Fabrication and Growth Mechanism of Nano-Octahedrons: A Study on Cu2O Core-Shell Structures for Oxygen Evolution Reaction

This paper explores the process and growth mechanism involved in fabricating nano-octahedrons. Using two key components, CTAB and hydrazine hydrate, leads to the formation of octahedral shapes. A Cu2O octahedral core–shell structure was successfully created by employing a coating technique, and its performance in the oxygen evolution reaction (OER) was thoroughly evaluated. Due to its high dispersibility in ammonia solution, Cu2O serves as an excellent representative for other components. Additionally, this chapter provides a detailed account of the production of octahedral nanocages through the ammonia etching method. Notably, the Cu2O nano octahedra demonstrate superior OER performance compared to commercial RuO2, exhibiting a significantly low overpotential of only 370 mV at 10 mA cm−2. These findings bear important implications for designing stable core/shell nanostructures and hollow structures by implementing appropriate chemicals while also deepening our understanding of the formation of octahedral shapes.

Due to their exceptional oxygen evolution reaction (OER) performance in alkaline environments, transition metal-based electrocatalysts have garnered significant interest among researchers. Nanostructuring is often employed to enhance the effectiveness of these catalysts. However, the practical implementation of these catalysts poses several challenges. [1][2][3] Using a polymer binder to attach the catalyst powder to the electrode increases electrical resistance, loss of active catalyst sites, and reduced mass transfer. Moreover, binders typically result in active sites only at loadings below 1 mg cm −2 . 4-6 High-current electrocatalysis causes rapid exfoliation of the covered catalyst. 7 Additionally, the low conductivity of catalysts necessitates the addition of conductive additives, such as nano-carbon compounds. [8][9][10] However, these additives are susceptible to etching or oxidation at high potentials, compromising electrocatalytic performance. The development of self-supporting electrodes can enhance the utilization of electrochemical materials. Well-structured materials effectively improve charge transfer rates and prevent aggregation of active materials. [11][12][13] A self-supporting catalytic electrode with a fine structure offers the following advantages: (i) Direct catalyst growth on the substrate without a binder reduces the possibility of interfacial bridging between active components. 14 (ii) The substrate exhibits higher electron-transfer capacity. 15 (iii) The fine structure facilitates the wide-scale release and transportation of gases. 16 (iv) The substrates and catalyst work synergistically. The morphology, dimensions, and microstructures are well-known determinants of metal oxides' chemical and physical characteristics. Thus, creating single-crystalline octahedral nanoparticles remains an exciting prospect, especially the ability to regulate particle dimensions within a tight size distribution range at room temperature. [17][18][19] Common wet-chemical processes for creating Cu 2 O nanoparticles include the hydrothermal technique, template method, and solution-phase synthesis method. 20 These processes often involve the addition of additives such as organic polymers and inorganic ions to control the size and shape of the products. 21 However, these additives are often expensive or environmentally harmful, posing challenges for their elimination. 22 This paper focuses on the synthesis of Cu 2 O nano-octahedra, achieved through a relatively minor modification to the synthesis process. The morphology and composition of the products will be studied using various methods, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The OER characteristics of these Cu 2 O nanooctahedral particles will be assessed using sodium hydroxide (NaOH) as the alkaline electrolyte. This research's findings may aid in designing future metal oxide nanostructures with exposed crystal surfaces for enhanced performance. The study also investigates a technique for etching cuprous oxide, as Cu 2 O exhibits solubility in ammonia solution, allowing for the creation of nanocages. Among the various structures produced, Cu 2 O nano octahedra demonstrate excellent performance with a low overpotential of 370 mV at 10 mA cm −2 , surpassing commercial RuO 2 .
Synthesis of Cu 2 O octahedrons.-In a typical procedure, 0.125 g of CuSO 4 ·5H 2 O powder was dissolved in 100 ml of distilled water, resulting in a 0.005 M bright blue aqueous CuSO 4 solution. The obtained solution was vigorously mixed with the addition of CTAB, maintaining a mole ratio of surfactant to Cu 2+ at 1:1. After 15 min of stirring, 0.42 ml of a 6 mol l −1 NaOH solution was titrated into the mixed solution. Subsequently, a red precipitate began to form after an additional 15 min of stirring. They were carefully added to the combined solution at room temperature to compare the effect of different concentrations of reducing agent solutions. The stirring process continued for about an hour before the experiment was terminated. The final products were collected through centrifugation and subjected to multiple cleaning cycles with ethanol and distilled water to ensure proper purification.
Characterization.-The phase composition and purity of the asprepared Cu 2 O microstructures were investigated using a Phillips X'pert Multipurpose X-ray Diffraction System (MPD) equipped with Cu-K radiation (λ = 1.54 Å). The X-ray diffraction system operated at 40 kV and 40 mA, and the measurements were conducted in the 2θ range of 20°-70°. For the scanning electron microscopy (SEM) study, an FEI Nova Nano SEM 230 field emission scanning electron microscope was utilized. Transmission electron microscopy (TEM) images were obtained using a Tecnai G2 z E-mail: anand@snnu.edu.cn 20 transmission electron microscope, operating at an accelerating voltage of 200 kV. To capture high-resolution transmission electron microscopy (HRTEM) images, a Phillips CM200 field emission transmission electron microscope was employed, operating at an accelerating voltage of 200 kV. UV-vis absorption spectra were obtained using a 1 cm quartz cell and a CARY 5 G UV-visible Spectrophotometer (Varian).
Characterization of the electrochemical.-All electrochemical measurements in this study were conducted using a three-electrode setup and equipment from the Shanghai Chenhua Electrochemical Workstation CHI760E and a Pine spinning disk system from the United States. The experiments were performed at a temperature of 25°C, utilizing a 0.1 M/1 M KOH solution as the electrolyte. To ensure the platinum-carbon electrodes were smooth and free of contaminants, they were initially polished using Al 2 O 3 powders of various sizes. A total of 5 mg of catalyst was weighed and then mixed with 900 ml of isopropanol, 100 ml of water, and 20 ml of Nafion. The mixture was sonicated, and 20 ml of the resulting solution was pipetted onto the platinum-carbon electrode in two separate drops for electrochemical testing. It is important to note that the temperature was maintained at a constant 25°C throughout the test.
Effect of reducing agent and surfactant.-The formation of octahedrons can be observed by adjusting the mol ratio of the reducing agent (N 2 H 4 ·H 2 O) to Cu 2+ . Figure 1 demonstrates the morphological transformation from wires to octahedrons as the concentration of the reducing agent is altered. In line with the information provided in the previous chapter, a nanowire structure is produced when the mole ratio of N 2 H 4 ·H 2 O to Cu 2+ is 1:1. Figure 1b shows a combination of wire-like and octahedron-like nanoparticles as the ratio increases to 5:1, and the wires begin to merge.
The presence of NH 4+ ions significantly impact the relative stability of the 100, 111, and 110 planes in Cu 2 O octahedrons, leading to a transformation in their growth rates along these directions. Contrary to the growth rate along the 100 directions, the presence of NH 4+ ions cause the "110" surfaces to initially increase in the area before decreasing. Similarly, the area of the "111" planes continue to grow during this period, resulting in an octahedral morphology. This finding indicates that the 111 planes exhibit the highest stability in the presence of NH 4+ ions. Meanwhile, the relative stability of the 110 planes falls within the range of the 100, 110, and 111 planes. Initially, the "110" and "111" planes increase in area, but ultimately, the crystal surface is composed solely of the most stable "111" planes. 23 Fig. 3d shows diffraction rings that can be indexed on a cubic cell, confirming that the produced nanowires consist solely of cubic Cu 2 O. These findings align with the characterization performed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM).
Etching of Cu 2 O octahedrons.-Hollow nanostructures have recently garnered significant attention due to their potential applications in various fields. Among these structures, the hollow octahedron is a three-dimensional configuration with unique characteristics such as low density, high specific surface area, and active phases. This makes them promising candidates for applications in photocatalysis. Reports indicate the successful creation of hollow In 2 O 3 spheres utilizing carbon spheres as templates. Notably, these hollow In 2 O 3 spheres exhibited stronger responsiveness compared to bulk particles. 27 The distinctive structure, large surface area, and increased surface-active sites of hollow spheres have also shown improved gas sensing characteristics, which have implications for enhanced photocatalytic efficiency. [28][29][30] This study presents a simple approach for creating porous hollow octahedrons using Cu 2 O as a sacrificial template based on the ammonia etching technique. The synthesis process offers advantages such as controllable synthesis parameters, the use of simple equipment, and quick reaction times. 31 This strategy can potentially be extended to creating hollow octahedrons using different metal oxides. The characterization of the produced structures was carried out using transmission electron microscopy (TEM). Additionally, Cu 2 O nanoparticles were treated with an ammonia solution to demonstrate their solubility in the gas phase. These findings contribute to the growing field of hollow nanostructures and provide insights into their potential applications in photocatalysis and gas sensing.
The slight variations in the synthesis conditions are shown in Table I. It suggests that SDS is advantageous for the sphere-like  Recent studies have highlighted the systematic control of Cu 2 O external factors, including its form, crystallinity, and structure. Specifically, selective ethanol adsorption on different surfaces of Cu 2 O octahedral particles has been observed to induce targeted etching on the 111 facet, resulting in branching patterns limited by etching. This phenomenon suggests that the formation of diverse  morphologies is influenced by the selective adsorption of capping agents onto specific crystal surfaces. [32][33][34] The capping agent, generated in an alkaline environment with a pH range of 7 to 13, is introduced into the system during particle development. The injected capping agent molecules tend to adsorb onto higher energy planes preferentially. This adsorption stabilizes the facets and impedes growth perpendicular to those planes, thereby altering the growth rates of distinct crystal orientations. 33 Additionally, it encourages the redistribution of surface energies on each Cu 2 O facet, facilitating the formation of well-defined Cu 2 O microcrystals and the exposure of extensive 111 surfaces. As a result, these novel Cu 2 O nanoparticle morphologies arise under a kinetic growth regime. However, understanding the growth behavior of 111 facets and achieving precise control over the morphology of Cu 2 O growth on 111 planes remains challenging. 34 Based on experimental findings, introducing a strong alkali during the precipitation reaction plays a significant role in the synthesis process.
Comparison of UV-visible absorption.- Figure 4 displays the absorption spectra of the samples, providing insights into the optical characteristics of the as-prepared Cu 2 O nanostructures. The Cu 2 O nanowires exhibit a modest absorption peak at approximately 520 nm. In contrast, the Cu 2 O nanospheres exhibit two prominent UV-visible absorption peaks at around 740 nm and 480 nm. The Cu 2 O nano-octahedron demonstrates a strong absorption band with a center wavelength of 590 nm. Interestingly, during the formation of the nanocrystals, a sequence of color variations in the solution was observed. 31,32,35 Upon adding NaOH to the aqueous mixture of CuSO 4 and SDS, Cu(OH) 2 and potentially Cu(OH) 4 2− species were formed, resulting in a pale blue solution. Subsequently, the addition of N 2 H 4 .H 2 O induced the formation of Cu 2 O nanocubes, causing the solution's hue to change from light blue to green within seconds. These observations highlight the optical differences among the various Cu 2 O nanostructures and provide valuable insights into their unique properties and potential applications.   Comparisons between the effects of nanospheres, nanowires, and nano-octahedra have shown that octahedrons composed entirely of {111} surfaces exhibit superior performance compared to other morphologies. [14][15][16][17] Catalytic performance.-Multiple experiments were conducted to evaluate the prepared catalysts' oxygen evolution reaction (OER) performance. The Cu 2 O nano octahedra exhibited an overpotential of 370 mV at 10 mA cm −2 , which is lower than that of other synthesized catalysts and comparable to RuO 2 , as evidenced by the linear sweep voltammetry (LSV) curves in 1 M KOH (Fig. 6a). Furthermore, the Cu 2 O nano octahedra displayed a Tafel slope of 99 mV dec −1 , which is significantly lower than that of the nanospheres (189 mV dec −1 ) and nanowires (136 mV dec −1 ), and comparable to the Tafel slope of RuO 2 (118 mV dec −1 ) (Fig. 6b). These findings suggest that the active sites present in Cu 2 O nano octahedra greatly enhance their catalytic performance. The synergistic coupling effect of Cu 2 O also contributes to the increased activity of the catalyst.
The stability of the catalyst was assessed in this study. Figure 7 illustrates the results of chronopotentiometry testing after 12 h in an alkaline solution, indicating the strong OER activity and remarkable stability of the Cu 2 O nano octahedra catalyst. Figure 8, calculated using the cyclic voltammetry (CV) data shown in Fig. 7, represents the electrochemically surface-active region of the catalyst. The Cu 2 O nano octahedra exhibit a double-layer capacitance (Cdl) value of 8.4 mF cm −2 , which is higher compared to the nanospheres (2.4 mF cm −2 ) and nanowires (6.9 mF cm −2 ). This indicates that the Cu 2 O nano octahedra catalyst possesses a larger electrochemical surface area and higher OER activity. These results highlight the enhanced stability and superior electrochemical performance of the Cu 2 O nano octahedra catalyst, making it a promising candidate for various electrochemical applications.
The electrical and catalytic properties of octahedral and hexapod Cu 2 O nanoparticles with 111 planes have been found to surpass those of cubic Cu 2 O particles with 100 orientations. This suggests that, in Cu 2 O particles, the 111 crystal orientations exhibit higher electrical conductivity than the 100 facets. [31][32][33]35 However, when it comes to detecting concentrations with physiological significance, electrodes fabricated using octahedral and hexapod Cu 2 O particles demonstrate lower detection limits. [27][28][29] In electrochemical sensing, the sensitivity is more significantly influenced by the detection limit 14,15 rather than the particle orientation. Therefore, while controlling the morphologies and surface orientations of different particles is possible, the physical and chemical characteristics of the materials themselves are crucial in determining their electrocatalytic performance. 30

Conclusions
This study has highlighted the significance of N 2 H 4 ·H 2 O in the synthesis of Cu 2 O nano-octahedrons and has presented a simple etching technique for Cu 2 O nanoparticles. The presence of NH 4+ ions in the medium promotes the stability of the 111 planes in Cu 2 O nanostructures. Additional N 2 H 4 ·H 2 O plays a crucial role in the formation of nano-octahedrons. In the presence of NH 4+ ions, both the "110" and "111" planes are more stable than the "100" planes. Initially, the areas of the "110" and "111" planes increase, but eventually, the crystal surface is dominated by the "111" planes. Ammonia solution is an effective and safe etching solution for creating Cu 2 O core-shell structures. It selectively removes the core while leaving the shell intact. The 111 surfaces of Cu 2 O nanoparticles have been identified as the active surface for the oxygen evolution reaction (OER). Cu 2 O nanooctahedra exhibit superior OER performance to commercial RuO 2 , with a low overpotential of 370 mV at 10 mA cm −2 . These findings contribute to our understanding of the formation of octahedral structures, particularly in selecting appropriate chemicals for synthesizing hollow structures and stable nanostructures. They provide valuable insights for designing and fabricating future nanostructures with improved performance and potential applications.