3.1. SnO2-rGO Composition and Manostructure
As mentioned, the solvothermal process to synthesize SnO
2-rGO composites was previously reported in the literature [
15]. When attempts were made to replicate this process in this work, however, only a modest loading of SnO
2 on the rGO was achievable. Films made from this material had a high resistance (~1 MΩ) and ca. 25% less response when compared with other strategies reported below. In a first attempt to improve the material homogeneity and particle loading, the proportion of reagents used for the SnO
2 synthesis (SnCl
2, sodium acetate, and sodium citrate) relative to the graphene oxide was doubled. This caused the formation of SnO
2 nanoparticles that were not attached to the rGO sheets, which was attributed to the rapid homogenous nucleation of SnO
2 at the beginning of the reaction. To avoid this situation, a two-step synthesis was employed that included the vigorous stirring of the reagents in solution at 60 to 70 °C prior to the higher temperature second reaction stage. This approach yielded better results in terms of SnO
2 loading, dispersion, and nucleation onto graphene sheets, as determined by TEM. After coating with inks made with the reaction product, the deposited films were annealed to further reduce the graphene oxide. The efficiency of this annealing is indicated by the XPS data in
Table 1. The high reduction rate of the graphene oxides contributed to achieving the room-temperature working/measuring environment. Additionally, the conductivity of the sensing materials was increased for films that were annealed, and much thinner films could be used without reaching the limit of electric resistance of our measuring system (~10
6 Ω), which hence resulted in an improved gas response.
XRD was used to characterize the structural composition of the SnO
2-rGO nanocomposite material. In
Figure 1, four major SnO
2 peaks can be seen in the XRD diffraction spectra centered at 2θ values around 27° (110), 33° (101), 52° (211), and 65° (112), indicating the successful of syntheses of the rutile-structured SnO
2 [
19,
20]. As expected, there was no visible diffraction peak from graphite at 10.4°, indicating an effective GO reduction and concomitant graphene sheet dispersion [
20,
21]. Moreover, the dispersion was enhanced by the deposition of nano-scaled SnO
2 crystals. It effectively suppressed the restacking of graphene layers. The latter is highly desirable when using graphene oxide-based materials in sensors because of the improved response afforded by the higher surface area/volume ratios [
15]. Such improved porosity is confirmed by the optical characterization that follows.
The SEM picture in
Figure 2A indicates that the microstructure of the resulting SnO
2-rGO is composed of an interconnected 3D network. Similar microstructures were observed previously for modified SnO
2-rGO derived from a freeze-drying process [
15]. Interestingly, we note from
Figure 2A that the material can preserve this microstructure even after being milled and re-dispersed in NMP during the ink preparation protocol used in this work. In the SEM picture in
Figure 2B and the TEM pictures in
Figure 2C,D higher magnification images of the same sample are shown. In these images, semi-spherical SnO
2 nanoparticles can be seen decorating the surface of the rGO sheets. Such nanoscale deposits appear as white spots in the SEM image and as black dots in the TEM pictures. Their distribution on the surface of the reduced GO is more uniform in
Figure 2D, where a two-step synthesis procedure was used, than in
Figure 2C, where a one-step synthesis was conducted. The compositional uniformity was confirmed by EDX measurements taken in parallel with the SEM characterization (
Figure 3). The spectrum in
Figure 3A, obtained from the area indicated in
Figure 3B, shows that C, O, and Sn are the major constituents of the material, as expected. Moreover, when Sn and C are marked, respectively, in red and green in a SEM-EDX map (
Figure 3B), it is clearly seen that the SnO
2 deposits are, in fact, well dispersed over the surface of the rGO sheets. Such improved loading and dispersion of the nanoscale deposited were obtained only with the two-step synthesis protocol.
High-resolution X-ray photoelectron spectroscopy (XPS) spectra were collected to determine the nature of the chemical bonding in the SnO
2-rGO nanocomposite material. Regions of the spectra for elements of interest were fitted and deconvolved.
Figure 4A shows that the C1s peak between 283 eV and 292 eV possesses contributions from three separate peaks, indicating the presence of C atoms involved in carboxyl, ether-like C-S, and C-C bindings, respectively. The O1s peak between 529 and 536 eV in
Figure 4B in turn indicates the presence of carbonate and hydroxide groups and metal oxide bonds. Finally, the Sn3d region of the spectrum in
Figure 4C displays two peaks centered at 487–488 eV and 496 eV, which are attributed to Sn 3d electrons with 5/2 and 3/2 spins, respectively, and are typical for SnO
2-graphene composites [
22].
All the XPS peaks and assignments in
Figure 4 were previously described in literature [
23] and afford a clear idea of the atomic bonding in the material. The oxygen peaks in
Figure 4B indicate both that SnO
2 has its chemical structure preserved, since metal oxide bonds are seen, and that graphene oxide is in fact not completely reduced in the conditions used, since carboxyl, hydroxyl, and ether groups are present [
23]. Moreover, from the absence of Sn-C around 192 eV in
Figure 4C, we conclude that SnO
2 is linked to rGO mainly through bonds between Sn and O atoms in the graphene structure and possibly through some oxygen (from SnO
2) to carbon bonds. The latter is advantageous for sensing, since Sn does not form direct bonds with C and is available to interact with CO. The presence of C-S links in
Figure 4A (the second peak in the spectrum) suggests the existence of some residual sulfuric acid (H
2SO
4), probably originating from the autoclave used during the synthesis (H
2SO
4 was used to wash the liners in the oven between experiments) or from synthesizing GO using Hummer’s method [
24]. The content of the sulfur elements presenting in the SnO
2/rGO samples is only 0.606%. All other chemical links between the different atoms in the structure are as expected.
XPS was also used to characterize the extent of graphene oxide reduction under different conditions. The carbon-to-oxygen (C:O) atomic ratios extracted from spectra from different samples (submitted to different treatments) are shown in
Table 1. From the comparison of GO (graphene oxide—line 1) with the samples SnO
2/r-GO one-step (line 3) and SnO
2/r-GO two-step (line 4), we can clearly see that GO reduction is concomitantly happening during the solvothermal process along with SnO
2 nanoparticle synthesis, since the C:O ratio for graphene oxide was increased from 2.4 to 5.0 (SnO
2/r-GO one-step) or 12.1 (SnO
2/r-GO two-step). Such an effect from reaction conditions is confirmed by a control sample (r-GO—line 2) in which GO was subjected to the same protocol used for the one-step synthesis of SnO
2/r-GO, but without SnCl
2 added to the medium. In this case, the C:O proportion rose to a similar level (5.5) as for the SnO
2/r-GO one-step.
The beneficial effect of the two-step solvothermal synthesis of SnO
2-rGO is also confirmed by the data in
Table 1. The SnO
2/r-GO two-step (line 4) presents a significantly higher C:O ratio than SnO
2/r-GO one-step (line 3), indicating that the additional stirring step not only improved the distribution of SnO
2 (see discussions for
Figure 2 and
Figure 3), but also facilitated the reduction of graphene. The SnO
2/rGO one-step samples results correspond to
Figure 2C and the SnO
2/rGO two-step sample results correspond to
Figure 2D. Additional post-treatment methods performed at 120 °C for 12 h or at 200 °C for 30 min were also used to improve the chemical reduction of GO. We can clearly see that 200 °C for 30 min allowed for a higher extent of graphene reduction, with the C:O ratio increasing to 16.6. Although such treatment is incompatible with one of the substrates tested here (PET–with an effective maximum process temperature of approximately 150 °C), it is suitable for sensor fabrication on polyimide (Kapton) films, and the shorter processing time is attractive for the overall fabrication protocol.
3.2. Inkjet-Printed Electrodes and Active Layer
The printing and coating techniques were used in this work to fabricate an electronic sensor for CO mainly due to their high-throughput and the possibility of making a device through a large-area scalable process [
6]. Interdigitated electrode structures (IDE) were used, since they afford an increased active area for relatively small resistance-based devices. In this case, inkjet is an appropriate printing method for allowing the high-resolution printing of flexible metal traces using commercially available nanoparticle-based inks [
25]. Arrays of high-resolution, flexible Ag IDEs were printed on PET and Kapton, with an example shown in
Figure 5A. The line-width and spacing for the digits were both around 100 µm, the thickness was between 300 and 400 nm, and the resistivity after sintering was 3.52 × 10
−5 Ω·cm (22.1% of pure Ag).
Inkjet printing was also explored for depositing the active layer of the devices. The SnO
2-rGO material was re-dispersed in NMP, producing a dark-blue dispersion with a viscosity of 1.93 cP and a surface tension of 38.6 mN/m that was stable for days. Although we were not able to filter this dispersion through 0.45 µm syringe filters, the same could be effectively jetted using 10 pL cartridges in the Dimatix printer at a frequency of 10 kHz, using a firing voltage of 16 V and the waveform shown in
Figure 5B. Additionally, shown in
Figure 5B is a stroboscopic optical image of a droplet ejected from one of the nozzles. The reproducible droplet distance from the nozzle in the stroboscopic imaging and the absence of “tailing” or satellite droplets indicates stable jetting at the target speed of 5–7 m/s, which is necessary for high fidelity electrode patterning.
A number of other parameters also had to be optimized for inkjet-printing a continuous layer of SnO
2-rGO on PET and Kapton. Initially, severe dewetting was observed during the printing of NMP dispersions, and this was mostly mitigated by the introduction of a surface treatment step with UV-Ozone for 5 min. A picture of a single layer of SnO
2-rGO printed on PET is shown in
Figure 5C—top. Similar films with one or two layers of SnO
2-rGO were also deposited on top of the active area of the IDEs previously printed (
Figure 5C—bottom). The thicknesses of such films, probably under 100 nm per layer, could not be reproducibly measured with a stylus-based profilometer. Thicker films with up to 10 layers were also deposited, but their homogeneity was dramatically reduced in comparison with the thinner ones. For the reason mentioned above, inkjet-printing is considered sub-optimal for the deposition of the active layer, and further optimizations would be necessary for that. Therefore, slot-die coating was explored to fabricate the SnO
2-rGO active layer.
3.4. CO Sensing
The flexible gas sensors fabricated here were tested against a concentration of 50 ppm of CO (balanced with N
2). The amount of 50 ppm CO is a threshold limit for the intoxication of humans [
26]. The response (S) values reported are defined as the decrease in resistance (initial film resistance, R
0, minus film resistance under CO exposure, R
f) divided by the original resistance, R
0 (
). Moreover, to facilitate comparisons, the sensing curves (shown in
Figure 7) were further normalized to R
0. Other important characteristics of the sensors are their response and recovery times, measured by the speed with which resistance readings transition between equilibrium values (within 10% of those), measured with or without exposure to CO.
The individual sensors with inkjet-printed active layers showed a relatively unstable response to CO. The inkjet-printed sensors were highly resistive when just a few layers (one or two) of SnO2-rGO were deposited for the active layer, and this high resistance was maintained even after thermal post-treatment. With an increased number of overprinted SnO2-rGO layers (ca. 10) in conjunction with sintering at 120 °C for 12 h, the resistance was lowered to ~10.2 MΩ. The film morphology, however, was inhomogeneous and the detection curves very irreproducible, with a noted response to temperature and humidity. New formulations based on different solvents and including additives are currently being explored in order to fabricate higher-performance, all-inkjet-printed sensors. Below, sensors with active layers fabricated by slot-die coating are discussed.
Figure 7A shows a sensing curve for an all-printed device in which the active layer was deposited by slot-die coating. A significant change in the resistance of the film was observed with exposure to CO, and this change could be repeatedly reversed by a subsequent exposure to N
2. The normalized response in this case was ~15%, which is higher than for similar non-printed CO sensors reported in the literature [
4], and the response and recovery times of 4.5 s and 12 s were observed. The response was reduced (ca. 12%) for slot-die-coated devices in which SnO
2-rGO material synthesized in a one-step process was used (results not shown). This result indicates that our additional synthesis step, known to promote the better loading, dispersion, and nucleation of SnO
2 nanodeposits onto graphene sheets (see
Section 3.1), also results in improved sensing properties. The sensing performance of the drop-cast devices, SnO
2/r-GO one-step slot-die-coated devices, and SnO
2/r-GO two-step slot-die-coated devices are presented in
Table 2.
The specificity of the all-printed sensor to CO was demonstrated primarily through a control measurement in which the device was tested in presence of H
2 (
Figure 7B). Upon H
2 exposure, a much smaller variation of resistance (ca. 4%) was recorded, demonstrating the higher reactivity of CO molecules towards SnO
2. A test for CO with a sensor having only rGO in the active layer (no SnO
2) showed sensitivities of only 2−4% (results not shown). The specificity was further attested to by additional sensor tests against pure N
2, CO
2, and air, where <1% variations in film resistance were observed. Altogether, these results are consistent with a sensing mechanism where CO molecules are first adsorbed on the surface of the SnO
2-rGO material and then oxidized to CO
2 through a reaction that consumes electrons and oxygen [
3]. This leads to an increase in the carrier concentration in graphene and a measurable change in resistance. Such a reaction is facilitated and can happen at room temperature due to the high surface area of junctions formed between the graphene molecules and SnO
2 [
13,
15].
In one last control measurement, the sensing response for CO was also measured for a sensor in which the active layer was deposited by drop-casting instead of slot-die coating (
Figure 7C). The response, response time, and recovery time for this device are, respectively, 7%, 6 s, and 14 s. This significantly lower performance is attributed to differences in thickness between the coated and cast devices. The cast device is six times thicker than the coated device, resulting in a reduced volume-to-surface area ratio and longer diffusion distances for CO to permeate the film. The fact that thinner film active layers result in a higher sensing response is also reported in literature [
27].