Hierarchical and Well-Ordered Porous Copper for Liquid Transport

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Functional Nanostructured Materials (including low-D carbon)

Hierarchical and Well-ordered Porous Copper for Liquid Transport Properties Control Quang N. Pham, Bowen Shao, Yongsung Kim, and Yoonjin Won ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02665 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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ACS Applied Materials & Interfaces

Hierarchical and Well-ordered Porous Copper for Liquid Transport Properties Control Quang N. Pham1†, Bowen Shao2†, Yongsung Kim3, and Yoonjin Won1,2* 1

2

Department of Mechanical and Aerospace Engineering

Department of Chemical Engineering and Materials Science University of California, Irvine. 3

Samsung Advanced Institute of Technology * [email protected]

KEYWORDS hierarchical structure, mass transport, inverse opals, wettability, electrochemical polishing, electrochemical oxidation.

ABSTRACT

Liquid delivery through interconnected pore network is essential for various interfacial transport applications ranging from energy storage to evaporative cooling. The liquid transport performance in porous media can be significantly improved through the use of hierarchical morphology that leverages transport phenomena at different length scales. Traditional surface engineering techniques using chemical or thermal reactions often show nonuniform surface nanostructuring within three-dimensional pore network due to uncontrollable diffusion and reactivity in geometrically complex porous structures. ACS Paragon Plus Environment

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Here, we demonstrate hierarchical architectures on the basis of crystalline copper inverse opals using an electrochemistry approach, which offers volumetric controllability of structural and surface properties within the complex porous metal. The electrochemical process sequentially combines subtractive and additive steps – electrochemical polishing and electrochemical oxidation – to improve surface wetting properties without sacrificing structural permeability. We report the transport performance of the hierarchical inverse opals by measuring the capillary-driven liquid rise. The capillary performance parameter of hierarchically engineered inverse opal (K/Reff = 10-2 µm) is shown to be higher than that of a typical crystalline inverse opal (K/Reff = 10-3 µm) by an order of magnitude owing to the enhancement in fluid permeable and hydrophilic pathways. The new surface engineering method presented in this work provides a rational approach in designing hierarchical porous copper for transport performance enhancements.

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MANUSCRIPT Introduction Hierarchical design in micro- and nanoporous media that are inspired by biological materials, such as wood1, sea sponges2,3, and bone4,5, draws interests toward fully exploiting individual transport property at various length scales6. The unique combination of high permeability, large surface area-tovolume ratio, and high active sites in hierarchical porous media promises performance breakthroughs for various applications, ranging from catalysis to energy storage and conversion7-11. The rational design and synthesis of hierarchical porous media for enhanced interfacial transport properties have been extensively demonstrated on microstructures such as pillar arrays12-14, plywood stacks15-16, and inverted scaffolds17-19. Recent advances in nanofabrication techniques further utilize template-assisted selfassembly approach to create inverse opal (IO) structures20-25, a relatively new class of three-dimensional (3D) porous media with spherical pore arranged in periodic order. The crystalline pore network and submicron characteristic length scale of IOs attract a myriad of mass transport applications that require well-ordered microscale fluid flow. Among the numerous explored transition metal oxide nanostructures used in metallic hierarchical architectural material, copper oxides remain extensively investigated26-28 owing to its nontoxicity, low-cost synthesis29, unique morphologies30-32, and intrinsic surface and catalytic properties29. However, due to the compactness of the pore packing volume, the incorporation of copper oxide nanostructures within copper IOs is a daunting challenge and has yet to be demonstrated. A major challenge in installing nanoscale oxide features onto complex pore network is to control surface properties without consequently hindering its mass transport performance. For instance, the additive growth of thick metal oxide nanostructures within porous media with sub-micron pore sizes may result in clogging of pores and inhibition of mass flow. Therefore, the successful installation of oxide features within 3D porous network for mass transport applications requires the fulfillment of three criteria: (1) conformal deposition of copper oxides along the curvature of the pore surfaces and uniform throughout the depth of the sample; (2) the deposition of an ultra-thin oxide film below 10% of the pore ACS Paragon Plus Environment

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diameter that can preserve the fluid-permeable pathways within IOs; and (3) a stable hydrophilic surface despite varying environmental conditions. A common approach in functionalizing copper surfaces has traditionally been through either thermal or chemical oxidation techniques33-35, which often produces nonhomogeneous oxidation in porous media due to the lack of uniform volumetric reactions. Such produced oxide film thickness ranging from submicron to the tens of micron has limited previous investigations14,32,36 to flat or structured surfaces with no reports on the oxidation of submicron copper porous media. In addition to the permeability within the fluid pathways, the surface hydrophilicity also facilitates mass transport through porous media that requires stable wetting properties under various external conditions for use in real-world applications. In order to satisfy the three criteria described above, we electrochemically demonstrate the hierarchical design of 3D crystalline copper IO through the combination of both subtractive (i.e., electrochemical polishing) and additive (i.e., electrochemical oxidation) fabrication techniques, which improves surface wetting properties without sacrificing structural permeability within the hierarchically ordered porous structure. Specifically, electropolishing demonstrates fine tunability of overall IO porosity for enhanced mass transport performances by increasing the interconnected window openings between spherical pores. Despite increasing the permeable pathways with electropolishing, the porous copper surface remains hydrophobic that is resistant to capillary-driven liquid delivery. Sequentially electrodepositing copper oxide nanofeatures and performing electrochemical oxidation of the surface significantly improves the wetting properties that facilitate mass transport. Due to the preceding electropolishing step, the increase in the interconnected window diameters prevents the deposited oxides film from clogging the permeable pathways and negatively affecting the liquid delivery performance. The series of electrochemical steps presented in this work will make strides in producing a new class of hierarchical 3D copper porous structures to control transport mechanics as well as understanding the changing nature of copper oxides as a functionalized nanostructure for a wide range of applications. The general creation of hierarchical porous copper includes the fabrication of copper IO (Figure 1a-c) and multiple electrochemical steps (Figure 1d-f). Crystalline copper IOs are fabricated using ACS Paragon Plus Environment

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templated-assisted vertical deposition process22 (see Method). Self-assembled opal template of 1 µm polystyrene spheres serves as the sacrificial mold while copper structural material electrodeposits into the empty spacings between the spheres. Removal of the templated opals reveals an inverted porous copper scaffold (~74% porosity37,38) with uniform pore sizes and narrow interconnected windows between adjacent pores, denoted as the “via,” which are formed from the templated sphere-to-sphere contact points. While the interconnectivity of pore network enables fluid-permeable pathways, the size of via diameters determines the structural permeability in relation to mass transport performance. When the copper oxide nanostructures electrodeposit into the porous copper framework, the interconnected via may significantly decrease in diameters or potentially close themselves, inhibiting transport and preventing uniform hierarchical nanostructuring from occurring throughout the volume of the porous structure. In response, we adopt the strategy of using electropolishing to increase the via diameter within the copper IOs and thus, the overall structural porosity, before proceeding with the electrodeposition of oxide nanostructures to induce hierarchical surface functionalization. The presented electrochemical process is performed in three steps: (step 1) electropolishing of porous copper38,39 to remove structural material through Cu → Cu2++ 2e-, (step 2) electrodeposition of cuprous oxide40 through 2Cu2+ + H2O + 2e- Cu2O + 2H+, and (step 3) oxidation of the cuprous oxide41 through Cu2O +H2O2 2CuO + H2O. All the electrochemical steps are conducted using pulsed potential to ensure adequate amount of time for ion diffusion through the copper IOs, resulting in conformal surface chemical reactions (see Methods for more electrochemical process details, see Supporting Information Figure S3 for guide in copper compositional diagram at each step). This study systematically investigates the effects of electrochemical process parameters on the modulation of the IO structural porosity, oxide film thickness, chemical compositing and wetting properties by using scanning electron microscopy (SEM, FEI Quanta), X-ray photoelectron spectroscopy (XPS, AXIS Supra), and goniometer (Kyowa Interface Science). In such systematic analysis of electrochemistry parameters, the majority of the samples undergoes all three electrochemical steps, and within which, one step is varied to isolate the interplayed parameters. Hereupon, we list the sequential steps undertaken in ACS Paragon Plus Environment

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the electrochemical process for clarity, and the step in which parameters are modulated will be denoted with an underline (e.g., samples undergone all three electrochemical steps with varied parameters in step 2 will be represented as step 1-2-3). Before introducing the electrochemical oxidation step, we investigate the electroetched IOs by modulating the electropolishing cycle numbers in step 1, which determines the degree of structural material removal. The nondimensionalized value of via diameter to the pore diameter dvia/dpore increases with increasing electropolishing cycle number (see Supporting Information Figure S9 for statistical distributions of diameter measurements). The overall structural porosity φ of the IOs can be obtained with dvia/dpore values using a numerical correlation derived through computational fluid dynamics (CFD) from our previous work25: φ = 0.5833(dvia/dpore) + 0.6633. The IO porosity ranges from ~75% to ~90% through electropolishing as presented in Figure 2a. Porosity can be further increased with additional electropolishing cycles (i.e. > 300 cycles) but risk structural damages to the IO (see Supporting Information Figure S6) as it reaches its maximum theoretical porosity38 of ~96%. The mechanical strength is expected to decrease with increasing electropolishing cycles due to the incremental thinning of structural material. However, the three-dimensional IOs that are electropolished below 300 cycles demonstrate relatively high structural integrity by remaining a monolithic architecture with no observable structural collapse or delamination. The surface wettability of the as-fabricated (see Supporting Information Figure S5) and electropolished IOs (see Figure 2b) reveals that the porous copper structures remain consistently hydrophobic. At 0 electropolishing cycle, the apparent contact angle is ~135°. Between 50 and 150 electropolishing cycles, the contact angles remain relatively constant at ~140° before increasing up to ~150° at 250 cycles. The slight increase but stable wettability at ≤ 150 electropolishing cycles can be contributed to the relatively unaltered “cut-level” along the surface, as seen from the top SEM images of the electropolished copper IO in Figure 4b. The copper IOs with ≥ 200 electropolishing cycles exhibit more broken pore ligaments such that the roughness of the microporous surfaces is comparatively higher than that at lower electropolishing cycles, which may have contributed to the observable increases in contact angle42. ACS Paragon Plus Environment

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The following surface functionalization technique deposits cuprous oxides (in step 2) and further oxidizes the surface (in step 3) with film thickness of ~50 nm that uniformly conforms to the active areas of the three-dimensionally-interconnected micron spherical pore structure. The various electrochemistry parameters (i.e., oxidation deposition cycle number, molarity of hydrogen peroxide electrolyte, and applied potential) are systematically tested within their respective processes in steps 2 and 3. Such electrochemistry parameters are rationally selected after a similar study using flat copper substrates, and detailed results are presented in Supporting Information Figure S12. In this process, although the electrodeposition of cuprous oxide produces a hydrophilic surface (step 2), we observe the instability of the surface energy with extended exposure to liquid wetting. The wettability of a freshly prepared sample (step 1-2) demonstrates an initial apparent contact angle of ~30°. As the sample is exposed to liquid by quickly dipping it in a bath of deionized (DI) water and drying it with compressed air, the subsequent contact angle of the surface significantly increases with each dipand-dry process due to the de-oxidation of cuprous oxide in aqueous environment43 (see red squares in Figure 3c). However, by introducing the additional step of electrochemical oxidation of the deposited cuprous oxides (step 3), the surface wettability of the samples (step 1-2-3) proves to be stable despite repeated liquid exposure (see blue triangles in Figure 3c). The observation of such consistent hydrophilicity motivates the use of a multi-step electrochemical oxidation process, which is presented for the first time within this work. To understand the effects of each step in the electrochemical oxidation process (step 1-2-3) on surface morphology and wettability, we first vary the oxide deposition cycles in step 2 from 10 and 90 to control the length of reaction time whereas the electrochemical oxidation in step 3 remains constant with applied potential and molarity at 0.9 V and 1 mM, respectively, which is denoted as step 1-2-3. Figure 3a shows the resulting copper IOs after cuprous oxide deposition. The increase in oxide deposition cycles causes an increase in oxide thickness δoxide from 20 nm to 60 nm, which is calculated based on the changes in via dimeter dvia before and after oxidation, as shown in Figure 3d. Furthermore, a minor decrease in contact angles from 30° to 20° corresponds with increasing oxide deposition cycle ACS Paragon Plus Environment

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numbers, suggesting that even small amount of electrodeposited copper oxide contributes to higher surface energies (see Supporting Information Figure S7a). Then, we modulate the combinations of applied potentials (0.6 – 0.9 V) and molarity of the hydrogen peroxide electrolyte (1 – 5 mM) in the electrochemical oxidation in step 3 while maintaining the oxide deposition cycles in step 2 constant at 90 cycles, which is denoted as step 1-2-3. The resulting variations of copper oxide morphology within copper IOs are shown in Figure 3b. This study suggests the combination of high potential and low concentration contributes to the formation of uniform, conformal nanoscale copper oxide crystals (~80 nm in size) along the pore curvature with minimal changes to the via diameters. Both lower electrical potential (i.e., lower than 0.75 V) and higher electrolyte concentration produces long copper hydroxide wires44 (2 - 4 µm in length), which overwhelm the porous structure such that no pores or via are visible (see Supporting Information Figure S8 for the chemical composition characterization). The conformal coating of nanoscale copper oxide crystals observed underneath the overgrown microwires suggests a continuing presence of the initial deposited oxide layer. The wettability measurement shows consistent hydrophilicity between 20° to 30° in contact angles in Supporting Information Figure S7b. This parametric study suggests an optimized combination of electrochemical oxidation with applied potential, electrolyte molarity, and oxidation cycle numbers of 0.9 V, 1 mM, and 90 cycles, respectively, to deposit conformal thin copper oxide film for the following electropolished and oxidized copper IOs for transport study. After separately demonstrating fine tunability in structural porosity and surface oxidation, we prepare the IO samples with varying electropolishing cycles to evaluate their liquid transport properties through capillary wicking measurement. In this process, the extent of electropolishing varies from 50 to 250 cycles by increments of 50 cycles while the oxidation parameters are kept constant at 0.9 V, 1 mM concentration, and 90 oxidation cycles. Figure 4 displays the structural morphology of the copper IO after fabrication, electropolishing (step 1), and electrochemical oxidation (after step 1-2-3). Additional cross-sectional SEM imaging confirms that electropolishing and oxidation occurs uniformly throughout the depth of the IOs (Supporting Information Figure S10). ACS Paragon Plus Environment

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The overall porosity increases with electropolishing cycles, as shown in Figure 2a, and subsequent oxidation at constant parameters generally decreases the porosity by ~2% -5% without closing the via (Figure 5a). By oxidizing the copper IOs, the surface wettability improves by decreasing the apparent contact angle from ~130° – 150° down to ~30° – 40° (see inset in Supporting Information Figure S11a). The hydrophilic wetting characteristics and tunable porosity promise liquid transport enhancement in hierarchically engineered copper IOs. The liquid transport can be evaluated by a figure of merit called the capillary performance parameter, defined as the ratio of permeability to the effective capillary pore radius K/Reff. We determine the performance parameter through capillary-driven liquid rise measurements25 (see Method and Supporting Information Figure S14). The incremental increase in electropolishing cycles from step 1-2-3 results in a general increase in capillary performance parameter K/Reff values from ~1 x 10-3 µm to 11 x 10-3 µm (Figure 5b). The deviations in K/Reff values for each sample are associated with the variances in liquid rise height across the width of the IO, which are caused by nonuniform capillary wicking. This may be contributed to the polycrystalline nature of IOs45, which presents additional transport resistance at grain boundaries between crystalline pore domains25. The K/Reff values for unpolished, but electrochemically oxidized (step 2-3), 1 µm pore diameter copper IOs (0 cycle) presented in this work agrees well with our previously finding25 for polycrystalline IO structure. By increasing the electropolishing cycles beyond 100 cycles (> 80% in porosity), the resulting K/Reff is comparable to that of single crystalline IO (i.e., porous media without grain boundaries), surpassing the upper bound of liquid transport in polycrystalline IOs with typical porosity of ~75%. While functionalization of copper promises to be effective in increasing the surface hydrophilicity for various applications ranging from drag reduction46 to heat transfer enhancement47,48, their functionality depends upon the durability of the copper oxide coating49. Hence, the changes in surface wettability examined in this study aim to elucidate the fundamental nature of copper oxide nanostructure as a durable hierarchical and functionalized structure in highly complex pore network. In this manner, we monitor the wettability of oxidized copper IOs as it changes with repeated liquid ACS Paragon Plus Environment

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exposure, time, and heating conditions to understand their wetting durability. The electrochemical steps provide a stable hydrophilicity at elevated temperatures under 75°C, and the additional oxidation using step 3 further stabilizes wetting properties after repeated liquid exposure. However, we observe a rapid increase in hydrophobicity with both temporal monitoring over a span of 10 days and heating above 75°C (see Supporting Information Figure S11 and S12), which requires additional surface treatments in order to fully use these materials under various conditions. The changes in wetting properties can be explained by surface chemistry changes as provided by detailed XPS analysis under such temporal and thermal conditions (see Supporting Information Figure S13 for details and XPS spectra).

Conclusion In conclusion, we report controllable modulation of structural porosity as well as surface wetting properties of hierarchically designed copper IOs for enhanced liquid transport through their pore networks using complimentary electrochemical steps in series. In this process, once electropolishing systematically modulates the structural porosity, electrodeposition of oxides and additional electrochemical oxidation enable thin conformal coating of oxide nanostructures along active surface areas, which improves the hydrophilicity of porous copper. The liquid transport through rationally designed copper IOs exhibits an enhancement in performance with increasing structural porosity for fluid to permeate with lower amount of hydraulic resistance. In order to understand the stability of copper oxides for various applications, we examine the fundamental nature of copper oxide nanostructure as a hierarchical and functionalized structure over time and under heating conditions, confirming stable hydrophilicity with repeated liquid exposure and heating under 75°C. We also note that surface wettability quickly becomes hydrophobic with time and at elevated temperatures due to changes in surface chemical composition. Additional work is needed to engineer a functionalized copper surface that remains durable in various conditions while still three-dimensionally conforms to complex pore curvatures without hindering transport mechanics throughout the network. The rational design of a


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multifunctional crystalline IOs through hierarchical architecture provides opportunities to optimize mass transport and diffusion in future classes of three-dimensionally interconnect pore structures.

METHOD Copper Inverse Opal Preparation The preparation of copper IO has been reported in our previous work25. A silicon wafer is coated with 20 nm of titanium as an adhesion layer and 80 nm of gold by electron beam evaporation (CHA Industries). The gold-coated substrate is functionalized in an aqueous solution of 1 mM 3-mercapto-1propanesulfonic (Sigma Aldrich) for over 24 hours. The colloidal suspension (0.6% wt/v) is prepared by dispensing polystyrene spheres with diameters of 1 µm (Thermo Fisher) in DI water. The substrate is submerged vertically in the suspension. A base heating of the colloidal suspension at ~55°C induces gentle convective mixing and prevents sphere sedimentation. As the solvent evaporates over time, the polystyrene spheres begin to self-assemble into crystalline arrangement at the thin-film region of the evaporating meniscus, and a film of highly-ordered opal forms (Supporting Information Figure S1a). The opal film is annealed in a radiant heat oven (Lab-Line) at ~98°C for 5 hours. Copper as a structural material is electrodeposited within the spacings between the sacrificial spheres using a three-electrode system with a current of 7.5 mA cm-2 in a galvanostatic mode controlled by a potentiostat (SP-300, BioLogic) (Supporting Information Figure S1b). Afterward, the polystyrene spheres entrapped inside the copper scaffold are dissolved using tetrahydrofuran (Sigma Aldrich) to reveal an inverted opal structure (Supporting Information Figure S1c). To ensure all organic residues and contaminants are removed, the IOs are cleaned with oxygen plasma (Harrick) at 50 W for 2 min, followed by immersion in 0.01 M of hydrochloric acid for 3 min to remove any oxygen species from the surface, and rinsed in DI water.

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Step 1. Electropolishing: Electropolishing method38,39 is used to systematically increase the via diameters and thus the copper IOs’ porosity based on the reaction: Cu → Cu2++ 2e- (see Supporting Information Figure S2 for schematic of electrochemical set-up). The electropolishing of copper IO anode is performed in a three-electrode system using Ag/AgCl as the reference and a piece of bulk copper as the counter-electrode. An aqueous solution of 0.5 M CuSO4 (Alfa Aesar) and 0.1 M H2SO4 (BDH Chemicals) is prepared as the electrolyte. To ensure adequate time for ion diffusion throughout the porous medium, pulse potential technique (0.65 V versus Ag/AgCl; 0.1 sec on and 10 sec off) is applied on the working electrode with various electropolishing cycle numbers ranging from 50 to 250 cycles. Step 2. Cuprous Oxide Electrodeposition: Copper oxide nanostructures with controllable thickness and morphologies can be grown directly on copper IO structures by first electrodepositing cuprous oxide using a pH-dependent reaction40: 2Cu2++H2O+2e- → Cu2O + 2H+. The electrodeposition uses a three-electrode system, which comprises of an electropolished copper IO, a carbon cloth, and a Ag/AgCl as the working, counter, and reference electrode, respectively. An aqueous electrolyte solution of 0.4 M CuSO4 (Alfa Aesar) and 1.6 M lactic acid (Thermo Fisher) is prepared where the pH values and electrical potential with a temperature condition (i.e., 65°C) are chosen using the Pourbaix diagram50. The pH value of the electrolyte is adjusted to 9 using sodium hydroxide. Pulsed process (-0.5 V versus Ag/AgCl; 0.1 sec on and 10 sec off) deposits cuprous oxide on copper IO surfaces. Step 3. Oxidation: The oxidization of cuprous oxide using hydrogen peroxide solution is based on the reaction41: Cu2O + H2O2 → 2CuO + H2O and is performed using the same electrochemistry setup as the previous cuprous oxide deposition process where the electrolyte is a solution of 1 M NaOH and H2O2 (Thermo Fisher). The pulsed oxidation (1 sec on and 10 sec off) of the cuprous oxide is performed at room temperature.


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Capillary Rise Measurement The capillary performance parameter is measured using a liquid rise test and defined as the ratio of permeability to the effective pore radius K/Reff, as derived using Washburn dynamics37 and can be expressed as a function of capillary rise height h in: h2 = (2σKt)/(µφReff), where t is the time that corresponds to h, µ is the viscosity of the liquid, φ is the structural porosity, σ is the liquid surface tension, and K is the permeability. The effective pore radius Reff = 0.5dporecos-1θ where θ is the static contact angle. Within the capillary wicking experiment, the sample is vertically lowered into a sealed chamber with a reservoir of DI water, using a motorized z-stage (see Supporting Information Figure S14 for schematic set-up). A parafilm top cover with a thin slit allows the IO sample to be lowered through while keeping the enclosure saturated to minimize evaporation. As the porous sample touches the DI water, the liquid immediately wicks up, and during which, the z-stage stops moving to initialize the measurement at time t = 0 sec. The capillary liquid rise is monitored with a camera at 120 fps. Postimage processing allows us to track the liquid rise over time, using ImageJ software. The effective capillary rise height h is averaged from approximately 30 measurements of the liquid rise height along the lateral width of the sample with the base of h starting at the top of the formed meniscus to the edge of the liquid rise. More details on capillary wicking measurements can be referred to by our previous paper25.

Surface Wettability Measurement Surface wettability is characterized by examining the contact angle of a water droplet on the surface. Using a dispensing system (Kyowa Interface Science), a ~15 nL sessile droplet of DI water dispenses onto the surface, during which, a high-speed camera (FASTCAM SA8, Photron with 100,000 fps) captures the droplet. An integrated software (FAMAS) calculates the evolution of the droplet contact angle using half-angle method. The dynamic contact angles are also examined using droplet volume changing method. As a droplet forms from the capillary tip and comes in contact with the surface, the droplet continues to gradually increase in volume, which produces a maximum contact angle (i.e., ACS Paragon Plus Environment

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advancing contact angle). Then as the droplet gradually decreases, we measure a minimum contact angle (i.e., receding contact angle). The expansion and contraction of the droplet is recorded with the high-speed camera. In addition, temperature-dependent contact angles are measured using a thermoelectric cooler module to control sample heating temperature, which is confirmed and monitored using a thermocouple attached to the sample.

ACKNOWLEDGMENT Q.N.P. is grateful for the financial support from the UCI Mechanical and Aerospace Engineering Department Graduate Fellowship and the Samsung Global Research Outreach Program. The characterization was performed at the Irvine Materials Research Institute at UCI.

AUTHOR CONTRIBUTION Y.W. proposed and supervised the project. Q.N.P. and B.S. contributed equally in material preparation and characterization. Q.N.P., B.S., Y.K., and Y.W. wrote the manuscript. All the authors participated in discussions of the research.

SUPPORTING INFORMATION The following information are available in the Supporting Information section: 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 ACS Paragon Plus Environment

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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

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Figure 1. Process schematic for inverse opal (IO) fabrication and electrochemical processes. a) Inverse opal is created by self-assembling spheres into a close-pack formation to form an opal template. b) Copper electrodeposits in between the sacrificial spheres. c) Removing the spheres reveals a copper IO with defined pore diameter dpore and via diameter dvia. Electrochemical process rationally modulates structural and surface properties and can be separated into three distinct steps. d) The as-fabricated IO is electropolished in step 1 to increase dvia. e) Afterward, cuprous oxide is electrodeposited onto the pore surfaces in step 2, followed by f) the electrochemical oxidation of the cuprous oxide in step 3.


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Figure 2: Structural porosity and surface wettability of electropolished IOs (step 1). a) The nondimensionalized ratio of dvia/dpore (unfilled blue circle) is used to quantify the structural porosity φ of the IOs using a numerical correlation derived from computational fluid dynamics (filled red circle). b) The contact angles of IOs after electropolishing remain hydrophobic motivating further surface functionalization to improve surface wettability for study in mass transport. The inset images show a capture of the droplet contact angle at 0 and 250 electropolishing cycles. Scale bars are 200 µm.


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Figure 3: Systematic analysis of the electrochemical oxidation process parameters. Top view scanning electron microscopic (SEM) images of oxidized IOs with a) varying cuprous oxide electrodeposition cycles after undergoing step 1-2-3, and b) varying applied voltage and electrolyte molarity during the oxidation of cuprous oxide after step 1-2-3. Scale bars for images and insets in are 1 µm. The underline in the step sequence denotes the step in which parameters are modulated. c) The contact angle of ACS Paragon Plus Environment

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oxidized surface after step 1-2 (red squares) and step 1-2-3 (blue triangles) with repeated surface wetting and drying. d) Copper oxide layer thickness δoxide is quantified based on the changes in the via diameter dvia before and after oxidation in copper inverse opals.

Figure 4: Structural characterization of electropolished and oxidized copper IOs. Top view SEM images show a) as-fabricated IOs, followed by b) incremental electropolishing cycles from 50 to 250 cycles (step 1). The IOs then undergo a two-step electrochemical oxidation process, with the final morphological results (step 1-2-3) shown in c). Scale bar is 1 µm.


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Figure 5: Structural porosity after oxidation and liquid delivery performance. The porosity in a) and the capillary performance parameter K/Reff of IOs in b) (after step 1-2-3) increase with extensive electropolishing. The IOs with 0 electropolishing cycles represent as-fabricated IOs with electrochemically oxidized surfaces (step 2-3).


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ACS Applied Materials & Interfaces 1 2 3 4 5 As-Fabricated 6 7 8 9 10 11 12 13 14 Process 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Electropolished

Oxidized


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a

Inverse Opal Fabrication

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Electrochemical Process

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

b

c


dvia dpore Opal template formation

Electrodeposit copper

d

Opal template removal

f

e

CuO

Cu2O Electropolish copper inverse opal (Step 1)


Electrodeposit oxide film (Step 2)

Electrochemical oxidation of cupric oxide (Step 3)

Fig. 2 Structural and surface wettability analysis with electropolishing process

Step 1: circle Step 2: square Step 3: triangle

a 1 2 3 4 5 6 7 8 9 10 11 12 b13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38


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Step 1

151.2°

133.9° 250 Cycle

0 Cycle


Electrodeposition Cycle Number

aPage 29 of 32 (Step 1-2-3)


1

Electrolyte Molarity (mM) 3 2

0.6

750mV Potential (V)

(Step 1-2-3)

1 2 3 4 5 6 b7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

0.75

0.9


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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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a31 of 32 Page As-Fabricated


b

Electropolished (Step 1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

50 Cycles

100

Oxidized (Step 1-2-3)

c


150

200

250

Fig.5 Overview characterization

Step 1: circle Step 2: square Step 3: triangle

a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 b15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

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Hierarchical and Well-Ordered Porous Copper for Liquid Transport

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