Photothermal Film Research Articles

Wearable Photo-Thermo-Electrochemical Cells (PTECs) Harvesting Solar Energy.

Solar induced thermal energy is a vital heat source supplementing body heat to realize thermo-to-electric energy supply for wearable electronics. Thermo-electrochemical cells, compared to the widely investigated thermoelectric generators, show greater potential in wearable applications due to the higher voltage output from low-grade heat and the increased option range of cheap and flexible electrode/electrolyte materials. A wearable photo-thermo-electrochemical cell (PTEC) is first fabricated here through the introduction of a polymer-based flexible photothermal film as a solar-absorber and hot electrode, followed by a systematic investigation of wearable device design. The as-prepared PTEC single device shows outstanding output voltage and current density of 15.0mV and 10.8Am-2 and 7.1mV and 8.57Am-2 , for the device employing p-type and n-type gel electrolytes, respectively. Benefiting from the equivalent performance in current density, a series connection containing 18pairs of p-n PTEC devices is effectively made, which can harvest solar energy and charge supercapacitors to above 250mV (1sun solar illumination). Meanwhile, a watch-strap shaped flexible PTEC (eight p-n pairs) that can be worn on a wrist is fabricated and the realized voltage above 150mV under light shows the potential for use in wearable applications.

Optical fibre taper-enabled waveguide photoactuators

Photoactuators have attracted significant interest for soft robot and gripper applications, yet most of them rely on free-space illumination, which requires a line-of-site low-loss optical path. While waveguide photoactuators can overcome this limitation, their actuating performances are fundamentally restricted by the nature of standard optical fibres. Herein, we demonstrated miniature photoactuators by embedding optical fibre taper in a polydimethylsiloxane/Au nanorod-graphene oxide photothermal film. The special geometric features of the taper endow the designed photoactuator with microscale active layer thickness, high energy density and optical coupling efficiency. Hence, our photoactuator show large bending angles (>270°), fast response (1.8 s for 180° bending), and low energy consumption (<0.55 mW/°), significantly exceeding the performance of state-of-the-art waveguide photoactuators. As a proof-of-concept study, one-arm and two-arm photoactuator-based soft grippers are demonstrated for capturing/moving small objects, which is challenging for free-space light-driven photoactuators.

Interfacial Solar Evaporation Toward Efficient Urine Purification and Resource Recovery with Polypyrrole-Based Photothermal Conversion Films

Interfacial Solar Evaporation Toward Efficient Urine Purification and Resource Recovery with Polypyrrole-Based Photothermal Conversion Films

Ternary Alloy Incorporated Janus Hydrogel Photothermal Film for Solar-Driven Water Evaporation, Desalination, and Wastewater Purification

Ternary Alloy Incorporated Janus Hydrogel Photothermal Film for Solar-Driven Water Evaporation, Desalination, and Wastewater Purification

Ternary Alloy Incorporated Janus Hydrogel Photothermal Film for Solar-Driven Water Evaporation, Desalination, and Wastewater Purification

Ternary Alloy Incorporated Janus Hydrogel Photothermal Film for Solar-Driven Water Evaporation, Desalination, and Wastewater Purification

Solar Desalination via Multilayers of Transparent Photothermal Fe3O4@Cu2–xS Thin Films

In contrast to a conventional single layer solar still, herein a multilayer system for solar light harvesting and water evaporation via transparent photothermal thin films is developed. Monodispersed Fe3O4@Cu2–xS nanoparticles are dispersed in a polymer solution and deposited on glass and quartz substrates as transparent thin films. These thin films are optically characterized with strong absorptions in the UV and NIR regions, and high average visible transmittance. The photon energies in the UV/NIR regions are converted to heat increasing the water evaporation rate, while the film high transparency makes it possible for light to pass through many layers for an enlarged solar harvesting surface area. A multilayer photothermal evaporation system (MPTES) is developed for water evaporation experiments using various photothermal films. Water evaporation rates are compared between Fe3O4@Cu2–xS and Fe3O4 against different numbers of layers in the MPTES. The photothermal and optical properties of Fe3O4@Cu2–xS thin films are correlated to the heating behaviors and saltwater evaporation rate. Due to high transparency and strong photothermal effects of the films, water evaporation is significantly enhanced via multilayers. The concept of the multilayer solar still will show promise in water desalination.

Yolk-like non-stoichiometric nickel sulfide-based Janus hydrogel photothermal film for enhanced solar-driven water evaporation and multi-media purification

Solar-driven interface water evaporation is a promising strategy for desalination and wastewater treatment. However, it remains a huge challenge to simultaneously achieve a high light-to-heat conversion efficiency (η) and multi-media evaporation applications. In this study, a highly efficient Janus hydrogel photothermal film was developed using yolk-like non-stoichiometric nickel sulfide (NiS2-x) microspheres and agar hydrogel. The NiS2-x immobilized in the agar hydrogel has full-spectrum absorption characteristics at 200–2500 nm, which can perform efficient light-to-heat conversion and regulate water transport channels. Additionally, the pure agar in the bottom can transport water effectively and avoid heat loss. By the pouring method, the Janus hydrogel film can be easily prepared into various shapes; hence, it can be adjusted depending on the environment in which it is used. The optimized Janus hydrogel film (Janus hydrogel-1) possessed good hydrophilicity and showed an excellent solar evaporation rate of 1.45 kg m-2h−1, and a high η of 97% under one-sun irradiation. Theoretical simulation results showed that the outstanding water evaporation performance of Janus hydrogel-1 was mainly due to its relatively free water transport channels. Janus hydrogel-1 can be used for water evaporation applications in various media, including seawater, heavy metal ion/organic wastewater, and domestic sewage. Our work highlights the great potential of Janus hydrogel-1 for realizing a highly effective solar energy-driven interface water evaporation and multi-media purification.

Fabrication of Photothermal Film for Deicing Process Based on Gold Nano-Aggregate Encapsulated Yolk-Shell Structure

Ice accumulation on vessels, airplanes, or on the off-shore plants surfaces causes major accidents and difficulties in operation. Therefore, highly efficient and environmentally friendly materials for use as deicing surfaces are in continuous demand. Accordingly, photothermal materials have gained enormous attention owing to their outstanding performance in the removal of ice. Herein, a gold nano-aggregate yolk-shell structure (GNA-YS) was introduced as a deicing material. GNA-YS was homogeneously dispersed in photocurable polyurethane acrylate and used to fabricate a highly efficient and sunlight-responsive GNA-YS film. Upon irradiation with an 810 nm light-emitting diode (LED) (135 mW cm−2), the temperature of the GNA-YS film increased by approximately 70 °C within 2 min, which rapidly (within 6 min) melted the accumulated ice. Moreover, the GNA-YS film exhibited a temperature increase of 15 °C in a refrigerator under LED illumination for 1 min. Additionally excellent stability was confirmed through repeated experiments. The newly developed deicing GNA-YS film has prospective applicability in vessels or airplanes.

Magnetic responsive and flexible composite superhydrophobic photothermal film for passive anti-icing/active deicing

The formation and accumulation of ice can lead to severe economic loss or even loss of life. To prevent these losses, there has been rapid development in icephobic materials in recent years. To deal with various icing conditions and different types of components, it is highly desirable to develop a surface that possesses both passive anti-icing and active deicing functions. However, so far there is no suitable solution for anti-icing material with adequate durability, low cost, and simplicity in fabrication that can also be turned into active deicing when necessary. Here, we demonstrate a magnetically responsive and flexible superhydrophobic photothermal film (PFe-PCS) consisting of polydimethylsiloxane (PDMS), iron powder (Fe), and candle soot (CS). The film displayed both passive anti-icing and active deicing performances by a simple approach without using fluorine-containing chemicals. The film consists of two superhydrophobic layers that can trap two air layers in the micro/nano structures. As a result, the freezing time is 4.7 times of that of bare substrate. Such designed surface structure not only displays a significant delay in water freezing time, but also minimizes heat loss by keeping the heat within the top surface during photothermal heating. The accumulated ice can be immediately melted in 237 s and the melted water can be rapidly rolled off. The PFe-PCS film can be applied to substrates with complex shapes including curved substrates via magnetic force. More importantly, the PFe-PCS film displayed excellent stability when exposed to strong acid, base, salt solutions and liquid nitrogen. The superhydrophobicity and anti-icing performances were well retained after 320 cycles sandpaper abrasion, 3 h water flow impact, and cyclic icing/deicing. This work originated the concept of double layered surface structure for both passive anti-icing and active de-icing. The film is robust and self-healing, making it potentially applicable for ice prevention and removal for various shaped components.

Synthesis of a Co-Sn Alloy-Deposited PTFE Film for Enhanced Solar-Driven Water Evaporation via a Super-Absorbent Polymer-Based "Water Pump" Design.

Solar-driven water evaporation is a promising solution to water pollution, the energy crisis, and water shortages. However, the approach in which the photothermal film is in direct contact with bulk water for water evaporation may lead to a large amount of heat loss, thereby reducing the light-to-heat conversion efficiency (η) of the photothermal film. Here, a highly efficient solar-driven water evaporation system was developed using a Co-Sn alloy-deposited Teflon (PTFE) film (Co-Sn alloy@PTFE) and super-absorbent polymers (SAPs) supported with a floating foam substrate. The Co-Sn alloy with full-spectrum (200-2500 nm) absorption characteristics is devoted to high light-to-heat conversion, while the porous PTFE with high mechanical performance can support the Co-Sn alloy. We used density functional theory to prove that the Co-Sn alloy had a strong adhesive force with PTFE without surfactants due to the high adsorption energy between the (101) crystal plane of the Co-Sn alloy and the hydroxyl group on the PTFE film. Importantly, via the SAP-based "water pump" design, we improved the η of the Co-Sn alloy@PTFE film to 89%, mainly because the SAP not only effectively performed water transportation but also markedly reduced the heat loss of the Co-Sn alloy@PTFE film. Our work highlights the strong potential of Co-Sn alloy@PTFE-based light-to-heat conversion systems for realizing highly effective solar energy-driven water evaporation.

Conjugated Organic Photothermal Films for Spatiotemporal Thermal Engineering.

With the growth of photoenergy harvesting and thermal engineering, photothermal materials (PTMs) have attracted substantial interest due to their unique functions such as localized heat generation, spatiotemporal thermal controllability, invisibility, and light harvesting capabilities. In particular, π-conjugated organic PTMs show advantages over inorganic or metallic PTMs in thin film applications due to their large light absorptivity, ease of synthesis and tunability of molecular structures for realizing high NIR absorption, flexibility, and solution processability. This review is intended to provide an overview of organic PTMs, including both molecular and polymeric PTMs. A description of the photothermal (PT) effect and conversion efficiency (ηPT ) for organic films is provided. After that, the chemical structure and optical properties of organic PTMs are discussed. Finally, emerging applications of organic PT films from the perspective of spatiotemporal thermal engineering principles are illustrated.

Broadband reflection cholesteric liquid crystal film fabricated by near-infrared photothermal response technology

ABSTRACT A polyvinyl alcohol–modified copper sulphide (PVA-CuS) nanoparticlephotothermal film with modulated photothermal response properties is fabricated. Aliquid crystal cell with modulated photothermal conversion function is fabricated by using the glass substrate with this photothermal film as one side of this liquid crystal cell. In the formula of cholesteric liquid crystal (CLC), there is achiral compound with temperature-dependent helical twisting power and aultraviolet (UV)-polymerisable monomer. Therefore, under the irradiation of the near-infrared laser, the temperature of the sample gradually increases due to the photothermal conversion function of the liquid crystal cell, which causes the pitch of the CLC to gradually increase. Meanwhile, the liquid crystallinity polymerisable monomer polymerises under the irradiation of UV light to form apolymer network, and fix the pitch at each temperature section, then form anon-uniform distribution of the pitch to achieve broadband reflection. The influence of heating speed and UV light intensity is investigated under different modulation modes of near-infrared laser.

Plasmonic photothermal film for defogging and anti-icing/deicing on PTFE

The heat generated by surface plasmon in metal nanoparticles is an interesting topic in recent research on nano-optics. In this work, Au/TiO2 plasmonic photothermal films are obtained on the Polytetrafluoroethylene (PTFE) surface by a simple fabrication method for defogging and anti-icing/deicing. The experimental data demonstrates that the temperature of these plasmonic photothermal films increased by more than 25°C under light illuminating at 3 suns (300mW/cm2). The evaporation and deicing process of a water droplet on these plasmonic photothermal films are observed and analyzed, which exhibits their excellent ability of defogging and anti-icing/deicing. Furthermore, the influence of the Au/TiO2 concentration on plasmonic photothermal effect is also studied. The results point out that the film with a much small amount of Au nanoparticles (~3%) has the best performance, which is discussed based on the surface plasmon resonance. The novel plasmonic photothermal films with a simple fabrication method, low production cost, good photothermal effect, less heavy metal ions pollution, and insulation property, have broad potential applications in defogging, anti-icing, and deicing, etc.

Broadband photodetectors based on enhanced photothermal effect of polymer encapsulated graphene film

In this work, a broadband photodetector is constructed by coating a photothermal layer on a thermistor. The photothermal effect effectively convert light energy into heat, which is then reflected by voltage variation of the thermistor. A highly efficient photothermal film is synthesized by encapsulating a reduced graphene oxide (rGO) film into a polydimethylsiloxane (PDMS) layer. Benefiting from the excellent photothermal effect, the photothermal detector based on the rGO/PDMS displays a good linear response, a high responsivity, a quick response, and a broad range of spectral response (400–1300 nm). The responsivity for simulated sunlight at 0.8 kW/m2 reaches a high value up to 176.9 V/W with response-recovery time of ~80 s. And laser emissions of 6.37 kW/m2 at 473, 532, 808 nm also can be detected at high responsivities of 15.6, 14.6 and 19.4 V/W respectively. This work not only provides a high performance photothermal film, but also develops a feasible strategy for broadband photodetectors.

Concentrated Acid‐Induced Dehydration of Fallen Leaves for Efficient, Sustainable, and Self‐Cleaning Solar Steam Generation

To develop solar steam generation (SGG) for water treatment, light absorbers have been widely synthesized by carbonization of plants at high temperature. However, the thermal treatment has high energy‐consumption and is environmentally unfriendly. Thus, it is urgent to explore new green approaches to convert biomasses to carbon materials for SSG. Herein, a concentrated acid‐induced dehydration method is developed to convert fallen leaves to carbon powders, fallen‐leaf photothermal film prepared by dehydration (FLPD). The resultant FLPD exhibits a strong absorption covering a wide spectrum. It also contains sufficient oxygen‐containing groups on the surface, leading to low water evaporation enthalpy. To meet the demands of practical applications, the FLPD membrane is modified with polyvinyl alcohol (PVA) to improve the mechanical stabilities and the surface wettability is further enhanced by an oxygen plasma treatment. An SSG device based on the modified membrane attains a competitive evaporation rate of 1.355 kg m−2 h−1 under 1 sun irradiation. The evaporation rate remains approximately constant when evaporating the seawater. Moreover, the salt deposited on the surface could be self‐cleaned due to the high wettability. Therefore, herein, it is demonstrated that concentrated acid‐induced dehydration is an effective and green approach to convert cheap biomasses to carbon materials for SSG applications.

Efficient-heat-utilization 3D T-shaped porous sponge assists 2D photothermal films to achieve self-acting salt rejection and extra evaporation under high-concentration brine

Solar-driven interfacial steam generation (SISG) technology for seawater desalination is a green solution to the current freshwater crisis. However, salt accumulation and heat loss in the SISG system under high-concentration brine still have not been solved simultaneously. Inspired by the 3D evaporation structure design, we proposed a strategy to achieve self-acting salt rejection and extra evaporation. The strategy is based on a 3D T-shaped porous sponge (3DTPS), which helps various 2D photothermal films achieving self-acting salt rejection and extra evaporation simultaneously. 3DTPS provides a large number of water exchange channels for the 2D photothermal film. Therefore, under one sun (1 kW m−2), even if the brine concentration is as high as 20 wt%, salt-free can be maintained. When the hot is transferred downward, the T-shaped structure enables the 3DTPS to expose its heated surface to a greater extent in the air. 3DTPS (D = 2.5 cm) is used as an additional evaporation structure for extra evaporation, resulting in an increase of evaporation efficiency for an extra more than 21%.

Tuning the wettability of solar absorbers towards high-efficiency solar vapor generation

As an environmentally-friendly fresh water production method, solar water evaporation plays an important role in solving the shortage of fresh water. Recently, various kinds of solar absorbers have been developed. However, most of solar absorbers are hydrophilic for the sufficient water supply, and less attention has been paid to the effect of absorber’s wettability on the evaporation performance. Herein, the effect of wettability of photo-thermal films on the evaporation rates was investigated. The evaporation of the full-wetting film (0.99 kg m−2 h−1) increased by 5.3% compared to the semi-wetting film (0.94 kg m−2 h−1) at 1 kW m−2. Conversely, the evaporation of the full-wetting film (8.77 kg m−2 h−1) decreased by 8.5% compared to the semi-wetting film (9.58 kg m−2 h−1) at 13 kW m−2. The systematic test indicated that the full-wetting film exhibited high evaporation performance at low optical power densities (1–5 kW m−2), and the performance of the semi-wetting film was better at high optical power densities (9–13 kW m−2). This work provides a new avenue to achieve high evaporation performance by tuning the wettability at different application environments.

Light angle dependence of photothermal properties in oxide and porphyrin thin films for energy-efficient window applications

The photothermal experiments on the incident light angle dependence are carried out using simulated solar light on thin films of both iron oxides (Fe3O4 and Fe3O4@Cu2-xS) and porphyrin compounds (chlorophyll and chlorophyllin). Fe3O4 and Fe3O4@Cu2-xS are synthesized using various solution methods that produce mono-dispersed nanoparticles on the order of 10 nm. Chlorophyll is extracted from fresh spinach and chlorophyllin sodium copper is a commercial product. These photothermal (PT) materials are dispersed in polymethyl methacrylate (PMMA) solutions and deposited on glass substrates via spin coating that result in clear and transparent thin films. The iron-oxide based thin films show distinctive absorption spectra; Fe3O4 exhibits a strong peak near UV and gradually decreases into the visible and NIR regions; the absorption of Fe3O4@Cu2-xS is similar in the UV region but shows a broad absorption in the NIR region. Both chlorophyll and chlorophyllin are characterized with absorption peaks near UV and NIR showing a “U”-shaped spectrum, ideally required for efficient solar harvest and high transparency in energy-efficient single-pane window applications. Upon coating of the transparent PT films on the window inner surfaces, solar irradiation induces the photothermal effect, consequently raising the film temperature. In this fashion, the thermal loss through the window can be significantly lowered by reducing the temperature difference between the window inner surface and the room interior, based on a new concept of so-called optical thermal insulation (OTI) without any intervention medium, such as air/argon, as required in the glazing technologies. Single-panes are therefore possible to replace double- or triple panes. As OTI is inevitably affected by seasonal and daily sunlight changes, an incident light angle dependence of the photothermal effect is crucial in both thin film and window designs. It is found that the heating curves reach their maxima at small angles of incidence while the photothermal effect is considerably reduced at large angles. This angle dependence is well explained by light reflection by the thin film surface, however, deviated from what is predicted by the Fresnel's law, attributable to non-ideal surfaces of the substrates. The angle dependence data provide an important reference for OTI that window exposure to the sun is greater at winter solstice while that is considerably reduced in the summer. This conclusion indicates much enhanced solar harvesting and heat conversion via optically insulated windows in the winter season, resulting in much lower U-factors.

Surface-ligand protected reduction on plasmonic tuning of one-dimensional MoO3−x nanobelts for solar steam generation

Sub-stoichiometric MoO3−x nanostructures with plasmonic absorption via creating oxygen vacancies have attracted extensive attentions for many intriguing applications. However, the synthesis of one-dimensional (1D) plasmonic MoO3−x nanostructures with widely tunable plasmonic absorption has remained a significant challenge because of their serious morphological destruction and phase change with increasing the concentration of oxygen vacancies. Here we demonstrate a surface-ligand protected reduction strategy for the synthesis of 1D MoO3−x nanobelts with tunable plasmonic absorption in a wide wavelength range from 200 to 2,500 nm. Polyethylene glycol (PEG-400) is used as both the reductant to produce oxygen vacancies and the surface protected ligands to maintain 1D morphology during the formation process of MoO3−x nanobelts, enabling the widely tunable plasmonic absorption. Owing to their broad plasmonic absorption and unique 1D nanostructure, we further demonstrate the application of 1D MoO3−x nanobelts as photothermal film for interfacial solar evaporator. The surface-ligand protected reduction strategy provides a new avenue for the developing plasmonic semiconductor oxides with maintained particle morphology and thus enriching their wide applications.

A Hydrogenated Metal Oxide with Full Solar Spectrum Absorption for Highly Efficient Photothermal Water Evaporation

Searching for cost-effective photothermal material that can harvest the full solar spectrum is critically important for solar-driven water evaporation. Metal oxides are cheap materials but cannot cover the full solar spectrum. Here we prepared a hydrogenated metal oxide (H1.68MoO3) material, in which H-doping causes the insulator-to-metal phase transition of the originally semiconductive MoO3. It offers a blackbody-like solar absorption of ≥95% over the entire visible-to-near-infrared solar spectrum, owing to its unusual quasi-metallic energy band, and high solar-to-heat conversion rate due to quick relaxation of excited electrons. Using a self-floating H1.68MoO3/airlaid paper photothermal film, we achieved a stable and high water vapor generation rate of 1.37 kg m-2 h-1, a superb solar-to-vapor efficiency of 84.8% under 1 sun illumination, and daily production of 12.4 L of sanitary water/m2 from seawater under natural sunlight. This thus opens a new avenue of designing cost-effective photothermal materials based on metal oxides.