6 mg/mL. An equally concentrated 1:1 mixture of these compounds was spincoated (4000 rpm) on a 100 nm polycrystalline Au(111) substrate (Georg-Albert-PVD) that was preliminary cleaned under an ozone-producing UV lamp for 15 min and rinsed with ethanol. The cross-linking was performed at 80 °C for 6 h, under argon protection (Figure 1), following the established
Figure 1. Preparation and structure of cross-linked PHFs. See the text for details.
procedure.22 The resulting PHFs were cleaned by ultrasonication, separated from the primary support, and transferred to a cleaned glass substrate for characterization and conductivity measurements, following the established procedure.24 The thickness of PHMs in the dry state was estimated to be ∼36 nm by ellipsometry (see below). NP Synthesis and Loading. Monodisperse, citratestabilized AuNPs with quasi-spherical shape were prepared by following the kinetically controlled strategy via the reduction of HAuCl4 by sodium citrate.30 Sodium citrate-stabilized, spherical 14428
DOI: 10.1021/acs.jpcc.5b03572 J. Phys. Chem. C 2015, 119, 14427−14433
Article
The Journal of Physical Chemistry C
indicates that varying RH value has a prominent impact on the membrane thickness, owing to the hydrophilic nature of the hydrogel. The thickness increased continuously with increasing RH. When the RH was increased from 15% to 80%, the membrane thickness was almost doubled, which showed that it swelled up to twice its original volume. Such a swelling is typical for hydrogel films and membranes and was, in particular, observed for a porous membrane of PEG-diacrylate and polyurethane.33 Pristine PHM: Conductivity. The variation in the resistance of pristine PHM measured as a function of RH is compiled in Table 1; the respective plot is given below, in the section describing the conductivity of the NP-loaded membranes. The resistance of the pristine membrane at 0% RH is quite high, amounting to ∼1.6 TΩ. Such a high value is understandable in view of the insulator character of the STARPEG precursors. The resistance decreases, however, by a factor of 100 at 50% RH, by a factor of 1000 at 70% RH, and by ∼5.5 orders of magnitude at 100% RH. This is an unprecedented response for a homogeneous material, which is comparable to the highest values reported so far for complex hybrid assemblies such as a humidity sensor based on multilayer graphene oxide/ polyelectrolyte nanocomposite films, viz., a factor of 2.65 × 105 in capacity response upon RH variation over the entire range.7 We speculate that the major reason for such a high response in the present case is a nanometer scale thickness of the hydrogel membrane, associated with the very high surface-to-volume ratio. Note that, generally, the charge carrier transport mechanism in hydrogels at low RH (
