*Article* **Preparation of CuCrO2 Hollow Nanotubes from an Electrospun Al2O3 Template**

**Hsin-Jung Wu 1,**†**, Yu-Jui Fan 2,**†**, Sheng-Siang Wang 1, Subramanian Sakthinathan 1, Te-Wei Chiu 1,\* , Shao-Sian Li 1,3,\* and Joon-Hyeong Park <sup>4</sup>**


Received: 17 July 2019; Accepted: 1 September 2019; Published: 3 September 2019

**Abstract:** A hollow nanostructure is attractive and important in different fields of applications, for instance, solar cells, sensors, supercapacitors, electronics, and biomedical, due to their unique structure, large available interior space, low bulk density, and stable physicochemical properties. Hence, the need to prepare hollow nanotubes is more important. In this present study, we have prepared CuCrO2 hollow nanotubes by simple approach. The CuCrO2 hollow nanotubes were prepared by applying electrospun Al2O3 fibers as a template for the first time. Copper chromium ions were dip-coated on the surface of electrospun-derived Al2O3 fibers and annealed at 600 ◦C in vacuum to form Al2O3-CuCrO2 core-shell nanofibers. The CuCrO2 hollow nanotubes were obtained by removing Al2O3 cores by sulfuric acid wet etching while preserving the rest of original structures. The structures of the CuCrO2-coated Al2O3 core-shell nanofibers and CuCrO2 hollow nanotubes were identified side-by-side by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The CuCrO2 hollow nanotubes may find applications in electrochemistry, catalysis, and biomedical application. This hollow nanotube preparation method could be extended to the preparation of other hollow nanotubes, fibers, and spheres.

**Keywords:** electrospinning; CuCrO2; hollow nanotube; Al2O3 template; one-dimensional structures

#### **1. Introduction**

One-dimensional (1D) nanostructure materials such as nanotubes, nanobelts, and nanofibers have attracted wide interest in nanoscience and technology [1]. Regulating the size and shape of synthesized nanomaterials is of great technological interest nowadays. Particularly, hollow nanostructures have received considerable attention due to their high surface areas and structural uniqueness, thus they have been extensively applied in many fields, such as sensors, dye-sensitized solar cells, catalysts, supercapacitors, photoelectrochemical cells, electronics, and biomolecule devices. Hence, different approaches have been used in the development of hollow nanotubes and nanofibers for large-scale synthesis [2,3]. One of such structural approaches is electrospinning which has been widely applied to synthesize nanofibers from a variety of oxide materials [4].

Electrospinning is a fiber formation method that uses self-repulsion effect, which induces an electrostatic charge on a precursor material to stretch the liquid in an electric field into fiber structure. The dimension of fiber diameter ranges from tens nanometer to few micrometers [5]. In the past few

years, it has been an effective method to prepare polymer-based nano- or microfibers. Different kinds of polymers have been successfully electrospun from melts or solutions into ultrathin fibers [6]. Up to date, the preparation of nanofibers with solid cross-sections has been studied [7,8].

P-type transparent conducting oxides with delafossite structure has been demonstrated with potential applications in various fields including organic photovoltaic (OPV) devices [9], perovskite solar cells [10], antibacterial surface [11], gas sensors [12], solid propellants [13], etc. The delafossite structure of copper-based catalysts also has great importance in catalytic steam reforming of methanol to hydrogen production and heterogeneous catalysis for chlorine production due to their high thermal stability, fine porous structure, high surface area, high selectivity, and excellent activity at low temperature. Besides, copper delafossite materials are more stable than Ru, Pd, Au, and Pt catalyst at the steam reforming process [14–16]. Cu-based delafossites have been reported including CuAlO2 [17], CuFeO2 [18], CuGaO2 [19], CuInO2 [20], CuScO2 [21], CuCrO2 [22], and Mg-doped CuCrO2 [23,24]. The chemical formula of delafossite structure is that of a ternary oxide A+B<sup>+</sup>3O2. According to the report, the delafossite structure of CuCrO2 has a wide bandgap of 3.1 eV and highest conductivity among all types of semiconductors [25]. Hence, CuCrO2 and CuAlO2 have drawn considerable attention in optoelectronic devices [26,27]. The delafossite material consists of two alternating sheets: a planar layer of triangular-patterned cations (A) and a layer of edge-sharing BO6 octahedrons flattened with respect to the c-axis. Depending on the orientation of layer stacking, two polytypes of delafossite oxide can be created. Considering the morphological effects, catalyst with hollow tube structure shows very promising potential because of the highly selective catalytic reaction. For example, ZSM-5/SiO2 hollow structure catalyst selectively increases the paraxylene from the 24% to 89.6% in xylene in methanol-to-aromatics conversion [28]. A single-wall carbon nanotube/iron tetraphenyl porphyrin composite sensor shows a selectively high response toward xylene among benzene and toluene [29]. Carbon nanotube pores (CNTP) show potential to be used as next-generation water purification technologies because CNTP provides high selectivity of water and anions [30]. Further, a porous hollow tube CeO2/Au@SiO2 nanocatalyst exhibited excellent catalytic activity toward 4-nitrophenol reduction [31]. Platinum (Pt) functionalized NiO hollow tube exhibited remarkable selectivity of C2H5OH sensing against CO and H2 gases [32]. The hollow structure of CuO@SiO2 exhibits excellent catalytic activities toward CO and NO oxidation compared with individual CuO and SiO2 [33]. Besides, carbon nanotube catalyst could raise the selectivity of H2 production rather than CO [34].

However, nanotube with hollow cross-sections are challenging to fabricate because of multi-step treatments (e.g., a template process) or specially designed instrumentation facilities (e.g., for co-electrospinning with coaxial capillaries) [35]. Nanofiber (7.85 m2/g) [36] or nanopowder structures (30.92 m2/g) [37], such as hollow nanotubes (136 m2/g), have a higher surface-to-volume ratio and higher porosity, which are favorable for adsorption in catalysis [38]. Hence, developing a simple approach to obtain hollow nanotubes is of great importance. [36,39]. In this study, the main objective was to explore the use of Al2O3 microfibers as a template to prepare a core-shell structure of Al2O3-CuCrO2 by immersion in Cu-Cr-O precursor solution. The alumina structure was then removed by etching in H2SO4 to form the CuCrO2 hollow nanotubes.
