Hierarchical and Well-ordered Porous Copper for Liquid Transport Properties Control

Transcription

1 Supporting Information Hierarchical and Well-ordered Porous Copper for Liquid Transport Properties Control Quang N. Pham 1, Bowen Shao 2, Yongsung Kim 3 and Yoonjin Won 1,2 * 1 Department of Mechanical and Aerospace Engineering, University of California-Irvine, Irvine, CA Department of Chemical Engineering and Materials Science, University of California-Irvine, Irvine, CA Samsung Advanced Institute of Technology *Correspondence and requests for materials should be addressed to Y.W. ( won@uci.edu) Table of Content SI 1. Process Schematic for Copper Inverse Opal Fabrication SI 2. Process Schematic for Electropolishing and Oxidation of Copper Inverse Opal SI 3. Oxidation Guide with Copper Phase Diagram SI 4. Motivation for a Two-step Electrochemical Oxidation Process SI 5. Surface Wettability of Copper Inverse Opal SI 6. Excessive Electropolishing of Copper Inverse Opals SI 7. Wettability of Electrochemically Oxidized Copper Inverse Opals SI 8. Copper Hydroxide Microwire Confirmation with XPS SI 9. Data Distribution for Diameters of Pore and Via after Electropolishing and Oxidation SI 10. Volumetric Electrochemical Processes Confirmation SI 11. Durability Analysis of Nanostructured Inverse Opal SI 12. Electropolishing and Oxidation of Flat Copper Substrates: Oxide Morphology and Thickness SI 13. Wettability and Chemical Composition of Nanostructured Oxides on Flat Copper Substrates SI 14. Schematic Set-up for Capillary Wicking Measurement S-1

2 SI 1. Process Schematic for Copper Inverse Opal Fabrication Figure S1. The fabrication of copper inverse opal a) utilizes the self-assembly of polystyrene spheres to form a sacrificial opal template through vertical deposition. After the opal film anneals in an oven to increase sphere-to-sphere contact area, b) copper is electrodeposited into the void spacing between spheres. c) By dissolving the opal template, a copper inverse opal structure remains. SI 2. Process Schematic for Electrochemical Steps Figure S2. The electrochemical process performed on copper inverse opals is separated into a series of three steps. a) Step 1: The electropolishing of as-fabricated inverse opal removes copper structural material from the porous medium and deposits onto the bulk copper cathode. b) Step 2: The oxidation of porous copper starts with the electrodeposition of cuprous oxide as a seed layer onto the inverse opal surfaces. c) Step 3: the electrochemical oxidation of the cuprous oxide film occurs in this step. S-2

3 SI 3. Oxidation Guide with Copper Phase Diagram Figure S3. The various composition of copper at different stages of the multiple electrochemical steps. The guide for different copper compound is shown for the copper materials after a) step 1: electropolishing, b) step 2: cuprous oxide electrodeposition, and c) step 3: electrochemical oxidation of the cuprous oxide. SI 4. Motivation for Electrochemical Oxidation Process After the electrodeposition of oxides in step 2, we additionally perform the electrochemical oxidation step in order to convert the cuprous oxide to copper oxide [1]. The step 3 is necessary to prevent the de-oxidation process of cuprous oxide in the aqueous environment after the step 2 [2]. As observed in Figure S4a, the initial apparent contact of the cuprous oxide coated flat surface from step 1-2 is ~30. However, when the sample is repeatedly wetted and dried (i.e., by quickly dipping the sample in a beaker of DI water and blow drying it with compressed air), the contact angle increases to ~70 (see red squares in Figure S4a). Following electrochemical oxidation of the cuprous oxide using step help the surface to remain consistently hydrophilic despite repeated wetting and drying (see blue triangles in Figure S4a). This motivates a two-step electrochemical oxidation process to ensure wetting stability. The resulting surface energy of oxidized samples as they separately undergo step 1-2 and step is also examined for its durability under heating environment. Both samples from step 1-2 and endure heating up to 95 C follow by relaxing back down to 25 C. Initially, both samples show a slight continual increase in contact angle until the temperature reaches above 75 C, in which the rising rate in contact angle rapidly increases toward the hydrophobic regime. This suggests a significant and permanent compositional shift in the copper surface chemistry at elevated temperatures beyond 75 C, such that the surface wettability fail to return to its original contact angle. S-3

4 Figure S4. The wetting durability of oxidized flat copper substrates through multiple electrochemical oxidation steps. a) The deposited cuprous oxide surface after step 1-2 (red squares) rapidly increases in contact angle with each succession of surface wetting and drying. The additional electrochemical oxidation of the cuprous oxide after step (blue triangles) results in a surface that remains consistently hydrophilic despite repeated wetting and drying. The temperature-dependent contact angle of b) cuprous oxide from step 1-2 and c) additional electrochemical oxidation from step shows rapid de-oxidation at elevated temperatures with increasing contact angle. SI 5. Surface Wettability of Copper Inverse Opal After the fabrication of copper inverse opals, the samples are plasma-cleaned to remove organic residues and immersed in 1 mm of HCl immersion. The surface wettability of the cleaned copper inverse opal is presented in Figure S5, which demonstrate the intrinsic hydrophobicity of the porous metal. Figure S5. Contact angle measurements of copper inverse opal after the surface cleaning. S-4

5 SI 6. Excessive Electropolishing of Copper Inverse Opals Figure S6. Surpassing the porosity limit of inverse opals through excessive electropolishing. When electropolishing is performed above 300 cycles, structural damages occur with few remaining intact spherical pores. Scale bar is 1 μm. SI 7. Wettability of Electrochemically Oxidized Copper Inverse Opals Figure S7. The surface wettability of electrochemically oxidized copper inverse opals after d) step and e) step The underline in the step sequence denotes the step in which parameters are modulated. S-5

6 SI 8. Copper Hydroxide Microwire Confirmation with XPS The chemical composition of the microwires found in Figure 3b and Figure S12a are examined using X-ray photoelectron spectroscopy (XPS). Figure S8 presents the main peaks for copper hydroxide (Cu(OH)2) in high-resolution Cu 2p3/2 and O 1s spectra, confirming the chemical composition of the microfeature. Figure S8. High-resolution X-ray photoelectron spectroscopy spectra at a) Cu 2p3/2 and b) O 1s for an oxidized copper surface with densely populated microwires. S-6

7 SI 9. Statistical Distribution for Pore and Via Diameters after Electropolishing and Oxidation Figure S9. Statistical distribution for the structural characteristics in copper inverse opals. After electropolishing process (step 1), the diameters of a) via and b) pore are presented for each electropolishing cycle number. The following electrochemical oxidation using step functionalizes the inverse opals at constant parameters (0.9 V in applied voltage, 1 mm in electrolyte concentration, and 90 oxidation cycles). The resulting measurements of c) via and d) pore diameters are also shown for each respective electropolishing cycle. S-7

8 SI 10. Volumetric Electrochemical Processes Confirmation Figure S10. Cross-section view of the copper inverse opal after series of electrochemical steps The uniformity throughout the depth of the inverse opals after the electrochemical process can be confirmed by examining the cross-section of the porous media. The schematic represents interconnected pores with surface nanostructuring throughout the complex structure. Scale bar is 2 μm. SI 11. Durability Analysis of Nanostructured Inverse Opal Temporal monitoring over a span of 10 days is conducted for oxidized copper IOs that have been electropolished for 0, 50, 150, and 250 cycles and are maintained under ambient environment (results are shown in Figure S11). While the initial contact angles of the electropolished and oxidized copper IOs are reasonably hydrophilic at ~30, the contact angles for the electropolished IOs rapidly increase to ~120 within 50 hours after initial sample oxidation and then remain at a constant level of hydrophobicity. The apparent contact angles of the oxidized IOs after 10 days are similar to that after electropolishing, suggesting that natural de-oxidation over time reverts the surface back to its intrinsic characteristics [3]. Compared to the electropolished IOs, the unpolished sample approaches its intrinsic hydrophobicity at a relatively slower rate, which can be contributed to the naturally lower surface roughness that is unaffected by the etching effect of electropolishing. The insets of contact angle images demonstrate the dramatic wettability changes of IO with 250 electropolishing cycles from 0 to 240 hr. The durability of copper oxides under external heating conditions is further examined to provide insights into the evolution of oxidation with temperature changes. In this study, the copper IO is heated using a thermal electric cooler module. While the contact angles remain relatively constant at ~50 as the sample heats up to 75 C, after which the surface becomes hydrophobic (see Figure S11b). Incrementally relaxing the temperature back down to 25 C fails to return the surface wettability back to its original state, suggesting that the changes in droplet contact angle are not due to temperature-dependency but rather are caused by permanent de-oxidation from external heating [4]. Once equilibrated, each temperature is maintained for approximately 10 min as contact angle measurements are conducted. The wettability measurement presents in Figure S10b is of a copper IO that has been electropolished for 250 cycles and oxidized one day earlier. A complimentary analysis using XPS reveals that surface chemical composition changes on oxidized flat copper substrates as they undergo temporal and thermal conditions (see SI 12 and 13 for further discussions). S-8

9 Figure S11. The durability of oxidized copper inverse opals measured by surface wettability. a) The contact angle of electropolished and oxidized inverse opals after step increases over time in ambient environment. The drastic change in surface wettability can be observed by the droplet contact angle of the 250 polishing cycle sample from 0 to 240 hr, as shown in the inset image from bottom to top, respectively. The contact angle of freshly oxidized inverse opals that undergo various degree of electropolishing are presented in the plot inset. b) The surface wettability of an oxidized copper inverse opal that has been polished for 250 cycles significantly changes in a heating environment, especially above 75 C to become hydrophobic. The surface hydrophobicity remains even as the temperature relaxes back down to 25 C. SI 12. Electropolishing and Oxidation of Flat Copper Substrates: Oxide Morphology and Thickness Due to the highly porous architecture of IOs, it is difficult to perform XPS to characterize surface chemical composition due to limited depth penetration (1-2 nm in depth). Therefore, we conducted oxidation on bulk copper substrates to examine the properties and chemical compositions of the nanostructured copper oxides on a flat surface. The preparation of oxidized flat copper substrates is outlined as follows. Flat copper substrates are polished with 220, 400, 600, and then 1000 sandpaper grits. The substrates are cleaned by ultrasonication in succession with DI water, ethanol, and acetone for 10 min each. In order to mimic the electrochemical processes that the IOs undergo as mention in SI 2 and 3, the preparation of flat copper substrates follow similar processes. That is, the substrate anode is electropolished using a three-electrode system (see Methods for details). Pulsed potential (0.65 V versus Ag/AgCl; 1 sec on and 1 sec off) applies for 75 cycles. After rinsing the substrates with DI water, cuprous oxide electrodeposits (step 1-2) on the flat copper surface S-9

10 using a similar three-electrode system as the one used in the oxide deposition of copper IOs (see Methods). A potential of -0.5 V is continuously applied for 75 sec. The cuprous oxide on the flat copper is further oxidized by applying pulsed potential technique (0.6 V versus Ag/AgCl; 0.1 sec on and 1 sec off). By varying the oxidization cycle number (10-90 cycles) as well as electrolyte molarity (1-50 mm) in step 1-2-3, their effects on the copper oxide morphology and surface wettability can be examined to elucidate the fundamental nature of copper oxide nanostructure as a hierarchical and functionalized structure. On the flat copper substrates, nanoscale cubic cuprous oxide crystals cover the surfaces while microwires overlaying above for certain combinations of oxidation cycle number and molarity (see Figure S11a for SEM imaging of oxide morphologies). At a given oxidation cycle number with low electrolyte molarity (1 mm), only cuprous oxide crystals with nanograss features are seen on the surface (see Figure S12b for quantification of nanograss length) while more concentrated solutions (> 10 mm) consistently produce microwires that aggregate as clusters and spikes. General measurements of the microwire dimensions (see Figure S12c) reveal that for a given cycle number with increasing electrolyte molarity, the nucleation of microwires initially appears large in length (~2 4 μm) but then decrease in size. Increasing the oxidization cycle number results in a longer duration of chemical reaction, which facilitates the initial large growth of copper oxide and copper hydroxide structures during the process as these features gradually cover sample surface. While increasing the molarity results in a decrease microwire length for longer oxidation cycles (> 50 cycles), the microwire forest becomes denser in its coverage. The wettability of such grown copper nano- and microstructures is monitored and has shown to significantly increase over a span of 10 days (Figure S12i and k), and above 75 C (Figure S12j). However, it should be noted that oxidation of flat copper substrates and copper IOs vary slightly. With a significantly higher surface area in copper IOs, a greater amount of cuprous oxide is exposed to the hydrogen peroxide solution during the oxidation process. Therefore, more microwire features can easily nucleate in copper IOs than it would on the flat copper surface with the same oxidation parameters. Thus, even at lower molarity, the microwires can still grow on the IOs. The thickness of the copper oxide and copper hydroxide grown on the surface can be estimated by using focus ion beam (FIB) to carefully mill out a cross-section of the substrates (Figure S11d). SEM images reveal an increasing overall oxide layer thickness due to the growth of copper hydroxide microwires (Figure S12e-h). S-10

11 Figure S12. Morphology characteristics of oxides on flat copper substrates. a) Top view SEM images of oxide nanostructuring for various combinations of electrolyte molarity and oxidation cycle, with the quantified length of the nanograsses and microwires display in b) and c), respectively. d) Focused ion beam is used to mill a surface cross-section to estimate the oxide layer thickness, and the cross-sections for each corresponding electrolyte concentration is shown S-11

12 in e-h). The wettability of electropolished and oxidized flat copper substrates with i) timedependency at 0 hr (bottom) and after 240 hr (top). Representative j) temperature- and k) timedependent contact angle measurements as monitored for changes. All scale bars are 1 μm. SI 13. Wettability and Chemical Composition of Nanostructured Oxides on Flat Copper Substrates The surface wettability and chemical composition of oxidized flat copper substrates are examined to elucidate the evolving nature of copper oxide as a functionalization group. As shown in SI 11, the surface contact angle of oxidized flat copper substrates is measured to demonstrate the changes in surface wettability over time and at elevated temperatures, suggesting that the surface chemistry has permanently been altered. To confirm such changes in surface composition, XPS examines the chemical characteristics of the ultra-thin oxide film (a representative XPS survey spectrum of a newly oxidized sample is shown in Figure S13a). As compared to the high-resolution Cu 2p3/2 and O 1s spectra of a freshly oxidized copper substrate (Figure S13b), both the temporally (Figure S13c) and thermally degraded (Figure S13d) samples exhibit significant changes in spectra. The peak fitting for the aged oxidized surface reveals two distinct peaks in the O 1s spectrum that represents the binding energies of CuCO3 and Cu(OH)2 as well as CuO. While the heated oxide surface also possesses similar peaks in the O 1s spectrum, they are not as distinct in their separation which may potentially be due to the shorter amount of exposure to the ambient environment. Another major characteristic in the thermally de-oxidized surface is the creation of the pure copper peak at ~931 ev that shifts the Cu 2p3/2 spectrum toward lower binding energy, confirming that the surface becomes more metallic in composition. Both the temporally and thermally degraded samples possess smaller proportion of CuO peaks, which may explain the decrease in surface wettability. The reactivity of the oxidized copper surfaces to the environment and external stimuli drastically alters the chemical compositions to decrease their wettability, which may make copper oxidation an unstable form of surface functionalization in ambient environment. S-12

13 Figure S13. XPS spectra of oxidized copper surfaces with time and heating conditions. a) XPS survey spectrum of a freshly oxidized copper substrate. High-resolution Cu 2p3/2 and O 1s spectra for b) fresh oxidizes, c) oxides after 10 days, and d) oxides after heating up to 95 C. S-13

14 SI 14. Schematic Set-up for Capillary Wicking Measurement Figure S14. Schematic set-up for capillary wicking measurements. An enclosed chamber is sealed to minimized evaporation. A thin slit on the chamber cover allows the inverse opal sample to be lowered by a motorized z-stage. As the inverse opal makes contact with the liquid reservoir, capillary wicking of the fluid through the porous material occurs and is captured by a camera. A representative evolution of a wicking result is shown through the image capture, with the white dashed line serving as a guide that follows the liquid rise height. REFERENCES (1) Chirizzi, D.; Guascito, M. R.; Filippo, E.; Malitesta, C.; Tepore, A. Talanta 2016, (2) Gerischer, H. J. Electroanal. Chem. Interfacial Electrochem. 1977, 82, (3) Zahiri, B., Sow, P.K., Kung, C.H. & Mérida, W. Adv. Mater. Interfaces 2017, 4, (4) Chang, F.-M., Cheng, S.-L., Hong, S.-J., Sheng, Y.-J. & Tsao, H-K. App. Phys. Lett. 2010, 96, S-14