Effects of Anodization-Assisted Electrodeposition Conditions on the Fabrication of CuO-Cu2O Coatings on Nanoporous Stainless Steel
Surface functionalization can be utilized as a useful tool to improve the antibacterial performances of medical devices. Furthermore, some antibacterial agents are coated or impregnated on the surface of 316LSS to prevent contamination of bacteria. Here, CuO-Cu2O coatings were prepared on 316L nanoporous stainless steel (NPSS) by anodization-assisted electrodeposition for applications in antibacterial materials. The present study investigated influences of HClO4 concentration, reaction temperature, and load voltage on the morphology, structure, and composition of the coatings. SEM images showed that appropriate nanopores with an average size of about 93 nm were formed on the stainless steel surface and then were successfully filled with CuO-Cu2O. The nanopore size increased with increasing application of electrolyte concentration. Both the diameter and depth of the nanopores increased with increasing voltage. The EDX result indicated that Cu and O were embedded in as-prepared CuO-Cu2O/NPSS. XRD analysis showed that the CuO-Cu2O/NPSS surface comprised CuO and Cu2O.
Stainless steel (SS) with low-cost, high mechanical strength and corrosion resistance, biocompatibility, good electrical conductivity, and commercial availability has attracted much attention due to its potential applications in industrial and biomedical materials [1, 2]. However, its surface properties that allow living microorganisms to exist on SS products do not meet the health criteria. Thus, the antibacterial properties of SS must be improved.
Recently, several researchers have investigated the modification of the SS surface with nanostructures and incorporated catalysts, such as oxide metals and noble metals, to improve its catalytic properties [3–5]. For example, Rezaei et al. described the use of nanoporous SS (NPSS) containing a porous Pd film as a novel electrocatalyst/electrode design for glycerol oxidation . Zhan et al. demonstrated the formation of nanopore arrays on the SS surface by anodization for visible-light photocatalytic degradation of organic pollutants . Metal-based nanoporous materials with large specific surface areas, high porosity, and versatile porous structure can significantly improve utilization of surface active sites and promote chemical reactions, such as potential applications in catalysis [8–10].
Accordingly, tremendous interest has been focused on the design and fabrication of antibacterial agents (e.g., silver, copper, and zinc) on surfaces of NPSS to prevent initial adhesion of bacteria. Among these agents, copper and its oxides received intense attention due to many advantages, such as stability, cost-effectiveness, and rapid and effective bacterial killing property [11–13]. Such advantages of Cu-containing NPSS can be used in biomedical devices, such as prosthetic joints and dental implants.
Anodization has shown its excellent capability in fabricating the highly ordered nanopore structure on the surface of the metal [14, 15]. The formation of the self-organized nanoporous anodic layer onto the stainless steel using anodization has been investigated in the literature [16–18]. From these literatures, we found that the nanostructured surface was consistent with a mixture of Cr and Fe oxides enriched in Cr2O3. Rezaei et al. decorated nanostructured gold within NPSS using anodic oxidation and galvanic replacement reaction, exhibiting excellent repeatability and reproducibility, long-term stability, and acceptable selectivity . Wang et al. examined the use of self-organized Cu-containing nanotubes and nanopores on Ti90−xCu10Alx (x = 0, 45) alloys, created by anodic oxidation of Ti90−xCu10Alx in NH4F-containing ethylene glycol electrolytes for the evaluation of their antimicrobial activity and cytotoxicity . Simultaneously, incorporation of appropriate elements or metal oxides into well-ordered nanostructures can enhance antimicrobial activity or other electrochemical properties. Electrochemical deposition has been proved as a controllable and efficient method to fill the nanopores with metals atoms. For example, Wu et al. showed that Au nanowire arrays were successfully deposited onto the anodized aluminum oxide template by a simple and efficient combined alternate current-direct current (AC-DC) electrodeposition . Ali and Maqbool showed that cobalt-nickel binary nanowires could be grown on nanoporous alumina membranes via AC electrodeposition . Electrodeposition of CuO-Cu2O on NPSS (CuO-Cu2O/NPSS) will create a strong antibacterial material.
In this study, CuO-Cu2O coatings were prepared on 316L nanoporous stainless steel (NPSS) by anodization-assisted electrodeposition. Then, CuO-Cu2O/NPSS was characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDX). The present study also investigated the influences of HClO4 concentration, reaction temperature, and load voltage on the morphology of nanoporous structures.
Perchloric acid (70% HClO4), ethylene glycol, Cu(CH3COO)2·H2O, and CH3COONa were purchased from Tianjin Tianli Chemical Reagent Co., Ltd. Double-distilled water was used throughout the experiments.
2.2. Anodization-Electrodeposition Treatment
Prior to anodization, AISI316L SS samples (Ф20 mm × 5 mm) were mounted on the resin to obtain an electrode with a square area of 1 cm2. SS electrode samples were grounded with No. 80–1500 emery papers, followed by suspensions of diamond paste until a mirror finish was obtained, and then ultrasonically cleaned in acetone and ethanol. Electrochemical anodization was carried out in a two-electrode setup, where SS acts as the working electrode and the platinum gauze as the counter electrode. The ethylene glycol electrolyte containing different concentrations of HClO4 was used as the electrolyte. A DC voltage stabilizer provided a constant potential between working and counter electrodes with the anodizing duration of different anodizing time. During anodization, the electrolyte was vigorously stirred or not stirred, and its temperature was maintained at 0°C. The anodized electrode was then ultrasonically washed in ethanol and distilled water for 10 min and dried in a nitrogen stream.
Electrodeposition was carried out using the Gamry Reference 600 electrochemical workstation, with the NPSS sample acting as a working electrode, a saturated calomel electrode (SCE) as the reference electrode, and the platinum mesh electrode as a counter electrode. CuO-Cu2O coatings were electrodeposited onto the surface of NPSS at a cathode potential of −1.0 V versus SCE for 3600 s. Influences of electrolyte concentration and deposition temperature on the morphology of CuO-Cu2O/NPSS were investigated. Table 1 lists the specific process numbers, electrolyte concentrations, and deposition temperature parameters. Figure 1 displays the preparation procedure of CuO-Cu2O/NPSS. CuO-Cu2O coatings were prepared on 316L nanoporous stainless steel (NPSS) by anodization-assisted electrodeposition.
2.3. Microstructural Characterization
Phases of NPSS and CuO-Cu2O/NPSS were identified by Rigaku DyMax2500 XRD with the 1.5410 Å Cu Kα line as an excitation source. Morphology, structure, and compositional distribution of NPSS and CuO-Cu2O/NPSS were characterized by a field-emission scanning electron microscope (JSM-7001F, JEOL), equipped with the energy-dispersive X-ray (QX-200, Bruker).
3. Results and Discussion
3.1.1. Effect of Electrolyte Concentration
Influences of three processing factors, namely, HClO4 concentration, temperature, and load voltage, on nanopores were investigated.
Figure 2 shows the microstructure of the nanoporous surface prepared in a HClO4 solution with different concentrations on SS under 50 V voltage at 0°C for 600 s. As shown in Figure 2, the pore size of nanopores formed by a 3 vol% electrolyte HClO4 solution is smaller than that of the 5 vol% electrolyte HClO4 solution, and nanopores exhibited characteristics of strip distribution [21, 22]. The nanoporous surface prepared in the electrolyte HClO4 solution at 5 vol% concentrations was distributed homogeneously and exhibited appropriate pore size. However, the microstructure of the nanoporous surface prepared in an electrolyte HClO4 solution at 7 and 9 vol% concentrations exhibited different degrees of pitting corrosion , and nanopore walls became thinner. Overall, the nanopore size increased with increasing application of electrolyte concentration, which is in line with expected outcomes and literature . In the following experiments, the 5 vol% electrolyte HClO4 solution was used as the electrolyte.
3.1.2. Effect of Load Voltage
Figure 3 illustrates the microstructures of the 316L stainless steel at different anodizing voltages. The nanopore structure can be formed on the surface of the 316L stainless steel (Figure 3). The load voltage is one of the most important influential factors in the formation of the nanoporous structure. Both the diameter and depth of the nanopores increase with increasing voltage. An ideal nanopores structure can be obtained at voltages of 40 and 50 V (see Figures 3(c) and 3(d)). It was found that the nanopore structure obtained at 50 V is slightly larger than that at 40 V, with estimated values of 93 and 87 nm, respectively. In the following experiments, SS samples were anodized at 50 V.
3.1.3. Effect of Anodizing Time
Figure 4 illustrates the microstructures of the 316L stainless steel in a 5 vol% electrolyte HClO4 solution at different times under 50 V voltage. Corrosion accelerated with increasing anodizing time. The color of the electrolyte turned rapidly from colorless to tan. Figure 4(a) indicates the nanopore structure prepared at 300 s. The homogeneous porous structure and some large pores were formed at an anodization time of 600 s (as shown in Figure 4(b)). Thin nanopore walls were detected, and irregular nanopore sizes were anodized at 1800 s. The microstructure of the nanoporous surface prepared at an anodization time of 3600 s showed different degrees of pitting. In the following experiments, the anodization time of 600 s was used as the anodizing time.
Anodic oxidation current was recorded each second during anodization at 3600 s. Figure 5 shows the resulting current against time plots. A sharp falling and rising of the current were noted at short anodization times (t < 120 s). The decline in the current may be ascribed to nucleation and growth of pores on the barrier layer. These observations support the conclusions derived from current against time responses recorded during anodization (Figure 4(b)) and indicate that anodizing at 600 s provides the best nanopore size.
3.1.4. Effect of Stirring
Figure 6 compares the results of anodic oxidation of SS with or without magnetic stirring. SEM images show relatively uniform pore size obtained by anodic oxidation with uniform magnetic stirring and larger pore size.
An appropriate nanoporous structure with a mean diameter of 93 nm was obtained on the SS surface by the following conditions: 5 vol% electrolyte HClO4 solution; 50 V load voltage; 600 s reaction time; reaction with uniform magnetic stirring.
Copper is easily electrodeposited at a low overpotential. In the present work, Cu was electrodeposited onto nanopores of the NPSS electrode at a cathode potential of −1.0 V versus SCE for 3600 s.
Figure 7(a) shows that numerous flower-like deposits were formed on the surface of NPSS under conditions of process A. As indicated in Figure 7(b), nanopores on the SS surface were observed, and the deposited CuO-Cu2O formed by electrochemical deposition incompletely covered the surface.
In addition to the formation of more uniformly distributed branched copper oxide, flower-like copper deposits were formed on the SS surface (as shown in Figure 8(a)). As displayed in Figure 8(b), the remaining nanopores were detected around copper deposits formed by electrodeposition, and the prepared porous CuO-Cu2O film does not cover the surface. Considering the effect of temperature, nanopores were still present around copper deposits formed by electrodeposition at 20°C. In the following experiments, 60°C was used as the deposition temperature.
As shown in Figure 9, the surface of SS nanopores is densely distributed with deposited CuO-Cu2O, and a regular cubic crystal structure of CuO exists on the NPSS surface.
Figures 10(a) and 10(b) show the morphological changes of the CuO-Cu2O/NPSS surface with different magnifications. CuO-Cu2O coatings have been successfully grown on the surface of NPSS. Analysis of coating morphologies revealed the copper dendritic crystals formed by electrodeposition. These coating morphologies agree with those recently reported . Formation of dendritic deposits is a primary characteristic of electrodeposition at overpotentials belonging to the plateau of limiting diffusion current density, whereas induction time of dendritic growth initiation depends on the overpotential of electrodeposition [26, 27]. The mechanism of typical copper dendritic structures is explained based on the diffusion-limited aggregation (DLA) model  because polycrystalline copper branches are a typical characteristic of DLA-like copper deposits. This result indicates that NPSS-assisted electrodeposition is an important technique for synthesizing metallic nanomaterials with controlled shape and size. Such a porous layer has a high surface area and excellent antibacterial properties. Figure 10(b) illustrates the EDX results of CuO-Cu2O/NPSS. Combined with XRD results, electrodeposition has formed copper oxide.
Appropriate CuO-Cu2O/NPSS was obtained by the following conditions: 0.1 M Cu(CH3COO)2·H2O + 0.1 M CH3COONa solution and 60°C deposition temperature.
3.3. XRD Analyses of CuO-Cu2O/NPSS Samples
Figure 11 shows the XRD spectra of CuO-Cu2O/NPSS. In Figure 11, the presence of Cu2O and CuO diffraction peaks can be seen. The appearance of the Cu2O phase agrees with a previous report . Zhu et al. reported that the nanoporous surface of the Ti-Cu amorphous alloy consists of TiO, Ti2O3, TiO2, and Cu2O . The presence of Cu2O and CuO phases was reasonable for the bactericidal effect.
In summary, a porous CuO-Cu2O film on NPSS was successfully fabricated by anodization-assisted electrodeposition. The present study investigated influences of HClO4 concentration, reaction temperature, and load voltage on the morphology of the nanoporous structure. An appropriate nanoporous structure with a mean diameter of 93 nm was obtained on the SS surface by the following conditions: 5 vol% electrolyte HClO4 solution; 50 V load voltage; 600 s reaction time; reaction with uniform magnetic stirring. Appropriate CuO-Cu2O/NPSS was obtained by the following conditions: 0.1 M Cu(CH3COO)2·H2O + 0.1 M CH3COONa solution and 60°C deposition temperature. XRD analysis shows that the CuO-Cu2O/NPSS surface comprises CuO and Cu2O.
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Conflicts of Interest
The authors declare no conflicts of interest.
The authors would like to thank the financial support from the National Natural Science Foundation of China (51171125 and 11172195), National Science Foundation for Distinguished Young Scholars of China (E010101), Shanxi Province Programs for Science and Technology Development (20110321051), Post-Doctoral Science Fund of China (2013M541209), Youth Foundation of Taiyuan University of Technology (2012L073), Natural Science Foundation for Young Scientists of Shanxi Province (2013021013-5), and Shanxi Province Nature Science Fund (2012011021-3).
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