nanomaterials Article A Scalable Solution Route to Porous Networks of Nanostructured Black Tungsten V. Vinay K. Doddapaneni 1 , Kijoon Lee 2,3 , Tyler T. Colbert 1 , Saereh Mirzababaei 2,3 , Brian K. Paul 2,3 , Somayeh Pasebani 2,3 and Chih-Hung Chang 1, *   Citation: Doddapaneni, V.V.K.;Lee, K.; Colbert, T.T.; Mirzababaei, S.; Paul, B.K.; Pasebani, S.; Chang, C.-H. A Scalable Solution Route to Porous Networks of Nanostructured Black Tungsten. Nanomaterials 2021, 11, 2304. https://doi.org/10.3390/ nano11092304 Academic Editor: Ion N. Mihailescu Received: 18 August 2021 Accepted: 31 August 2021 Published: 5 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, OR 97331, USA; doddapav@oregonstate.edu (V.V.K.D.); tycolbert94@gmail.com (T.T.C.) 2 School of Mechanical, Industrial and Manufacturing Engineering, Oregon State University, Corvallis, OR 97331, USA; leekij@oregonstate.edu (K.L.); mirzabas@oregonstate.edu (S.M.); brian.paul@oregonstate.edu (B.K.P.); somayeh.pasebani@oregonstate.edu (S.P.) 3 Advanced Technology and Manufacturing Institute (ATAMI), Corvallis, OR 97330, USA * Correspondence: chih-hung.chang@oregonstate.edu; Tel.: +1-541-737-8548 Abstract: This paper studied the feasibility of a new solution-processed method to manufacture black tungsten nanostructures by laser conversion of tungsten hexacarbonyl precursor on the Inconel 625 substrate under argon atmosphere at ambient pressure. The results show that sublimation of the precursor can be prevented if the decomposition temperature (>170 ◦ C) is achieved using the laser heating method. Three different laser powers from 60–400 W were used to investigate the role of laser parameters on the conversion. It was found that lower laser power of 60 W resulted in a mixture of unconverted precursor and converted tungsten. Higher laser powers >200 W resulted in α-W (BCC) in one step without further heat treatment. Different oxygen concentrations from 0.5 ppm to 21 vol% were used in the laser canister to investigate the effect of oxygen concentration on the conversion. It was found that the hard vacuum (>10 −4 torr) or hydrogen is not necessary to obtain α-W (BCC). The solar absorptance varied from 63–97%, depending on the amount of precursor deposited on the substrate and oxygen content in the laser canister. This solution-based laser conversion of tungsten precursor is a scalable method to manufacture tungsten coatings for high-temperature applications. Keywords: nanostructures; black tungsten; solution-based; solar absorber 1. Introduction Tungsten, the highest melting point element known in the periodic table, with low thermal expansion coefficient, good thermal and electrical conductivity, wear-resistance, and chemical stability, has a wide range of applications, such as ohmic contacts, intercon- nects, solar thermal absorbers, IR reflectors, diffusion barriers, and high-strength metal matrix composites [1–4]. One growing use of W is as a high-temperature, absorber layer to increase the solar absorptivity and decrease the thermal emissivity of solar thermal collec- tors or receivers for various energy applications [1,5–7]. Particularly, creating plasmonic nanostructured metals (nanorods, nanoparticles, nanoporous films) is of great interest as they augment the absorption of electromagnetic radiation over a tunable range of UV to near IR [8,9]. Different synthesis methods have been used to manufacture W films and coatings capable of modifying the solar absorptance of these surfaces. Gesheva et al. [10] deposited black W films by chemical vapor deposition (CVD) of tungsten hexacarbonyl (W(CO) 6 ) under hydrogen atmosphere for photothermal solar energy conversion applica- tions. Shah et al. [9] manufactured a spectrally selective solar absorber by laser sintering of W micro and nanoparticles on stainless steel (SS) substrate with a solar absorptance of 83% and emissivity of 11.6%. Sibin et al. [4] manufactured IR-reflective W films by magnetron sputtering to control the thermal emittance of stainless steel substrates. Gao et al. [1] produced surface textured W with enhanced absorption of 74% and reduced Nanomaterials 2021, 11, 2304. https://doi.org/10.3390/nano11092304 https://www.mdpi.com/journal/nanomaterials