Ultralight Aerogels Research Articles

Bioarchitectonic Nanophotonics by Replication and Systolic Miniaturization of Natural Forms.

The mimesis of biological mechanisms by artificial devices constitutes the modern, rapidly expanding, multidisciplinary biomimetics sector. In the broader bioinspiration perspective, however, bioarchitectures may perform independent functions without necessarily mimicking their biological generators. In this paper, we explore such Bioarchitectonic notions and demonstrate three-dimensional photonics by the exact replication of insect organs using ultra-porous silica aerogels. The subsequent conformal systolic transformation yields their miniaturized affine 'clones' having higher mass density and refractive index. Focusing on the paradigms of ommatidia, the compound eye of the hornet Vespa crabro flavofasciata and the microtrichia of the scarab Protaetia cuprea phoebe, we fabricate their aerogel replicas and derivative clones and investigate their photonic functionalities. Ultralight aerogel microlens arrays are proven to be functional photonic devices having a focal length f ~ 1000 μm and f-number f/30 in the visible spectrum. Stepwise systolic transformation yields denser and affine functional elements, ultimately fused silica clones, exhibiting strong focusing properties due to their very short focal length of f ~ 35 μm and f/3.5. The fabricated transparent aerogel and xerogel replicas of microtrichia demonstrate a remarkable optical waveguiding performance, delivering light to their sub-100 nm nanotips. Dense fused silica conical clones deliver light through sub-50 nm nanotips, enabling nanoscale light-matter interactions. Super-resolution bioarchitectonics offers new and alternative tools and promises novel developments and applications in nanophotonics and other nanotechnology sectors.

Ultralight aerogels via supramolecular polymerization of a new chiral perfluoropyridine-based sulfonimidamide organogelator.

Chiral and enantiopure perfluorinated sulfonimidamides act as low-molecular weight gelators at low critical gelation concentration (<1 mg mL-1) via supramolecular polymerization in nonpolar organic solvents and more heterogenic mixtures, such as biodiesel and oil. Freeze-drying of the organogel leads to ultralight aerogel with extremely low density (1 mg mL-1). The gelation is driven by hydrogen bonding resulting in a helical molecular ordering and unique fibre assemblies as confirmed by scanning electron microscopy, CD spectroscopy, and computational modeling of the supramolecular structure.

Photothermal self-floating aerogels based on chitosan functionalized with polydopamine and carbon nanotubes for removal of arsenic from wastewater

Although wastewater disposal in evaporation ponds may be a cost-effective strategy for wastewater management, it overlooks water recycling and can possibly pose significant threats to the surrounding environment. The development of photothermal materials with functional groups capable of adsorbing toxic contaminants offers a promising opportunity for detoxification and potentially solar-driven recycling of wastewater disposed in evaporation ponds. In this study, ultralight aerogels based on chitosan (Ch), a natural biodegradable polymer, functionalized with mussel-inspired polydopamine (PDA) and carbon nanotubes (CNT) were fabricated through a facile approach and examined for the removal of arsenic from water. Three aerogels, namely chitosan, CNT/Ch, and PDA@CNT/Ch were fabricated and characterized using SEM, FTIR, TGA, XPS, and BET surface area analysis. Adsorption isotherms, thermodynamics, and kinetics were systematically investigated, revealing the feasibility of the fabricated aerogels for arsenic removal from aqueous solutions. The UV–Vis and photothermal analysis indicated excellent light-absorption and light-to-heat conversion performance of the functionalized aerogels. Arsenic adsorption by aerogels occurred rapidly and reached equilibrium within 30 to 60 min and was well-fitted by pseudo second-order kinetics model. The Langmuir model well described the adsorption isotherm, and the maximum adsorption capacities were found to be 31.5, 36.6, and 38.7 mg/g at neutral pH for chitosan, CNT/Ch, and PDA@CNT/Ch, respectively. The adsorption mechanism was studied by FTIR and XPS analysis and the adsorption pathway was evaluated using intraparticle diffusion model. This study broadens the scope of utilizing aerogels for the detoxification and potential recycling of wastewater from diverse sources and effluents disposed in evaporation ponds.

Porous Carbonaceous Aerogels Composed of Multiscale Carbon-Based Units for High-Performance Microwave Absorption.

Structural engineering is definitely a promising and effective approach to develop excellent microwave absorbing materials with quantities of advantages. Especially, when carbon materials act as the constituents, the fabricated absorbers are available to gain more prominent absorption performance. However, extra high conductivities and the widespread aggregations and stacking of low-dimensional carbon materials always detrimentally affect the impedance matching and weaken the attenuation capacity, inevitably confining their further absorption applications. Herein, by introducing the amorphous chiral carbon nanocoils to overcome the challenges and achieve the strategies of structure optimization and multicomponent recombination, the reduced graphene oxide/carbon nanocoil/carbon nanotube aerogels were successfully synthesized by a successive hydrothermal method and freeze-drying strategy. The as-obtained aerogels possess a porous architecture that contribute to the extraordinary impedance matching and multiple reflections, which integrate the multifarious dielectric loss mechanisms of diverse carbon materials simultaneously. Benefiting from the tricomponent synergistic effect, the ultralight aerogels reach an outstanding microwave absorption property with an extremely low filler content of only 6 wt %. This work provides a helpful approach to design hierarchical absorbers consisted by multidimensional carbon materials for fantastic microwave absorption.

Scalable Ultralight Wood-Inspired Aerogel with Vertically Aligned Micrometer Channels for Highly Efficient Solar Interfacial Desalination.

An ultralight material that simultaneously combines remarkably rapid water transportation, highly efficient photothermal conversion, and excellent thermal insulation is highly desired for solar-driven interfacial desalination but was challenging. In this work, inspired by the unique natural structure of wood, we developed an ultralight aerogel by ice-templated synthesis as an integrated interfacial evaporator for solar-driven water production. The interior features vertically aligned biomimetic microscale channels facilitating rapid transportation of water molecules, while an improved photothermal interface allows high solar absorption and conversion via nonradiative relaxation and molecular vibrations. The biomimetic aerogel is ultralight with a density as low as 0.06 g/cm3, especially its fabrication is size- and shape-programmable as a whole and easily scalable. Additionally, the outstanding thermal insulation of the aerogel focuses heat precisely at the evaporation interface, reducing ineffective heat loss, while the uniformly distributed large-sized channels promote the dynamic convection of high concentration salt ions on the evaporator surface. Consequently, the evaporator shows broadband light absorption of 92.7%, leading to a water evaporation rate reaching 4.55 kg m-2 h-1 under 3 simulated solar irradiations, much higher than that of other reported evaporators with randomly distributed pores. This work provides new insight into advanced hybrid aerogels for highly efficient and durable solar-driven interfacial desalination systems.

Antioxidant cellulose nanofibers/lignin-based aerogels: a potential material for biomedical applications

BackgroundLignin is a naturally occurring and aromatic biopolymer with well-known antimicrobial and antioxidant properties. Thus, in this work, the use of cellulose nanofibers (CNF) and lignin to produce ultra-light aerogels for biomedical applications was studied. Aerogels containing varying amounts of lignin (0–30 wt%) and different concentrations of the crosslinking agent Fe3+ (25–100 mM) were developed.ResultsThe different bioaerogels were fully characterized and their physical, mechanical and bioactive properties analyzed. It was observed that the bioaerogels soluble fraction tends to decrease as the lignin content increases for the different Fe3+ concentrations, due to lignin–CNF interactions through hydrogen bonds. The bioaerogels containing lignin showed remarkable radical scavenging activity as the DPPH concentration decreased with time. This confirms the benefits of including lignin in bioaerogels to impart antioxidant properties. To study the suitability of the produced bioaerogels for controlled drug release, the release of tetracycline (TC) was studied. All of the bioaerogels released TC in a sustained manner for 6 h and presented similar profiles. However, the bioaerogels containing higher concentrations of crosslinker showed a higher release of TC. The TC loading conferred clear antimicrobial activity against S. aureus as expected, unlike the insignificant antimicrobial activity of the bioaerogels without TC. The biocompatibility of the samples was demonstrated for all materials produced (with and without TC loading) by the Kruskal–Wallis test with multiple comparisons. After observation of cell morphology, no significant differences were evident suggesting that the CNF–lignin bioaerogels present optimal biocompatibility for use in the biomedical and pharmaceutical industry.ConclusionsThe CNF–lignin bioaerogels presented in this work highlights their promising application as biomedical applications, such as wound dressings due to their biocompatibility, antimicrobial and antioxidant properties, as well as their swelling and solubility properties.Graphical

Thiadiazole-Derived Covalent Organic Framework Macroscopic Ultralight Aerogel.

Shaping covalent organic frameworks (COFs) into macroscopic objects for practical application remains a huge challenge. Herein, a new thiadiazole-derived COF macroscopic ultralight aerogel (NNS-VCOF) was prepared through acid-catalyzed aldol condensation between 2,5-dimethyl-1,3,4-thiadiazole and a tritopic aromatic aldehyde derivative. NNS-VCOF aerogel shows extremely low density (ca. 0.020 g cm-3), excellent mechanical properties (compression modulus of 16.65 kPa), thermal insulation properties (low thermal conductivity of 0.03270 W m-1 K-1 at 25 °C), and flame retardancy (quickly self-extinguishing after ignition) due to its three-dimensional sponge-like architecture and special nitrogen heterocyclic framework. To our delight, NNS-VCOF aerogel not only can be used as an outstanding macroscopic material but also shows efficient photocatalytic hydrogen evolution properties in a powder state because of the superhydrophilicity and appropriate optical properties.

Aqueous Processable Two-Dimensional Triazine Polymers with Superior Photocatalytic Properties.

Efficient and scalable production of high-quality and processable two-dimensional (2D) polymers are highly desired but have not yet been reported. Herein, we demonstrate a convenient noncovalent functionalization strategy for producing highly uniform, aqueous processable and semiconducting 2D triazine polymers. Experimental and theoretical analysis reveal that the aromatic amphiphilic 1-pyrenebutyrate can adsorb and intercalate into the interlayer of bulk crystalline covalent triazine framework (CTF) through noncovalent π-π stacking interaction between the pyrene moiety and the porous basal plane of 2D triazine polymer layer, which greatly facilitate the exfoliation of CTF in water in large scale. The as-prepared highly water-dispersible single-layer/few-layer 2D triazine polymer nanosheets can be easily processed into ultralight aerogels with a density of 5-15 mg cm-3, which can be further shaped into mechanically strong films upon simple compression. This noncovalent functionalization not only improve the dispersibility and processability of 2D triazine polymer, but also optimize its band structure and promote the photogenerated carrier separation via an interesting surface molecule doping effect, thus resulting in a remarkable photocatalytic hydrogen evolution rate of 1249 μmol h-1 (24980 μmol g-1 h-1) and apparent quantum efficiency up to 27.2% at 420 nm for the 2D triazine polymer, outperforming most metal-free photocatalysts ever reported.

Aqueous Processable Two‐Dimensional Triazine Polymers with Superior Photocatalytic Properties

AbstractEfficient and scalable production of high‐quality and processable two‐dimensional (2D) polymers are highly desired but have not yet been reported. Herein, we demonstrate a convenient noncovalent functionalization strategy for producing highly uniform, aqueous processable and semiconducting 2D triazine polymers. Experimental and theoretical analysis reveal that the aromatic amphiphilic 1‐pyrenebutyrate can adsorb and intercalate into the interlayer of bulk crystalline covalent triazine framework (CTF) through noncovalent π‐π stacking interaction between the pyrene moiety and the porous basal plane of 2D triazine polymer layer, which greatly facilitate the exfoliation of CTF in water in large scale. The as‐prepared highly water‐dispersible single‐layer/few‐layer 2D triazine polymer nanosheets can be easily processed into ultralight aerogels with a density of 5–15 mg cm−3, which can be further shaped into mechanically strong films upon simple compression. This noncovalent functionalization not only improve the dispersibility and processability of 2D triazine polymer, but also optimize its band structure and promote the photogenerated carrier separation via an interesting surface molecule doping effect, thus resulting in a remarkable photocatalytic hydrogen evolution rate of 1249 μmol h−1 (24980 μmol g−1 h−1) and apparent quantum efficiency up to 27.2 % at 420 nm for the 2D triazine polymer, outperforming most metal‐free photocatalysts ever reported.

Ultra-broadband shielding of cellulose nanofiber commingled biocarbon functional constructs: A paradigm shift towards sustainable terahertz absorbers

Terahertz (THz) spectrum and technology are envisioned to be a highly promising solution for enabling 6G and beyond wireless networks. This demand coincides with an urgent need to develop efficient electromagnetic interference shields to alleviate electromagnetic pollution in the THz range. Fully bioderived THz shields could be a sustainable solution to transcend towards a biocarbon economy. Herein, we report an environmentally benign and facile approach to fabricating 3D porous ultra-light aerogels and flexible nanopapers from cellulose nanofibers and highly conductive biocarbon fully derived from biomass. These functional constructs showcased excellent broadband THz shielding performance in both sub THz (W band) and THz region (0.4–2.0 THz). The THz shielding effectiveness of a 600 µm thick nanopaper and 3.00 mm aerogel was found to be 46 dB and 70 dB, with superior THz absorption behaviour and minimal reflection of THz signals. The electronic structure and charge transfer properties were examined under an external electric field based on density-functional theory. The results show that the effect of the external electric field induces intramolecular charge transfer from the lower-graphitic sheet to the upper-graphitic sheet of the nanocomposite suitable for THz excitation. Moreover, these sustainable absorbers could deliver similar THz shielding performance compared to graphene foam (74 dB), exemplifying their humungous potential as advanced functional materials for futuristic THz devices.

Aerogels containing ionomers and microwave assisted growth of carbon nanostructures on carbon urchins for multifunctional electromagnetic interference shielding

Hierarchal 3D porous carbon materials show huge potential for EMI shielding because of their adjustable pore structure, tunable morphology and high electrical conductivity. However, poor graphitic nature and improper contact between materials lead to reduce the inherent conductivity, which supress the EMI shielding performance. In this work, we designed a hierarchical Carbon–Carbon nanostructure, which resembles a ‘sea-urchin’, exhibiting high shielding performance. A facile strategy of microwave assisted growth of carbon nanofiber (CNF) on the ‘self-assembled’ carbon urchin (CU) holds the rank of hierarchy, which enhance the electrical contact between the urchins. In order to obtain a 3D construct, a classical Ionomers pair-PEDOT:PSS was chosen as the matrix and CNF@CU was infused and subjected to freeze-drying to design ultra-light aerogel. The morphological assessment reveals that CNF@CU in the aerogel offers trapping centers by providing multiple interfaces. The aerogels exhibited two orders of jump in electrical conductivity and showed excellent thermal stability. The high limiting oxygen index value begins to suggest that these aerogels can extend their utility to flame retardant materials. The EMI shielding properties of the construct were studied in the X-band (8.2–12.4 GHz). The aerogel with 1.7 wt% of CNF@CU exhibited −50.8 dB of shielding efficiency (representing 99.999% blocking). The excellent EMI performance achieved here is mainly due to the conductive losses, dipole polarization, interface polarization, and wave trappers. These wave trappers are extremely beneficial for shielding EM radiations through multiple scattering. The utility of these materials were further evaluated using a Bluetooth module, wherein the aerogel potentially blocked all the transmitting Bluetooth signals confirming the quick shielding quality of the proposed aerogel. Taken together, this work provides a facile approach for designing and developing lightweight aerogel based EMI shielding materials.

High hydrophobic ZIF-8@cellulose nanofibers/chitosan double network aerogel for oil adsorbent and oil/water separation

Ultralight aerogels with low bulk density, highly porous nature, and functional performance have received significant focus in the field of water pollution treatment. Here, high-crystallinity, large surface-aera metal frame-work (ZIF-8) was efficiently utilized to assist in the preparation of ultralight yet highly oil and organic solvent adsorption capacity, double-network cellulose nanofibers/chitosan-based aerogels through a physical entanglement and scalable freeze-drying approach. After chemical vapor deposition with methyltrimethoxysilane, a hydrophobic surface was obtained with a water contact angle of 132.6°. The synthetic ultralight aerogel had low density (15.87 mg/cm3) and high porosity (99.01 %). Moreover, the aerogel had a three-dimensional porous structure, which endowed it with high adsorption capacity (35.99 to 74.55 g/g) for organic solvent, and outstanding cyclic stability (>88 % of the adsorption capacity after 20 cycles). At the same time, aerogel removes oil from various oil/water mixtures by gravity alone and has excellent separation performance. This work holding excellent properties in terms of convenient, low-cost, scalability to manufacture environmentally friendly biomass-based materials for oily water pollution treatment.

Superelastic and Ultralight Aerogel Assembled from Hemp Microfibers

AbstractAerogels with both high elastic strain and fast shape recovery after compression have broad application potentials as thermal regulation, absorbents, and electrical devices. However, creating such aerogels from cellulosic materials requires complicated preparation processes. Herein, a simple strategy for scalable production of hemp microfibers using a top‐down method is reported, which can further be assembled into aerogels with interconnected porous structures via ice‐templating technique. With density as low as 2.1 mg cm−3, these aerogels demonstrate isotropic superelasticity, as exhibited by their fast shape restoration from over 80% compressive strain. Due to the high porosity (99.87%) and structural tortuosity, these aerogels show a low thermal conductivity of 0.0215 ± 0.0002 W m−1 K−1, suggesting their potential in thermal insulation application. Certain hydrophobic modification using silane derivative further endows these aerogels with reduced water affinity. Overall, the proposed strategy to prepare bio‐based microfibers using scalable technology, as well as the assembled aerogels, provides new insights into the design and fabrication of multifunctional bio‐based aerogels for value‐added applications.

Ultralight aerogel sphere composed of nanocellulose-derived carbon nanofiber and graphene for excellent electromagnetic wave absorption

Ultralight aerogel sphere composed of nanocellulose-derived carbon nanofiber and graphene for excellent electromagnetic wave absorption

Fabrication of an Ultralight Ni-MOF-rGO Aerogel with Both Dielectric and Magnetic Performances for Enhanced Microwave Absorption: Microspheres with Hollow Structure Grow onto the GO Nanosheets.

An ultralight Ni-MOF-rGO aerogel which possess the merits of not only broad bandwidth and strong absorption but also lightweight and thin matching thickness is fabricated through a hydrothermal treatment, freeze-drying, and annealing procedure. The Ni@C microspheres are dispersed randomly and evenly on the graphene oxide (GO) nanosheets, which can be proved through SEM and TEM results. The electromagnetic parameters of the composite can be adjusted by changing the mass ratio of the MOF and GO to endow the material with both good impedance matching and superior electromagnetic wave absorption performances. Consequently, the resulting composite shows outstanding microwave absorption performance, which achieves strong absorption (-51.19 dB) and broad effective absorption bandwidth (6.32 GHz) with a thickness of 1.9 mm while the filling content is only 2 wt %. In addition, the multiple loss mechanisms of the Ni-MOF-rGO aerogel are illustrated, including conduction loss, dipolar polarization, interfacial polarization, magnetic resonance, and eddy current loss. In a word, the extraordinary microwave absorption performance is ascribed to the synergistic effects of the unique multiple layered structure of GO and the hollow core-shell structure of the Ni@C microsphere. This work demonstrates that the ultralight aerogel with excellent electromagnetic wave absorption performance is a promising strategy for microwave absorption application.

Study on preparation and performance of advanced aerogel foamed concrete with ultra-light aerogel

Aerogel foamed concrete has been acknowledged as one of the most promising energy efficient building materials with high thermal insulation performance and low density in applications of green buildings and nearly zero energy buildings. However, the practical economy of aerogel foamed concrete is still relatively poor because of the greater amount of high-price ultra-insulating aerogel additive needed in the foamed concrete. Therefore, based on the premise of maintaining the high thermal insulation performance and low density of foamed concrete, reducing the amount of aerogel additive has become a key issue to improve the economy and promote the application of high-performance aerogel foamed concrete. This study proposed an equal volume replacement method based on using an ultra-light SiO2 aerogel instead of conventional SiO2 aerogel to reduce the amount of aerogel additive in foamed concrete while maintaining a high thermal insulation performance and low density. Two kinds of aerogel foamed concrete were prepared by filling light density (170 kg/m3) and ultra-light density (75 kg/m3) adiabatic aerogel powder into foamed concrete. An experiment was conducted to study the influence of different aerogel densities on the thermal conductivity, compressive strength and density of foamed concrete. The research results indicate that the density and thermal conductivity can be effectively reduced by filling ultra-light aerogel powder compared with traditional aerogel foamed concrete with an acceptable compressive strength. The results also show that the mass dosage of aerogel can be significantly reduced by using ultra-light aerogel powder. Specifically, for a foam volume ratio of 70 % and aerogel volume ratio of 13 %, the density and thermal conductivity of the ultra-light aerogel foamed concrete were reduced by approximately 10 %, which resulted in a cost reduction of 49 %. By applying the ultra-light aerogel foamed concrete in the construction of an exterior wall of a building, the heat loss through the wall can be reduced by 9 %. This study provides a significant reference for the preparation of high-performance foamed concrete and accelerates the application of super adiabatic aerogels in the building energy conservation industry.

Lipophilic Modified Hierarchical Multiporous rGO Aerogel-Based Organic Phase Change Materials for Effective Thermal Energy Storage.

In the field of thermal energy storage, organic phase change materials (PCMs) are widely used as functional materials to boost thermal applications. However, there is often a tradeoff between constructing shape-stable composite PCMs with high enthalpy value and those with low leakage rates. Here, we proposed a promising scheme to address this issue. A novel hydrogel consisting of reduced graphene oxide (rGO) and covalent organic framework (COF) was prepared via hydrothermal methods, and the rGO-COF ultralight aerogel with a hierarchical porous structure was formed after freeze-drying. The rGO-COF aerogel shows excellent absorption ability and affinity for organic solvents. It can sufficiently adsorb the molten organic PCMs, such as polyethylene glycol (PEG), and synthesize shape-stable composite PCMs with excellent leak resistance. The COF grown on the surface of rGO has a superior affinity for PEG, so rGO-COF aerogel shows an outstanding PEG loading rate of up to 96.1 wt %, which is 1.7 wt % higher than that of rGO aerogel. In addition, the COF effectively reduces the subcooling of PEG/rGO-COF with 20.3%, compared to PEG/rGO. Meanwhile, the prepared PEG/rGO-COF exhibits extremely high enthalpy and relative enthalpy efficiency (164.6 J/g, 97.4%). This demonstrates that a promising direction was highlighte for the preparation of high-enthalpy shape-stable composite organic PCMs in this work.

Scalable manufacturing of light, multifunctional cellulose nanofiber aerogel sphere with tunable microstructure for microwave absorption

Thanks to the high strength-to-weight ratio and aspect ratio, large specific surface area, and sustainability, cellulose nanofibers (CNFs) have incomparable advantages in constructing multifunctional aerogels. However, efficient utilization of the nanofibrous microstructure for developing aerogels of high-performance is extremely challenging due to the severe agglomeration of CNF. Here, a novel type of ultralight aerogel spheres composed of fibrous or sheet-like CNF-derived nanocarbon are manufactured via a facile, scalable drop-freezing followed by a pyrolysis approach. The nanostructured carbons of tunable morphologies are accomplished, endowing the aerogel spheres with abundant hierarchical pores, large specific surface area, and three-dimensional conductive skeletons with ultralight nature. In addition to the high-efficiency adsorption of the organic dye, the aerogel spheres enable a controllable and excellent electromagnetic wave absorption. The aerogels composed of coexisting fibrous and layered nanocarbons display an effective absorption bandwidth of 6.16 GHz, and a minimum reflection loss of −66.3 dB at the thickness of 2.14 mm, and a merely 1 wt% filler loading, which significantly outperforms that of other carbon-based absorbers. This work thus suggests a new avenue for designing high-performance, sustainable multifunctional biomass-based aerogels, demonstrating extensive application potentials in adsorption, electromagnetic absorption, and aerospace.

Ultralight aerogel textiles based on aramid nanofibers composites with excellent thermal insulation and electromagnetic shielding properties

Ultralight, flexible and robust aerogel fibers based on aramid nanofiber/modified carbon nanotubes are prepared via a facile wet spinning method following with a freeze-drying technique. The aerogel textiles based on the strong aerogel fibers exhibit excellent thermal insulation properties within a wide range of temperature (−30–200 °C), which can be attributed to their ultralow density (5–12 mg/cm3) and high specific areas (123–219 m2/g). Moreover, the optimized aerogel textiles (with a thickness of 0.92 mm) show high-efficient electromagnetic shielding performance with a value exceeding 30 dB in the X-band. Overall, those aerogel textiles exhibit great potential for applications as thermal insulation and electromagnetic shielding equipment under harsh environments.

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents