Structures, methods of making, and methods of use

Abstract

The present disclosure provides for a nanostructured black nickel film, methods of making a nanostructured black nickel film, structures including the nanostructured black nickel film, and the like. The nanostructured black nickel film can include a structure including a plurality of nickel coated columnar structures. Coatings, including the nanostructured black nickel film, can be used in electrodes, solar cells, electrolysis, and catalysis.

Description

T|H Docket: 222112-2530STRUCTURES, METHODS OF MAKING, AND METHODS OF USECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to U.S. Provisional Application Serial No. 63 / 722,286, having the title “STRUCTURES, METHODS OF MAKING, AND METHODS OF USE”, filed November 19, 2024, and claims priority to U.S. Provisional Application Serial No. 63 / 701 ,095, having the title “STRUCTURES, METHODS OF MAKING, AND METHODS OF USE”, filed on September 30, 2024, the disclosure of which is incorporated herein by reference in its entirety.BACKGROUND

[0002] The hydrogen evolution reaction (HER) is critically important in electrochemical water splitting. Unfortunately, the high cost and scarcity of some material used in HER limit its large scale application in electrochemical hydrogen production. Thus, there is a need to overcome this and other obstacles present with present technologies.SUMMARY

[0003] The present disclosure provides for a nanostructured black nickel film, methods of making a nanostructured black nickel film, structures including the nanostructured black nickel film, and the like.

[0004] In an aspect, the present disclosure provides for methods of making a substrate having an antireflective coating. In an aspect, the present disclosure provides for a method of making a substrate having an antireflective coating, comprising: disposing a templating substrate having a first side and a second side opposite the first side into a solution, wherein the solution has a monolayer of silica nanoparticles disposed on the surface of the solution; removing the templating substrate from the solution, wherein as the templating substrate is removed from the solution, a first monolayer of silica nanoparticles is disposed on a surface of the first side of the templating substrate to form a first coated templating substrate; etching the first coated templating substrate to form a plurality of columnar structures on the first side of the first coated templating substrate to form an etched templating surface; removing the monolayer of silica nanoparticles from the etched templating surface; disposing a layer of gold onto the plurality of columnar structures of the etched templating surface to form a plurality of gold coated columnar structures; disposing the etched templating substrate including the plurality of gold coated columnar structures into a solution comprising a nickel compound, wherein a nickel layer is formed on the plurality of gold coated columnar structures to form a structure including a plurality of nickel-gold coated columnar structures on the etched templating substrate; separating the etched templating substrate from theT|H Docket: 222112-2530 structure including the plurality of nickel-gold coated columnar structures; and removing the layer of gold from the nickel-gold coated columnar structures to form a structure including a plurality of nickel columnar structures. In an embodiment, the present disclosure provides for a structure made for the method as described above and herein.

[0005] In an aspect, the present disclosure provides for a structure comprising: nanostructured black nickel film, wherein the nanostructured black nickel film has a first surface and a second surface on the side opposing the first surface, wherein the first surface is textured and the second surface is smooth, wherein the first surface is an antireflective surface, wherein the first surface includes a plurality of nickel coated columnar structures have a spacing of about 100-400 nm between adjacent nickel coated columnar structures and wherein the nickel coated columnar structures have a diameter of about 100-400 nm, wherein the spacing plurality of nickel coated columnar structures is uniformly distributed and arranged in a periodic hexagonal order. In an aspect, the first surface has an average specular optical reflectance of about 1.5 to 2.5% over the entire visible spectral range, wherein the second surface has a mirror-like smooth surface that has an average specular optical reflectance of about 80 to 90%.

[0006] In an aspect, the present disclosure provides for a structure comprising: nanostructured black nickel film, wherein the nanostructured black nickel film has a first surface and a second surface on the side opposing the first surface, wherein the first surface is textured and the second surface is smooth, wherein the first surface is an antireflective surface, wherein the first surface has a cellular structure with uniform pore size, wherein the cellular structure is about 100 to 400 nm, wherein the pores uniformly distributed and arranged in a hexagonal order periodically.

[0007] Other compositions, apparatus, methods, features, and advantages will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional compositions, apparatus, methods, features and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Further aspects of the present disclosure will be more readily appreciated upon review of the detailed description of its various embodiments, described below, when taken in conjunction with the accompanying drawings.

[0009] Figure 1 is a schematic illustration depicting the templating procedures for fabricating nanostructured black Ni films.T|H Docket: 222112-2530

[0010] Figure 2A is a typical top-view SEM image of an LB-assembled monolayer colloidal crystal of 200 nm silica nanospheres. The inset shows the Fourier transform of a 10 x 10 pm2region. Figure 2B is a magnified SEM image of the sample in Figure 2A.

[0011] Figure 3A is a photograph of a 4-inch-sized silicon wafer with templated moth-eye nanopillars on its surface. Figure 3B is a typical top-view SEM image of the sample in Figure 3A. The inset shows the Fourier transform of a 10 x 10 pm2region. Figure 3C illustrates a magnified SEM image of the same sample. Figure 3D illustrates a typical side-view SEM image of the same sample.

[0012] Figure 4A is a photograph of the smooth side of a templated black Ni film. Figure 4B is a photograph of the textured side of the same sample.

[0013] Figure 5A illustrates a typical side-view SEM image of a nanostructured black Ni film. Figure 5B illustrates a magnified SEM image of the sample in Figure 5A. Figure 5C illustrates a typical top-view SEM image of the silicon mold after removing the templated black Ni film. Figure 5D illustrates a magnified SEM image of the sample in Figure 5C.

[0014] Figure 6 illustrates an energy-dispersive X-ray spectrum of the textured surface of a black Ni film.

[0015] Figure 7 illustrates a normal-incidence specular reflection spectra obtained from 3 random locations on the smooth and textured surfaces of the black Ni sample shown in Figures 4A-4B.

[0016] Figures 8A-8D illustrate an electrocatalytic HER activity of Ni samples (smooth and textured surfaces) in an acidic solution. Figure 8A linear sweep voltammograms of a black Ni film's smooth and textured surfaces in 0.5 M H2SO4solution at a scan rate of 10 mV S'1. Figure 8B illustrates tafel plots with corresponding Tafel slopes of the samples. Figure 8C illustrates cyclic voltammograms of the smooth and textured surfaces of Ni films in 0.5 M H2SO4solution at a scan rate of 50 mV s1. Figure 8D illustrates charging current density as a function of scan rate; slope of line fit corresponds to double layer capacitance (Cai) of each sample.DETAILED DESCRIPTION

[0017] Before the present disclosure is described in greater detail, it is to be understood that this disclosure is not limited to particular embodiments described, and as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present disclosure will be limited only by the appended claims.

[0018] Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value inT|H Docket: 222112-2530 that stated range, is encompassed within the disclosure. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the disclosure, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure.

[0019] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure, the preferred methods and materials are now described.

[0020] As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present disclosure. Any recited method can be carried out in the order of events recited or in any other order that is logically possible.

[0021] Embodiments of the present disclosure will employ, unless otherwise indicated, techniques of chemistry, material science, and the like, which are within the skill of the art.

[0022] The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to perform the methods and use the coatings disclosed and claimed herein. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in °C, and pressure is at or near atmospheric. Standard temperature and pressure are defined as 20 °C and 1 atmosphere.

[0023] Before the embodiments of the present disclosure are described in detail, it is to be understood that, unless otherwise indicated, the present disclosure is not limited to particular materials, reagents, reaction materials, manufacturing processes, or the like, as such can vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. It is also possible in the present disclosure that steps can be executed in different sequence where this is logically possible.

[0024] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.General discussionT|H Docket: 222112-2530

[0025] The present disclosure provides for a nanostructured black nickel film, methods of making a nanostructured black nickel film, structures including the nanostructured black nickel film, and the like. The nanostructured black nickel film includes a structure including a plurality of nickel coated columnar structures. Coatings such as nanostructured black nickel film can be used in electrodes, solar cells, electrolysis, and catalysis. A surface of the nanostructured black nickel film has a high surface area, is antireflective, and is very dark or black in appearance. In this regard, black metal films, such as nanostructured black nickel films, with low light reflection from their surface area are of great technological importance in many electrochemical applications, ranging from efficient electrode materials for solar cells to photocatalysis.

[0026] The present disclosure provides for a templating nanofabrication technology for making black nickel films that include a plurality of nickel coated columnar structures, where the black nickel films can be electrocatalytic active. In an aspect, a monolayer silica colloidal crystal is assembled by a Langmuir-Blodgett technique that are used as sacrificial templates in fabricating wafer-sized, subwavelength-structured nanopillar arrays on silicon substrates. A layer of gold is deposited on the nanopillar array. Electroless nickel plating, which leverages a straightforward auto-catalytic chemical reaction, is applied to deposit conformal medium-phosphorus Ni coatings on the templated gold coated silicon nanopillars. Separation of the electroless Ni coatings from the silicon molds and removal of the gold layer leads to self-standing black Ni films with periodic arrays of nanopores on their surfaces as well a plurality of nickel coated columnar structures, which is a negative image of the templating silicon nanopillars.

[0027] The resulting black nickel coatings exhibit ultra-low light reflection for a broad range of visible and near-IR wavelengths. In a particular aspect, the ordered nanopores and the plurality of nickel coated columnar structures enable a refractive index gradient at the air / metal interface and can significantly suppress specular reflection (e.g., about 90% or more) for a smooth electroless Ni film to about 1% or less for a black Ni sample over a broad range of visible and near-infrared wavelengths. In an aspect, the nanotextured Ni films (e.g., the nanopores and the plurality of nickel coated columnar structures) can greatly enhance the electrocatalytic performance of hydrogen evolution reaction in an acid solution (e.g., about 0.5 M H2SO4). Extensive electrochemical measurements based on linear sweep voltammograms and cyclic voltammograms reveal that nanostructured Ni surfaces can notably reduce electrode overpotentials, while simultaneously increasing the double layer capacitances and electrochemically active surface areas. These black Ni films that possess super dark surfaces and enhanced electrocatalytic performance can be used in technological applications in electrochemical and photoelectrochemical hydrogen production. In a particular aspect, the nanostructured black nickel film has been electrochemicallyT|H Docket: 222112-2530 characterized to demonstrate an improvement of these coatings with large surface areas in electrocatalytic activities for hydrogen evolution reaction (HER). Additional details are provided in Example 1.

[0028] In general, the present disclosure provides for a method of making nanostructured black nickel films that include a plurality of nickel coated columnar structures. The method includes disposing a monolayer of silica nanospheres (silicon dioxide (SiO2)) on a templating substrate (e.g., silicon substrate). The templating structure is etched to form subwavelength-structured nanopillar arrays on silicon substrates. The silica nanospheres can be removed from the substrate using an acidic solution. Subsequently, a thin layer of gold is disposed on the columnar structures. The structure is exposed to a solution including a nickel compound to form a layer of nickel on the gold layer to form a nickel-gold layer on the plurality of columnar structures. The nickel-gold layer can be separated from the templating structure (e.g., using a HF solution), where the nickel-gold layer includes a negative image of the nanopillar arrays. The gold layer can be removed (e.g., using a typical gold etchant, such as gold etchant TFA from Transene)) to produce a structure including the plurality of nickel coated columnar structures (e.g., nanostructured black nickel film). The nanostructured black nickel films can then be further processed and used in various structures such as electrodes and the like. Additional features are described in Example 1 .

[0029] In an aspect, the nanostructured black nickel film can have a thickness of about 100 nanometers to 1 cm or about 50 nm to 1 pm. The length and width can be varied and can be on the scale of micrometers to centimeters or more. The nanostructured black nickel film has a first surface and a second surface on the side opposing the first surface. The first surface is textured (e.g., negative image of the nanopillar array) and the second surface is smooth.

[0030] In an aspect, the nanostructured black nickel film can include a plurality of nickel coated columnar structures having a spacing of about 100-400 nm or about 200 nm between adjacent nickel coated columnar structures (e.g., measured at the base of the columnar structure). In an aspect, the nickel coated columnar structure can have a diameter of about 75-400 nm, about 100-400 nm or about 200 nm at the base of the nickel coated columnar structure. In an aspect, the nickel coated columnar structures taper from the base of the nickel coated columnar structures to the tip. The columnar structure with the spacing is uniformly distributed and arranged in a hexagonal periodic order. In an aspect, the first surface can have a cellular structure with uniform pore size, where the cellular structure of the pore depth is about 75-400 nm, about 100 to 400 nm or optionally about 200 nm. The pores can be separated by about 100 to 400 nm or optionally about 200 nm. In an aspect, the first surface includes the plurality of nickel coated columnar structures and the pores as described above and herein.T|H Docket: 222112-2530

[0031] In an aspect, the nanostructured black nickel film includes the plurality of nickel coated columnar structures as well as area under the plurality of nickel coated columnar structures (as illustrated in Figure 1). The dimensions of the plurality of nickel coated columnar structures are described above and herein and the thickness in the area under the plurality of nickel coated columnar structures can be about 10 nanometers to 1 cm or about 50 nm to 1 pm (e.g., measured from the base of the columnar structure to the surface of the second side of the nanostructured black nickel film). The total thickness of the nanostructured black nickel film can be about 100 to 1 cm or about 150 nm to 1 pm.

[0032] In an aspect, the nanostructured black nickel film includes a cellular structure with uniform pore size. Under the base of the pore is an area and the thickness in the area can be about 10 nanometers to 1 cm or about 50 nm to 1 pm. The total thickness of the nanostructured black nickel film can be about 100 to 1 cm or about 150 nm to 1 pm.

[0033] In an aspect, the first surface is an antireflective surface. In particular, the first surface has an average specular optical reflectance of about 1 .5 to 2.5%, about 2%, or about 1.95% over the entire visible spectral range (e.g., about 400-800 nm). The second surface can have a mirror-like smooth surface that has an average specular optical reflectance of about 80 to 90% or about 85%.

[0034] Now having described features of the present disclosure generally, additional description is provided below. In an aspect, the method of making a substrate having an antireflective coating includes disposing a substrate having a first side and a second side opposite the first side into a solution. In an aspect, the substrate is made of a material selected from: glass, sapphire, a silicon wafer, or a polymer-based substrate. In an embodiment, the substrate can include a silicon substrate, a gallium arsenide (GaAs) substrate, a gallium antimonide (GaSb) substrate, indium phosphide (InP), gallium nitride (GaN), and the like. In an embodiment, the silicon substrate can include a single crystal silicon substrate, a multi-crystalline substrate, or an amorphous silicon substrate. In an aspect, the substrate can be flat or substantially flat (e.g., a sheet of glass such as a glass slide or larger (e.g., centimeter to meter range in length and / or width)). In an embodiment, the substrate can have a thickness of about 2 pm to 1000 pm and the length and width can vary depending upon the desired use or application.

[0035] In an aspect, the solution can be water, ethanol, butanol, or a mixture thereof, and in particular the solution is water. The solution has a monolayer of silica nanoparticles disposed on the surface of the solution. In an embodiment, silica nanoparticles can be made separately mixed with ethylene glycol, toluene, benzene, or a mixture thereof (in particular ethylene glycol) and then the mixture is disposed on the surface of the solution after the substrate is within the solution. The substrate can be removed from the solution. As the substrate is removed from the solution, a first monolayer of silica nanoparticles is disposedT|H Docket: 222112-2530 on the first side of the substrate and a second monolayer of silica nanoparticles is disposed on the second side of the substrate (the second monolayer of silica nanoparticles can be removed prior to additional processing) to form a first coated substrate. The monolayers of silica nanoparticles self-assemble on the first surface and the second surface and uniformly distributed and arranged in a hexagonal periodic order.

[0036] The first silica nanoparticle monolayer is comprised of about 100 to 400 nm (e.g., diameter) silica nanoparticles or about 200 nm silica nanoparticles. In an aspect, the silica nanoparticles are spherical or substantially spherical. The silica nanoparticles are to form the silica monolayer are the same diameter or substantially (e.g., within about 5% or 10% mean diameter) the same diameter.

[0037] The first coated substrate can be processed to remove excess material (e.g., silica nanoparticles) or the silica nanoparticles from the second side. Then the solution that included the silica nanoparticles can be removed from the surface. For example, if the solution included ethylene glycol, then the ethyl glycol can be removed so that the silica nanoparticles remain on the substrate.

[0038] After the substrate surface is etched to produce the nanopillar array, the silica nanoparticles can be removed using HF and then a gold layer formed on the nanopillar array. In an aspect, the gold can be disposed used chemical deposition or another deposition technique. In an aspect, the gold layer can have a thickness of about 40-100 nm or about 60 nm.

[0039] The gold coated substrate can be disposed into a solution including a nickel compound. The nickel compound can be a solution of nickel sulfate or nickel salt. A nickel layer is formed on the gold coated substrate to form a nickel-gold coated substrate. The nickel layer can have a thickness of about 10 nanometers to 1 cm or about 50 nm to 1 pm. The thickness of the nickel layer will vary based on the substrate. For example, the thickness of the nickel layer in the nanopillar array is less than the area beyond the base of the nanopillar array, which is illustrated in Figure 1 .

[0040] The templating substrate and the plurality of nickel-gold coated columnar structures can be separated from one another using a hydrofluoric acid solution. The plurality of nickel-gold coated columnar structures can then be treated to remove the gold layer to form the plurality of nickel columnar structures, which is referred to as a nanostructured black nickel film.EXAMPLES

[0041] Now having described the embodiments of the disclosure, in general, the examples describe some additional embodiments. While embodiments of the present disclosure are described in connection with the example and the corresponding text andT|H Docket: 222112-2530 figures, there is no intent to limit embodiments of the disclosure to these descriptions. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of embodiments of the present disclosure.Example 1

[0042] The hydrogen evolution reaction (HER) is critically important in electrochemical water splitting [1-7], which is a promising technique for producing clean hydrogen fuels from renewable energy sources like photovoltaic panels. Platinum-based electrocatalysts are most efficient in catalyzing HER in acidic media [8-10], Unfortunately, the high cost and scarcity of platinum limit its large scale application in electrochemical hydrogen production [11 ,12], Non-precious-metal-based electrocatalysts have therefore been extensively investigated [10,13-15], Inexpensive nickel-based catalysts, especially nickel-phosphorus alloys produced by simple and straightforward electroless plating techniques, have shown outstanding electrocatalytic performance [13,16-26], To efficiently catalyze HER, nickel- based electrocatalysts should have large number of active sites and intrinsic activity of each active site [10,18], Nanostructured nickel-based catalysts, which are produced by controlling electroplating or electroless plating conditions, or by using various top-down and bottom-up nanofabrication technologies, have been comprehensively developed to meet these preferable electrochemical characteristics [23,27-34], Besides electrocatalysis, nickel-based electrodes have also been broadly utilized in carrying out various photoelectrochemical reactions, such as photoelectrocatalytic (PEC) water splitting, photoelectrocatalyzed wastewater remediation and energy production, and PEC reduction of CC2to synthetic fuels [35-44], To greatly improve energy efficiencies of these PEC processes, nanostructured black Ni films exhibiting high electrocatalytic performance and minimal light reflection are highly desirable.

[0043] Inspired by the hexagonal arrays of nanonipples found on the corneal lenses of moths’ eyes, which render night camouflage by significantly suppressing light reflection over a broad range of wavelengths and incident angles, dark moth-eye antireflection coatings have been extensively exploited on a large variety of substrates ranging from semiconductors (e.g., silicon) to polymers [45-58], Subwavelength-structured moth-eye nanopillars fabricated by various top-down (e.g., photolithography) and bottom-up (like colloidal lithography) approaches enable a gradual transition in effective refractive index at the substrate surface

[0058] , This index gradient can greatly reduce optical reflection and enhance light transmission and absorption by eliminating abrupt index change at a smooth optical surface.

[0044] We have developed a simple colloidal lithography technique in fabricating waferscale broadband antireflection coatings on crystalline silicon substrates for highly efficientT|H Docket: 222112-2530 solar cells [55,57,59], In this approach, self-assembled silica nanoparticles are used as structural templates in patterning moth-eye nanopillars directly on the surface of silicon wafers. Here we combine this scalable bottom-up technology with electroless plating to enable black Ni films possessing superior broadband antireflection properties. Synergistically, these nanostructured metal surfaces with well-defined geometries are expected to significantly boost active surface areas, promising as efficient catalysts for important electrocatalytic and photoelectrocatalytic applications. Systematic electrochemical measurements in acidic media have confirmed the greatly enhanced electrocatalytic hydrogen evolution reaction activity of the templated black Ni films with super dark surfaces. Experimental section

[0045] Langmuir-Blodgett (LB) assembly of monolayer colloidal crystals on silicon wafers

[0046] Monolayer colloidal crystals composed of hexagonally close-packed silica nanospheres with 200 nm diameter were first assembled using the simple LB assembly technique

[0057] , Monodispersed silica nanospheres synthesized by the standard Stober method were purchased from Particle Solutions LLC (Alachua, Florida). The as-synthesized nanoparticles were purified by multiple centrifugation-redispersion cycles (at least 6 times) using 200-proof ethanol (Pharmaco Products). The purified silica nanospheres were redispersed in ethylene glycol and the particle volume fraction of the final colloidal suspension was adjusted to 0.20. The nanoparticle suspension was slowly dropped on the surface of still water in a large glass petri dish with a pre-immersed 4-inch-sized silicon wafer (n-type, (100), test grade, University Wafer, Inc.). Silica nanospheres will rapidly assemble into iridescent monolayer colloidal crystals floating on the water surface. The silicon wafer was then withdrawn at a steady rate of ~ 12.5 mm / min using a syringe pump (KD Scientific 780-230). The floating silica colloidal crystals will uniformly transfer onto the silicon wafer surfaces.

[0047] Templating nanofabrication of moth-eye nanopillars on silicon wafers

[0048] The LB-assembled silica nanospheres were then used as sacrificial templates in patterning periodic moth-eye nanopillars on silicon wafer surfaces. A standard chlorine reactive ion etch (RIE) process operating at 5 mTorr chamber pressure, 20 SCCM chlorine flow rate, and 80 W for 30 min was performed to selectively etch the exposed silicon wafer. The templating silica nanospheres were dissolved in a 2.0 vol.% hydrofluoric acid aqueous solution for 60 s. After drying with compressed air, a black silicon wafer resulted.

[0049] Replication of black Ni films using templated silicon nanopillars and electroless plating

[0050] A 60 nm thick gold layer was sputtered onto the templated black silicon wafer using an SPI Module Sputter Coater. The wafer was broken into smaller pieces (~ 1 cm x 2T|H Docket: 222112-2530 cm) using a diamond wafer scriber. Techni EN 8200 electroless nickel plating solution was purchased from Technic, Inc. The Au-coated black silicon sample was immersed in 200 mL of the electroless plating solution maintained at 90 °C for 30 min under continuous stirring. The sample coated with a mirrorlike nickel film was rinsed with deionized water, followed by immersing it in a 2.0 vol.% hydrofluoric acid aqueous solution. After separating from the templating silicon mold, the final black Ni film was rinsed with deionized water and blow- dried using compressed air.

[0051] Sample characterization

[0052] Scanning electron microscope (SEM) and energy-dispersive X-ray (EDX) analysis were carried out using FEI Nova NanoSEM 430. All samples except for the conductive Ni films were coated with a thin layer of gold (5 nm thick) before SEM imaging. Normal-incidence specular reflection spectra were measured using an Ocean Optics HR4000 high-resolution vis-NIR spectrometer, a tungsten halogen lamp (LS-1) as the light source, and an R600-7 reflection probe. A silicon wafer with sputtered aluminum (1000 nm thick) was used as the high-reflection standard, while an Ocean Optics STAN-SSL low- reflectivity specular reflectance standard was utilized in calibrating the absolute reflectivity of the black Ni films.

[0053] Electrochemical measurements

[0054] All electrochemical measurements were carried out using a potentiostat (CH Instruments, model 760E). The hydrogen evolution reaction electrocatalytic activity and electrochemical double layer capacitance (Cd,) were determined in a conventional three- electrode electrochemical cell in an acidic solution of 0.5 M H2SO4. The templated black Ni films with either textured or smooth surfaces exposed to the electrolyte solutions were used as the working electrodes. O-rings with an internal diameter of 0.49 cm (O-ring Service Kits™, 568-110) were utilized in defining the exposed areas of the Ni electrodes. A graphite rod and an Ag / AgCI electrode (in saturated KCI) were used as the counter and reference electrode, respectively. Before recording the electrochemical activity of Ni films, the electrolyte solutions were purged with nitrogen gas for 5 min. Linear sweep voltammetry (LSV) was conducted at a scan rate of 10 mV s1to evaluate the HER catalytic performance of the Ni films. All potentials reported were converted from versus Ag / AgCI to versus reversible hydrogen electrode (RHE) using the equation: E(gHE) = E(Ag,AgCl)+ E^Ag / AgCV)+ (0. 059 x pH) to exclude the influence of pH. The current density was calculated using the surface area defined by the O-rings. Cyclic voltammetry (CV) was performed at various scan rates ranging from 50 to 400 mV s1in a non-faradaic region in the acidic media. The electrochemical double-layer capacitance was calculated as: Cdl= ic / v, where icand v denote charging current and scan rate, respectively. The electrochemically active surfaceT|H Docket: 222112-2530 area (ECSA) of both textured and smooth Ni films were estimated using the equation:ECSA = Cdl / Cs, where Csis the specific capacitance and a typical value of 0.04 mF cm2as reported in previous work [60,61] was used.Results and discussion

[0055] The schematic illustration in Figure 1 depicts the templating procedures for fabricating nanostructured black Ni films. Monolayer silica colloidal crystals assembled on a silicon wafer are first applied as sacrificial templates in a standard chlorine RIE process to make periodic arrays of moth-eye nanopillars on the surfaces of silicon wafers. After removing the templating silica nanoparticles by a brief hydrofluoric acid wash, a thin layer of gold (60 nm thick) is sputtered on the wafer surface. This gold layer can then catalyze the electroless nickel plating reaction to form a conformal Ni coating over the templated silicon nanopillars. Another hydrofluoric acid wash can separate the electroless plated Ni film from the silicon mold. The gold film can then be removed (not shown in Figure 1). The process produces a self-standing black Ni film possessing a nanotextured black surface and a smooth mirrorlike surface.

[0056] Monolayer silica colloidal crystal templates are first assembled on 4-inch-sized silicon wafers using the simple and scalable Langmuir-Blodgett technique

[0062] , which leverages high surface tension of water (72.75 mN nrr1at 20 °C) and strong capillary forces between neighboring particles

[0063] to spontaneously organize floating nanospheres on water surface. Figure 2a shows a typical top-view SEM image of a LB-assembled monolayer colloidal crystal consisting of 200 nm silica nanospheres. The long-range hexagonal ordering of the crystal is clearly shown by the image and the Fourier transform of a larger 10 * 10 pm2region (see the inset of Figure 2a). The SEM image also reveals that crystalline imperfections, such as point defects and grain boundaries, are abundant over the sample. The magnified SEM image in Figure 2b further illustrates the uniformity of the silica nanospheres and the two-dimensional (2D) feature of the self-assembled monolayer colloidal crystals.

[0057] The LB-assembled silica nanospheres are then used as sacrificial templates in patterning biomimetic moth-eye nanopillars directly on silicon wafer surfaces [55,57], In a standard chlorine RIE process, the etching rate of silica nanospheres is significantly smaller than that of silicon, leading to the preferential etching of the exposed (i.e., unprotected) silicon wafer and the final formation of tapered silicon nanopillars. The templating silica nanoparticles are finally removed by a brief hydrofluoric acid wash. Figure 3a presents a photograph of a 4-inch-sized silicon wafer with the top surface being RIE-processed. The etched region exhibits uniform black appearance over all viewing angles, while the unetched top small area retains the original highly reflective state of the polished wafer. Figures 3bT|H Docket: 222112-2530 and 3c show typical top-view SEM images of the dark region in Figure 3a. By comparing Figure 3b with Figure 2a and the corresponding Fourier transforms in the insets, it is evident that the original geometries of the colloidal crystal template, including hexagonal ordering, lattice constant, and even crystalline imperfections, are faithfully transferred to the templated silicon nanopillars. The tapered geometry of the nanopillars can be clearly seen from the side-view SEM image in Figure 3d. By using the top-view and side-view SEM images, the average width and height of the silicon nanopillars are measured to be 143 ± 29 nm and 1110 ± 67 nm, respectively, indicating an average aspect ratio of ~ 7.8. Like natural motheye gratings, these tapered silicon nanopillars with large aspect ratios can create a smooth refractive index transition at the wafer / air interface and thus can effectively suppress light reflection from the silicon substrate over a broad range of wavelengths and incident angles [47,64],

[0058] A conformal coating of 60 nm thick gold layer is then sputtered on the templated silicon nanopillars to catalyze the subsequent electroless nickel plating reaction. The state- of-the-art Techni EN 8200 electroless nickel plating solution, which leads to rapid deposition (with a typical plating rate of ~ 20 pm hr1) of lead and cadmium-free, medium phosphorus (5.0 - 8.0 wt.%), and bright Ni coatings, is used in this work. The main components of this solution are nickel sulfate and sodium hypophosphite, and the auto-catalytic reaction leading to the electroless nickel plating is: 2Ni2++ 8H2PO2+ 2H2O - 2 / Vi(s) + 6H2PO3+ 2H++ 2P(s) + 3H2(<J). Compared with traditional electroplating processes, which typically require expensive instruments and precisely controlled process parameters, the electroless plating technique leverages this simple metal-catalyzed chemical reaction to routinely deposit high- quality nickel-phosphorus alloy coatings on the templated black silicon wafers. Importantly, the electroless plating reaction is auto-catalytic, meaning that once a nickel layer forms on the surface, it will act as a catalyst to continue this reaction. Therefore, a thinner gold or silver catalytic layer (as thin as 10 nm) can produce nearly identical electroless Ni films. Due to the poor interfacial adhesion between the thin gold layer and the silicon nanopillars, dilute hydrofluoric acid solution can easily penetrate into the interfacial areas and dissolve the intrinsic oxide layer covering the silicon nanopillars, leading to the easy delamination of the final black Ni film from the silicon mold. Figure 4a displays a photograph of the bright side of an electroless plated black Ni film replicated from the moth-eye silicon nanopillars. This smooth surface exhibits typical metallic luster and mirrorlike appearance. In sharp contrast, the replicated surface looks super dark from any viewing angle (Figure 4b).

[0059] The microstructures of the templated black Ni films are characterized by scanning electron microscope. Figures 5a and 5b show typical side-view SEM images of the dark surface of the sample in Figure 4b. By comparing these images with those in Figure 3b-3d, it is apparent that the porous nanostructures on the black Ni film surface are faithful replica ofT|H Docket: 222112-2530 the templating silicon nanopillars. The original hexagonal ordering is retained throughout the templating nanofabrication steps. One noticeable difference is the introduction of tiny cracks on the film surface, which are possibly caused by the buildup of excess internal stresses in the metal films during the rapid electroless plating process

[0065] , Resembling moth-eye nanopillars, the templated Ni nanopores enable a refractive index gradient at the film surface, resulting in efficient broadband antireflection performance and super dark appearance of the nanotextured films. Figures 5c and 5d present typical top-view SEM images of the silicon mold after removing the replicated black Ni film from its surface. Besides some metal residuals, the silicon nanopillars maintain their original geometries and long-range hexagonal ordering. They can be reused as structural templates in fabricating new black Ni films.

[0060] Energy-dispersive X-ray spectroscopy is used in quantifying the elemental compositions of the electroless plated black Ni films. Figure 6 shows a typical EDX spectrum of the textured surface of the sample in Figure 4b. The corresponding weight and atomic percentages of different elements are summarized in Table 1. Carbon and oxygen atoms in the sample come from various complexing agents added in the electroless plating solution, such as sodium citrate (NasCeHsOy^FTO), sodium acetate (CH3COONa), and lactic acid (CH3CHOHCOOH)

[0066] , Small amount of silicon possibly originates from the silicon mold. The weight percentage of phosphorus (6.53 wt.%) agrees with its expected amount of this medium-phosphorus electroless Ni alloy. The gold catalytic layer used in triggering the initial electroless Ni plating reaction contributes to the presence of Au atoms in the final film.

[0061] Table 1. Compositions of different elements in electroless plated black Ni film analyzed using EDX spectroscopy.

[0062] The drastically different reflective properties of the smooth and textured surfaces of the electroless plated Ni films are quantitatively characterized by vis-NIR opticalT|H Docket: 222112-2530 spectroscopy. Figure 7 compares normal-incident specular reflection spectra obtained from 3 random locations on the bright and black surfaces of the sample shown in Figure 4. The high uniformity in optical performance for both surfaces can be clearly seen by the overlapped spectra taken from different locations across the sample. The average reflectivity from the smooth surface is 91 .9% for wavelengths from 450 to 950 nm, while it greatly drops to only 0.86% for the black textured surface in the same wavelength range. These reflectivity values favorably agree with the shining bright and super dark appearances of the two surfaces of the electroless Ni sample in Figure 4.

[0063] The electrocatalytic performance of the hydrogen evolution reaction on the electroless plated Ni films in an acidic (0.5 M H2SO4) solution was evaluated using a traditional three-electrode electrochemical cell. Figure 8a shows the linear sweep voltammograms of the HER activity on the smooth and textured Ni surfaces in the potential range from 0 to -0.41 V (vs. RHE). The nanotextured Ni film exhibits a small overpotential (201 mV at 10 mA cm2) compared to that of the smooth surface (407 mV at 10 mA cm2), indicating drastic enhancement of HER performance by the textured film. The Tafel slopes of both surfaces shown in Figure 8b (114 mV decade1for the smooth side vs. 68 mV decade1for the textured side) implies faster HER kinetics on the textured Ni film. Another significant improvement in the HER electrocatalytic performance is indicated by the cyclic voltammograms of the smooth and textured Ni surfaces (Figure 8c) in the non-Faradaic range (0.014 to 0.086 V vs. RHE) at the scan rate of 50 mV s1. The much larger charging current observed at the textured Ni surface is caused by larger double layer capacitance. Figure 8d reveals that the charging current has a linear relationship with the scan rate. The double layer capacitances of the textured and smooth Ni surfaces in the acidic solution can be estimated based on the slopes to be 4.02 mF / cm2and 0.161 mF / cm2, respectively. Based on the Cm values, ECSA per apparent surface area (1 cm2) in 0.5 M H2SO4solution increases from 4.0 cm2of the smooth Ni surface to 100.5 cm2of the textured film, representing a 25-fold enhancement. Table 2 summaries the important electrocatalytic properties of HER on the smooth and textured Ni surfaces in the acidic solution.

[0064] Table 2. Comparison of electrocatalytic performance of hydrogen evolution reaction on the smooth and textured Ni surfaces in 0.5 M H2SO4solution.T|H Docket: 2221 12-2530Conclusions

[0065] In conclusion, we have developed a bottom-up templating technique for fabricating bioinspired black Ni films. Wafer-scale moth-eye nanopillars can be patterned on the surfaces of silicon wafers by using LB-assembled, monolayer, close-packed silica colloidal crystals and chlorine RIE. Faithful replication of the templating silicon nanopillars is enabled by a simple and scalable electroless Ni plating process leveraging an auto-catalytic chemical reaction. The templated nanoporous Ni films can create a smooth transition in effective refractive index across the air / Ni film interface, resulting in super dark Ni surfaces with less than 1 % average specular reflectivity over a broad range of visible and NIR wavelengths. Compared with the counterpart smooth Ni surfaces produced in the same electroless plating batch, the nanotextured films can greatly enhance the electrocatalytic performance of the hydrogen evolution reaction in acidic media by reducing overpotentials and increasing double layer capacitances and electrochemically active surface areas.References

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[0132] It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a concentration range of “about 0.1% to about 5%” should be interpreted to include not only the explicitly recited concentration of about 0.1 wt% to about 5 wt%, but also include individual concentrations (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within the indicated range. In an embodiment, “about 0” can refer to 0, 0.001 , 0.01 , or 0.1. In an embodiment, the term “about” can include traditional rounding according to significantT|H Docket: 222112-2530 figures of the numerical value. In addition, the phrase “about ‘x’ to ‘y’” includes “about ‘x’ to about ‘y’”.

[0133] It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations, and are set forth only for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiments of the disclosure without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure.

Claims

T|H Docket: 222112-2530CLAIMS1. A method of making a substrate having an antireflective coating, comprising: disposing a templating substrate having a first side and a second side opposite the first side into a solution, wherein the solution has a monolayer of silica nanoparticles disposed on the surface of the solution; removing the templating substrate from the solution, wherein as the templating substrate is removed from the solution, a first monolayer of silica nanoparticles is disposed on a surface of the first side of the templating substrate to form a first coated templating substrate; etching the first coated templating substrate to form a plurality of columnar structures on the first side of the first coated templating substrate to form an etched templating surface; removing the monolayer of silica nanoparticles from the etched templating surface; disposing a layer of gold onto the plurality of columnar structures of the etched templating surface to form a plurality of gold coated columnar structures; disposing the etched templating substrate including the plurality of gold coated columnar structures into a solution comprising a nickel compound, wherein a nickel layer is formed on the plurality of gold coated columnar structures to form a structure including a plurality of nickel-gold coated columnar structures on the etched templating substrate; separating the etched templating substrate from the structure including the plurality of nickel-gold coated columnar structures; and removing the layer of gold from the nickel-gold coated columnar structures to form a structure including a plurality of nickel columnar structures.

2. The method of claim 1 , further comprising the following step prior to the separating the etched templating substrate from the structure including the plurality of nickel-gold coated columnar structures step: disposing the etched templating substrate including the plurality of nickel-gold coated columnar structures into a hydrofluoric acid solution.

3. The method of claims 1 or 2, wherein the first self-assembled silica nanoparticle monolayer is comprised of silica nanoparticles having a diameter of about 100 to 400 nm, wherein each silica nanoparticle has about the same diameter.

4. The method of any one of claims 1 to 3, wherein the first self-assembled silica nanoparticle monolayer is comprised of silica nanoparticles having a diameter of about 200 nm.T|H Docket: 222112-25305. The method of any one of claims 1 to 4, wherein the templating substrate is made of a material selected from: glass, sapphire, a silicon wafer, or a polymer-based substrate.

6. The method of any one of claims 1 to 5, wherein the gold layer has a thickness of about 40-100 nm.

7. The method of any one of claims 1 to 6, wherein the gold layer has a thickness of about 60 nm.

8. The method of any one of claims 1 to 7, wherein the nickel compound is nickel sulfate.

9. The method of any one of claims 1 to 8, wherein the nickel film has a thickness of about 10 nm to 500 nm.

10. The method of any one of claims 1 to 9, wherein the plurality of nickel coated columnar structures have a spacing of about 100 to 400 nm between adjacent nickel coated columnar structures.11 . The method of any one of claims 1 to 10, wherein the nickel coated columnar structures have a diameter of about 100 to 400 nm.

12. The method of any one of claims 1 to 11 , wherein the plurality of nickel coated columnar structures have a spacing of about 200 nm between adjacent nickel coated columnar structures and wherein the nickel coated columnar structures have a diameter of about 200 nm.

13. A structure made for the method of any one of claims 1 to 12.

14. A structure comprising: nanostructured black nickel film, wherein the nanostructured black nickel film has a first surface and a second surface on the side opposing the first surface, wherein the first surface is textured and the second surface is smooth, wherein the first surface is an antireflective surface, wherein the first surface includes a plurality of nickel coated columnar structures have a spacing of about 100-400 nm between adjacent nickel coated columnar structures and wherein the nickel coated columnar structures have aT|H Docket: 222112-2530 diameter of about 100-400 nm, wherein the spacing of the plurality of nickel coated columnar structures is uniformly distributed and arranged in a periodic hexagonal order.

15. The structure of claim 14, wherein the first surface has an average specular optical reflectance of about 1 .5 to 2.5% over the entire visible spectral range, wherein the second surface has a mirror-like smooth surface that has an average specular optical reflectance of about 80 to 90%.

16. The method of claims 14 or 15, wherein the plurality of nickel coated columnar structures have a spacing of about 200 nm between adjacent nickel coated columnar structures and wherein the nickel coated columnar structures have a diameter of about 200 nm.

17. A structure comprising: nanostructured black nickel film, wherein the nanostructured black nickel film has a first surface and a second surface on the side opposing the first surface, wherein the first surface is textured and the second surface is smooth, wherein the first surface is an antireflective surface, wherein the first surface has a cellular structure with uniform pore size, wherein the cellular structure is about 100 to 400 nm, wherein the pores uniformly distributed and arranged in a hexagonal order periodically.

18. The structure of claim 17, wherein the first surface has a cellular structure with uniform pore size, wherein the cellular structure is about 200 nm.

19. The structure of claims 17 or 18, wherein the first surface has an average specular optical reflectance of about 1 .5 to 2.5% over the entire visible spectral range, wherein the second surface has a mirror-like smooth surface that has an average specular optical reflectance of about 80 to 90%.

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