1. Introduction

Ruthenium (Ru)-based materials such as ruthenium oxide (RuO₂) and metallic Ru have been widely utilized in photocatalysis, photovoltaics, fuel cells and battery applications. RuO₂ has been considered as one of the most efficient photocatalysts used in oxygen evolution reactions (OER) in photoelectrochemical (PEC) water oxidation [1]. In dye-sensitized solar cells, RuO₂ and Ru substitute for the conventional and expensive platinum (Pt) counter electrode by successfully reducing triiodide ions (I₃^-) to iodide ions (I^-) [2, 3]. The alloy compound of Pt and Ru enhances the electrocatalytic oxidation of methanol and formic acid [4]. In metal-air batteries, RuO₂ and Ru remarkably reduce the overpotential during the discharge and charge processes, leading to a higher cyclability and durability of the battery devices [5–7].

In the photo- and electrocatalytic reaction mechanism involving the charge carrier generation, the integration of carbons such as graphite, graphene and carbon nanotubes (CNTs) with catalysts is inevitable to lower the resistance at the interfaces and to improve the charge carrier mobility [8–11]. In addition, carbonaceous materials can modify the optical properties of catalysts [12] and participate in the electrocatalysis by itself [13]. Chen et al. demonstrated that the growth of N-doped CNTs on the surface of LaNiO₃ via the chemical vapor deposition (CVD) method promoted the oxygen reduction reaction (ORR) activity of LaNiO₃ by 5.8-fold in terms of current density during the half-cell test in 0.1 M KOH [10]. After 75 cycles of Zn-O₂ battery operation, the device containing LaNiO₃ and N-doped CNT core-corona structures required 0.19 V and 0.52 V less overpotentials during discharge and charge, respectively compared with LaNiO₃ only. It suggests that combining photo- or electroactive materials with a carbon moiety in one entity can trigger a synergistic effect in both catalytic activity and stability.

The catalytic properties are also dependent on the configuration as well as the designing of synthetic protocols of a material. In this respect, we suggest a new concept to embed the hybrid materials of Ru and CNTs which are coupled via plasma-enhanced chemical vapor deposition (PECVD) in the structure of inverse opals (IOs). Based on the interconnected periodic pores, three-dimensionality in network and structural robustness, the utilization of IO structures has a positive impact on photonics, photo- and electrocatalysis by improving the light-matter interaction and charge transport properties [14, 15]. Since CNTs were grown by PECVD in this study, the material quality is sufficiently high for the practical application in nanophotonics, optoelectronics, and in photo- and electrocatalysis [16–18]. Taking advantage of the aforementioned benefits of CNT-grafted Ru IO structures, we expect to develop an advanced electrode platform for energy conversion and storage devices.

2. Experimental details

2.1 Preparation of freestanding RuO₂ IO

Polystyrene (PS) spheres were synthesized by the initiation of 27.5 mL of styrene monomer (Sigma-Aldrich) using 0.14 g of potassium persulfate (PPS; Sigma-Aldrich) in 200 mL of N₂-purged H₂O at 60 °C. After 30h, the as-synthesized PS spheres were washed by dialysis with deionized water. In order to fabricate the PS opal structure, 0.1 mL of PS sphere solution was dropped onto Ni mesh (diameter: ½ inch) and dried in air, followed by a thermal-annealing on a hot plate at 95 °C overnight. Then, 40 mg of RuCl₃∙xH₂O (Sigma-Aldrich) in 1 mL of ethanol was reacted with 35 μL of 5 M NaOH aqueous solution. After the filtration of Ru(OH)_x precipitates by centrifugation, a small portion of O₂-plasma treated PS opal structure on Ni mesh was immersed into the RuO₂ precursor solution. The RuO₂ precursor solution infiltrated PS opal structure was calcined at 550 °C in air for 2.5h to remove PS spheres and to obtain the rutile phase of the RuO₂ frameworks.

2.2 N-doped CNT growth on RuO₂ IO

As a catalyst for the synthesis of CNTs, Fe thin layer was deposited using an electron-beam evaporator on RuO₂ IO structures. Afterwards, N-doped CNTs were grown via PECVD technique with a stream of H₂ (80 sccm), NH₃ (20 sccm) and C₂H₂ (5 sccm) gases at 750 °C under a chamber pressure of 4 Torr and a plasma potential of 490 V. For comparison, the Ru IO structure was prepared in the absence of NH₃ and C₂H₂ gases during the PECVD process. The analysis of N-dopants along the CNTs can be found in reference [19].

2.3 Instruments and characterization

Scanning electron microscope (SEM) images were obtained using the JEOL JSM-6700F and the Hitachi S-4800. X-ray diffraction (XRD) measurements were performed by a Rigaku D/MAX-2000 equipped with Ni filtered Cu KR radiation (λ = 1.5418 Å). Raman spectra were recorded by a LabRam HREvo 800 (HORIBA Jobin Yvon) at λ_ex = 633 nm.

3. Results and discussion

Figure 1 depicts the fabrication process of RuO₂ IO and N-doped CNT-grafted Ru IO structures. PS spheres were prepared by emulsion polymerization as a removable core material to generate three-dimensionally periodic macropores. The PS opal structure was obtained via self-assembly of PS spheres on Ni mesh, providing interconnected spaces for the infiltration of RuCl₃ ethanolic solution. Sodium hydroxide (NaOH) was applied to increase the pH of the RuCl₃ solution. The solution protects metal substrates such as Ni mesh from corrosion. Finally, the freestanding RuO₂ IO structure was fabricated after the calcination step, followed by N-doped CNT growth on the surface of RuO₂ IO via the PECVD technique. Hydrogen (H₂) gas, streamed during the CNT synthesis, converted RuO₂ into metallic Ru.

 

Fig. 1 Schematic illustration of the fabrication process of the N-doped CNT-grafted Ru IO structure. Step1: PS opal structure is prepared by drop-casting the PS sphere solution onto Ni mesh; Step2: The PS opal structure is immersed into RuCl₃ solution to fill the gaps between the PS spheres; Step 3: Calcination process produces RuO₂ IO structure; Step 4: PECVD process is carried out for the growth of CNTs onto the RuO₂ IO.

Download Full Size | PDF

SEM measurements were carried out to observe the changes in the structural properties of the as-prepared materials after the consecutive synthetic processes. Figure 2(a) shows that opal structure consisting of hexagonally packed PS spheres was prepared. PS spheres with a uniform size of ~800 nm were used in this study. Figures 2(b) and 2(c) confirm that PS opal structure was reversely developed into IO structures possessing multilayered and periodic macropores surrounded by interconnected frameworks. The size of the macropores was smaller than that of the initial PS spheres due to the shrinkage during the calcination step and it was decreased to ~500 nm. As revealed by Fig. 2(d) and 2(e), the IO structures were successfully covered with a sufficient amount of CNTs. CNT growth was mostly observed on the surface of the IO frameworks and the macropores are remained unblocked. The IO structure was preserved after the PECVD process, indicating that RuO₂ IO was stable against gas streams at high temperatures exceeding ~750 °C. Therefore, the additional Fe introduced onto the IO structure was responsible for the CNT formation. The length of CNTs tethered to the IO structure reached a few micrometers. Since Fe film with a thickness of ~2 nm was deposited via electron-beam evaporation mediated the binding between the IO structure and the CNTs, the adhesion between them would be higher than in the physical mixture of the IO structures and the CNTs.

 

Fig. 2 SEM image of (a) PS opal structure. SEM images of RuO₂ IO structures. (b) and (c) were measured at low and high magnification, respectively. SEM images of N-doped CNT-grafted Ru IO structures taken at (d) top-down position and (e) side position

Download Full Size | PDF

XRD and Raman spectroscopy results are shown in Fig. 3. Figure 3 confirmed the composition of the structures in Fig. 2(b) and 2(c) and Fig. 2(d) and 2(e). The red profile peaks in Fig. 3(a) indicate that the Ru precursor infiltrated the gap between PS spheres was crystallized in the mixture form of tetragonal and cubic RuO₂, which are relevant to the circle and triangle marks, respectively. Compared with the diffraction peaks obtained from commercial RuO₂ powder (black line) which can be assigned to a pure tetragonal RuO₂ phase, the XRD peaks of RuO₂ IO structure revealed binary phase characteristics. This could be a result of the fabrication process of RuO₂ IO using RuCl₃ solution coupled with NaOH. When the RuO₂ IO structures were placed in the PECVD chamber under an H₂ gas stream, RuO₂ was reduced and converted into metallic Ru as represented by the square marks in the yellow and blue profiles of Fig. 3(a) [3]. The formation of RuO₂ and metallic Ru could also be confirmed by Raman spectroscopy. The peaks at around 503, 618, 687 cm⁻¹ were derived from the first order phonon bands, E_g, A_1g and B_2g, respectively [3].

 

Fig. 3 (a) XRD and b) Raman spectra obtained from commercially available RuO₂ powder (black line, Sigma-Aldrich), RuO₂ IO (red line), Ru IO (yellow line) and N-doped CNT-grafted Ru IO (blue line) structures grown on Ni mesh. The asterisk, circle, triangle and square marks in Fig. 3(a) are assigned to the signals from Ni, tetragonal RuO₂, cubic RuO₂ and metallic Ru, respectively. In Fig. 3(b), the four samples exhibited E_g, A_1g and B_2g bands and only N-doped CNT-grafted Ru IO showed D, G and 2D bands.

Download Full Size | PDF

CNT growth on the RuO₂ IO structure was detected by XRD. The diffraction peak at 26.3° of the blue profile in Fig. 3(a) can be indexed to the (002) planes of the CNTs [5]. This indicates that the CNTs are composed of graphitic carbons [20]. As shown in the blue spectrum of Fig. 3(b), Raman spectroscopy further confirmed the generation of CNTs. Only CNT-grafted Ru IO structures showed characteristic bands at around 1329, 1574 and 2646 cm⁻¹ which can be assigned to the D, G and 2D bands of CNTs. D and G bands arise from the structural defect in sp² carbon and the stretching between sp²-bonded carbon atoms in graphitic materials [21]. It supports the formation of disordered graphitic CNTs. The sp² graphitic feature of the CNTs can also be recognized by the 2D band [21, 22]. The appearance of the 2D peak in the Raman spectrum indicates that the CNTs grafted onto Ru IO possess the intrinsic tube structure with three-dimensionally stacked graphene-like shells [23, 24].

4. Conclusions

We successfully demonstrated the fabrication of N-doped CNT-grafted Ru IO structures as confirmed by SEM, XRD and Raman spectroscopy. RuO₂ IO provided an excellent reaction site to grow CNTs without any collapse in structure. Since the composition of frameworks of IO structure can be varied from metals, semiconductors to transition metal oxides, a further development of diverse combination of CNT/IO with a different functionality may be advantageous for certain applications such as energy conversion and storage, optoelectronics and catalysis.