Physiochemical properties of three-dimensional printed electrode (3DE)

The fabrication process for the 3DE utilizing the FDM 3D printing technique provides a time efficient and low-cost approach to mass producing electrode materials. Compared to commercial electrodes such as copper, aluminum, and carbon electrodes, the architecture and surface area of the 3DE can be easily customized to suit a particular application, as shown in Fig. 1a. The printing process for the 3DE was fully automated, with a high degree of precision (with 0.4 mm nozzle size), which made it possible to complete the printing process for eight 8 electrodes in just 30 min. Furthermore, using a conductive filament as the printing material can facilitate the application of 3DEs in electronic fabrications, which would allow the printed thermoplastic material to conduct electricity without any post-treatment. The detailed characterization of the Black Magic filament was not carried out because it has been available in the market as a commercial end product28,29. The graphene content in the commercial conductive filament (from Black Magic) is around 8%, and the chemical composition of the filament consists of just carbon and oxygen, according to the investigation of the physiochemical properties of the same conductive filament conducted by Foster et al.28. Although the 3DE could conduct electricity, the surface resistance was too high, which hindered its application in electronic devices. Therefore, a thin layer of gold was sputtered onto the surface of the 3DE to reduce the surface resistivity (Fig. 1b).

Figure 1 Physiochemical characterization. (a) Optical image of 3D printing process, (b) 3D printed electrode used throughout study. (c) FESEM image of 3DE/Au electrode, and (d) corresponding magnified cross-sectional area. Full size image

The surface uniformity of the gold sputtered 3DE (3DE/Au) was investigated via a field emission scanning electron microscope (FESEM) analysis. The surface of the 3DE/Au is shown in Fig. 1c, where it is clearly seen that the extruded filament is around 0.4 mm and printed in a uniform array. In the cross section of the 3DE/Au shown in Fig. 1d, a thin layer of gold can be noticed on the electrode surface with a lighter contrast. Furthermore, polymer nanowires and several crystalline binders were embedded within the Black Magic filament. The chemical compositions of the 3DE/Au and 3DE were determined via energy dispersive X-ray (EDX) analyses within ESI Figure S1, which shows that significant carbon and oxygen peaks predominate throughout the spectra. This indicates that the electrical conductivity of Black Magic filament was mainly contributed by the of organic-based graphene materials. The presence of gold on the 3DE/Au could also be detected in the EDX spectra.

Electrodeposition of Ppy/rGO nanocomposite on 3DE/Au electrode

To the best of our knowledge, this is the first time that 3DE and 3DE/Au were used as novel electrodes for the deposition of a Ppy/rGO nanocomposite via the electrodeposition method. From the FESEM images (Fig. 2a), we can clearly see that the surface of the 3DE/Au was fully covered with Ppy/rGO, with noticeable pores scattered across the electrode surface. Moreover, the Ppy nanoparticles were found to be distributed on the rGO surface through the magnified FESEM image (Fig. 2b). An EDX analysis was carried out to justify the presence of the Ppy/rGO nanocomposites on the 3DE/Au surface, as shown in Fig. 2c. It is clear that carbon, nitrogen, and oxygen are the most predominant peaks, indicating that the deposited nanocomposites were Ppy/rGO. Furthermore, an intense peak for Au was observed, indicating that a thin layer of Ppy/rGO nanocomposites was deposited on the 3DE/Au electrode, which allowed the gold from the electrode surface to be identified within a single spectrum.

Figure 2 FESEM image of (a) Ppy/rGO nanocomposite on the electrode surface and (b) its corresponding magnified image. Corresponding (c) EDX and (d) Raman analysis of Ppy/rGO nanocomposite. Full size image

The Ppy/rGO deposition process was initiated by the application of an oxidation potential (+0.8 V vs. Ag/AgCl), wherein the pyrrole monomers were electrochemically oxidized to start the polymerization. The neutral monomer was oxidized to form radical cations with a delocalized radical state and later formed a complex with the pTS anions through the electrostatic charge attraction. The coupling of the pyrrole monomers formed a larger dimer, which was then immediately reoxidised to form a cation. The propagation of the polymer chains continued as more newly form radical cations were introduced to the dimeric chain, and eventually formed the Ppy nanocomposites. Interestingly, the presence of GO with a negatively charge surface provided an alternative anchoring site for the pyrrole monomer. The GO was then subsequently reduced to rGO upon the release of electrons from the pyrrole radical cations. The free electrons released from the formation of radical cations reduced the GO into rGO, and the polypyrrole nanoparticles were attached on the rGO surface through π-π interaction, hydrogen bonding, and Van der Waal forces30,31.

To validate the interaction between the Ppy and rGO, Raman spectroscopy was carried out, and the spectra are shown in Fig. 2d. Two significant GO peaks can be observed at 1356.27 cm−1 and 1597.67 cm−1, which are assigned to the D-band and G-band, respectively. The D-band of GO is related to the surface defect of the hexagonal lattice, whereas the G-band corresponds to the sp2 carbon bond stretching32,33. For pure Ppy, the significant peaks observed at 1582.52 cm−1 and 1378.14 cm−1 correspond to the C=C backbone stretching and ring stretching mode of Ppy, respectively. Moreover, the C-H bond deformation peak can be found at 1056.50 cm−1 in the same spectrum. In addition, the two small peaks located at 971.40 cm−1 and 934.17 cm−1 correspond to the ring deformation of the radical cation (polaron) and bication (bipolaron), respectively34,35. The results of an evaluation of the significant peaks for the Ppy/rGO nanocomposites are represented in Table 1, and are compared to those for the GO and pure Ppy. Overall, it is clear that the characteristic peaks of pure Ppy can be found on the Raman spectrum of the Ppy/rGO nanocomposites, which verified the presence of Ppy within the nanocomposites. Moreover, by taking into account the D/G band ratio (I D /I G ), the atomic sp3/sp2 carbon ratio can be calculated to measure the graphitic disorder of the GO material36. Interestingly, the calculated I D /I G ratio for the Ppy/rGO nanocomposites (0.80) was lower than that for pure GO (0.99), which showed that the graphitic disorder of the Ppy/rGO was less than that of pure GO. In addition, the shifting of the G-band from 1597.67 cm−1 to 1575.40 cm−1 further proved that the GO was rGO during the electrodeposition process37,38.

Table 1 Raman analysis of GO, pure Ppy and ppy/rGO nanocomposites. Full size table

3DE/Au based solid-state supercapacitor

The electrical function of the 3DE made using different materials was shown in ESI Figure S2. Ppy/rGO nanocomposite was electrodeposited on top of the 3DE made by Black Magic filament and ordinary PLA filament. Due to the electrical insulator nature of ordinary PLA filament, a thin layer of gold was sputtered on top of the 3DE made from PLA filaments before electrodeposition. Since the ordinary PLA filament has no electrical conductivity, the Ppy/rGO was deposited only on the thin layer of sputtered gold on the electrode surface. The CV profile of the 3DE made by ordinary PLA filament was fully contributed by the Ppy/rGO on the thin gold layer. Due to the limited reactive surface area for active material deposition, the CV performance of 3DE made by ordinary PLA filament was very small. On the other hand, the area under the CV curve of the 3DE made from Black Magic filament was significantly larger than the 3DE made from ordinary PLA filament. This is because the conductive nature of the Black Magic filament increases the reactive surface area of the 3DE for electrodeposition of Ppy/rGO nanocomposite.

A fully freestanding supercapacitor was developed utilizing two Ppy/rGO decorated 3DE/Au electrodes, with a PVA-KOH gel electrolyte sandwiched between them, as shown in Fig. 3. The potential of this novel 3DE/Au electrode was evaluated through the electrochemical performance of the as-fabricated solid-state supercapacitor. For comparison, Ppy/rGO decorated 3DE electrodes were also tested under the same procedure. After the creation of the solid-state supercapacitors, cyclic voltammetric (CV) analyses were carried out over a potential range of +0.0 V to 1.0 V, at a scan rate of 50 mV s−1. The CV profile (Fig. 4a) shows the general capacitive property of the supercapacitor, and the area under the curve is indicative of the capacitance of the system. Overall, the 3DE/Au-based supercapacitor had a larger current response than the 3DE, because the presence of the gold layer facilitated the electrical conductivity and ionic transport during the operation of the charge storage mechanism. Next, the galvanostatic charge/discharge (GCD) cycling of the solid-state supercapacitors was carried out to evaluate their specific capacitance, C sp . A typical shark fin GCD profile was found for the 3DE/Au-based supercapacitor (Fig. 4b), indicating good charge/discharge properties with a minor IR drop. The specific capacitance was calculated to be 98.37 Fg−1 at a current density of 0.5 Ag−1, which was approximately three times that of the 3DE-based supercapacitor (32.46 Fg−1). The Nyquist plots for both supercapacitors in a frequency range of 0.1 Hz to 300 kHz are illustrated in Fig. 4c. A small semicircle can be noticed in the high-frequency region of the 3DE/Au-based supercapacitor, indicating a small charge transfer resistance (R ct ) at the electrode/electrolyte interface. Furthermore, the equivalent series resistance (ESR), including the contact resistance at the active/current collector interface, ionic resistance of the electrolyte, and intrinsic resistance of the substrate, could be deduced from the intercept at the real axis (Z re ). The presence of the gold layer reduced the R ct value of the 3DE from 15.4 Ω to 1.3 Ω, as well as the ESR value (from 27.5 Ω to 24.7 Ω). These EIS results further demonstrated the enhanced electrical conductivity and ionic mobility of the 3DE/Au-based supercapacitor. In addition, the 3DE/Au-based supercapacitor exhibited good cycling stability (Fig. 4d) over the first 100 charge/discharge cycles (a capacitance retention of 81.94%). However, because of the organic-based 3DE/Au electrodes and evaporation of the electrolyte from the PVA-KOH gel matrix, the capacitance retention drastically dropped to 12.15% after 1000 charge and discharge cycles. The Ppy/rGO layer on the 3DE/Au surface shriveled (ESI Figure S3) due to insufficient electrolyte support from the PVA-KOH gel layer. Consequently, the overall reactive surface area of the electrode was greatly reduced, resulting in poor capacitance performances.

Figure 3 Schematic illustration of solid-state supercapacitor fabrication. Full size image

Figure 4 Electrochemical performance of 3DE/Au-based supercapacitor. (a) Cyclic voltammogram analysis over 1.0 V potential range at scan rate of 50 mV s−1, (b) corresponding galvanostatic charge/discharge profile at current density of 0.5 Ag−1, (c) Nyquist plot, and (d) cyclic stability profile of as-fabricated supercapacitor. Full size image

3DE/Au-based photoelectrochemical sensor utilizing cadmium sulfide nanoparticles

The application of 2D and 3D nanomaterials in electronic applications has received much interest from a profusion of material chemists studying the exploration and exploitation of their distinctive characteristics. We next discuss how this 3DE/Au electrode was utilized for the photoelectrochemical (PEC) sensing of copper ions using cadmium sulfide as an active semiconductor39,40. The most common current collector for a PEC sensor is indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO) glass25,41. These optically transparent electrode materials provide high optical transmittance values, which allow a greater photoelectrical response compared to a glassy carbon electrode (GCE)42. As outlined in the introduction, one of the objective of this research was to find a possible substitute for the currently available electrode materials. We therefore investigated the potential application of the 3DE/Au as an electrode material for a PEC sensor with the aim of providing important insights into its photoelectrochemical properties.

Initially, the cadmium sulfide nanoparticles were synthesized through a facile solvothermal method with cadmium acetate and thiourea as the precursor reagents. Upon application, Nafion was mixed with the as-synthesized CdS nanoparticles as a binder to produce a yellowish paste. The paste was then applied to the surfaces of both the 3DE/Au and 3DE electrodes to form PEC sensors (Fig. 5). The FESEM images of the electrode surfaces are shown in Fig. 6a. The high mechanical stability of Nafion acted as an excellent binder to hold the CdS nanoparticles on the electrode surface43. The small cluster of CdS nanoparticles found on the magnified electrode surface (Fig. 6b) was a result of the self-agglomeration of the nanoparticles due to their high surface energy nature44. However, this phenomenon had no effect on the chemical composition or performance of the as-fabricated PEC sensor. This could be verified by the significant profile from the EDX and Raman analyses (Fig. 6c,d). Despite the electrodeposition of the Ppy/rGO nanocomposite, the thickness of the CdS layer was not easily manipulated by the doctor blade application. Therefore, only Cd and S peaks could be observed to be dominant in the EDX profile, with a small noticeable peak of C from the Nafion binder. This also showed that the nanoparticles deposited on the electrode surface were purely CdS with no contamination. The Raman analysis further showed the purity of the CdS nanoparticles, which had two distinctive peaks at 299.4 cm−1 and 601.1 cm−1, corresponding to the first-order (1LO) and second-order (2LO) longitudinal optical phonon modes of CdS, respectively27.

Figure 5 Schematic illustration of PEC sensor using 3DE/Au electrode. Full size image

Figure 6 Characterization of CdS nanoparticles. (a) FESEM image and (b) magnified image of CdS deposited 3DE/Au electrode surface. Corresponding (c) EDX and (d) Raman analysis of the CdS nanoparticles. Full size image

Photoelectrochemical sensing of copper ion

The photoelectrochemical properties of the 3DE/Au- and 3DE-based PEC sensors were evaluated prior to their application as a copper sensing platform. Surprisingly, the PEC performance of the as-fabricated sensor showed a remarkably good result in response to simulated light. Although the electrode material had an opaque structure, the conductive nature of the Black Magic filament and Au layer made it possible to transport the photo-excited electrons from the CdS nanoparticles and convert them into an electrical signal. As shown in Fig. 7a, the photocurrent intensity of the 3DE/Au-based PEC is apparently higher than that of the 3DE-based sensor, with values of 724.1 μA and 309.1 μA, respectively. The photocurrent generated was tremendously high compared to those of the previously reported CdS-modified electrodes27 and was reproducible after several on-off light illumination cycles. In addition, the presence of the Au layer in the 3DE/Au electrode enhanced the photocurrent stability throughout the illumination cycles, as shown in ESI Figure S4. Upon light illumination, the photo-excited electrons transferred from the valance band (VB) to the conduction band (CB) of the CdS nanoparticles, and then to the current collector. Because of the highly conductive Au coating on the 3DE/Au surface, the electron mobility was greatly enhanced and provided a rapid and stable photocurrent response compared to the bare 3DE. The high and stable photocurrent response of the 3DE/Au electrode could thus provide a possible alternative to an ITO/FTO glass electrode in a PEC sensing platform.

Figure 7 Photoelectrochemical performance. (a) Time-based photocurrent response of 3DE and 3DE/Au-based PEC sensor, measured in KCl:TEA electrolyte at bias potential of 0.1 V. (b) Effect of Cu2+ on the photocurrent response of 3DE/Au-based PEC sensor as concentration increases from 0.01 μM to 80 μM. (c) Linear relationship between the Cu2+ concentration and photocurrent response, where I 0 and I represent photocurrent intensities without and with the presence of Cu2+, respectively. Full size image