**4. Conclusions**

In the current study, focused on the copper substrate, two types of membranes with nanoscale needle structure and the micro/nanoscale feathery structure were successfully made by oxidative reaction and in situ chemical replacement reaction. The prepared membranes were used for systematically separating not only oil/water mixture consisting of single component but also multicomponent mixtures. We also tested the permeation pressure and liquid flux of each prepared membranes and studied the effects of microstructure and surface wettability on these two experimental performances. In comparison, while separating the single component oil/water mixture, it was observed that the separation efficiencies of the prepared membranes were above 99.8%, the Ag-CS membrane exhibited higher efficiency as compared to the Cu(OH)2-CS membrane after 10 repetitions of the experiment. As a result of the combined effect of generated surface structure and surface wettability, the Cu(OH)2-CS membrane showed a higher permeation pressure and supported larger amounts of oil/water mixtures, but in turn, the Ag-CS membrane had higher liquid flux and separated the oil/water mixture more quickly. In the separation for multicomponent oil/water mixture, the prepared membranes all showed good performance with high separation efficiency, and the results of separating multicomponent oil were somehow in between single component oils. The TOC test result of Cu(OH)2-CS membrane and Ag-CS membrane were 31.2% and 17.7% higher than average of the two oils probably because some new oil droplets created by mutual dissolution passed through the membranes. Nevertheless, the prepared membranes still performed well and maintained high efficiencies in separation, thus proving that the prepared membranes, especially the Ag-CS membrane with a feathery micro/nanocomposite structure, could be an exciting application in the field of oil/water separation.

**Supplementary Materials:** The video about the separation process as the supplementary material is available online at http://www.mdpi.com/1996-1073/12/23/4564/s1.

**Author Contributions:** The contributions of each author to the research are as follows: Conceptualization, J.Y., C.C., B.Q., J.W., M.A.Q.; Methodology, J.Y., B.Q., J.W., and M.A.Q.; Software, J.Y. and C.C.; Resources, J.Y., and M.A.Q.; Writing-Original Draft Preparation, J.Y.; Writing-Review & Editing, B.Q., J.W. and M.A.Q.

**Funding:** This research was funded by the project of National Natural Science Foundation of China, China (No. 51776168, No.51636006, No. 51611130060), the Natural Science Foundation of Jiangsu Province, China (No. BK20171236), Funds of International Cooperation and Exchange of the National Natural Science Foundation of China (Research collaboration NSFC-VR) (No. 51961135102), Shaanxi Creative Talents Promotion Plan-Technological Innovation Team (2019TD-039), The Fundamental Research Funds for the Central Universities (Creative Team Plan No.cxtd2017004 in Xi'an Jiaotong University), and the Fundamental Research Funds for the Central Universities, China (No. xjj2017086). The APC was funded by National Natural Science Foundation of China (51776168).

**Conflicts of Interest:** All authors have read and approved the final submitted manuscript. No conflicts of interest are involved in this work.
