2D MXenes: the lowest-emissivity black materials

Black materials with low infrared absorption/emission are rare in nature but highly desired in numerous areas, such as solar-thermal energy harvesting, infrared camouage, and anti-counterfeiting. Due to the lack of spectral selectivity in intrinsic materials, such counter-intuitive properties are generally realized by constructing complicated subwavelength articial nanostructures with precise nanofabrication techniques. Here, we report that 2D Ti3C2Tx MXenes embrace both a low emissivity (down to 10%) and a high solar absorptance (up to 90%), yielding the best spectral selectivity (8.2) and the highest solar-thermal eciency among the reported intrinsic solar absorbing materials. We demonstrate their appealing potentials in the aforementioned areas. Moreover, the spectral selectivity relies on both the nanoake orientations and terminal groups, providing great tunability. First-principles calculations suggest more potential low-emissivity MXenes such as Ti2CTx, Nb2CTx, and V2CTx. This work opens the avenue to further exploration of a family of low-emissivity black materials with over 70 members.

and therefore precise lithography or high-vacuum deposition techniques generally have to be used in their fabrication, which inherently restricts their cost reduction. For these reasons, it is preferable if there was a exible intrinsic material with strong, broadband solar absorption but low IR emissivity. In fact, prior to the exploration of metamaterials/metasurfaces, materials with intrinsic spectral selectivity such as W, HfC, SnO 2 , In 2 O 3 , ZrB 2 , and TiB 2 were investigated [30][31][32] , but their innate solar absorptance (<70%) and spectral selectivity were generally well below the generally desired threshold value for e cient solar harvesting (Supplementary Table 1). The lack of ideal intrinsic low-emissivity black materials has been a longstanding challenge for decades 21,30 .
In this context, we resorted to a new family of two-dimensional (2D) materials, MXenes, composed of transitional metal carbides and nitrides. Since they were rst reported in 2011 33 , MXenes have brought substantial bene ts and new opportunities to a variety of areas ranging from energy storage 34,35 , and catalysis 36 to electromagnetic shielding 37,38 , and photothermal conversion [39][40][41] . Speci cally, the outstanding ability of Ti 3 C 2 T x (T denotes the -O-, -OH, -F and other terminal groups) MXenes in electromagnetic wave absorption has enabled both the electromagnetic interference shield effects (in the microwave region) 37,38 and photothermal effects (in the visible and near-IR regions) [39][40][41] . However, the interaction between the MXenes and electromagnetic waves in the mid-IR region (>2.5 μm), which dominates the radiative heat transfer near room temperature, is rarely explored.
In this study, for the rst time, we discovered that unlike the strong absorption in visible, near-IR, and microwave regions, the black Ti 3 C 2 T x MXene lms show strong re ection up to 90% for the mid-IR wavelengths, resulting in a quite low emissivity. Associated with its high absorption (90%) across the solar spectrum, it achieves a record-high spectral selectivity (8.2) among reported intrinsic solar absorbing materials, and thus the highest solar-thermal conversion e ciency (78% under 1 sun, at 100°C ), to the best of our knowledge. We demonstrated the great potential of this low-emissivity black material in applications that are extremely challenging for traditional rigid metamaterials, including solarthermal conversion on exible high-porosity substrates, multispectral camou age coatings, and anticounterfeiting. Excellent intrinsic spectral selectivity is offered by Ti 3 C 2 T x MXene in a free-standing form as well as coatings on versatile substrates, even on porous rough surfaces, showing much wider applications than metamaterial selective absorbers. The optical properties of the Ti 3 C 2 T x MXenes are highly tunable due to the dependence on the terminal groups and the orientation of nano akes. Further, rst-principles calculations show other commercially available MXenes such as Ti 2 CT x , Nb 2 CT x , and V 2 CT x are also potential low-emissivity black materials, opening the avenue to further exploration of a family of low-emissivity materials with over 70 members.

Results
Low emissivity and high solar absorptance of MXenes. As widely acknowledged, polished metals generally possess high light re ection over an ultra-broad band due to their high damping constant and low refractive index (Fig. 1a). For instance, as shown in Fig. 1b, the stainless steel (SLS) has both a low solar absorptance of 38% and a low IR emissivity of 9% at 100 °C. In contrast, most of the black materials such as carbon-based absorbers have strong UV-visible-IR absorption (Fig. 1c). A black absorber made of carbon nanotubes (CNT) shows not only a as high as 95%, but also a near-unity of 93% (Fig. 1d). The blackbody-like absorber is not an ideal solar absorber, since most of the harvested solar energy will be dissipated via the massive thermal re-radiation (at 100 °C), leading to a low solarthermal e ciency h solar-th of only ~32% under the illumination of 1 sun (1 kW m -2 ) near room temperature.
Unlike metals and blackbodies, a free-standing 15-mm-thick Ti 3 C 2 T x MXene lm exhibits a high of 90% comparable to that of the CNT absorber, while beyond the solar spectrum, its absorption rapidly declines to a rather low level due to the increasing mid-IR re ection (Figs. 1e, f). Its is only around 17% at 100 °C, suggesting excellent spectral selectivity ( ). Moreover, the of the Ti 3 C 2 T x lm can be further reduced to as low as 10% by controlling the surface morphology as discussed later, which is even close to that of some polished metals (9% for W and SLS) (Fig. 1h). To visually demonstrate the low of the Ti 3 C 2 T x lm, it was placed on a hot plate with a constant temperature of 100 °C, and characterized by an IR imager (the default emissivity was set as 95% in this work). Despite the identical real temperature of 100 °C, the Ti 3 C 2 T x lm appears much colder than the CNT absorber in the IR image, and as cold as the polished SLS with a comparably low (Fig. 1i). The h solar-th (1 sun, 100 °C) of the low-emissivity Ti 3 C 2 T x lm can reach a record-high value of 78% for intrinsic black materials, far higher than that of the CNT absorber (32%). To the best of our knowledge, both the and h solar-th of the Ti 3 C 2 T x MXene are the highest among all the intrinsic solar absorbing materials ( > 50%) reported so far (Supplementary Table 1). By virtue of the higher h solar-th , its measured surface temperature rise (62 °C) is higher than that of the CNT absorber (50°C ) under 1-sun illumination in the open air (Fig. 1g).
Fabrication and characterizations of Ti 3 C 2 T x MXene lms. The free-standing black Ti 3 C 2 T x MXene lm with great exibility in this work was prepared by a typical process reported previously 35,37 , which includes chemically etching of the Al atoms from Ti 3 AlC 2 phases, nano akes delamination by centrifugation, and vacuum-assisted ltration. The TEM image of the prepared Ti 3 C 2 T x powder after delamination veri es that 2D nano akes with few layers were attained (Fig. 2a). The cross-sectional SEM image of the ltrated lm shows that it has a 15 μm thickness and is stacked by well-aligned nano akes ( Fig. 2b). Both the X-ray diffraction (XRD) and the X-ray photoelectron spectroscopy (XPS) were performed to characterize the vacuum-ltrated Ti 3 C 2 T x lm (the top side). The XRD pattern with a pronounced peak at 2θ = 6.5° is assigned to the (002) plane of the Ti 3 C 2 T x MXene (Fig. 2c), which is well consistent with the results in the previous works 37,42  Both the top and bottom surfaces of the Ti 3 C 2 T x lm show strong light absorption for the wavelengths from 0.3 to 1.2 μm and dramatically reduced absorption for longer wavelengths ( Fig. 2g and Supplementary Fig. 2). The superior performance can be maintained without notable decay after longterm (120 hours) thermal annealing in both the vacuum (< 7×10 -2 Torr) at 400 °C and the ambient air at 200 °C ( Supplementary Fig. 3). Interestingly, the absorption of the top surface is slightly higher than that of the bottom over the whole spectral range. The and @100 °C of the top are 90% and 17%, while those of the bottom are 82% and 10%, respectively. As a result, although with the same surface temperature (~100 °C), the bottom (39 °C) appears a bit colder than the top (45 °C) in the IR image (Fig. 2h). The surface SEM images reveal that the bottom surface that adhered to the lter membrane during ltration is atter than the top (Fig. 2i). Their 3D morphologies given by a surface pro ler also quantitatively verify the roughness of the bottom surface (R a = 215 nm) is around half of that for the top surface (410 nm) (Supplementary Figs. 4 and 5). This difference in morphology is attributed to the different boundary conditions of the two surfaces during vacuum-assisted ltration. The stacking of nano akes in the bottom was assisted by the at lter membrane and strong vacuum pressure, while the top surface was formed under weaker constraints. In other words, some of the nano akes on top were not oriented parallel to the lter membrane during ltration and formed a rougher surface. Here we infer that the inplane and cross-plane optical properties of the 2D MXene nano akes differ from each other.
Consequently, the variations in the 2D nano akes orientation will lead to the discrepancy in absorption properties between the two surfaces.
To test this hypothesis, we simply coated the Ti 3 C 2 T x solution on two different substrates including silicon and glass by drop-casting to obtain more randomly oriented nano akes. The absorption intensity of both the two lms over the UV-visible-IR range is stronger than that of the vacuum-ltrated lm, indicating larger and ( Supplementary Fig. 6a). From the cross-sectional SEM image of the lm on silicon ( Supplementary Fig. 6b), we observed that the majority of nano akes are stacked together with random orientations, which is different from the well-aligned architecture of the vacuum-ltrated lm (Fig.   2a). There are a lot of bumps on the lm surface and the roughness is quite large (Inset of Supplementary Fig. 6b). These phenomena con rm the strong dependence of the optical properties on the orientation of nano akes. The fundamental reason is that the in-plane permittivity of the 2D MXenes differs from the cross-plane permittivity. Actually, for the MXene lms prepared by different approaches, variations in the geometry and therefore in the electrical conductivity were also reported in prior works 43,44 . Even with randomly oriented nano akes, the lms still offer a much lower of 45% than the CNT absorber. Therefore, accompanied by a high of 92%, their h solar-th is still superior to CNT absorbers.
Potential applications of low-emissivity black MXenes. Porous structures are indispensable components to a lot of solar-thermal applications, such as steam generation 1 , seawater desalination 39 , and smart textiles 45 . However, current high-performance selective solar absorbers are usually realized by constructing arti cial metamaterials/metasurfaces in nanoscale (multilayer nano lms or nanophotonic structures) on dense at substrates [2][3][4]24,26 , which will lose their effectiveness when coated on porous substrates owing to the break of resonant conditions. For instance, we coated a previously reported high-performance metamaterial absorber (Ag lm/TiN nanoparticles/SiO 2 ) on a highly porous Nylon 66 membrane shown in the inset of Fig. 3a. The metamaterial absorber only offers a low of 45%, much lower than the values (~95%) on dense substrates 3 . In contrast, the intrinsic Ti 3 C 2 T x sustained its high of 85% and low of 25% even on highly porous membranes (Fig. 3a), because its optical properties are dominated by the material itself instead of the subwavelength microstructures. As a result, the h solar-th of the Ti 3 C 2 T x -Nylon 66 absorber (68% under 1 sun, at 100 °C) is superior to that of its metamaterial-based counterpart (42%).
Under the COVID-19, discarded face masks made from long-lasting plastics have been becoming a disaster to the environment. Moreover, the disposal of masks without disinfection may cause further virus transmission. The e cient Ti 3 C 2 T x absorber on porous Nylon 66 is promising to be used in disinfecting of face masks under solar illumination and make masks reusable. As shown in Fig. 3b, under the exposure of 1 sun, the N95 face mask with Ti 3 C 2 T x -Nylon 66 was rapidly heated to the critical temperature of 70 °C for virus inactivation within 1 min 46,47 . After 10 min exposure, the surface temperature of the face mask reached around 90 °C, while that of the metamaterial-based mask was only 72 °C due to the lower h solar-th .
In short, intrinsic MXene absorbers are compatible with both dense and porous substrates, and can also work in the form of a free-standing lm, which signi cantly expands application scenarios of selective solar absorbers. Moreover, unlike metamaterials, there is no need to employ time-consuming, expensive nanofabrication techniques for the preparation of such intrinsic absorbers, which make them more attractive in large-scale applications.
Besides the solar-thermal conversion, the low-emissivity black MXenes can bene t many other areas such as multispectral camou age and anti-counterfeiting. In conventional IR camou age coatings that are mostly made of metallic micro powders such as aluminum, both the high glossiness and high brightness limit their compatibility with visible and even near-IR light 22,48 . Figs. 3c and d show the optical and IR images of a person who wore a white T-shirt with black Ti 3 C 2 T x coating (0.16×0.128 m 2 ) at night. The temperature reading of the area with Ti 3 C 2 T x coating in the IR image was only 25 °C and much lower than the body temperature (~36 °C), appearing nearly as cold as the environment (23 °C). This result demonstrates that low-emissivity Ti 3 C 2 T x could be implemented as IR camou age coatings to conceal human bodies from IR detection. Meanwhile, as shown in Fig. 3c, the black Ti 3 C 2 T x coating on the white T-shirt was invisible at night, allowing the covered objects to blend in with the dark environment. In other words, the low-emissivity black Ti 3 C 2 T x can overcome the issues of conventional IR camou age coatings and be an appealing alternative in both IR and visible camou age.
Counterfeiting is causing tremendous losses in both the security and property of customers, companies, and governments. Anti-counterfeiting features are increasingly demanded to prevent valuable items such as brands, tickets, quick response (QR) codes, banknotes, and con dential documents from being replicated. Currently, most anti-counterfeiting technologies are enabled by photoluminescence 49 and magnetic response 50 . Here we demonstrated an alternative technology against counterfeiters by using IRmetameric security inks composed of low-emissivity MXenes. In Fig. 3e, the black anti-counterfeiting word "HKUST" was written on a white (polyvinylidene uoride) PVDF membrane using both the commercial black paint (for "HK") and the Ti 3 C 2 T x MXene solution (for "UST"), which cannot be told apart by naked eyes because of the similar spectral features in visible light. However, the "HK" almost disappeared under the IR thermal imager due to the small contrast in of the PVDF (~95%) and the black paint (~92%), while the "UST" can still be clearly observed because of the much lower of the Ti 3 C 2 T x (Fig. 3f).
Theoretical investigations. To explore the underlying mechanisms of the spectral selectivity in the Ti 3 C 2 T x , and search for more potential low-emissivity MXenes, we investigated their optical properties by the density functional theory (DFT) calculations. As shown in Fig. 4a, bulk Ti 3 C 2 T x lms made of layered akes with and without different terminal groups (-OH, -O, and -F) under the normal incidence of light were calculated. Referring to the Fresnel equation, the surface light re ection (R) from materials in the air at normal incidence can be obtained by where ω is the angular frequency and is the permittivity. Then, for optically thick materials with no transmission, the light absorption can be expressed as Large ε 1 or ε 2 values lead to high surface re ection, and therefore low absorption/emission. On the contrary, to achieve low re ection and therefore high absorption, ε 1 nd ε 2 should approach 1 and 0 (ε 2 > 0). It should be noted that at normal incidence (wave vector along the z-direction), the electric component of light orients in the x-y plane (Fig. 4a). Hence, it is the in-plane permittivity that dominates the re ection. Interestingly, for the four kinds of Ti 3 C 2 T x , both the in-plane ε 1 and ε 2 are relatively smaller in the UV-visible-NIR range but increase to quite large levels in the mid-IR region (Figs. 4b, c). As a result, they offer wavelength selectivity in light absorption as expected (Fig. 4d). In particular, both the Ti 3 C 2 (OH) 2 and the Ti 3 C 2 F 2 exhibit high solar absorptance and low IR emissivity. Moreover, the DFT results verify that the in-plane permittivity of the Ti 3 C 2 T x MXenes is different from that along the zdirection. This well explains why the absorption properties of the lms strongly depend on the orientations of nano akes. As shown in Fig. 4g and Supplementary Fig. 7, for Ti 3 C 2 T x with various terminal groups, their absorptance calculated from the in-plane permittivity shows much better spectral selectivity than that from the z-direction, indicating that well-aligned nano akes parallel to the substrate are preferable, which is consistent with the experiments. Due to the differences in composition, the absorption spectra of the Ti 3 C 2 T x given by the DFT calculations do not exactly agree with the experimental data. However, they validate the great spectral selectivity, and demonstrate the dependences on terminal groups and nano ake orientations. Apart from the Ti 3 C 2 T x , the DFT calculations suggest that other MXenes such as Ti 2 CT x , Nb 2 CT x , and V 2 CT x are also promising low-emissivity black materials ( Fig.   4h and Supplementary Fig. 8).

Discussion
In summary, we for the rst time report on a group of 2D MXenes as black materials with low emissivity. It is demonstrated that the free-standing Ti 3 C 2 T x lm prepared by vacuum-assisted ltration offers both a high solar absorptance up to 90% and a low IR emissivity down to 10%, yielding the highest spectral selectivity for intrinsic solar absorbing materials reported so far. As a result, under 1 sun illumination in the open air, the MXene lm achieves a high temperature rise around 62 °C with respect to the ambient air. By comparison, the temperature of the CNT absorber with a higher emissivity only increases by 50 °C, due to the higher photo-thermal conversion e ciency provided by the lower emissivity. Both our experiments and DFT calculations reveal that the absorption/emission properties of the MXenes strongly rely on the orientation of the nano akes, and the terminal groups. Speci cally, lower emissivity is obtained in those lms with nano akes well-aligned parallel to the substrates, and -OH and/or -F terminal groups. The highly selective, exible, black MXenes show great potential in solar-thermal energy conversion, IR camou age, thermal insulation, and anti-counterfeiting.

Methods
Fabrication of the MXene lms. The T i3 C 2 T x colloidal solution was prepared by the liquid-phase delamination of Ti 3 AlC 2 powder. In detail, 1.98 g of lithium uoride (LiF) (Alfa Aesar, 98.5%) was added to 35 mL of 9 M HCl aqueous solution. Then, 2 g of Ti 3 AlC 2 powder was added to the mixture. After etching for 24 h at 35 °C, the solution was washed and centrifuged with deionized water until the supernatant reached a pH value of 6. Next, to delaminate the MXene, 1 g etched MXene was dispersed into 0.5 L DI water, and deaerated with Ar, followed by sonication for 1 h. The mixture was then centrifuged for 1 h at 3500 rmp, and the supernatant with dark green color was collected. After that, a certain amount of MXene dispersion solution was vacuum ltrated through a hydrophilic Celgard 3501 membrane, and the obtained MXene membrane was further dried under 60 °C in the vacuum oven, and a membrane with a thickness of about 15 μm was obtained. The concentration of the solution was determined by testing the weight of the dried MXene lm.
Material characterizations. The morphology of the Ti 3 C 2 T x nano akes was observed by transmission electron microscopy (TEM, JEM-2010F, Jeol). The phase identi cation of the MXene lm was conducted by using an X-ray diffractometer (XRD, PANalytical) with Cu Kα radiation at 45 kV and 40 mA. The surface and cross-section morphologies were characterized by scanning electron microscopy (SEM, JSM-7100F, Jeol) equipped with energy dispersive X-ray spectroscopy (EDX). The composition of the lm was characterized by X-ray photoelectron spectroscopy (XPS, PHI 5600 multi-technique system, Physical Electronics). A 3D optical pro ler (NPFLEX, Bruker) was used to characterize the morphology and roughness of the lm. The surface roughness was obtained by analyzing 307,200 data points with a vertical resolution of 0.15 nm.
Optical measurements. The UV-visible-NIR (0.3-2.5 μm) re ectance (R) and transmittance (T) spectra of the samples were measured using a spectrometer (Lambda 950, Perkin Elmer) equipped with a 150 mm integrating sphere. A white BaSO 4 plate was used as a standard reference in the UV-visible-NIR re ectance measurements. The mid-IR (MIR, > 2.5 μm) R and T spectra were measured using a Fourier transform infrared spectrometer (FTIR, Vertex 70, Bruker) with an integrating sphere. A gold lm was used as a standard reference in the mid-IR re ectance measurements. The absorptance (A) spectra were directly derived from 1-R-T.
Thermal measurements. To measure the temperature rise under solar illumination, the MXene lm and reference samples were attached to the surface of polystyrene foam. A solar simulator (Oriel Sol2A, Newport) using a xenon lamp was used to provide standard and stable 1-sun power (1 kW m -2 ). T-type thermal couples, which were connected to a data acquisition device (NI9213, National Instrument), were attached on the backside of samples to measure the steady-state temperatures. The temperature data were recorded every two seconds. A thermal imager (Ti25, Fluke) was used to taken IR photos of the samples. Solar absorptance and thermal emissivity calculation. The spectrally averaged solar absorptance is de ned as 2 Here E solar (λ), E b (λ,T), α(λ), and ε(λ) represent the spectral solar power (AM 1.5G), the blackbody emission at T, the absorptance, and the emissivity at the wavelength λ, respectively. I solar is the total solar irradiance (AM 1.5G, 1 sun or 1 kW m -2 ), and σ is the Stefan-Boltzmann constant. Solar-thermal conversion e ciency calculation. The solar-thermal energy conversion e ciency under 1 sun can be obtained by 2 where T is the operating temperature, and T 0 is the ambient temperature.
Thermal stability tests. Thermal annealing cycles (24 hours × 5 cycles) in ambient air at both 100 and 200 °C were conducted by placing the samples on a hot plate (KW-4AH, Chemat Technology Inc.). Thermal annealing cycles (24 hours × 5 cycles) in vacuum at both 300 and 400 °C were conducted in a quartz tube furnace (OTF-1200X, MTI Corporation). The samples placed in a ceramic crucible were put in the quartz tube and sealed. The sealed quartz tube was evacuated to reach a vacuum atmosphere (<7×10 -2 Torr). Afterward, the samples were heated up to 300 °C (or 400 °C) at a rate of 10 °C/min and annealed for 24 h cycle -1 .

Data availability
The data sets generated and analyzed during the current study are available from the corresponding author upon reasonable request. Figure 1 Light-matter interaction of metals, carbon-based blackbodies, and MXenes. a, The high solar re ectance, high mid-IR re ectance, and low IR emissivity of metals. c, The high solar absorptance, high mid-IR absorptance, and high IR emissivity of carbon-based blackbodies. e, The high solar absorptance, high mid-IR re ectance, and low IR emissivity of MXenes. b,d,f, Absorptance spectra of a polished stainless steel (SLS) sheet, a CNT black absorber, and a Ti3C2Tx MXene lm, as well as the AM 1.5G solar spectrum and the radiation spectrum of a blackbody at 100 °C. g, Temperature vs. time of the CNT and MXene absorbers, and the air temperature under 1 sun. h, Comparison of solar absorptance and IR emissivity of intrinsic materials including metals (Au, Ag, Al, Cu, W, and SLS), radiative cooler materials (SiO2 and CaCO3), semiconductors (Si, Ge, CdTe, GaAs, and Co3O4)30, black materials (carbon-based and polymers), TiB232, ZrB232, HfC31, and Ti3C2Tx MXenes. i, IR photographs of the SLS, CNT, and MXene on a hot plate with a constant temperature of 100 °C. Characterizations of the free-standing Ti3C2Tx MXene lm fabricated by vacuum-assisted ltration. a, TEM image of few-layer Ti3C2Tx nano akes. b, Cross-sectional SEM image of the free-standing lm. c, XRD pattern of the lm. Inset: Photograph of the exible black lm (top side). d, XPS spectrum of the lm. e,f, High-resolution XPS spectra of O 1s and F 1s of the lm. g, Absorptance spectra of the bottom (attached to the lter membrane) and top sides of the vacuum-ltrated lm. h, IR photographs of the two sides placed on a hot plate (100 °C). i, Surface SEM images of the two sides.

Declarations
who wore a white T-shirt with the black Ti3C2Tx coating at night. Inset: An optical photograph at daytime. e,f, Optical and IR photographs of the word "HKUST" written using commercial black paint (for "HK") and Ti3C2Tx MXene (for "UST") solutions.