Super Black Coating on the Commercial Black Anodized Al(6061) by Direct and Scalable CVD–Growth of Carbon Nanofibers

Using carbon‐based super black coatings on optical devices can achieve superior stray light suppression for applications in astronomy. For the first time, the work presents carbon nanofiber‐based black coating on commercial anodized Al(6061), which facilitates the development of a highly effective route to directly integrate the carbon‐based material on the common substrate for optical baffles. The scalable and available structural engineering effect is synergized with the anodized Al(6061) coating with black dye composed of nickel catalyst and the intrinsic broadband light absorption of the CVD‐grown carbon material to ultimately achieve a superior broadband light absorber. Nickel catalysts embedded in anodized Al(6061) offer a practical pathway for carbon nanofiber growth through CVD without additional stacked catalysts. The CVD‐growing mechanism and CNF nanostructures are demonstrated through TEM and EDS element mapping, SEM, and Raman spectroscopy. CNF‐grown Al(6061) substrates offer above 99% broadband light absorption and low light reflectance below 1% in UV–vis–NIR and mid–IR ranges. This facile approach has been useful for super black coating on Al(6061)‐based complicated sculptures, such as concave substrate and an optical baffle. These results have demonstrated a facile method that can significantly impact the industrial scaling‐up of high‐quality, super‐black coating on spaceborne devices.


Introduction
[3][4][5] When applied on optical baffles or vanes, super black coating plays an essential role in eliminating stray lights that strike star trackers, which are the optical systems used by satellites.This super black coating has to meet the requirements for optical baffle usage, space environment, and industrial production, such as the uniform deposition on complicated shapes and shape edges of the baffles. [6]Aluminum and its alloy are the most used material for optical baffle assemblies due to their lightweight, the fact that they are the richest earth-abundant material, low cost, outstanding mechanical properties, and processability; therefore, to avoid degradation, it is important for the development of super black coating on such substrates to consider the proper temperatures and atmosphere. [4,6,7]irect integration of the black coating onto the functional aluminum substrates has enabled strong mechanical bonding and broadband ultra-black absorption with aluminum substrates.This kind of blackening coating is typically attributable to the inherent qualities of carbon materials, combined with the structural effect of the nano-and microcavities formed during anodization.10] Carbon materials possess remarkable broadband light absorption properties, particularly the vertically aligned carbon nanotube array (VACNTA), because of the -band optical transitions and a porous structure that lowers Fresnel reflectance. [9,11,12]The super black layer based on VACNTA on the textured aluminum shows a low reflectance of 1 × 10 −5 . [7]Nevertheless, these results were demonstrated in a laboratory environment and obtained while using a specially designed apparatus in an oxygen dehydrogenation reaction.
In space applications, the scalability of super black coating is crucial to its implementation in the most recent cutting-edge commercial advancements.The most common method for this is to apply a black anodized coating that is achieved by a series of anodizing, coating, and sealing steps to aluminum alloy substrates. [13,14]Commercial black anodized Al(6061) exhibits high reflectance in the near-infrared and infrared ranges, but it shows enhanced absorption in the visible range, which is attributed to its nano and microporous structures as well as black dyes containing carbon black, nickel (Ni), and chromium (Cr). [6]17][18][19][20][21][22] Carbon nanofibers (CNFs) are perfect for integration into aluminum alloys because of their remarkable chemical and physical characteristics along with their low growth temperature under 600 °C.There has yet to be any attempt to combine CNFs on the aluminum alloy for use in broadband light absorbers.
In the current work, we propose a novel strategy that is expected to have a substantial industrial impact by directly integrating the carbon nanofibers to blacken the black anodized Al(6061), thus presenting broadband light absorption for stray light suppression for the optical baffles.Carbon nanofibers are grown on porous Al(6061) using a chemical vapor deposition (CVD) method that is assisted by Ni-containing black dyes.The strategic development of the CVD method enables the thermal decomposition of the carbon feed source at high temperatures, as a result of which denser and taller carbon nanofibers are deposited on the substrate at lower temperatures.It is shown here that the outstanding light absorption of carbon nanofiber synergized with the structural pore effect of the anodized Al(6061) does affect the broadband optical properties in the ultraviolet-visible-nearinfrared (UV-vis-NIR) and mid-infrared (mid-IR) ranges.Our simple approach facilitates the large-scale production of super black coatings on the standard Al(6061) alloys for use in space applications.

Results and discussion
Figure 1a depicts the engineering strategy used to achieve a high yield of carbon deposition on the substrate with a low melting point by enabling the higher thermal decomposition of the ethylene-based carbon source (C 2 H 4 ) in the first zone at a temperature of 750 °C as well as CNF growth on the substrate at lowtemperature zone 2 at a temperature of 550 °C at atmospheric pressure along with gas flow rates of C 2 H 4 , hydrogen (H 2 ), and argon (Ar) as 400, 200, and 400 sccm, respectively.[25] Denser and taller CNF fibrils using such an approach have been observed when compared with CNF growth at the same temperature of the substrate and gasphase decomposition in top-view field emission scanning electron microscope (FE -SEM) images presented in Figure S1 (Supporting Information).Figure 1b illustrates the CNF growth that is achieved on nano and microporous structures of commercial anodized Al(6061) activated by the Ni catalyst, including in the black-dye coating for the anodized Al(6061).The present work shows that the black anodized Al(6061) substrate has a dual nanoand microporous-based structural engineering effect for light absorption and CNF growth that is assisted by Ni active catalysts within the black-dye coating of the commercial anodized Al(6061) with a barrier oxide layer in between to protect the Ni catalyst.
The atomic structures of the CNF growth assisted by Ni catalyst are studied by transmission electron microscopy (TEM) and TEM-energy dispersive X-ray spectroscopy (EDS), and they are depicted in Figure 2. Figure 2a shows the TEM image of CNFs that confirms their fishbone structure, which is facilitated by the use of the Ni catalyst.The average diameter and length of CNF are 30 and 100 nm, respectively.In comparison, the Ni catalyst's average diameter is 15 nm, as presented in both the inset of Figure 2a and the high magnification of TEM in Figure 2b.The inset in Figure 2b reveals that the lattice spacing of the Ni catalyst is ≈0.202 nm, which can be presented as the (111) Ni crystal plane, and this is confirmed by X-ray diffraction (XRD) patterns in Figure S2 (Supporting Information). [26]The elemental composition characterization by EDS in Figure 2c-f also confirms that C and Ni are distributed throughout the entire structure experiencing CNF growth assisted by the Ni catalyst.The carbon nanofiber microstructure is further confirmed by the Raman spectra presented in Figure 2g, which show three representative broadband peaks of carbon nanofibers centered at 1346, 1598, and 2835 cm −1 , which are respectively denoted as D (e.g., defects), G (e.g., crystalline graphitic carbon), and G ′ (e.g., overtone of D peak, more vitality with less graphitic sheets and defects). [27,28]he intensity ratio between the G peak and D peak is 1.42, implying a higher degree of graphitization of the CNFs, which is  associated with the high-quality growth of CNFs. [29]All the atomic structure-based and microstructure analyses have demonstrated the successful growth of CNF activated by Ni catalyst embedded inside the commercial black anodized Al(6061) without the need to include further steps of additional catalyst deposition, thus enabling process simplification for further scaling-up.
Figure 3a provides simple illustrations of the CNF growing process.Further studies are carried out using the FE-SEM technique to elucidate the process by which CNFs are grown from a bare black anodized Al(6061) b), Ni catalyst is annealed un-der H 2 c), and CNF is grown on the substrate d), as shown in Figure 3b-d at different magnifications.As presented in the low and high-resolution top-view SEM images, the bare black anodized Al(6061) contains innumerable micro and nanoporous sides, which were fully absorbed and filled with a thin film of the Ni-contained black dye, and then broken into Ni nanoparticles during the catalyst annealing.In Figure S3d,e (Supporting Information), EDS presents the distribution of the Ni element to the substrate.Following the introduction of C 2 H 4 , a carbon source gas, carbon nanofibers with high density and an average length of a few 10 μm were grown, and these covered the whole area as shown in EDS-SEM element mapping, and few Ni catalysts remained on it, as shown in Figure S3f (Supporting Information).The results confirm that embedded Ni is an available and effective catalyst for successfully growing carbon nanofibers.
The optical properties of the substrates grown with varying growth times are presented in Figure 4 using UV-vis-NIR (wavelength ranges of 250-1500 nm) and Fourier transform infrared spectroscopy (FT-IR) (wavelength ranges of 1600-20 000 nm). Transmittance and reflectance were measured, after which absorption was calculated as described previously.The transmittance was negligible for all substrates for both wavelength ranges.Higher absorption corresponds to lower reflectance; it is therefore worth focusing on absorption.The bare black anodized Al(6061) shown in Figure 4a presented good absorption, with an average absorption of 96% in the range from 250 to 670 nm, which then dropped down in NIR, which showed very high re-flectance, to an average of 66%, as can be seen in the NIR results presented in Figure 4b.As mentioned earlier, due to the combination of micro/nanocavities of the anodized aluminum and the light absorption nature of the black dye material, the black anodized Al(6061) has demonstrated good optical properties in the UV-vis but very odd behavior at the NIR.By directly integrating the CNFs on it, the light absorption was significantly enhanced up to a maximum of 99.8% and with a mean value of 99.5% in UV-vis-NIR.In that wavelength range, the light absorption curves of the growing times of 15 and 30 min were similar to each other, i.e., above 99% in the UV-vis-NIR range.A similar trend was seen for the 45-and 60-min growth with the same average light absorption, both of which exhibited a value of 99.3%.Due to the detector change in the UV-vis-NIR instrument, the artificial peak at ≈860 nm could be ignored.All the grown CNF-based samples show similar light absorption in the wavelength range from 860 to 1200 nm.Over 1200 nm, the longer growth time samples outperformed those with a shorter growth time.In general, a shorter growing time leads to better light absorption in the UVvis range but a reduced effect for the NIR range, which conflicts with the longer growing time.The shorter growth time is believed to have induced shorter carbon nanofibrils, which cannot cover the whole micro/nanoporous side, thus leading to a high possibility of a dominant structural effect for the light absorption in the UV-vis range, as the longer growing time induced longer carbon nanofibers with higher density that covered the whole substrate, thus providing the best absorption in the NIR with the most influence of the intrinsic light absorption of carbon nanostructures, as discussed above, which will be proven in later discussion of the CNF-grown-black anodized Al(6061) microstructures at different growth times.
This effect can be seen in the mid-IR of the 1600-20 000 nm wavelength range using FT-IR, as presented in Figure 4d.Mostly, the light absorption of CNF-grown black anodized Al(6061) substrate in the mid-IR outperformed that of the bare black anodized Al(6061).The black anodization by itself was shown to lead to good light absorption in the mid-IR with significant maximum values at the narrow wavelength ranges of 2820-3255 nm and 7485-9737 nm due to the random size of its microcavity structures being equivalent to wavelengths. [9,10]At the 2820-3255 nm wavelength range, our CNF-grown substrates reached the maximum light absorption of 99.9%.On the other hand, at the wavelength range of 7485-9737 nm, the mean light absorption of the bare black anodized was 99.5%, which was higher than the cases of the 15, 30, and 60 min growth times and lower than the case of the 45 min growth time, as a maximum value of 99.9%.The microcavity size of the bare substrate can be explained to be compatible with such a wavelength range, and growing the CNFs on it reduces the size of the microcavity until the longer CNF fibrils and their length can be comparable with the wavelength range, which is why the maximum value of light absorption was achieved with a growth time of 45 min.The more the growth time increases, up to 60 min, the longer the CNF fibrils are, to the point that they exceed the wavelength range, which reduces the light absorption.However, the 60 min-grown substrates showed the best light absorption in the further wavelength range of 9737-20 000 nm.We have not considered extending the growth time, which has caused the lower light absorption at the UV-vis-NIR in this scope.Therefore, our 1h-grown sample showed the best light absorption of ≈99% from the UV-vis-NIR to THz, which

CNF-based black coating on black anodized Al(6061) 250-15001600-20 000 99.399 This work
Spray Al-CNT matrix coating on an aluminum plate 300-25002000-50000 98.590.1 [15]   Spraying CNT-based coating on an aluminum plate 350-800850-2400 97.496 [16]   Black ceramic coating on Al7075 400-2000 96.6 [17]   ITO-CNTs and Al 2 O 3 -In 2 O 3 -P 2 O 5 -SiO2 glass coating on Al(6061) 200-25003000-20000 9394 [18]   Al-doped ZnO/Al 2 O 3 -SiO 2 -V x O y coating on Al-70Si 200-2500 93.9 [19]   Black zeolite coating on Al(6061) 200-2500 93 [20]   Ni-P alloy electroless plating on aluminum alloy 450-800 98.8 [21]   Black nickel-cobalt coating on aluminum alloy 200-2300 92 [22]   was the most consistent performance in broadband light absorption.Our proposed explanation above can be confirmed by the morphology obtained using FE-SEM and the corresponding EDS in Figure 5, which represent the CNFs growing on the black anodized Al(6061) with a shorter growth time of 15 min and a longer growth time of 60 min.As can be seen in Figure 5a, a shorter growth time induced a shorter CNF fiber, as shown in the higher magnification FE-SEM image; a shorter fiber cannot guarantee coverage of the whole microcavities, which are vividly exposed, as can be seen in the low magnification FE-SEM image.On the other hand, the longer growth time of 60 min with more prolonged and denser CNF fibrils covers the whole microcavity embedded substrate, as presented in Figure 5b.Moreover, the summary of the element composition mapping EDS in Figure 5c,d shows a lower C weight percentage of 44.8% with a 15-min growth time, which is lower than that associated with a 60-min growth time, i.e., 95.7%.This indicates that a shorter growing time provides the best performance in the UV-vis, while a longer growing time is dominant at a higher wavelength of NIR and at mid-IR.
Table 1 compares this work's broadband light absorption to that obtained by other research focusing on the super black coating on the aluminum alloy.We took the average broadband light absorption of the most optimal growing condition of 1 h for comparison with the current industrial state-of-the-art methods for achieving super black coating on the aluminum alloy, such as black anodized coating, plasma electrolytic oxidationbased black coating, a black solution-based spraying approach, or electroless plating coating, as described previously.Our absorptions are 99.3% and 99% in the wavelength ranges from 250 to 1500 nm and 1600-20 000 nm, respectively, in comparison with the best absorption of carbon nanotube(CNT)-based spray coating of 98.5% in the range of 300-2500 nm and that of 98.8% of nickel-phosphorus (Ni-P) electroless plating in a narrow range of 450-800 nm.To our knowledge, the average light absorption of our super black coating developed herein has surpassed the corresponding absorptions reported by other literature over the wide wavelength range from ultraviolet to terahertz.Once again, it is essential to confirm the synergized effect of the structural engineering effect of the porous structure of the anodized aluminum substrate and the intrinsic material property of the carbon nanofiber-based carbon nanostructure, to achieve the best performance in broadband light absorption, as we have done in our approach.
Figure 6 illustrates the scalable feasibility of CNF-based super black coatings for commercial black anodized Al(6061) with arbitrary shapes such as concave substrate a) and even in the optical baffle b).Photographs vividly show the super dark coating of black anodized Al(6061) after CNFs have been deposited on them, and the edge of the sculptures could not be seen due to the fact that the incident light was blocked by the black coating.Further, as demonstrated in Figure S4 (Supporting Information), our super black coating on the concave substrate has withstood a dropping test at a height of 1 m.These results show the promise of the industrial fabrication of super black coating for commercial anodized Al(6061) for use in space applications such as optical detectors for astronomy and stray light suppression.

Conclusion
In summary, we have successfully fabricated a super black coating with high quality and high yield of CNFs on commercial black anodized Al(6061).The most optimized condition of 1 h growth showed light absorption exceeding 99% in the broadband from UV-vis-NIR to mid-IR, which outperformed the current state-of-the-art method for super black coating at the commercial level of the black anodized Al(6061).Interestingly, the black anodized Al(6061) plays a dual role by providing a textured structure for light-trapping and multiple-scattering sides as well as available and embedded Ni catalyst for the CNF growth, ultimately having a synergized effect for superior black coating.Further, our showcase of the super black coating on the arbitrary substrates, even the optical baffle assemblies, has provided an industrial perspective for using such a scalable approach for space applications.Notably, the good electrical conductivity of the CNFs can later serve as an anti-static function for long-term stability in the space environment.

Experimental Section
Materials: The commercial black anodized Al(6061) substrates (weight fraction: 1.7% Ni, 0.6% Cr, 3.9% S, 0.5% Mg, 10.9% C, 53.3% O, and balance Al, confirmed by the EDS mapping in Figure S3d, Supporting Information) along with their 3D shape-based sculptures, including a concave substrate and optical baffle assemblies, were purchased from CHTech Co., Ltd (Korea).All gases used for the CVD system were purified to 99.9%.
Al(6061) substrates were cleaned using isopropanol alcohol, ethanol, and de-ionized water in ultrasonication for 15 min each and then dried using an air gun.Prior to synthesis, the commercial black anodized Al(6061) was annealed in an oven at 80 °C for 10 h to form a uniform layer of Ni catalyst in the black dye-coated anodized Al(6061), which is necessary to enhance the efficient growth of carbon nanostructures. [32,33]The substrate was vertically loaded inside zone 2 of the CVD chamber, after which zone 1 was heated to 750 °C for 1 h in Ar gas (400 sccm) and H 2 gas (200 sccm), and the temperature of zone 2 was 550 °C.Then, the substrate was kept at that temperature for 10 min to allow catalyst annealing under mostly H 2 gas flow, followed by C 2 H 4 gas flow (400 sccm) to initiate CNF growth.The C 2 H 4 flowing time, defined as a growing time, was studied to determine its effect on the optical properties.Between runs, air baking (400 sccm of air flow) is employed at 800 °C for 1 h to remove carbon residues on the CVD chamber wall.
Characterization of Catalyst and CNF: The atomic structures of CNFs and catalysts were analyzed using transmission electron microscopy (Hitachi HF5000 Cs-STEM/TEM equipped with X-ray energy dispersive spectroscopy (EDS)) at a driving voltage of 200 kV.The particle size distribution of the Ni catalyst in the TEM image was analyzed using ImageJ.Powder x-ray diffraction (XRD) patterns of the carbon nanofibers and Ni catalyst were obtained by a Bruker D8 Discover diffractometer at 40 kV.The surface morphologies of the growing process of the CNFs on the black anodized Al(6061) were obtained using an ultra-high-resolution field emission scanning electron microscope (FE-SEM) (Hitachi SU8230 with an AMETEK EDAX Octane Elect Super silicon drift detector for EDS mapping at an acceleration voltage of 20 kV, an amp time of 3.84 μs, a resolution of 125.3 eV, and an acquisition time of 200 s and EDAX APEX Advanced software for analysis of all EDS mapping data).Raman spectra with a 532 nm laser excitation (FEX Micro-Raman spectrometer) were used to confirm the formation and quality of CNFs on the substrates.
Characterization of the Optical Properties: The optical properties of all CNF-grown black anodized Al(6061) substrates having a size of 5 cm × 5 cm × 0.2 cm were assessed using PerkinElmer LAMBDA 1050+ UV/Vis/NIR spectrophotometers for the UV-vis-NIR range (250-1500 nm) and the PerkinElmer Frontier FT-IR for the infrared range (1600-20 000 nm).The total reflectance was measured using an integrated sphere with a light incident angle of 8°and normalized with the white reflectance standard for the UV-vis-NIR range.The specular reflectance was collected utilizing a Pike integrated sphere accessory with a light incident angle of 45°and a gold mirror-based reflectance reference for the FT-IR.Previous studies have shown that reflectance measurements obtained using such standard instruments have high reliability. [8,34]The transmittance and the reflectance were measured, while the absorption, A, was calculated from 1 -R -T, with R and T corresponding to the reflectance and transmittance, respectively.

Figure 1 .
Figure 1.a) Fast-heating strategy for thermal decomposition of carbon feedstock (e.g., C 2 H 4 ) in zone 1 to grow CNF on anodized Al(6061) at a lower temperature at zone 2 along with a photograph of the as-made substrate and b) schematic illustration of CVD growth of CNFs on the Ni catalystembedded anodized Al(6061).

Figure 2 .
Figure 2. TEM images obtained under different magnifications a,b), EDS element mapping of CNFs with nickel as a catalyst c-f), and Raman spectra of grown CNFs g).

Figure 3 .
Figure 3. Simplified schematic illustrations of the CNF growth mechanism a), FE-SEM top-view images with different magnifications of bare black anodized Al(6061) b), a substrate after catalyst annealing with nanoparticle Ni catalyst c), and a CNF-grown substrate d).

Figure 4 .
Figure 4. Optical properties of the CNF-grown black anodized Al(6061) prepared with different growing times compared to the bare black anodized Al(6061) using UV-vis-NIR a-c) and FT-IR d-f).

Figure 5 .
Figure 5. FE-SEM top-view images a,b) demonstrate the growth of CNF at 15 and 60 min respectively, along with corresponding simplified elemental mappings c,d) of the EDS.

Figure 6 .
Figure 6.Photographs of super black coating on complicated sculptures with shape edges of concave shape a) and optical baffle b) before and after deposition of carbon nanofibers.

Table 1 .
Comparison of broadband light absorption of our results with other studies examining black coating on aluminum alloy in the literature.