Recent Research Progress on the Preparation and Applications of Metallic, Semiconducting, and Carbon-Based Photothermal Nanomaterials

Abstract

Energy obtained by green ways with releasing environmental pollution is still a challenge for sustainable development for model society. Among energy technologies, photothermal conversion by using solar energy has become a new field and a hot topic in recent years. Based on the exploration of nanomaterials in the past decades, photothermal nanomaterials by using nanomaterials bring new chances for expending the utilization of green energy with high efficiency, mainly including metal semiconductors and carbon nanomaterials. Their modulated structure for enhancing light absorption, accelerating transformation of photon into heat, and located heat management were also considered important for promoting the utilization of solar energy and therefore, the strategies for designed and controllable preparing of photothermal nanomaterials were also summarized. The applications of photothermal nanomaterials were also reviewed to reveal the new chances for energy conversion engineering or promoting the solar energy utilization of solar energy in some cold regions or somewhere with low solar irradiation.

1. Introduction

Today, energy shortage is a serious issue to the modern society and green energy is expected for our sustainable development [1,2]. Solar energy has been considered abundant for decades and consequently technologies were explored for utilizing this clean energy with high efficiency, such as photothermal power generation, photocurrent conversion, photocatalysis, photochemical reaction, solar-driven interfacial evaporation, and so on [3,4,5]. Although numerous achievements have been obtained in the past years, the harvesting and utilization of solar energy still face challenges due to its inherent intermittency, fluctuation, and low energy density. Hence, advanced materials, such as nanomaterials, were designed and prepared for the utilization of solar energy in limited conditions and to bring new applications. Among them, photothermal conversion technology is considered a promising application strategy of solar energy, performing unique advantages in the fields of space heating, power generation, and seawater desalination [6,7]. Traditional solar thermal materials often have issues such as narrow spectral absorption ranges, weak light absorption, and poor thermal management. Consequently, these materials cannot fully utilize solar energy during photothermal conversion, resulting in consistently low efficiency levels [8]. With the rapid development of nano-functional materials in the past decades, it has become possible to directly utilize solar energy through photothermal processes due to the tunable optical properties of nanomaterials (such as light absorption, paths for photon transferring, and so on), presenting huge potential applications [9,10]. Therefore, photothermal nanomaterials exhibit a great chance for green energy generation and application, and thus the studies on such nanomaterials with photothermal conversion properties have attracted lots of attention [11].

Compared with traditional photothermal materials, nano-photothermal materials rely on the nanoscale effect to achieve significant improvement in conversion into heat [12,13]. In the past decades, several nanomaterials were designed and prepared with enhanced photothermal conversion capacities for solar energy utilization [11,13]. Generally, based on the differences in electronic structure and photothermal conversion mechanisms, nano-photothermal materials can be classified into three main categories of metal, semiconductor, and carbon nanostructure. These mechanistic differences reveal the unique optical and thermal properties of each material, which can be applied to practical applications in various photothermal systems according to their distinctive characteristics.

Metallic materials primarily achieve photothermal conversion through surface plasmon resonance (SPR) between metals and dielectrics. When incident light irradiates the metal surface, electrons in the metal are excited and undergo transitions from lower to higher energy levels, generating large number of hot electrons. These hot electrons then transfer their energy to the metal lattice via electron–phonon coupling, thereby heating the material itself. The accumulated heat is subsequently dissipated into the surrounding environment through thermal transport, ultimately realizing efficient photothermal conversion [14]. Some metals (Au, Ag, Pd, just name a few) in nanoscale perform high local surface plasmon under light irradiation, promoting the light-absorbed intensity and accelerating the energy conversion. For alloy or hetero nanostructures, the coupling of different carrier behaviors would significantly promote their photothermal conversion [15].

Owing to the unique band structure, semiconductor materials achieve photothermal conversion primarily through electron excitation and non-radiative recombination of electron-hole pairs. As photothermal materials, their most defining feature is the tunable bandgap structure. To address practical demands in diverse photothermal systems, the structure (including band gap, interface, carrier concentration, and so on) of semiconductor nanomaterials can be modulated through methods such as interface modification and elemental doping to enhance photothermal conversion efficiency [16,17,18].

The high efficiency of photothermal conversion in carbon-based materials stems from their strong coupling between delocalized π-electron systems and lattice vibrations. Carbon materials can excite electrons from π orbitals to π* orbitals by absorbing incident light, then convert the absorbed energy into thermal energy through molecular thermal vibrations, thereby exhibiting superior photothermal conversion efficiency [14]. The material sources for preparing carbon photothermal nanomaterials are so abundant that such kinds of photothermal materials are usually obtained at a low cost. In addition, carbon nanomaterials (such as graphene, carbon nanotubes, porous carbon nanomaterials, etc.) absorb light across broad ranges due to their hybrid structures; many efforts are therefore focused on increasing the absorption intensity and improving the utilization of light energy [19].

Hence, reasonable design and preparation of nanomaterials are the key paths for obtaining photothermal materials. What is more, the small size of photothermal nanomaterials provides high specific surface area and paves the way for constructing light-trapping structures of hollow or porous morphologies [17]. By constructing light-trapping structures, the transmission distance of the incident light would be prolonged in the material so that the multiple diffuse reflection and absorption of light enhanced the total absorptance improving the photothermal conversion [20]. In addition, nanomaterials with anisotropic morphology, such as one-dimensional carbon nanotubes and two-dimensional graphene, exhibit directional thermal conductivity during photothermal conversion achieving efficient directional transmission of photogenerated heat and avoiding material crystal destruction or chemical degradation caused by local overheating, thus ensuring the long-term operation stability of photothermal systems [21].

Based on the excellent properties resulting from the size effects of nanomaterials, photothermal conversion has been introduced into multi-dimensional and cross-field application systems [22]. For example, in recent years, solar-driven interface evaporation with photothermal nanomaterials achieved seawater desalination and deep purification of industrial wastewater at high evaporation rates [23]. For power generation with green ways, photothermal conversion/photovoltaic hybrid concepts were also proposed and photothermal nanomaterials were loaded on the thermoelectric devices for obtaining electric power via photothermal-thermoelectric process [24]. Combining the heat management, photothermal nanomaterials were also employed in infrastructures protection (deicing, etc.), reducing energy consumption of indoor-temperature modulation, and energy supply at cold regions [25,26].

Nano-scale effects bring new functions or enhancements to photothermal materials and thus enlarge the applications in energy conversion. Hence, by structure modulations, photothermal nanomaterials are expected to break out the bottleneck of traditional photothermal performance. This paper reviews recent studies on photothermal nanomaterials, including their preparation, structure modulations, and typical applications, aiming to provide theoretical and technical reference for the industrialization of photothermal nanomaterials and the global energy transition.

2. Photothermal Nanomaterials: Preparation and Photothermal Conversion Tailoring

The methods for designed and controlled preparation of nanomaterials have been explored in the past decades, paving the way for applications with targeted functions [27,28,29,30,31]. Based on the controlled strategies for constructing nanocrystals, photothermal nanomaterials were prepared effectively [32,33,34], thus strongly supporting the applications in energy conversion. Owing to the photothermal conversion properties of nanomaterials depending on their sizes, morphologies, electronic structure, surface or interface, and so on, the preparation and modulation methods of photothermal nanomaterials are the keys for converting solar energy into heat. In this section, the reported strategies for tailoring photothermal nanomaterials were reviewed, including metals and alloys, semiconductors, and carbon.

2.1. Metallic Nanomaterials

Metals exhibit high photothermal conversion efficiency owing to their high density of free electrons and thus rapid conversion rate with high light absorption. When scaled down to the nanoscale, certain metals exhibit strong localized surface plasmon resonance (LSPR). This effect significantly enhances their light absorption and photothermal conversion efficiency, while also allowing spectral tunability. Specifically, Au, Ag, Pd, etc. have been widely prepared for photothermal conversion application [35]. Au nanocrystals were prepared for photothermal therapy or medical imaging in the past decades [36,37,38] and the strategies were designed and explored for accurately tailoring Au nanocrystals [39,40,41], including morphologies and sizes. For instance, the largest all-alkynyl-protected rod-shaped trimetallic alloy nanocluster, {Au₉Ag_126−xCu_x (PhC=C)₆₈(BF₄)}⁴⁺, was synthesized via a direct reduction precursors strategy (Figure 1A), using phenylacetylene (PhC≡CH) ligand with diverse metal-coordination as the protective agent [42]. The metallic cluster core features a concentric core–shell heterostructure arranged as Au₃Ag₃₄@Au₆Ag₆₄@(AgCu)₂₈. Time-dependent density functional theory (TD-DFT) calculations revealed the 0.29 eV of energy gap between HOMO and LUMO, indicating that the electronic structure of the cluster lies at the critical transition boundary between molecular and metallic states. Consequently, the Au₃Ag₃₄@Au₆Ag₆₄@(AgCu)₂₈ cluster demonstrated remarkable exceptional photothermal conversion performance and cycling stability [9]. AuNCs@TBN-Superphanes were prepared via encapsulating Au cluster with a TBN-Superphane supramolecular cage, exhibiting a broad absorption range from 500 to 2500 nm and remarkable photothermal conversion under 808 nm [13]. In addition, alloyed metal nanocrystals or heterostructures were synthesized to modulate the LSPR absorption. As recently reported by Zheng et al., AuAg alloyed nanosheets were achieved via a seed-mediated synthesis method with precise controlling of Ag/Au ratio and morphology preserving of Au nanosheets. By this approach, a continuous blue-shift in the in-plane dipole plasmon resonance was facilely modulated from 1000 nm (NIR-II) to 700 nm (NIR-I) because the incorporation of silver modulates the plasmonic response by altering the free electron density and the local dielectric environment, thereby enabling effective optical tuning (Figure 1B) [42]. Due to the similar lattices of metallic Au and Ag, their heterostructures were usually prepared by using seed-mediated process. Chen et al. employed Au nanorods (NRs) as cores and epitaxially grew silver shells on the surface by precisely controlling the addition of AgNO₃, resulting in Au@Ag core–shell nanostructures with tunable aspect ratios [43]. The Ag shell broadened the LSPR absorption and thus absorption of the Au@Ag NRs covered a wider visible light range. Based on the enhanced light absorption, the optimized Au@Ag NRs performed a high photothermal conversion with a steady-state temperature of 48.1 °C under one sun irradiation. Furthermore, metallic Pd was grown onto the surface of Au@Ag NRs to form Au@AgPd NRs, which performed an expanded plasmon resonance absorption and enhanced photothermal efficiency. The studies demonstrated that it is an effective way to obtain photothermal metal reagents by constructing metallic heterostructure or alloy with appropriate compositions and suitable structure (Figure 1C). Pd@Ag core–shell nanoplates perform tunable SPR over a wider spectral region relative to pure Ag nanoplate and outstanding photothermal properties, serving NIR-SER substances and photothermal reagents for cancer therapy [44]. AuAg alloy nanoplates prepared via galvanic replacement enhanced tunable NIR absorption from the synergetic LSPR from triangular shape, performing a high photothermal conversion of 78.5% [45]. NIR-resonance plasmonic absorption of PtSb alloy nanocrystals can be optimized for compositions due to the strong p-d orbital hybridization, thus demonstrating excellent properties of photothermal conversion and activities on reactive oxygen species production for photothermal-catalytic therapy [15].

Based on the preparation and tailoring, self-assembly or space-confined strategy of metal nanocrystal also enhanced the photothermal conversion with LSPR coupling [46,47,48]. The plasma band of Ag nanoparticles is narrow and limits their applications. Nevertheless, black Ag nanocrystals grown via space-confined seed process presents high plasma resonance coupling due to the assemblage with widely distributed interparticle distances, improving utilization of solar energy for photothermal steam generation [49].

2.2. Semiconductor Nanomaterials

Generally, semiconductor nanomaterials exhibit strong light absorption and photon trapping capabilities within specific spectral ranges due to their ability to form electron-hole pairs with energies exceeding the bandgap. As these pairs relax toward the bandgap edge, significant thermal effects can be generated within the system, making them important candidate materials in the field of photothermal conversion. The bandgap width of semiconductor nanomaterials directly affects the range of the material’s optical response. Reducing the bandgap width of semiconductor material broadens its spectral absorption range, thereby enhancing its photothermal conversion efficiency. Moreover, the bandgap and electronic structure of semiconductors can be tailored through various strategies, such as constructing heterointerfaces, introducing defects, doping with cations, and controlling crystal phases. These modifications improve their photothermal performance by strengthening the light absorptions with broadened range and accelerating the energy conversion processes. The reported semiconductor nanomaterials currently used for photothermal applications are primarily metal oxysulfides and titanium compounds, such as copper sulfide nanocrystals, titanium oxide nanoarrays, and so on. Specifically, copper sulfide nanocrystals possess both semiconductor and plasmonic-enhanced photothermal conversion regents, effectively combining the non-radiative carrier recombination of low-dimensional semiconductors with LSPR response characteristics [32,50]. The phase of copper sulfide nanocrystals synthesized via chemical methods can be precisely controlled by manipulating the reducing environment and the reaction temperature [51]. For instance, CuS prepared by using sulfur precursor (sulfur dissolved in oleyamine and n-octylamine) would transform into Cu₇S₄ by heating at 80–240 °C in oleylamine while Cu_1.81S could be obtained via a similar path by using tert-dodecyl mercaptan. The phase controlling and photothermal conversion investigation results showed that the unique S-S double bond and semimetallic band gap of CuS enable a strong LSPR effect resulting in much higher photothermal conversion efficiency (Figure 2). Hollow core copper sulfide (H-Cu₂S) nanostructures were prepared by the hydrothermal reaction of copper sulfide via a Kirkendall effect process [52]. The hollow core and hierarchical shell structures promote photon capture by prolonging light-matter interaction path through multiple internal scatterings, achieving an exceptional photothermal conversion efficiency of 86.8%. Cu₂S/CuO@Cu nanoarray grows with size control onto the copper substrate in situ by two-step impregnation method and performs high photothermal conversion rapidly [53]. The strategies for tailoring composition and morphologies pave the way for tailored prepared copper sulfide photothermal nanomaterials with high performance.

Cr-doped TiO₂ treated in ammonia atmosphere would result in defects with Cr-N co-doping so that the process could be used in modulated defect state [54]. The Cr-N dopants of CrN-TiO₂ can control the nitridation temperature, and the doped Cr and N form Cr-N structural units in TiO₂ lattice forming a delocalized intermediate band to significantly enhance photo-generated electron–phonon interactions and energy conversion. Nanolamellar λ-Ti₃O₅@SiO₂ could be tailored via suppressing grain growth by SiO₂ layer during hydrogen reduction at high temperature [55]. The confined grain growing preserves the high-specific-surface-area layered morphology of λ-Ti₃O₅@SiO₂ and then results in the narrower bandgap of λ-Ti₃O₅, achieving a maximum solar absorption rate of 92.2% and a photothermal conversion efficiency of 91.7%. Additionally, other nanostructures such as nanosheets and nanotubes with specific morphologies were controlled to increase specific surface area and light absorption efficiency. For instance, TiO₂ nanotubes were grown on the surface of a titanium substrate, followed by the fixation of rigid plasma-sputtered TiN particles using a polyvinylidene fluoride (PVDF) binder via electrospinning. The resulting devices based on the TiN and porous structure of Ti nanotubes exhibited high photothermal conversion and anti-icing at low temperature (−23 °C) and low solar irradiance (0.35 sun illumination) [56] (Figure 3A). A regional phase transition on the MoS₂ (1T@2H MoS₂) was prepared by heat treatment using MoS₂ powder with high concentration of S vacancies obtained via electrochemical ultrasonic method. In the 1T@2H MoS₂ heterostructure, metallic 1T phase coexists with the semiconducting 2H phase, which can be optimized by high-temperature annealing conditions and affects the localized surface plasmon resonance (LSPR) of the metallic 1T phase and the non-radiative electron-hole recombination dynamics of the semiconducting 2H phase [57]. Therefore, the synergy of region phase connection results in high-efficiency absorption across the full spectrum and then demonstrates a photothermal conversion efficiency exceeding 90%.

Quantum dots with modification including elemental doping and surface modulation would also perform high photothermal conversion with enhanced light absorption capacity. A confined tin-based perovskite CsSnI₃ was encapsulated by SiO₂ shell so that the quantum dots exhibit high photothermal conversion efficiency (>99.4%) in near-infrared region and stably work in high temperature, high humidity and intense light (Figure 3B) [58].

2.3. Carbon-Based Photothermal Nanomaterials

Carbon materials possess a distinctive electronic structure with conjugated π-electron systems through sp² hybridized carbon lattices, exhibiting ultra-broad spectral absorption capabilities across the ultraviolet to microwave region. Upon absorbing light energy, the electrons in π-electron systems of carbon materials convert photon into heat rapidly through internal electronic transitions and molecular thermal vibrations. Furthermore, carbon materials possess key properties such as low density, high light absorption and high thermal conductivity, making them ideal for use in fields including solar water evaporation technology, photothermal therapy, environmental remediation and optoelectronic sensors. On the other hand, although carbon materials have some advantages for photothermal conversion and application, the controllable constructions of photothermal structures still face great challenges, such as the difficulty in precise tailoring of π-electron system during the preparation process. Moreover, aiming at the applied details, the heat management is also an important issue, which depends on the designed carbon structure. Therefore, the strategy for constructing carbon structures with high photothermal conversion properties has been expected in the past years [59,60].

In the past years, carbon materials with diverse shapes and structures have been designed and fabricated. Their properties are tuned through strategies such as surface modification via etching or by forming composites with other materials, which effectively regulate their physical microstructure. These microstructures generally would enhance light absorption and thus promote photothermal conversion. For example, a novel black carbon nanotube (CNT)-based acrylamide-acrylic acid copolymer gel (GEL-CNT-ILS) was synthesized via radiation-induced polymerization [61]. Based on the unique crosslinked polymer network structure, the porous photothermal carbon nanomaterial accelerates the water transport and promote interfacial water evaporation (4.18 kg·m⁻²·h⁻¹) with high light absorption and photothermal conversion capability. The synergistic interaction of carbon nanotubes and ionic liquids also significantly brings outstanding electrical conductivity (3.67 mS·cm⁻¹) and thermoelectric conversion capabilities (5.62 mV/K). A zero-dimensional functional nanocarbon (N-CQD) was prepared via a hydrothermal process for constructing composite materials with two-dimensional nanostructures (rGO), and the photothermal conversion capability was found to depend on the ratio of N-CQD and rGO [62]. The optimized samples achieve a photothermal conversion efficiency (η) of 82.64%, and a 49.82% increase in thermal conductivity (0.88 W·m⁻¹·K⁻¹), demonstrating potential practical applications.

Constructing carbon nanomaterials with special structure was considered an effective strategy for tailoring their photothermal conversion. Hollow carbon spheres (HCSs) were prepared by employing a hard template to encapsulate polyethyleneimine (PEI) via wet impregnation, creating PEI@HCS precursors [63]. Then, a series of carbon structures were obtained by annealing PEI@HCS at different temperatures, and the resulting carbon materials were employed as CO₂ adsorbents which released the gas by thermal input generated from photothermal conversion (Figure 4A).

In addition, introducing functional groups or chemically modifying carbon materials are general ways to alter their electronic structure for expanding absorption range of specific wavelengths and enhancing photothermal conversion efficiency. For instance, mesoporous carbon nanomaterial (MCN) surface was modified by coating up-converting Y₂O₂S:Yb³⁺/Er³⁺ so that the resulting composites would convert 980 nm laser light into visible light, boosting photothermal conversion efficiency from 59.48% to 82.86% [64]. After combining with light converting materials, the MCN/Y₂O₂S:Yb³⁺/Er³⁺ performs rapid photothermal response within 150 s under 280 mW cm⁻² 980 nm laser irradiation, demonstrating potential for cancer cell ablation (Figure 4B).

In short, methods of preparing nanomaterials for photothermal conversion were explored for constructing special units to enhance light absorption, accelerate the energy conversion, and improve thermal management or employment. These achievements provide necessary foundation for the photothermal conversion application and the employment of solar energy.

3. Applications of Photothermal Nanomaterials

Generations and utilizations of green energy via photothermal conversion perform some supplements on clean energy consumptions. So far, there have been lots of photothermal nanomaterials that have been explored for wide applications, such as steam generation/interface evaporation, photothermal-thermoelectric power generation, photothermal catalysis, anti-icing or deicing [65,66,67,68,69,70], providing new paths for solving the problems that traditional solutions are difficult to deal with. In this section, the recent reports on the applications of photothermal nanomaterials were reviewed to reveal the new chances of materials exploration and energy conversion engineering.

3.1. Photothermal for Green Energy Generation

Owing to their unique surface plasmon resonance effects, tunable optical properties, and exceptional thermal/electrical conductivity, metal nanomaterials simultaneously trap light and rapidly transfer the energy into heat, and thus perform high photothermal conversion effectively which provides heat management for applications, such as cutting-edge solar cell technologies and so on [71,72,73]. Generally, metal clusters or nanocrystals were considered useful compositions for enhanced photothermal conversion. For example, a system of Au₃Ag₃₄@Au₆Ag₆₄@(AgCu)₂₈ cluster in DMF is irradiated under a 660 nm laser with 0.5 W cm⁻² of power density and the temperature rise to 51.5 °C in 5 min with photothermal conversion efficiency (PCE) of 84.7%, demonstrating remarkable exceptional photothermal conversion performance and cycling stability [9]. Using nanoimprint polymer masks and photo-induced electroplating, high-aspect-ratio Ag nanowire (Ag NWs) grids were successfully fabricated on ITO-free tunnel oxide passivated contact (TOPCon) silicon cells with 6% of surface coverage and the visible light transmittance was over 95% (Figure 5) [74]. By spatially resolved light-induced current (SR-LBIC) and optical simulations, the nanowire grid was revealed to exhibit lower shading effects in the visible spectrum than geometrically predicted with enhanced NIR light capture. Therefore, such silver nanowire meshes enable boosting the external quantum efficiency (EQE) of cells and effectively enhance the performance of underlying silicon cells by extending the optical path length, demonstrating significant value in silicon/perovskite tandem cell applications. Ag NWs/polyethylene glycol (PNF) composite films were found the temperature rapid increases to 72.8 °C under 120 mW cm⁻² of illumination (Figure 6), demonstrating high photothermal conversion efficiency. Furthermore, the composite shows exceptional stability and flexibility, including resistance to strong acids and alkalis, inherent flame retardancy, and the ability to withstand tens of thousands of folding cycles with good recovery, which presents their capacities for portable photothermal energy applications in extreme environments, such as wilderness and aerospace applications [75].

Semiconductors with tunable bandgap provide a path to modulate their light absorptions and the transferring of excited electrons and thus enhance their photothermal conversion [76,77]. Moreover, after modulation via band engineering techniques such as element doping and defect introduction, light absorption range of semiconductors could be adjusted to enhance broad spectral response. Meanwhile, efficient synergistic processes like photothermal effects and thermoelectric conversion would also be utilized due to photo-generated carriers, facilitating multifunctional conversion of light energy. The excellent photothermal conversion of multi-scale pyramidal composited by CuS nanowires and rGO provides a possibility for generating power by combining with thermoelectric generators (TEGs) with maximum power generation of 5.55 W m⁻² under sunny conditions [78]. With interface evaporation, the water-electricity cogeneration prototype of CuS-rGO composite offers a viable solution for the simultaneous supply of freshwater and electricity in remote areas. The CuS nanowires construct a porous structure, promoting the multiple refraction for enhanced light absorption and providing path for accelerating water transportation. Furthermore, the narrow bandgap of CuS also enhances the non-radiative relaxation of electron-holes pair while the π electrons in rGO facilitate the thermal vibration in the lattice, resulting in photothermal conversion with high efficiency. Although the photothermal surface and the water transport channel are key for the integration of electricity generation and water evaporation, photothermal conversion capacity is the core of the composites.

In photovoltaics, semiconductors convert absorbed light into charge carriers. However, a substantial portion of this energy is inevitably lost as heat through non-radiative recombination, resulting in energy loss and reduced conversion efficiency. Hence, collecting or utilizing the side energy is significant and photovoltaic-solar thermal hybrid system is a possible way to promote solar utilizing. Based on semi-permeable perovskite photovoltaic (SPV) modules, a hybrid system was constructed by converting the high energy photon into current and low energy photon into heat [79]. The hybrid system generates high-temperature thermal energy up to 900 °C in a modeled large-scale high-concentration ratio with 12.3% of SPV electrical power and 27.9% of solar efficiency, while the SPV PEC increases to 14.3% with a heat temperature of 450 °C with a lower energy efficiency (24%) (Figure 7). Considering the thermal energy for driving steam turbines to generate electricity or be stored directly as a high-temperature heat source, the system provides a high-efficiency solution for harvesting the full solar spectrum. By utilizing low-grade thermal energy from photothermal conversion, the technologies combined with thermoelectricity attracted more and more attention [80,81]. Biomimetic photothermal materials were constructed on TiN/TiO₂ nanotube arrays to accelerate light absorption and energy conversion into thermal at the local interface, creating a temperature difference to the surrounding environment, which achieves a power output of up to 2.74 W m⁻² [82].

Nanomaterials with photothermal conversion properties play a significant role in energy applications, and numerous new materials with excellent performance have been explored in recent years (

Table 1. List of photothermal materials with both photothermal-thermoelectric (PT-TE) conversion capabilities.

Materials	Electrical Conductivity (S∙m−1)	PT Conversion Efficiency (%)	Thermal Conductivity (W m−1 K−1)	PT-TE Properties	Ref.
NdFeB@Ag	104	-	0.76	Conversion efficiency: 78.45%. Voltage: 3V.	[83]
Ag-MWCNTs/PW@CNS	1.39	73.9	0.433	TE efficiency: 81.6%	[84]
LA@MOF-C/GO	-	90	1.36	TE efficiency: 90% Voltage: 2.2V	[85]
TiN/TiO2@carbon cloth	-	92.18	-	Power outputmax: 2.74 W/m2	[82]
CuS-rGO	-	97.6	-	Power outputmax: 1.32 W/m2	[78]
Ag@Cu2S	~3 × 104	-	0.85	S(µV K−1): ~220	[86]
Cu2S/CuO	~2.3 × 104	-	-	Seebeck coefficient: ~220µV K−1	[87]
GEL-CNT-ILS *	0.367	-	-	Seebeck coefficient: ~5620µV K−1	[61]
GCPC	1.68 × 103	-	-	TE efficiency: 90%; Voltage: 2V	[88]
PGC-CF **	∼26	85.7	0.3216	Thermoelectric conversion efficiency(%): 81.5% (3V)	[89]
2LrGO@LIG/MA	307.9	94.10	0.831	Thermoelectric conversion efficiency(%): 99.1% (3V)	[90]

* GEL-CNT-ILS: carbon nanotube (CNT)-based acrylamide-acrylic acid copolymer gel; ** PGC-CF: partially graphitized porous carbon framework.

). When integrated into structures like transparent electrodes for solar cells, metal nanocrystals enhance light capture and thus improve device performance. Semiconductors with bandgap engineering enable broad spectral absorption and facilitate multifunctional systems. By combining photothermal conversion with thermoelectric generators or photovoltaic modules, green power can be generated in a cost-effective manner.

3.2. Photothermal Materials for Interface Evaporation

Interface evaporation is one of hot applications of photothermal materials due to its ability to produce fresh water at low cost and green energy consumption, which is also further explored to apply in environmental fields such as waste water treatment. Nanomaterials also have been investigated for applications on the interface evaporation with enhanced performance from size effects.

Metallic nanomaterials have been employed in photothermal interfacial evaporation or solution heating in recent years because of the high light absorption and rapid energy conversion [91,92]. For example, 3D aluminum particle plasmonic blackbody materials with porous structures exhibit an average absorption efficiency exceeding 96% across the broad solar spectrum ranging from 400 to 2500 nm, and hence, the corresponding energy transfer efficiency reaches as high as 90%, enabling practical solar-powered seawater desalination with a four-fold reduction in salinity, offering a viable solution for portable solar desalination systems [93]. Concurrently, the porous alumina layer encapsulating the nanoparticles effectively confines generated heat to the evaporation interface, significantly reducing thermal losses to the bulk water, and the hydrophilic insulating structure promotes mass transport of water. To address the issue of self-floating photothermal materials and efficient thermal localization for practical applications, a fully dielectric-insulated 3D plasmonic metal nanocrystals (i-Au/NPT) was successfully constructed by self-assembling gold nanoparticles within a nanoporous alumina template and removing the continuous gold film from the surface using ion beam stripping technology. This structure leverages the plasmonic effects of Au nanoparticles for broadband light absorption and photothermal conversion. Based on this synergistic light-heat-mass management, the self-floating evaporator achieves an evaporation efficiency of up to 80.5% under one sun illumination—approximately 20% higher than conventional plasmonic evaporators without requiring any external insulating floats, it offers new insights for developing high-performance, compact solar photothermal conversion devices [94]. By using metal nanocrystals, high performance of interface evaporation would be obtained, but the cost of plasmonic metals limited their wide applications.

Copper or cuprous sulfide nanocrystals are excellent photothermal conversion materials with strong NIR light absorption and are viewed as a kind of potential candidate for interface evaporation [95,96]. Aiming to enhance both solar thermal conversion efficiency and water transport, a multi-scale pyramidal CuS-rGO composite solar thermal structure was fabricated, demonstrating an impressive solar thermal conversion efficiency of up to 97.6% (Figure 8) and generated steam in Shenzhen Bay with an average daily water production of 12.1 kg m⁻² [78]. By constructing a thermal cycle system without external energy input, a TiN/TiO₂ nanotubes array achieves a high seawater desalination capacity of 2.02 kg m⁻² h⁻¹ under 1 Sun (Figure 9).

Carbon materials process the unique sp²-sp³ hybridized electronic structures which are considered key electronic characteristics for light absorption and energy conversion. In addition, carbon nanomaterials also possess some advantages for application on a large scale, such as low density, high optical absorption coefficient, and high thermal conductivity, demonstrating significant potential for low-density and high-efficiency energy applications [97,98]. For example, graphene oxide with modifications can be used as the raw material for the photothermal conversion layer for solar evaporation applications. During photothermal conversion, the surface temperature of carbon materials exposed to light would rise rapidly and thus the localized temperature enhances heating water molecules, providing the thermal energy required for evaporation and thereby achieving high evaporation [97,98]. As reported, a Janus-type layered porous carbon-graphene aerogel (CrGOA) enables the rapid photothermal response under low-light conditions and thus exhibits an ultra-high water evaporation rate of 3.66 kg·m⁻²·h⁻¹ with a steam generation efficiency of 96.9% under one-sun illumination [99]. The CrGOA also possesses high salt tolerance and the evaporation rate of CrGOA reached 2.95 kg·m⁻²·h⁻¹ in 16% saline water without salt accumulation, indicating its suitability as a solar-driven interfacial evaporator for practical applications. A solar osmotic vaporization desalination system of carbon nanotubes (CNTs)/graphene nanosheets (GNSs)-PVA films reaches 3.02 kg m⁻² h⁻¹ under 1 sun irradiation with an excellent stability in 10-day testing [100]. Leveraging the high-efficiency photothermal conversion and high thermal conductivity, a MOF-derived nanoporous carbon demonstrates rapid desorption kinetics. This enables a rapid-cycle water harvesting (0.18 L kg⁻¹ h⁻¹) under 30% relative humidity per solar cycle, facilitating the development of high-yield, solar-driven atmospheric water harvesting (AWH) systems for advanced freshwater generation [94].

By enhancing photothermal conversion and vapor generation rates by optimizing light absorption, interfacial evaporation has made significant strides for obtaining fresh water (

Table 2. List of photothermal materials for solar interface evaporator.

Material	Light Intensity	Evaporation Rate (kg·m−2·h−1)	Efficiency (%)	Stability	Refer.
Au@Ag-Pd/PS Janus nano-micro structures	1 sun	3.04	99.1	-	[43]
3D multiscale LM/PAN evaporator *	1 sun	2.66	96.5	-	[101]
CrN-TiO2 (2D evaporator)	1 sun	2.07	94	-	[54]
CrN-TiO2 (3D evaporator)	1 sun	4.59	94.7	4.50 kg m−2 h−1 over 6 cycles	[54]
1 T@2H MoS2	1 sun	1.27	90.52	10 cycles in 10 h	[57]
λ-Ti3O5 2D evaporation	1 sun	1.79	76.8	-	[55]
λ-Ti3O5 3D evaporation	1 sun	6.41	91.7	10 cycles in 10 h	[55]
Bi2Se3/Cu2-xS	1 sun	3.67	95.2	maintaining at 98.86% after 8 h test	[102]
CrGOA **	1 sun	3.66	96.90	-	[98]
3S-HCNs ***	1 sun	2.4	86	10 cycles in 10 h	[103]
SMoS2-porous hydrogel	1 sun	3.297	93.4	15 days	[104]
MnO/C nanoparticles	1 sun	2.38	98.4	-	[105]
Nanoporous black Au film	1 sun	1.51	94.5	1.50 kg m−2 h−1 over 18 cycles	[106]
CMP2-HDA-A	1 sun	1.59	95.3	1.54 kg m−2 h−1 across 10 cycles in 6 h	[107]
Biomimetic PDMX/HPP aerogel	1 sun	2.62	93.6	2.4–2.7 kg m−2 h−1 in 10 cycles in 10 h	[108]
Lignin-based carbon/melamine foam	1 sun	1.54	95.88	1.54 kg m−2 h−1 for 12 cycles	[109]
SA/MWCNTs@PPy/MWCNTs-NH2@PU	1 sun	2.40	92.76	2.08 kg m−2 h−1 after 18 days	[110]
PAN@CuS	1 sun	2.27	83.9	-	[111]
SM-Ti3C2Tx/PVA Aerogels	0.5 sun	0.92	88.52	1.8–1.9 kgm−2 h−1 in 30 h	[112]
Wood/ZIF-8@PDA	1 sun	2.7	86	10 cycles in 10 h	[113]
PVA/GO hydrogel (3D evaporator)	1 sun	3.71	~90	3.70 kg m−2 h−1 after 10 days	[114]
Anti-fouling CuZnSnSe nanocarambolas	1 sun	1.528	86.4	30 cycles over 30 days without decay	[115]
MoN1.2-rGO-HSs	1 sun	2.6	-	2.4 kg m−2 h−1 after 8 h	[116]
Bifunctional photothermal membrane (Co-N-C/CF)	1 sun	1.88	87	5 cycles in 5 h	[117]
Cu@C–N	1 sun	1.94	89.4	-	[118]

* LM/PAN: liquid metal/polyacrylonitrile; ** CrGOA: carbon-graphene composite aerogel; *** 3S-HCNs: triple-shell hollow carbon nanospheres.

). Besides noble metal nanocrystals, carbon-based and semiconductor nanomaterials were expected for wide application on a large scale with a balance of performance, scalability, and cost. However, challenges remain in long-term stability under real conditions, salt rejection durability, and large-scale manufacturing. A lot of future work should focus on integrated system design, ecological impact assessment, and improving the energy efficiency of complete water-production cycles to transition from laboratory prototypes to deployable sustainable water solutions.

3.3. Other Photothermal Applications of Nanomaterials

Coupling functional nanomaterials directly with thermoelectric modules provides a significant reference for the synergistic utilization of multiple low-grade heat sources, such as solar energy and industrial waste heat, to achieve distributed energy–water co-generation. Integrating photothermal nanomaterials into traditional energy systems are effective strategies to convert or utilize solar thermal energy directly and overcome the defects of solar energy applications.

For instance, photothermal conversion paved the path for utilizing solar thermal for cold regions, reducing energy costs and carbon emissions associated with winter heating. Large-scale, finely ordered nanowire arrays were fabricated using an anodic aluminum oxide (AAO) template on surfaces of thermoplastic metallic glass and performed exceptional broadband light absorption with low reflectance (~0.6%) in the visible and near-infrared spectrum (Figure 10). Such nanowire arrays can reach 160 °C at a rate of 28.75 °C/s with 56% of photothermal conversion efficiency [119], paving the ways for applications in photothermal electric power generation. The carbon nanowalls were composites with silicon carbide nanowire (SiCNWs) aerogel to obtain excellent solar energy absorption and energy conversion efficiency, and then injecting polyethylene glycol into the SiCNWs layer, the device (CNW&ND@S-A/PEG) with the dual-functional was prepared. [120]. The reported device exhibits an absorption rate of 92% in the visible light spectrum while the bottom composite phase-change material has a thermal conductivity of 1.13 W m⁻¹·K⁻¹ and an enthalpy of storage of 157.0 J g⁻¹, which is expected to be used in building insulation for heating in cold regions. Meanwhile, carbon nanomaterials with excellent photothermal capacities are suitable for anti-icing and bring large opportunities for infrastructure. Huang et al. [88] fabricated a sandwich-structured anti-icing/deicing coating by sequentially assembling a conductive sheet of densely mixed CNTs/graphene, a flexible PDMS encapsulation layer, and a top composite layer of polymer filled with TiN/SiO₂ nanoparticles. What is more, the synergistic interaction between TiN nanoparticles with strong light absorption and carbon nanomaterials (graphene and CNTs) achieved nearly 100% light absorption efficiency, enhancing photothermal conversion efficiency and further reducing power consumption (Figure 11). Under 0.8 Sun light illumination, the coating only requires a low voltage of 1.8 V to significantly increase the surface temperature, preventing ice formation and efficiently removing ice/frost.

4. Summary and Outlook

Nanomaterials engineered for efficient photothermal conversion have emerged as a pivotal technology in the energy sector. Their core function lies in transforming sunlight into usable heat, which is leveraged across diverse applications such as solar-driven water desalination, low-grade thermal energy harvesting, and building-integrated heating in cold regions. These materials also contribute substantially to sustainability by reducing net energy consumption and enhancing the efficiency of existing energy infrastructures. By providing decentralized thermal energy solutions, they mitigate reliance on fossil fuels, alleviate pressure on global energy supply chains, and facilitate the adoption of cleaner living practices, thereby acting as a critical indirect enabler in the transition towards a low-carbon future.

To realize this potential and address persistent efficiency challenges in practical applications, a key strategy lies in engineering heterostructures or composites from conventional nanomaterials. These advanced architectures enhance photothermal performance through synergistic coupling effects between components. Moving forward, elucidating the intricate energy conversion mechanisms at the interfaces within these composites will be essential to guide the rational design and development of the next generation of photothermal nanomaterials.