Yong Wangab,
Guozhu Chen
c,
Mohamed Chakera and
Dongling Ma
*a
aInstitut National de la Recherche Scientifique, 1650 Boulevard Lionel Boulet, Varennes J3X 1P7, Canada. E-mail: dongling.ma@inrs.ca
bGanjiang Innovation Academy, Chinese Academy of Sciences, Ganzhou 341119, China
cSchool of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China
First published on 18th August 2025
Solar steam generation, a sustainable and cost-effective water purification technique, has emerged as a promising solution to the global freshwater shortage. Plasmonic photothermal nanomaterials (NMs) have recently garnered enormous attention owing to their strong light–matter interactions and high photothermal conversion efficiency. This review begins by outlining the fundamentals of the plasmonic effect. Subsequently, we classify the current solar steam generation systems and discuss the critical parameters governing their performance. Recent advancements in plasmon-empowered NMs are then summarized according to five major classes: metals, metal nitrides, metal chalcogenides, metal oxides, and MXenes. Furthermore, this review highlights four primary applications of plasmon-driven solar steam generation. Finally, it discusses existing challenges in this research field and provides perspectives on future research directions. This comprehensive review offers valuable insights into the rational design and fabrication of plasmonic NMs for efficient solar steam generation and can thus serve as a guide for future development in this field.
The key to the effectiveness of any solar steam generation system lies in the photothermal material, which absorbs solar energy and converts it into heat, driving water evaporation. To date, numerous photothermal nanomaterials (NMs) have been explored, including carbon-based NMs, plasmonic NMs, semiconductors, and organic polymers.12–17 Among these materials, plasmonic photothermal NMs have aroused widespread interest owing to their strong light–matter interactions, which enable enhanced solar harvesting and efficient photothermal conversion. Several advantages make plasmonic NMs more promising for efficient solar steam generation: (i) enhanced solar absorption arising from multiple optical transition processes; (ii) tunable absorption spectra, allowing for optimal spectral matching with solar radiation; (iii) locally strong light–matter interactions and electromagnetic fields, minimizing energy loss; and (iv) robust and efficient photothermal conversion capabilities, conducive to improving solar utilization. Despite these advantages, the full potential of plasmon-driven solar steam generation remains largely untapped. Therefore, a thorough analysis of existing plasmonic photothermal materials used in solar water evaporation is essential to guide future research and development in this field.
Recently, significant advancements have been made in the areas of plasmonic photothermal NMs and system designs for solar steam generation, including advanced plasmonic materials for enhanced photothermal conversion, novel plasmonic system configurations for improved energy harvesting, and new strategies in plasmonic material optimization. This review aims to provide an updated and in-depth analysis of these recent developments in plasmonic photothermal NMs for solar steam generation, offering a fresh perspective on the current state-of-the-art. As illustrated in Fig. 1, the reported plasmonic photothermal NMs for solar steam generation are mainly classified into five categories: (1) plasmonic metals; (2) plasmonic metal nitrides; (3) plasmonic metal chalcogenides; (4) plasmonic metal oxides; and (5) plasmonic MXenes. Additionally, Fig. 1 highlights five key design principles for plasmonic solar absorbers that should be holistically optimized to improve their solar steam generation performance: (1) solar absorption; (2) photothermal efficiency; (3) water transportation; (4) low-cost and stability; and (5) thermal management.
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Fig. 2 Schematic illustration of the two types of plasmonic nanostructures, as excited by the electric field (Eo) of incident light with wavevector (k). In (a), the nanowire has one dimension much larger than the wavelength of light. In this case, light coupled to the nanostructure will excite the free electrons to create a propagating surface plasmon that can travel along the surface of the metal nanostructure. In (b), the nanostructure is smaller than the wavelength of light, and the free electrons can be displaced from the lattice of positive ions (consisting of nuclei and core electrons) and collectively oscillate in resonance with the light. This is known a localized surface plasmon resonance (LSPR). Reproduced with permission.24 Copyright 2011, American Chemical Society. (c) Schematic describing the photothermal conversion process. |
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Fig. 3 The effect of various important factors on optical properties of plasmonic NMs. (a) Typical ultraviolet-visible-near infrared (UV-Vis-NIR) spectra of Ag, Au and Cu NPs obtained by LASiS (laser ablation synthesis in solution). Reproduced with permission.45 Copyright 2009, Royal Society of Chemistry. (b) Dependence of nanobubble-induced LSPR blue shift on Au NP diameter. Reproduced with permission.37 Copyright 2013, American Chemical Society. (c) Typical absorption spectra of Au NPs with different shapes. Reproduced with permission.38 Copyright 2015, Royal Society of Chemistry. (d) Experimental spectra for different compositions of 60 nm Au/Ag alloy NPs. Reproduced with permission.39 Copyright 2014, Royal Society of Chemistry. (e) Surface plasmon absorption spectra of Au nanorods of different aspect ratios. Reproduced with permission.40 Copyright 2006, American Chemical Society. (f) Absorption spectra of Au nanosphere dimers of varying interparticle spacing for incident light polarized parallel to the interparticle axis. Reproduced with permission.41 Copyright 2013, Optical Society of America. (g) Extinction measurements on ∼8 nm TiN NPs before and after air annealing at 150 and 250 °C. Reproduced with permission.43 Copyright 2017, American Chemical Society. (h) UV-Vis-NIR absorption spectra of CuS nanoplatelets and CuS–MWCNT hybrids deposited on glass substrates and CuS nanoplatelets deposited on a carboxyl functionalized glass substrate. Reproduced with permission.44 Copyright 2016, Royal Society of Chemistry. |
The LSPR also varies with NP coupling. Fig. 3f shows that the simulated absorption spectra of Au nanosphere dimers (nanosphere diameter: 50 nm) exhibit a redshift from ∼530 to ∼620 nm as the interparticle distance decreases from 100 to 2 nm.41 Additionally, the LSPR intensity is significantly enhanced under parallel polarization, and a distinct resonance feature appears when the distance is reduced to 2 nm. Plasmonic coupling via near-field interactions results in spectral splitting and broadening, which can enhance the solar thermal conversion efficiency.21 Moreover, it is well known that the LSPR peak positions of plasmonic NMs shift with the local refractive index of the surrounding dielectric media, enabling their application as sensors. Recently, Shen et al. fabricated a periodic array of Au mushrooms, consisting of the Au cap, photoresist pillar, and Au film, as a plasmonic biosensor for detecting alpha-fetoprotein and cytochrome c by measuring the local refractive index changes through LSPR.42
Moreover, for plasmonic metal nitrides, the degree of oxidation (e.g., TiNxOy) has great influence on their LSPR characteristics, as their LSPR originates from free electrons within the partially filled d-band of metal-like nitrides. TiN NMs, in particular, typically display a broad LSPR band in visible and NIR regions, with both the peak position and intensity undergoing a redshift and weakening, respectively, as the oxidation degree increases (Fig. 3g).43 Semiconductors, such as metal oxides and metal chalcogenides, demonstrate defect-dependent LSPR properties.25 Their free charge carrier density can be easily tuned via varying the stoichiometry. Notably, the LSPR of plasmonic semiconductors is more sensitive to environments and can be quenched. For instance, our group observed that the LSPR of plasmonic CuS underwent significant changes when hybridized with multi-walled carbon nanotubes (MWCNTs) (Fig. 3h).44 This phenomenon was attributed to the charge transfer between CuS and MWCNTs, which nearly completely quenched the LSPR. In brief, by strategically tuning the size, morphology, structure, defect concentration, and composition, it is possible to enhance and broaden the solar absorption capabilities of plasmonic NPs, thereby improving solar-thermal performance.
Material | Irradiation wavelength (nm) | Incident power (W cm−2) | Efficiency (%) | Ref. |
---|---|---|---|---|
Au nanorods | 808 | 2 | 50.0 | 51 |
Au nanoshells | 808 | 2 | 25.0 | 51 |
TiN NPs | 808 | 2.27 | 38.1 | 52 |
Cu2−xSe NPs | 800 | 2 | 22.0 | 53 |
Cu9S5 NPs | 980 | 0.51 | 25.7 | 54 |
Carbon dots | 635 | 2 | 36.2 | 55 |
MoO3−x quantum dots | 880 | 2 | 25.5 | 56 |
CuS/SiO2 nanocapsules | 980 | 1 | 31.2 | 57 |
Pt spirals | 1120 | — | 52.5 | 58 |
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The corresponding photothermal conversion efficiency (η) can be further calculated as follows:59
![]() | (2) |
Moreover, a continuous and sufficient water supply is also crucial for maximizing the evaporation capacity of solar absorbers.59 In floating systems, water is continuously transported to the evaporation interface by the capillary wicking effect of porous structures. Recent studies have demonstrated that micro-sized pores are particularly effective for efficient capillary water transport.63 Additionally, the hydrophilic properties of solar absorbers facilitate the uniform delivery of water molecules across the entire evaporation interface. However, excessively small pores and superhydrophilic surfaces can strongly bind adsorbed water, potentially reducing the evaporation rate.62 Therefore, optimizing the pore size and hydrophilicity is essential to achieve rapid water replenishment and enhance the evaporation rate.
Effective thermal management is also critical, as it addresses heat loss, which primarily arises from downward thermal conduction to bulk water, upward thermal convection to the environment, and upward irradiative energy loss. During solar steam generation, the dissipation of generated heat to bulk water through NM or water conduction is the primary cause of heat loss. To mitigate it, insulated solar absorbers should be developed to inhibit the heat loss to bulk water. As for upward thermal convection and radiation losses, they are inherently present, and minimizing them without compromising other factors remains challenging. However, it has been reported that low-temperature evaporation or the use of spectrum-selective plasmonic absorbers can effectively reduce radiative losses.7,13 In addition, three-dimensional (3D) solar absorbers with low thermal conductivity can effectively localize the generated heat at the evaporation interface, further reducing heat loss.64
Finally, the economic viability and stability of plasmonic NMs warrant significant attention for their practical applications. The long-term functionality of solar steam generation systems depends not only on the mechanical and chemical robustness of the NMs but also on their thermal stability under continuous irradiation. Performance degradation can result from various factors, including the dissolution, detachment, agglomeration, oxidation, and corrosion of plasmonic NPs, as well as the breakage of plasmonic films or NP assemblies. Recent advancements, such as the design of plasmonic core@shell structures, and the integration of plasmonic NMs with organic materials or other cost-effective photothermal NMs, have significantly improved the stability, sustainability, practicability, and photothermal performance. Nevertheless, the development of durable plasmonic composite photothermal NMs with exceptional photothermal performance for long-term, efficient solar steam generation still requires rational design strategies and substantial systematic optimization.
Absorber | Evaporation rate (kg m−2 h−1) | Conversion efficiency (%) | Solar density (sun) | Ref. |
---|---|---|---|---|
Au/SiO2 core–shell NPs | — | 24.0 | 1.4 | 60 |
Black Au membrane | 0.67 | 42.0–57.0 | 1 | 61 |
Au thin film | — | 44.0 | 1 | 72 |
Black Au film | 1.51 | 94.5 | 1 | 75 |
Au/AAO | ∼0.95 | 63.9 | 1 | 76 |
Au/poly(p-phenylene benzobisoxazole) nanofibre | ∼1.42 | 83.0 | 1 | 78 |
Au film/airlaid paper | — | 77.8 | 4.5 | 81 |
Au nanorods/biofoam | — | 76.3 | 0.51 | 101 |
Au nanoflowers | 2.24 | 90.9 | 1 | 102 |
Au NPs | — | 58.9 | 3.5 | 100 |
Au/polystyrene nanofiber | 1.85 | 98.7 | 1 | 115 |
Au@cellulose nanofibers | 1.72 | — | 1 | 116 |
Au nanoflowers | ∼1.36 | 85.0 | 1 | 117 |
Au gyroid | 1.29 | 74.0 | 1 | 118 |
Au/ceramic | 1.26 | 87.0 | 1 | 119 |
Au/CuS | 0.9 | 62.1 | 1 | 120 |
Au–CuS/GMs-80 | 1.54 | 88.8 | 1 | 120 |
Au NR@Cu7S4 | 2.35 | 95.5 | 1 | 121 |
Cu7S4–MoS2–Au/PDMS | ∼3.82 | 96.6 | 1 | 122 |
Au@Bi2MoO6-carbon dots | 1.69 | 97.1 | 1 | 123 |
Au/GO | 1.34 | 84.1 | 1 | 124 |
Au/ZnO | 0.82 | — | 1 | 125 |
Au@PPy/sanded cotton fabrics | 1.33 | ∼93.4 | 1 | 126 |
Au/PA6-GO | 1.10 | 75.6 | 1 | 127 |
Au/polymeric aerogel | 2.70 | 79.3 | 1 | 128 |
Black Ag nanostructure | 1.32 | 95.2 | 1 | 74 |
Porous black Ag film | 1.42 | 92.6 | 1 | 103 |
Ag/PPy | 1.55 | 92.6 | 1 | 104 |
Ag–PSS–AG/AG | 2.10 | 92.8 | 1 | 105 |
Ag@PDA | 2.08 | 97.0 | 1 | 107 |
Porous polyaniline nanofibers/Ag/carbon cloth | 2.21 | 98.0 | 1 | 129 |
Ag/black TiO2/carbon porous layered foams | 1.79 | ∼81.7 | 1 | 130 |
Ag-BSC/bamboo shoot | 1.51 | 86.8 | 1 | 106 |
TiO2/Ag aerogel | ∼1.49 | 93.8 | 1 | 111 |
Polypyrrole-Ag/AgCl | 1.73 | 95.2 | 1 | 131 |
Ag/PPy/poly(ionic liquid)s hydrogel | 1.37 | 88.7 | 1 | 132 |
Ag/carbon cloth | 1.36 | 92.8 | 1 | 133 |
Ag/MgFe2O4@surface-carbonized wood | 1.55 | 88.6 | 1 | 134 |
Wood-SnS-AgNPs | 1.53 | 90.1 | 1 | 135 |
Ag/GO-PW@SiO2 | 1.09 | 95.7 | 1 | 136 |
Ag nanoworms-Janus membrane | ∼0.77 | ∼51.1 | 1 | 137 |
Ag@polyacrylonitrile nanofiber membrane | 1.34 | 76.0 | 1 | 138 |
Carbonized melamine foams@Ag | 2.39 | 119.5 | 1 | 139 |
Bacterial cellulose/GO-Ag | 2.36 | 94.5 | 1 | 140 |
AgNPs@C/melamine foam | 1.62 | 91.3 | 1 | 141 |
Ag-NSP@BFP | 2.77 | 105.2 | 1 | 142 |
Ag/red phosphorus | 1.75 | 94.0 | 1 | 143 |
Ag/Cu2O | 1.35 | ∼49.0 | 1 | 144 |
rGO/AgNPs | 1.21 | 86.8 | 1 | 145 |
rGO–Ag nanowires | 2.02 | 91.0 | 1 | 146 |
Ag/CuO-rGO | 2.60 | 92.5 | 1 | 147 |
AgNPs@C3N4/GO membrane | 1.13 | 77.3 | 1 | 148 |
Ag@TiO2 | ∼1.00 | 68.6 | 1 | 149 |
Chitosan/lignin-C@Ag aerogel | ∼3.57 | ∼92.1 | 1 | 150 |
Ag/nanotubular TiO2 | 0.82 | ∼60.0 | 1 | 151 |
Ag/diatomite | 1.39 | 92.2 | 1 | 152 |
Ag-PDA@wood | 1.58 | 88.6 | 1 | 153 |
Cu@C/CLS | 1.54 | 90.2 | 1 | 65 |
Cu/G | 1.29 | 82.0 | 1 | 68 |
Cu/B–TiO2 | 1.94 | 98.6 | 1 | 154 |
Cu/carbon cells | 2.08 | 93.4 | 1 | 70 |
Cu@C | 1.51 | 94.6 | 1 | 155 |
NC@Cu | 2.76 | 137.1 | 1 | 71 |
Cu@C–N | 1.94 | 89.4 | 1 | 156 |
Cu cauliflower-like nanostructure | — | >60.0 | 1 | 113 |
Cu NPs/cotton fabric | 1.73 | 98.0 | 1 | 157 |
Cu NPs/laser-induced graphene | 2.29 | — | 1 | 158 |
Cu NPs/C–TiO2/SiO2 | 1.50 | 92.2 | 1 | 159 |
PVA/sodium alginate–Cu–CuOx/NC polyurethane | 1.99 | 89.5 | 1 | 160 |
Cu@PPy nanowire aerogel | 2.09 | 97.6 | 1 | 161 |
Plasmonic Pd/wood | 1.00 | 68.5 | 1 | 64 |
Pd/sponge | 2.02 | 131.0 | 1 | 162 |
Al NP/AAM | ∼0.92 | 58.0 | 1 | 66 |
Coral-like plasmonic black Al | 2.40 | 166.5 | 1 | 114 |
Bi–C/carbon felt | 1.50 | 91.9 | 1 | 163 |
Ni-NCNTs/KF | 3.22 | 90.5 | 1 | 164 |
PPy@Ni sponge | 1.71 | 85.3 | 1 | 165 |
Ni@C@SiO2 | 1.67 | 91.2 | 1 | 67 |
Co–N–C/CF | 1.88 | 87.0 | 1 | 166 |
In/microporous membrane | — | 71.6 | 1 | 69 |
AgCu@BT/BC | 1.40 | 95.7 | 1 | 167 |
Cu–Au core–shell NPs | 1.02 | 66.0 | 1 | 36 |
Au/Ag-PFC fibers | 1.40 | 86.3 | 1 | 168 |
Au/Ag–cellulose–polyethyleneimine | 1.31 | 82.1 | 1 | 169 |
Au–Ag alloy nanocorals | 2.32 | 64.0 | 1 | 170 |
Ag/Cu@sawdust biochar | 1.49 | 90.4 | 1 | 171 |
Ag–Ni/cellulose | 1.87 | 93.8 | 1 | 172 |
Ag/Au-GO | 1.00 | 63.0 | 1 | 173 |
Ti3C2Tx/Ag@SiO2 gel | 1.42 | 98.0 | 1 | 174 |
Ag/Ti3C2Tx/ANFs | 2.21 | 92.0 | 1 | 175 |
Ag–Cu-rGO | 1.89 | 90.2 | 1 | 176 |
Recently, Deng's group fabricated a self-assembled plasmonic Au thin film composed of Au NPs (diameter: ∼18 nm) for solar steam generation (Fig. 5a–d).72 This Au film exhibited broad light absorption throughout the entire visible region. By floating the Au film on the water surface to reduce heat loss, the photothermal conversion efficiency reached ∼44.0% under laser irradiation at 10.18 W cm−2. To further address heat loss and the limited physical stability of the Au film, they subsequently loaded the Au film onto an air-laid paper (Fig. 5e).81 Owing to the enhanced surface roughness, improved capillary force, and low thermal conductivity of this double-layer absorber, a conversion efficiency of 77.8% was achieved under 4.5 sun irradiation. Furthermore, they studied the size-dependent effects of Au NPs on solar steam generation, observing that 10 nm Au NPs showed superior performance than their larger counterparts due to their higher absorption capability.100 By loading 10 nm Au NPs onto highly scattering 200 nm polystyrene NPs, the system achieved a photothermal efficiency of ∼58.9% under laser irradiation at 35.36 W cm−2. More recently, Zhang et al. fabricated a self-supporting Au film with high porosity and a hierarchically porous structure by dealloying a Cu99Au1 precursor for solar steam generation.75 This multiscale-structured Au film exhibited excellent hydrophilicity, broadband absorption, and efficient water transport, resulting in a water evaporation rate of 1.51 kg m−2 h−1 and a conversion efficiency of 94.5% under 1 sun irradiation. Furthermore, Zhu et al. developed a highly efficient, broadband plasmonic Au/anodic aluminum oxide (AAO) membrane using physical vapor deposition (PVD) (Fig. 5f–i).76 The PVD process enabled the self-assembly of Au NPs into a thin layer on the top surface and within the inner pores of the nanoporous AAO. Owing to its high solar absorption, localized heating, and porous structure, the Au/AAO absorber achieved an evaporation rate of ∼0.95 kg m−2 h−1 with the efficiency of ∼63.9% under 1 sun illumination.
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Fig. 5 (a) A photograph of a floating plasmonic Au film with a diameter >3 cm at the air–water interface. (b) Scanning electron microscopy (SEM) images of self-assembled film of Au NPs under low and high magnifications. (c) Absorption spectra of the film of Au NPs and the aqueous solution of Au NPs. (d) Optical image (d1) of an assembled Au film within a cuvette and IR side-view image (d2) of the same sample at 10.18 W cm−2 laser illumination. Optical image (d3) of the solution of Au NPs within a cuvette and IR side-view image (d4) of the same sample at 10.18 W cm−2 laser illumination. Reproduced with permission.72 Copyright 2014, Wiley-VCH. (e) Schematic illustration of the structure of airlaid-paper-based AuNP film. Reproduced with permission.81 Copyright 2015, Wiley-VCH. (f–i) Schematic, processes, and photographs of plasmonic Au-deposited nanoporous template (Au/NPT) absorbers. (f) Schematic of an ideal plasmonic absorber. (g) Self-assembly of Au NPs on nanoporous templates to form plasmonic absorbers. (h) Digital camera images of a 1-inch-diameter bare nanoporous template sample and a ≤90 nm thick Au/NPT sample. (i) Cross-sectional SEM image of the Au/D-NPT sample with the average pore size D ∼365 nm and effective Au film thickness ∼85 nm. Reproduced with permission.76 Copyright 2016, AAAS. |
Additionally, numerous bio-derived and bio-inspired strategies have been developed to leverage natural structure features and their related benefits. For instance, Tian et al. fabricated a 3D plasmonic biofoam solar absorber by integrating Au nanorods with a biofoam (Fig. 6a).101 This system exhibited a conversion efficiency of 76.3% under laser illumination at 5.1 W cm−2, owing to its tunable LSPR wavelength, low thermal conductivity, and efficient solar absorption. Furthermore, Zhu et al. fabricated Au nanoflowers with broad solar absorption and outstanding photothermal effects by a tea-assisted method (Fig. 6b–f).102 They further assembled these Au nanoflowers with polyurethane and cellulose nanocrystals, yielding a 3D porous solar absorber. Under 1 sun irradiation, this absorber achieved a high evaporation rate of 2.24 kg m−2 h−1 and a high photothermal efficiency of ∼90.9%. Despite the fabrication of numerous plasmonic absorbers by loading plasmonic Au NPs onto various supports, the high cost of Au NPs remains a significant barrier to their widespread adoption in solar steam generation.
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Fig. 6 (a) Schematic illustration showing the fabrication of plasmonic foams (two photographs in bottom right show the aerogel before and after dense loading of Au nanorods). Reproduced with permission.101 Copyright 2016, American Chemical Society. (b–f) Synthesis and characterization of Au nanoflowers. (b) Schematic of Au nanoflowers synthesis. (c) UV-Vis-NIR absorption spectra of Au nanoflowers with the increasing feeding concentration of HAuCl4. (d and e) Transmission electron microscopy (TEM) images of Au nanoflowers, with inset showing high-resolution TEM (HRTEM) image. (f) Elemental mapping of Au nanoflowers. Reproduced with permission.102 Copyright 2022, Springer Nature. |
Owing to their more intense and localized LSPR absorption, plasmonic Ag NPs exhibited ten times more thermal generation than Au NPs.108 Furthermore, the lower cost of Ag NPs makes them a promising alternative to Au NPs economically. Recently, various Ag-based structures have been designed and fabricated to broaden the light absorption band and improve the evaporation performance. For instance, Chen et al. fabricated a plasmonic black Ag nanostructure with broadband absorption by a confined seeded growth method (Fig. 7a–e).74 Initially, Au seeds were anchored onto iron oxyhydroxide (FeOOH) nanorods within the confined space of resorcinol-formaldehyde (RF) nanoshells. Subsequently, after removing the FeOOH template, rod-shaped RF nanotubes decorated with Au seeds on their inner walls were obtained. The seeds then grew, gradually reducing interparticle spacing and enhancing plasmonic coupling, ultimately forming Ag NP assemblages with varying interparticle spacings within the tubular spaces. The interparticle coupling within these assemblages significantly enhanced plasmonic coupling, resulting in broad solar absorption. Upon 1 sun illumination, the black Ag absorber showed a water evaporation rate of ∼1.32 kg m−2 h−1 with a photothermal efficiency of 95.2%. In addition, Yu et al. fabricated a porous black Ag film by dealloying an Al99Ag1 alloy (Fig. 7f).103 This Ag film exhibited high porosity, low density, excellent hydrophilicity, and broadband solar absorption, achieving a water evaporation rate of 1.42 kg m−2 h−1 and a photothermal efficiency of 92.6%, along with high cycling stability over 30 cycles under 1 sun.
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Fig. 7 (a) Space-confined seeded growth of black Ag nanostructures for solar steam generation. TEM images of (b) FeOOH nanorods with surface modified with Au seeds, (c) FeOOH/Au nanorods overcoated with a layer of RF, (d) hollow RF nanorods containing Au seeds on the inner surface prepared by removing FeOOH, and (e) RF nanorods with AgNP assemblages upon growth of Ag on Au seeds. Reproduced with permission.74 Copyright 2019, American Chemical Society. (f) Schematic illustration showing the one-step dealloying method for preparing the NP-Ag film. Reproduced with permission.103 Copyright 2022, Springer Nature. (g) Diagram of loading Ag/PPy on the substrate. The substrate was coated with PPy firstly, surface deposition with Ag NPs by immersing into an AgNO3 solution secondly. Reproduced with permission.104 Copyright 2019, Elsevier. (h) Fabrication procedures of Ag–PSS–AG/AG device, including agarose gelation process and silver reduction process. Reproduced with permission.105 Copyright 2019, Wiley-VCH. (i) The fabrication process of the bamboo shoot-based bilayer evaporator. Reproduced with permission.106 Copyright 2022, Elsevier. (j) The fabrication procedures of the plasmonic Ag@PDA wooden flower. Reproduced with permission.107 Copyright 2020, Elsevier. |
Organic polymer-polypyrrole (PPy), a photothermal organic material, shows excellent natural broadband solar absorption and high stability.109 Furthermore, it can serve as a bridge between Ag NPs and various substrates to enhance their photothermal performance. For example, Xu et al. reported a photothermal absorber by coating a Ag/PPy layer onto a cellulose substrate (Fig. 7g).104 The interwoven fibrous structure on the cellulose substrate provided interconnected channels for water transport and steam evaporation. This absorber achieved a solar absorption of 95.4%, with a water evaporation rate of 1.55 kg m−2 h−1 and a photothermal efficiency of 92.6% under 1 sun irradiation. Additionally, Sun et al. reported a plasmonic double-layered absorber consisting of a top layer of Ag NPs–poly(sodium-p-styrenesulfonate)–agarose gel (Ag–PSS–AG) and a bottom layer of agarose gel (AG) (Fig. 7h).105 The synergetic effect between the two layers enhanced solar absorption and water transport, enabled the absorber to achieve a water evaporation rate of 2.10 kg m−2 h−1 with a photothermal efficiency of 92.8% under 1 sun.
In addition, integrating a 3D macroscopic structure with a microporous structure can effectively improve the photothermal performance.110 For example, Gao et al. fabricated a 3D porous TiO2/Ag photothermal aerogel for solar steam generation by incorporating electrospun TiO2/Ag nanofibers into chitosan with a freeze-drying process.111 This absorber achieved a vaporization rate of ∼1.49 kg m−2 h−1 with an evaporation efficiency of 93.8% and a parallel photocatalytic hydrogen generation rate of ∼3260 μmol m−2 h−1 under 1 sun, attributed to its desirable broadband absorption, aligned microchannels for mass transport, the plasmonic photothermal effect, and the confined thermal heating effect. Chen et al. synthesized a solar absorber composed of Ag microspheres deposited on bamboo shoot porous carbon (Ag-BSC) and bamboo shoot (Fig. 7j).106 The Ag-BSC was fabricated via pyrolysis, and the double-layer solar absorber was formed by coating Ag-BSC onto a bamboo shoot. Leveraging the broadband solar absorption of Ag-BSC and the superior thermal management of bamboo shoot, the absorber achieved a water evaporation rate of 1.51 kg m−2 h−1 with a photothermal efficiency of 86.8% under 1 sun. In another study, Chen et al. deposited Ag@polydopamine (Ag@PDA) NPs onto a 3D porous wooden flower for solar steam generation (Fig. 7i).107 The plasmonic-incorporated wooden flower exhibited an enhanced solar absorption of 98.65% due to the synergetic effect of plasmonic Ag@PDA NPs and the wooden flower. With abundant capillary microchannels and outstanding thermal management, the Ag@PDA absorber achieved a water evaporation rate of 2.08 kg m−2 h−1 and a photothermal efficiency of 97.0% under 1 sun. Despite the enhanced performance of plasmonic Ag-based photothermal absorbers, potential concerns regarding their poor chemical stability, toxicity and relatively high cost should be considered when plasmonic Ag NPs are applied for large-scale solar steam generation.
Cu is also an attractive plasmonic metal for solar steam generation, owing to its LSPR effect in the visible region, low cost, and high earth abundance.112 Recently, Fan et al. fabricated a cauliflower-like hierarchical Cu nanostructure on a Cu surface using a laser direct writing process.113 This nanostructure, with its broad light absorption ranging from the UV to NIR regions, achieved a photothermal efficiency of >60.0% under 1 sun. However, the poor chemical stability of Cu has greatly limited its application. To address this issue, graphene has been proven to be effective in enhancing the stability of Cu. For example, Xu et al. synthesized a self-floating absorber of Cu nanodot/N-doped graphene urchins (Cu/G) through a space-confined thermal treatment of Cu carbodiimide (Fig. 8a).68 The Cu/G absorber exhibited a solar absorption of 99.0%, attributed to the intensively hybridized LSPR effect of graphene-stabilized Cu nanodots. Furthermore, the absorber showed a water evaporation rate of 1.29 kg m−2 h−1 with an efficiency of 82.0% and excellent long-term stability over 7 days under 1 sun.
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Fig. 8 (a) Schematic of the plasmon-enhanced solar desalination process: the graphene matrix serves as highway for hot electrons photoexcited by Cu LSPR and maintains the Cu surface in the metallic state. Nitrogen doping provides a hydrophilic surface which is beneficial for water infiltration and transport. While the urchin-like structure is responsible for the self-floating property, enabling the temperature increase locally at the water–air interface and thus leading to more efficient solar desalination. Reproduced with permission.68 Copyright 2018, Elsevier. (b) Schematic illustration for the synthesis of a Cu@C/CLS structure. Reproduced with permission.65 Copyright 2021, American Chemical Society. (c) Schematic illustration of the preparation of the Cu@NC photothermal membrane. (d) TEM and (e) HRTEM images of Cu@NC. Reproduced with permission.71 Copyright 2022, Royal Society of Chemistry. (f) Schematic illustration of thin-shell-stabilized plasmonic Cu-based NPs. (g) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and (h) energy-dispersive X-ray spectroscopy (EDS) mapping images of Cu2.5Au1 NPs. (i) EDS spectra of Cu2.5Au1 NPs before and after the etching test with HNO3 (1.0 M) for 30 min. Reproduced with permission.36 Copyright 2021, American Chemical Society. |
More recently, core@shell structures have been shown to effectively inhibit surface oxidation and enhance the stability of Cu NPs. For example, Ren et al. synthesized a hybrid absorber of graphene encapsulated Cu NPs/3D carbonized loofah sponges (Cu@C/CLS) via a pyrolysis method (Fig. 8b).65 The Cu@C NPs were formed in situ on the CLS surface via the pyrolysis of metal–organic frameworks and natural loofah sponges. Owing to the efficient solar absorption and local heating effect of Cu NPs, the Cu@C/CLS absorber showed a high evaporation rate of 1.54 kg m−2 h−1 and a conversion efficiency of 90.2% under 1 sun. Moreover, the ultrathin graphene provided robust protection, enabling the composite to maintain a stable evaporation performance over 7 days. In another study, Meng et al. synthesized a core–shell nanostructure of Cu@N-doped carbon (Cu@NC; denoted as NC@Cu in the original reference) via thermally treating CuO@PDA (Fig. 8c–e).71 The Cu@NC membrane exhibited high solar absorption across the full spectrum and a synergistic effect between the NC and Cu NPs, resulting in a water evaporation rate of 2.76 kg m−2 h−1 and a photothermal efficiency of 137.1% under 1 sun. This evaporation efficiency beyond the theoretical limit resulted from cold evaporation on the side surface, which lowered its temperature below ambient levels and allowed the device to draw energy from the surrounding environment. The membrane also exhibited excellent reusability during 4 days of solar desalination tests. Furthermore, our group synthesized ultrastable Cu@Au core@shell NPs through a seed-mediated process (Fig. 8f–i).36 The Cu@Au NPs, featuring a thin and completely covered Au shell, exhibited significantly enhanced chemical stability, even in harsh environments (HNO3 solution). Detailed characterization and analysis revealed that the external Au layer, along with its compactness and coverage, play a critical role in achieving excellent stability. Importantly, we proved that the thin Au shell has no considerable effect on plasmon dynamics and heat transfer coefficients. As a result, these Cu@Au NPs achieved a water evaporation rate of 1.02 kg m−2 h−1 and a photothermal efficiency of 66% under 1 sun, alongside high chemical stability. Despite these recent advancements in developing high-performance and stable Cu NP-based absorbers in research labs, there are still challenges in practical applications as the large-scale implementation of these core@shell NPs and long-term stability under various conditions are yet to be demonstrated.
In addition to the three most known plasmonic metals (Au, Ag, and Cu) which exhibit typical plasmon resonances in the visible range, other plasmonic metals have also been studied. For instance, Zhou et al. developed a photothermal absorber via self-assembling Al NPs within an AAO membrane (Al NP/AAM) (Fig. 9a and b).66 This absorber showed a pronounced LSPR red shift from the UV to visible and IR regions, attributed to plasmon hybridization of close-packed NPs and surface oxidation of Al NPs. Moreover, the absorber offered unique advantages, including self-floating, broad solar absorption (>96.0%), localized heat at the water surface, and effective desalination. Under 1 sun, it achieved a water evaporation rate of ∼0.92 kg m−2 h−1 and a photothermal efficiency of ∼58.0%. Li et al. fabricated a coral-like plasmonic black Al absorber through a laser writing approach (Fig. 9c–f).114 The laser microfabricated Al (LM Al) achieved a high solar absorption (>92.6%) and a dramatic temperature increase to >90.6 °C under 1 sun. Under solar illumination, the absorber exhibited a water evaporation rate of 2.40 kg m−2 h−1 and a photothermal efficiency of 166.5%. In another study, Zhu et al. reported a 3D mesoporous plasmonic wood absorber via decorating Pd NPs in natural wood (Fig. 9g).64 This plasmonic wood achieved nearly full-spectrum solar absorption (≈99.0%) due to the LSPR effect of Pd NPs and the waveguide effect of natural wood. Moreover, its micro/nanochannels facilitated efficient water transport via capillary action. Owing to these features, the absorber exhibited a water evaporation rate of ∼1.0 kg m−2 h−1 and a photothermal efficiency of ∼68.5% under 1 sun. Additionally, Yang et al. fabricated a photothermal absorber consisting of plasmonic Ni@C@SiO2 core–shell NPs (Fig. 9h and i).67 Leveraging the LSPR effect of Ni NPs (around 500 nm), the broad solar absorption of C, and the protective function of SiO2, the absorber exhibited a water evaporation rate of 1.67 kg m−2 h−1 and a photothermal efficiency of 91.2% under 1 sun. In Table 2, we have summarized various plasmonic metal-based absorbers employed in solar steam generation. However, it is important to note that the high cost, low earth abundance, and/or poor stability of these plasmonic metals present significant challenges for their practical application in solar steam generation.
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Fig. 9 (a) Fabrication of Al NP/AAM. Aluminium foils served as the source materials for the entire fabrication process, AAM fabricated by anodic oxidation, and the Al NP/AAM structure formed after the NP deposition. (b) Optical photographs of the aluminium foil, AAM sample and Al NP/AAM structure observed from the AAM side. Reproduced with permission.66 Copyright 2016, Springer Nature. (c) Schematic of the fabrication process and solar absorption mechanism of black Al. Incident light: yellow; reflected light: red; absorbed light: green. SEM images of the surface morphologies of (d) bare and (e) LM Al surfaces. (f) SEM image of the cross-sectional view of surface micro-/nanostructures on LM Al substrate. Reproduced with permission.114 Copyright 2021, American Chemical Society. (g) Design of plasmonic wood. (g1) A tree transports water from the bottom upward and absorbs sunlight for photosynthesis. (g2) After NP decoration, the natural wood is cut perpendicular to the growth direction of the tree and turns black due to the plasmonic effect of the metal NPs. (g3) After metal NP decoration, light can be guided into the wood lumen and be fully absorbed for steam generation. (g4) Schematic of plasmonic effect of two adjacent metal NPs. (g5) Zoomed-in schematic illustrating the water transport along microchannels in wood. The cell wall is composed of many nanofibrous cellulose (NFC). Reproduced with permission.64 Copyright 2017, Wiley-VCH. (h) Synthetic scheme of Ni@C@SiO2 core@shell NPs and (i) corresponding TEM images of the particles at each stage. Scale bars are all 100 nm. Reproduced with permission.67 Copyright 2020, Wiley-VCH. |
Absorber | Evaporation rate (kg m−2 h−1) | Efficiency (%) | Solar density (sun) | Ref. |
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TiN/bio-carbon foam | 1.47 | 92.5 | 1 | 83 |
TiN/AAO | 1.10 | 78.0 | 1 | 84 |
ZrN/AAO | 1.27 | 88.0 | 1 | 84 |
HfN/AAO | 1.36 | 95.0 | 1 | 84 |
TiN/ceramic fiber wool | — | 80.0 | 1 | 85 |
TiN/AAO | — | 92.0 | 1 | 86 |
Thermally insulated TiN/PVDF | 1.34 | 84.5 | 1 | 87 |
TiN nanocavity arrays | 15.00 | 76.0 | 14 | 88 |
TiN/nanoporous AAO | ∼1.61 | 87.7 | 1.21 | 89 |
TiN/semi-rGO | 1.76 | 99.1 | 1 | 52 |
TiN/PVA | 3.80 | 95.3 | 1 | 186 |
TiN@PVA/PVDF | 0.94 | 64.1 | 1 | 182 |
TiN/PVDF | 1.01 | 66.7 | 1 | 187 |
Porous TiN nanospheres | 1.68 | 93.4 | 1 | 188 |
Hierarchical TiN nanotube mesh | 3.40 | ∼85.4 | 2.5 | 183 |
TiN/polyimide aerogel | 2.97 | 94.5 | 1 | 189 |
MoS2/Mo5N6/C-aerogel | 1.89 | 73.3 | 1 | 190 |
To date, plasmonic TiN nanostructures have been extensively explored for solar steam generation through integration with various supporting materials. For instance, Kaur et al. fabricated a plasmonic absorber by chemically immobilizing TiN NPs, synthesized via a thermal plasma method, onto ceramic fiber wool (CW) using siloxane linkages of (3-aminopropyl)triethoxysilane (APTES) (Fig. 10a).85 Due to its broadband solar absorption, self-floating feature, and highly porous structure, the absorber containing 300 mg of TiN exhibited a photothermal efficiency of ∼80.0% under 1 sun. In a later study, they also developed a solar absorber via loading TiN NPs onto AAO, in which the TiN NPs converted solar energy into heat and the AAO served as a water transporter.86 By adjusting the pore diameter and TiN NP layer thickness, the TiN/AAO absorber achieved a photothermal efficiency of 92.0% under 1 sun. In addition, Guo et al. fabricated a porous TiN/bio-carbon foam (TBCF) absorber by depositing TiN NPs onto carbonized wood (Fig. 10b).83 They identified an optimal absorber composition exhibiting maximal solar absorption, leading to a water evaporation rate of 1.47 kg m−2 h−1 and a photothermal efficiency of 92.5% under 1 sun. Furthermore, our group fabricated a high-efficiency plasmonic photothermal absorber consisting of TiN NPs and two-dimensional (2D) semi-rGO nanosheets by a microwave reduction method (Fig. 10c and d).52 The semi-rGO nanosheets not only served as support materials for plasmonic TiN NPs but also enhanced water evaporation and supply. Meanwhile, the TiN NPs improved solar absorption and hydrophilicity. The layered structure of TiN/semi-rGO effectively inhibited heat loss and enhanced solar energy utilization. As a result, the evaporation rate and photothermal efficiency of the optimal absorber of TiN/semi-rGO-25% reached ∼1.76 kg m−2 h−1 and 99.1% under 1 sun, respectively.
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Fig. 10 (a) Schematic represents the fabrication process of TCW (TiN NPs-CW) structure with possible interactions scheme between CW fibers and TiN NPs. OH group modified CW as well as TiN NPs are functionalized with coupling agent APTES separately. TiN NPs immobilized onto TCW shows steam generation after illumination. Reproduced with permission.85 Copyright 2017, American Chemical Society. (b) Setup for the measurement of solar water evaporation performance, schematic diagram of solar water evaporation process for the sample TBCF, and heat transfer diagram in TBCF during vapor generation process. Reproduced with permission.83 Copyright 2018, Springer Nature. (c) Schematic illustration of the fabrication of plasmonic TiN/semi-rGO nanohybrid solar absorber. (d) TEM images of TiN/semi-rGO-25% composite at different magnifications. Inset is the corresponding selected area electron diffraction (SAED) pattern. Reproduced with permission.52 Copyright 2023, Wiley-VCH. (e) Schematic diagram of electrospinning. (f) SEM and (g) TEM images of TiN/PVA electrospinning membrane. Reproduced with permission.182 Copyright 2020, Elsevier. (h) Schematic of water evaporation and steam generation by the TSA (TiN-based solar absorber) under moderate light concentration in a custom-made PTFE (polytetrafluoroethylene) cell. (i) Schematic structure of the TSA: TiN nanocavities (250 nm thickness), Ti2N thermal layer (∼1 μm), and rest of the Ti substrate. (j) Top-view SEM image of TiN nanocavities. (k) HRTEM-EDS elemental mapping for Ti (blue) and N (red). Reproduced with permission.88 Copyright 2021, Elsevier. |
Besides inorganic materials, organic materials have also been used as supports for TiN NPs. For example, Zhang et al. reported a double layered TiN–poly(vinyl alcohol)/polyvinylidene fluoride (TiN–PVA/PVDF) membrane via an electrospinning coating and heat crosslinking process (Fig. 10e–g).182 The TiN–PVA photothermal coating layer, consisting of PVA nanofibers with TiN nanodots, could reduce heat and mass transfer resistances because of its porous structure, interconnected pores, and good hydrophilicity. Meanwhile, the hydrophobic PVDF support effectively prevented the membrane wetting. This membrane achieved a water evaporation rate of 0.94 kg m−2 h−1 with a photothermal efficiency of 64.1% under 1 sun. In addition, modifying the structure of plasmonic TiN NMs can also improve the photothermal performance. For instance, Ren et al. synthesized a hierarchical TiN nanotube mesh (HTNM) via thermal treatment of a TiO2 nanotube mesh, pre-prepared through electrochemical anodic oxidation of a Ti mesh.183 They found that the solar absorption and photothermal efficiency were influenced by the thermal treatment temperature. The HTNM exhibited a water evaporation rate of ∼3.40 kg m−2 h−1 with a photothermal efficiency of ∼85.4% under 2.5 suns. In another study, Mascaretti et al. presented the scalable preparation of ultrathin TiN nanocavity arrays for solar steam generation (Fig. 10h–k).88 By combining anodization and thermal nitridation processes, they achieved broadband solar absorption confined within the nanocavities, leveraging the plasmonic effect and cavity resonances. The plasmonic array exhibited a water evaporation rate of ∼15.00 kg h−1 m−2 and a photothermal efficiency of ∼76.0% under 14 suns. Despite the outstanding photothermal performance of plasmonic metal nitrides, more work is still needed to understand their stability during solar steam generation, as their plasmonic effects are highly dependent on material purity. Additionally, the synthetic processes for metal nitrides need further improvement to reduce fabrication costs and optimize the size and morphology, thereby further improving their photothermal performance. At present, TiN NMs are mainly fabricated by laser ablation184 or direct high-temperature nitridation of TiO2 powders,185 which often result in broad size distribution and particle agglomeration.
Absorber | Evaporation rate (kg m−2 h−1) | Efficiency (%) | Solar density (sun) | Ref. |
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Cu7S4 nanocrystals | — | 60.5 | 1 | 90 |
Cu2−xS nanowires | 0.95 | 66.0 | 1 | 91 |
Cu9S5/PVDFM | ∼1.17 | 80.2 | 1 | 92 |
Cu2−xS/S/rGO | 2.64 | ∼95.6 | 1 | 191 |
Cu2−xS NRs–PVA gel | 1.27 | 87.0 | 1 | 196 |
MXene/Au@Cu2−xS | 2.02 | 96.1 | 1 | 197 |
CuS/polyethylene | 1.02 | 63.9 | 1 | 93 |
Carbon nanoparticle/CuS/polyurethane | 1.62 | 93.8 | 1 | 198 |
CuS/SCM | 1.09 | 68.6 | 1 | 94 |
CuS/PVDFM | 1.43 | 90.4 | 1 | 195 |
CuS hollow nanospheres | ∼1.30 | 85.1 | 1 | 199 |
Cu2−xSe NPs | 1.44 | ∼90.6 | 1 | 200 |
Cu2−xSe@PDAs | 2.71 | — | 1 | 192 |
Cu2−xTe nanowire | 1.40 | 81.0 | 1 | 193 |
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Fig. 11 (a) Schematic of reaction pathways for sulfur and 1-octadecene and colored 1-octadecene-sulfur precursor. TEM images of (b) monodisperse disk-like Cu7S4 nanocrystals with yellowish ODE-Sulfur precursor and (c) monodisperse spherical Cu7S4 nanocrystals with orange ODE-Sulfur precursor. Reproduced with permission.90 Copyright 2016, Wiley-VCH. (d) Schematic illustrations show the laser thermal preparation of Cu2−xS/S/rGO nanocomposites and interfacial solar water purification. Reproduced with permission.191 Copyright 2022, Springer Nature. (e) The solar-driven evaporation device and the structure diagram of Cu2−xSe@PDA. Reproduced with permission.192 Copyright 2022, Springer Nature. (f) A schematic illustrating the fabrication process of the bpCu2−xTe (broadband plasmonic Cu2−xTe nanowire) membrane. The broadband plasmonic Cu2−xTe nanowires are deposited onto a cellulose membrane by a vacuum filtration process, forming a double-layer structure. Insets: the inset (f1) shows a digital photograph of the bent bpCu2−xTe membrane, demonstrating its robust flexibility; the inset (f2) shows a water contact angle of 31° on the bpCu2−xTe membrane. (g) Representative cross-sectional SEM image of a selected area of the bpCu2−xTe membrane. Reproduced with permission.193 Copyright 2021, Elsevier. |
Among various plasmonic Cu2−xS NMs, metallic covellite CuS exhibits a broad and strong LSPR peak in the visible and NIR regions, attributed to the high density of free holes in its valence band.194 Unlike other plasmonic semiconductors, these free holes are primarily associated with disulfide bonds bridging CuS4–CuS3–CuS4 units, rather than cation vacancies, thereby allowing LSPR to be regulated by the reversible formation or cleavage of disulfide bonds without changing the composition and phase.30 It has also attracted tremendous attention for solar steam generation. For instance, Tao et al. fabricated a 3D hierarchical CuS structure by a hydrothermal method.195 When combined with a PVDF membrane support, the CuS/PVDFM absorber showed broad and high light absorption across the solar spectrum. Under 1 sun irradiation, the floatable absorber showed a water evaporation of 1.43 kg m−2 h−1 and a photothermal efficiency of 90.4%, along with excellent durability and reusability over 20 cycles. In addition, same as plasmonic Cu2−xS NPs, Cu2−xSe and Cu2−xTe NPs exhibited strong solar absorption and photothermal performance mainly in the NIR range. Cheng et al. fabricated a solar absorber composed of PDA-modified Cu2−xSe nanocomposites (Cu2−xSe@PDAs) with a core@shell structure (Fig. 11e).192 The optimal Cu2−xSe@PDA30 absorber, benefiting from broadband absorption and efficient water transport, showed a water evaporation rate of 2.71 kg m−2 h−1 under 1 sun. In another study, Chen et al. reported a plasmonic Cu2−xTe membrane with broadband solar absorption (Fig. 11f and g).193 This membrane presented outstanding solar absorption of ∼95.9% over the full spectrum, with high porosity, excellent hydrophilicity, and low thermal conductivity, enabling a water evaporation rate of 1.40 kg m−2 h−1 and a photothermal efficiency of 81.0% under 1 sun. These studies highlight the outstanding solar absorption abilities of plasmonic metal chalcogenides, making them promising, cost-effective photothermal nanomaterials. However, further studies are needed to assess their toxicity, stability, structural composition, surface ligands, and morphological regularity. In particular, as mentioned above, the plasmonic properties of these materials are highly sensitive to charge carrier densities, which can be affected by oxidation in air or changes in surface ligands. Thus, particular attention needs to be paid to their chemical stability and interactions with their immediate environments when exploring plasmonic metal chalcogenides for plasmon-related applications.
Absorber | Evaporation rate (kg m−2 h−1) | Efficiency (%) | Solar density (sun) | Ref. |
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WO3−x nanorods | 1.28 | 82.5 | 1 | 96 |
WOx nanosheets | 1.10 | 78.6 | 1 | 203 |
W18O49/rGO | 1.34 | 86.5 | 1 | 204 |
WO3−x/Ni foam | 1.50 | 88.0 | 1 | 205 |
WO2.72/polylactic acid fiber | 3.81 | ∼81.4 | — | 206 |
W18O49@PDMS | 1.15 | 80.7 | 1 | 210 |
W18O49 nanowires/carbon foam | ∼1.69 | — | — | 211 |
Cu/W18O49@graphene | 1.41 | 88.6 | 1 | 212 |
WO3−x/Ag/PbS/NF | 1.90 | 94.0 | 1 | 213 |
MoO3−x nanobelts | 0.99 | 62.1 | 1 | 95 |
MoO3−x nanoparticles | 4.14 | 90.7 | 1 | 214 |
Flower-like MoOx | ∼1.26 | 85.6 | 1 | 97 |
Nanoflower-like MoO3−x | 1.51 | 95.0 | 1 | 207 |
Ni-G-MoO3−x | 1.50 | 95.0 | 1 | 208 |
TiN/MoO3−x | 2.05 | 106.7 | 1 | 209 |
For WO3−x, by optimizing its morphology and surface nanostructures, its light-harvesting performance can be significantly enhanced. For example, Ming et al. synthesized 2D oxygen-deficient WOx nanosheets by introducing oxygen vacancies into WO3, which were used as nanofluids for solar steam generation (Fig. 12a and b).203 Owing to the LSPR effect, the WOx nanosheets showed broad and intense solar absorption across the entire solar spectrum. Under 1 sun, the WOx nanofluids achieved a water evaporation rate of 1.10 kg m−2 h−1 and a photothermal efficiency of ∼78.6%. In another study, Chala et al. fabricated a WO2.72/polylactic acid fiber membrane via melt electrospinning for use as a photothermal absorber.206 Due to its strong NIR photo-absorption and floatability on water, the fiber membrane showed a water evaporation rate of 3.81 kg m−2 h−1 and a photothermal efficiency of 81.39% under a 150 W infrared lamp. Li et al. developed a bilayer absorber by assembling WO3−x nanorods on wood for solar steam generation (Fig. 12c–e).96 The bilayer absorber showed a high solar absorption of ∼94.0%, attributed to the broad absorption of WO3−x nanorods and the multi-light scatting effect of wood pores/channels. It achieved a water evaporation rate of 1.28 kg m−2 h−1 and a photothermal efficiency of 82.5% under 1 sun. In addition, they also fabricated a 1D/2D W18O49/rGO bilayer absorber composed of W18O49 nanofibers and rGO nanosheets (Fig. 12f and g).204 The optimized bilayer absorber achieved a water evaporation rate of 1.34 kg m−2 h−1 and a photothermal efficiency of 86.5% under 1 sun. Furthermore, Wang et al. fabricated a 3D hierarchical WO3−x/Ni foam (WO3−x/NF) absorber via a hydrothermal-annealing route (Fig. 12h).205 Owing to the channel vapor escape and light-harvesting caused by the multi-scattering effect of 3D hierarchical porous NF and the LSPR effect of WO3−x, the WO3−x/NF absorber exhibited a broad solar absorption of 95.0% across the full spectrum. Under 1 sun, the WO3−x/NF absorber achieved a water evaporation rate of 1.50 kg m−2 h−1 and a photothermal efficiency of ∼88.0%.
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Fig. 12 (a) and (b) Structure characterization of the 2D defective WOx (WOAr2) nanosheets. (a) SEM image and (b) low-magnification TEM image. Reproduced with permission.203 Copyright 2018, Elsevier. (c–e) Characterization of the WO3−x nanorod-decorated wood evaporator. (c) The photo images of the WO3−x NR-decorated wood evaporator. (d) SEM image of the wood surface and the inset shows the SEM image of WO3−x nanorods on the surface of the wood. (e) SEM image of the long channels in wood. Reproduced with permission.96 Copyright 2019, Elsevier. (f) SEM image and (g) TEM image of 1D/2D W18O49/rGO-0.01 heterostructure. Reproduced with permission.204 Copyright 2019, Elsevier. (h) Fabrication process of the hierarchical WO3−x/NF. Reproduced with permission.205 Copyright 2021, Elsevier. |
Sub-stoichiometric MoO3−x represents another promising class of plasmonic metal oxide for solar steam generation and has also been widely developed. For example, Lu et al. fabricated a flower-like oxygen-defected MoOx hierarchical nanostructure composed of atomically thick nanosheets using a one-pot hydrothermal method for solar steam generation (Fig. 13a–d).97 The assembled plasmonic MoOx architecture exhibited a broad solar absorption of ∼90.0%. When MoOx was loaded onto the polytetrafluoroethylene (PTFE) membrane, the absorber achieved a water evaporation rate of ∼1.26 kg m−2 h−1 with a photothermal efficiency of 85.6% under 1 sun. Huang et al. also synthesized a photothermal absorber of nanoflower-like plasmonic MoO3−x (Fig. 13e and f).207 The 3D MoO3−x nanoflowers, benefiting from oxygen vacancy-induced LSPR effects and their flower-like structure, showed broadband solar absorption of 97.0%. As a result, the MoO3−x absorber exhibited a water evaporation rate of 1.51 kg m−2 h−1 and a photothermal efficiency of 95.0% under 1 sun. In another study, Li et al. fabricated plasmonic MoO3−x nanobelts with tunable solar absorption by the PEG-400 protected reduction method (Fig. 13g and h).95 During synthesis, PEG-400 acted as both a reducing agent to introduce oxygen vacancies to control plasmonic absorption and a surface-protecting ligand to maintain the initial morphology of the MoO3−x nanobelts. The optimal MoO3−x nanobelts, with their broadband absorption and special 1D nanostructure, showed a water evaporation rate of 0.99 kg m−2 h−1 with a photothermal efficiency of 62.1% under 1 sun. Furthermore, Gong et al. reported a plasmonic absorber composed of graphene and MoO3−x covered porous Ni (Ni-G-MoO3−x) through combining chemical vapor deposition and hydrothermal methods (Fig. 13i).208 Owing to the LSPR effect of plasmonic MoO3−x, the absorber exhibited improved solar absorption (96.0%). The Ni-G-MoO3−x absorber, with its superhydrophilic solar absorbing layer, showed a water evaporation rate of 1.50 kg m−2 h−1 with a photothermal efficiency of 95.0% under 1 sun. Recently, our group developed a 3D dual plasmonic TiN/MoO3−x composite as a photothermal absorber for solar steam generation (Fig. 13j).209 This 3D composite features a biomimetic urchin-like structure composed of plasmonic TiN NPs and MoO3−x nanorods. Owing to its high hydrophilicity, efficient water transport, high surface area, and potential multiple light scattering effects, the composite absorber achieved a water evaporation rate of ∼2.05 kg m−2 h−1 and a photothermal efficiency of 106.7% under 1 sun. The dual plasmonic nanostructure and photothermal stability of this composite were further demonstrated using photon-induced near-field electron microscopy combined with electron energy-loss spectroscopy and advanced in situ laser-heating TEM, respectively. Although plasmonic oxygen-deficient metal oxides exhibit outstanding solar absorption, their photothermal performance can be further improved by regulating their morphology, size, and composition.
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Fig. 13 (a) SEM image and (b) and (c) TEM images of the MoOx hierarchical nanostructure. (d) Elemental mapping of the MoOx hierarchical nanostructure. The vertical scale (bottom) is 5 nm. Reproduced with permission.97 Copyright 2018, Wiley-VCH. (e) Schematic illustration of the fabrication process of MoO3−x samples. MoO3−x was prepared from Mo powder by a solvothermal method. (f) Photographs of the products synthesized at different temperatures varying from OVs (oxygen vacancies)-enriched M160 (black powder) to oxygen-recovered M550Air (white powder). Reproduced with permission.207 Copyright 2019, Elsevier. (g) Schematic illustration of the PEG-400 surface protected reduction process for the formation of 1D MoO3−x nanobelts. (h) Low-magnification TEM images of the MoO3−x nanobelts synthesized in the presence of 0, 100, 200, 500 and 1000 μL of PEG-400. Reproduced with permission.95 Copyright 2020, Springer Nature. (i) SEM images of Ni-G-MoO3−x. Reproduced with permission.208 Copyright 2021, Elsevier. (j) Schematic illustration, SEM image, and TEM image of the dual plasmonic TiN/MoO3−x composite with a 3D urchin-like structure. Reproduced with permission.209 Copyright 2024, American Chemical Society. |
Absorber | Evaporation rate (kg m−2 h−1) | Efficiency (%) | Solar density (sun) | Ref. |
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Ti3C2Tx/PANI | 2.65 | 93.7 | 1 | 218 |
PDA@Ti3C2Tx | ∼1.28 | 85.2 | 1 | 217 |
Ti3C2Tx–TiOX | 2.09 | 92.0 | 1 | 221 |
Ti3C2Tx/polyethyleneimine/PDA/melamine foam | 1.47 | 88.7 | 1 | 222 |
Ti3C2Tx anchored structure | 2.48 | 89.3 | 1 | 223 |
Ti3C2Tx/FCuS | 1.34 | 93.0 | 1 | 224 |
Ti3C2Tx/carbon nanotube | 2.10 | 93.4 | 1 | 225 |
Ti3C2Tx/rGO hydrogel | 3.62 | 91.0 | 1 | 219 |
Ti3C2Tx/cellulose | 1.44 | 85.8 | 1 | 226 |
Ti3C2Tx nanoflakes/copper indium selenide | 1.43 | ∼90.0 | 1 | 227 |
MXene/PDA@TiO2/Fe3O4@C22-HMC | 2.09 | 94.4 | 1 | 228 |
HAP/PDA-modified Ti3C2Tx/PAM/PVA aerogel | 2.62 | 93.6 | 1 | 220 |
Carbonized MXene/PDA foam | ∼1.60 | 89.8 | 1 | 229 |
Ti3C2Tx/diatomite-modified coconut husk | 2.10 | ∼90.6 | 1 | 230 |
GO/Ti3C2Tx aerogel | 1.27 | 90.7 | 1 | 231 |
Aligned reduced graphene oxide/Ti3C2Tx | 2.09 | 93.5 | 1 | 232 |
Ti3C2Tx/rGO/MoS2 | 1.33 | 90.1 | 1 | 233 |
MoS2/Ti3C2 aerogel | 2.75 | — | 1 | 234 |
Ti3C2–MnO2@luffa sponge | 1.36 | ∼85.3 | 1 | 235 |
Cu3BiS3/Ti3C2 | 1.32 | 91.9 | 1 | 236 |
Chitosan-Ti3C2-PDA-Ag | 2.31 | 90.8 | 1 | 237 |
Ti3C2TxAuNFs | 1.59 | 97.8 | 1 | 238 |
Porphyrin-Ti3C2Tx | 1.41 | 86.4 | 1 | 239 |
SiO2/Ti3C2TX/poly(tetrafluoroethylene) | 1.53 | 85.6 | 1 | 240 |
Poly(lactic acid)/Ti3C2Tx@MoS2 | 1.39 | 91.0 | 1 | 241 |
Chitosan/lignin aerogel-carbonized lignin@MXene | 2.35 | 88.2 | 1 | 242 |
MXene/cellulose composite cloth | 1.34 | 89.6 | 1 | 243 |
Ti3C2Tx/aramid nanofiber aerogels | 1.48 | 93.8 | 1 | 244 |
Ti3C2/wood aerogel | 2.00 | 92.6 | 1 | 245 |
Ti3C2TX/carbon aerogels | 1.48 | 92.3 | 1 | 246 |
Ti3C2TX/cellulose nanofibers/luffa aerogels | 1.40 | 91.2 | 1 | 247 |
Ti3C2Tx/Au@Cu2−xS | 2.02 | 96.1 | 1 | 197 |
Ti3C2Tx/La0.5Sr0.5CoO3 | 2.26 | 92.3 | 1 | 248 |
Perovskite/Ti3C2/PVA hydrogels | 1.98 | 90.0 | 1 | 249 |
Foamy Ti3C2 membrane | 1.54 | 87.1 | 1 | 250 |
Ti3C2Tx-based hydrogel-coated cotton fabrics | 1.65 | 95.0 | 1 | 251 |
PVA/Ti3C2Tx/p-g-C3N4 | 1.85 | 93.5 | 1 | 252 |
Ti3C2Tx flexible Janus membrane | 1.34 | 81.5 | 1 | 253 |
PPy/Ti3C2Tx-PDA-fabric | ∼1.49 | 90.6 | 1 | 254 |
Recently, the integration of plasmonic MXenes with organic polymers to form MXene-based composites has been widely reported. For instance, Zhao et al. synthesized PDA@MXene microspheres for solar steam generation via a hydrogen bond-induced self-assembly method (Fig. 14a).217 By combining PDA, known for its excellent light absorption, with plasmonic Ti3C2Tx MXene, the PDA@MXene microspheres achieved a synergistic solar absorption capacity of ≈96% in the range of 250–1500 nm. Moreover, the excellent hydrophilicity of the microspheres is conducive to rapid water transport and vapor escape. As a photothermal film, the PDA@MXene microspheres exhibited a water evaporation rate of ∼1.28 kg m−2 h−1 and a photothermal efficiency of 85.2% under 1 sun. In another study, Ding et al. combined plasmonic MXene with a photothermal polymer of polyaniline (PANI) to fabricate MXene/PANI non-woven fabrics (MPs) by a wet-spinning process (Fig. 14b–d).218 The porous structure and crumpled micro-surface of the MXene/PANI fabrics enabled a high water evaporation rate of 2.65 kg m−2 h−1 and a photothermal efficiency of 93.7% under 1 sun.
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Fig. 14 (a) Schematic illustration for the fabrication of the PDA@MXene photothermal layer. Reproduced with permission.217 Copyright 2020, Springer Nature. (b) Schematic illustration of interfacial solar vapor evaporation of MXene/PANI fabrics. (c) Solvent exchange process for obtaining Ti3C2Tx organic solvent dispersion. (d) Schematic illustration of preparation of MPs. Reproduced with permission.218 Copyright 2023, Royal Society of Chemistry. (e) Schematic illustration of the SSG (solar steam generation) technology caused by the synergy of tailored water states in confined space and concave pyramid-shaped surface topography of Ti3C2Tx MXene/rGO-embedded hybrid hydrogels. Reproduced with permission.219 Copyright 2021, American Chemical Society. (f) The preparation and surface modification of MXene nanosheets with in situ polymerized PDA molecules. (g) Fabrication of the biomimetic PDMX/HPP aerogel with vertically aligned channels for solar energy-driven water evaporation. (h) Digital images of the lotus stem, and the transverse and longitudinal directions of the lotus stem. Reproduced with permission.220 Copyright 2023, Wiley-VCH. |
Furthermore, Lu et al. designed a hybrid hydrogel embedded with 2D nanostructures for solar steam generation by simultaneously infiltrating Ti3C2Tx and rGO nanosheets into a polymer network composed of PVA and chitosan (Fig. 14e).219 The fabricated Ti3C2Tx/rGO hydrogel absorber, featuring surface patterns, reduced the evaporation enthalpy and induced a Marangoni effect, achieving a water evaporation rate of 3.62 kg m−2 h−1 and a photothermal efficiency of 91% under 1 sun. In another study, Wang et al. fabricated a lotus-stem-inspired hydroxyapatite (HAP) nanowires/PDA-modified Ti3C2Tx MXene/polyacrylamide (PAM)/PVA aerogel (PDMX/HPP) with vertically aligned channels for solar steam generation (Fig. 14f and g).220 Benefiting from the vertically arranged pore structure of ∼20–70 μm diameter, excellent mechanical properties, outstanding hydrophilicity and water transport, strong water absorption capacity and reduced evaporation enthalpy, the aerogel achieved a water evaporation rate of 2.62 kg m−2 h−1 with a photothermal efficiency of 93.6% under 1 sun.
However, in general, plasmonic MXenes face several challenges. First, their synthesis typically involves complex processes and the use of hazardous chemicals, which poses challenges in large-scale practical applications. Second, MXenes have poor stability and are prone to corrosion and oxidation, further limiting their practical applications. Specifically, for solar steam generation, the strong interlayer interactions inherent to the 2D structure of MXenes make it difficult for them to maintain monodispersity, thereby compromising their photothermal performance. Therefore, improving the colloidal and chemical stability of 2D MXenes is a key challenge to advance their practical applications.
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Fig. 15 (a) Experimental setup for solar desalination. (b) The measured salinities (the weight percentage of Na+) of the four simulated seawater samples before and after desalination. (c) The measured concentrations of four primary ions in an actual seawater sample before and after desalination. The purple (green) shaded area refers to the overall typical salinity achieved by traditional membrane (distillation) desalination process, respectively. Reproduced with permission.66 Copyright 2016, Springer Nature. (d) The overall heavy metal ions removal performance of the L2.5-Cu1.0-PC hydrogel before and after solar distillation (including the contribution from adsorption). Reproduced with permission.70 Copyright 2021, Royal Society of Chemistry. (e) Model water purification performance of a Cu@C/CLS device. Absorption spectra of methylene blue solution and methyl orange solution before purification and the corresponding condensed water after purification. Reproduced with permission.65 Copyright 2021, American Chemical Society. (f) Schematic diagram of the integral prototype for condensate collection and triboelectric energy generation. (f1) The condensate collected under focused sunlight after the first 60 min. Schematic diagram of triboelectric nanogenerator (TENG) for (f2) water flowing down the wall and (f3) water swinging in the round bottom vessel. Reproduced with permission.117 Copyright 2018, Wiley-VCH. (g) Schematic of the closed-loop solar autoclave showing (I) the steam generation module, (II) the connection module, and (III) the sterilization module. Reproduced with permission.99 Copyright 2013, PNAS. |
Firstly, the high cost (e.g., materials and fabrication) and/or limited stability of plasmonic NMs greatly hinder their large-scale practical applications. Plasmonic photothermal NMs are highly attractive owing to their unique and strong light–matter interactions and localized electromagnetic fields. However, to enable a viable technology, they must exhibit robust, long-term chemical and physical stability; otherwise, performance degradation will take place unexpectedly, which is extremely detrimental to large-scale processes such as desalination or wastewater treatment. Regarding synthesis, the complicated preparation processes and the need for tightly controlled reaction conditions seriously restrict synthesis scalability. For example, we successfully synthesized highly stable laboratory-grade plasmonic Cu@Au NPs under a nitrogen atmosphere by precisely controlling the reaction conditions. However, attempts to scale up the synthesis resulted in NPs with poor uniformity and low yield. Achieving large-scale production of such NPs without compromising their structure and properties remains a great challenge. Since cost-effective materials and fabrication, scalable production and high stability have always been the essential prerequisites for widespread implementation, considerable efforts are required to address these issues.
Secondly, the integration of plasmonic and carbon-based photothermal materials offers a promising avenue to further improve solar steam generation performance. Carbon-based materials, derived from natural sources (e.g., wood) or synthetic sources (e.g., polymers), have been widely employed in solar steam generation due to their numerous advantages, including excellent solar absorption, low cost, high stability, lightweight characteristics and porous structures. The porous nature of these materials enhances solar absorption by plasmonic NPs or carbon materials through multiple light scattering events, meanwhile facilitating efficient water transport to the absorber surface. Combining plasmonic NPs with carbon-based materials not only enhances the solar absorption capacity and photothermal efficiency but also reduces costs, as the porous structure enables uniform dispersion of plasmonic NPs on the substrate, thereby increasing their utilization efficiency. Therefore, the key to advancing photothermal performance lies in the effective and robust integration of two exceptional photothermal materials.
Thirdly, the rational structural design of plasmonic NMs is crucial for achieving superior photothermal performance. As discussed above, the photothermal conversion efficiency highly relies on the SPR properties. Various morphologies, such as nanospheres, nanorods, nanowires, nanoflowers, and nanosheets, have been developed to tailor their SPR behavior. Among these, spherical-shaped NPs have been found to be the most effective in photothermal conversion, and further, their assembly often leads to even higher performance. How can the correlation of their delicate structural features with SPR properties contribute to future structural design? What synthetic and assembly routes need to be further developed towards the final goal? How to maximize the benefits from plasmonic NPs with as least as possible expensive materials? Significant efforts are required to address these questions, which can offer valuable insights. Although some fundamental understanding of plasmonic photothermal phenomena has been gained, current knowledge is still limited. Theoretical simulations and machine learning are expected to provide valuable guidance for accelerating future research in this field.
Finally, systematic theoretical and experimental investigations need to be closely integrated to comprehensively understand the influence of solar absorbers on the solar steam generation performance. These investigations should include not only light absorption ability, evaporation rate, and water supply efficiency but also thermal management, taking into consideration downward heat conduction losses, radiation losses to the surroundings, and convective heat losses. Experimental data should be fed back into the design loop and theoretical models timely modified. Additionally, environmental factors such as temperature, humidity, and wind speed interference also affect the evaporation performance and long-term stability of solar absorbers. By gaining a deeper understanding of these factors, a unified standard can be established in this field, enabling rigorous comparison across different materials and further, more effective development of future solar thermal systems.
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