Fabrication of Copper Nanowires Through Multi-step Anodizing, Electrochemical Barrier Layer Thinning and Electrodeposition

Nanoporous anodic aluminum oxide (AAO) is one of the most commonly employed templates for nanofabrication since the pore diameter, interpore distance, thickness of the oxide, barrier layer and walls can be precisely controlled by a proper choice of anodizing conditions. For the fabrication of metallic nanowires (NW), it is necessary to decrease the barrier layer thickness. In addition, to achieve sufficient electrical contacts at the bottom of the pores, a thin layer of Au has to be deposited either by sputtering or by electrodeposition (ED) using a cyanide bath. Researchers at Military University of Technology, Poland and Delft University of Technology, The Netherlands have proposed a methodology that combines multi-step anodizing, electrochemical barrier layer thinning (BLT) and ED for the fabrication of Cu NW.

Commercial purity aluminum alloy (AA 1050 alloy) was degreased and electropolished (EtOH:HClO₄ 4:1, 0 °C, 20 V, 120 s, Pt grid cathode). A multi-step anodizing protocol was employed to obtain nanoporous anodic aluminium oxide (AAO) templates with a desired nanoporous structure. Mild anodization (MA) in 0.5 M H₂SO₄ with 20 vol.% ethylene glycol (EG) at 0 °C, 20 V and 60 min (Fig. 1, reaction I) was carried out as the first step. The voltage was increased up to 45 V with 0.5 V steps for each 5 s and hard anodizing was performed at 45 V for 1 h (Fig. 1, reaction II). To thin down the bottom of the barrier layer, mild anodizing was carried out in 0.3 M oxalic acid, at 30 °C, 45 V and 30 min (Fig. 1, reaction III). Electrochemical barrier layer thinning (BLT) of multi-step anodized Al alloy was performed in 0.3 M oxalic acid (Fig. 1, reaction IV). A step-wise decrease in voltage and the duration of each voltage step was varied and the suitable conditions for BLT were optimized. For effective opening of the pores at the bottom, the Al alloy at the base was chemically etched using 0.1 M CuCl₂ in HCl. The applicability of the membranes formed using a combination of MA, HA and BLT was ascertained through electrodeposition (ED) of Cu using 0.3 M CuSO₄ and 0.1 M H₃BO₃ at -0.3 V vs. Ag/AgCl for 30 min (Fig. 1, reaction V). The Cu nanowires (NW) were liberated from the AAO template by chemical etching in 5% H₃PO₄ at 30 °C for 45 min (Fig. 1, reaction VI).

Fig. 1 Schematic representation of the various stages involved in the fabrication of Cu nanowires

MA in 0.5 M H₂SO₄ with 20 vol.% EG at 0 °C, 20 V and 60 min enables the formation of a protective oxide layer. The presence of this oxide layer as well as a steady step wise increase in voltage (0.5 V steps for each 5 s) up to 45 V prevents destruction of the Al alloy anode by the high current density avalanche generated during HA at 45 V for 1 h. The MA/HA combination though improved ordering of nanoporous structure in the resultant AAO, the thickness of the barrier layer at the bottom is a critical issue. MA in 0.3 M oxalic acid,    at 30 °C, 45 V and 30 min decreased the thickness of the barrier layer by a reasonable extent, which is suitable for subsequent BLT process. Since the conditions of anodization are mild, the interpore distance and ordering of the pores are maintained. A step-wise decrease in voltage enables BLT of the anodized Al alloy in 0.3 M oxalic acid. Thinning of the barrier layer is effective at selective voltage step and time (Un+1 = 0.75.Un; Δt = 60 s). Under such conditions of BLT, the hexagonal honeycomb-like morphology is maintained at the bottom of the pores (Fig. 2(a)). The applicability of the AAO formed using a combination of MA, HA, MA and BLT is confirmed by ED of Cu NW with a high aspect ratio (Fig. 2(b)). The successful ED of Cu NW confirms efficient BLT and sufficient electrical contact at the electrolyte–aluminum interface that could have facilitated the reduction of Cu²⁺ ions at bottom of the pores.

Fig. 2 FE-SEM micrographs of (a) bottom side of the AAO (after removal of Al alloy using 0.1 M CuCl₂ in HCl) indicating the effectiveness of BLT performed at U_n+1 = 0.75 Un; Δt = 60 s; and (b) ED Cu NW

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: W.J. Stepniowski et al., Journal of Electroanalytical Chemistry, 809 (2018) 59–66.

Development of a hierarchical micro/nano-porous structure on acupuncture needles for the treatment of colorectal cancer

Acupuncture is considered to be an effective therapy for treating functional disorders, pain relief, drug abuse and psychiatric disorders. The treatment mechanism involves the release of endogenous opiates and neurotransmitters, which are mediated through electrical stimulation of the central nervous system. In order to increase the intensity of the stimuli, it is necessary to use thicker needles and/or deeper insertion of the needles. However, a similar effect could be achieved by increasing the surface area of the needles. Researchers at the Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu Haany University, Republic of Korea and Flux Photon Corporation, USA, have fabricated a porous structure with hierarchical micro/nano-scale conical pores on the surface of conventional stainless steel acupuncture needles.

Conventional stainless steel acupuncture needles (CN) (length: 8 cm; diameter: 0.18 mm) were anodized in ethylene glycol medium modified with the addition of 0.2 wt. % NH₄F + 2.0 vol. % deionized water at 20 V for 30 min to form a porous structure with hierarchical micro/nano-scale conical pores (PN). The morphological features of CN and PN are shown in Fig. 1. The CN possess a smooth surface (Fig. 1(a)) whereas a hierarchical micro/nano-scale porous surface topology is developed after anodization (Figs. 1(b), 1(c) and 1(d)). The surface area of PN (1.03 m²∙g⁻¹) is ~ 25 times higher than CN (0.04 m²∙g⁻¹).

Fig. 1 Morphological features of (a) conventional acupuncture needle (CN);  and (b, c, d) nanoporous acupuncture needle (PN); (c, d) high resolution images.

The surface modified stainless steel acupuncture needles (PN) enhanced the therapeutic effects of colorectal cancer (CRC) treatment in rats.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: Bo Ram Lee et al., Enhanced Therapeutic Treatment of Colorectal Cancer Using Surface-Modified Nanoporous Acupuncture Needles, Scientific Reports, 7: 12900,  DOI:10.1038/s41598-017-11213-0