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Proc Natl Acad Sci U S A
Proc Natl Acad Sci U S A
PNAS
Proceedings of the National Academy of Sciences of the United States of America
0027-8424
1091-6490
National Academy of Sciences

38507451
202317192
10.1073/pnas.2317192121
videoVideoresearch-articleResearch ArticlechemChemistry410
Physical Sciences
Chemistry
Integration of photothermal water evaporation with photocatalytic microplastics upcycling via nanofluidic thermal management
Meng Xiangyu a 1
Wang Xin b 1
Yin Kuibo c 1
Jing Yao a
Gu Liuning d
Tao Zequan a
Ren Xinchuan a
Tang Mingyu a
Shao Xinxing d
Sun Litao c
Sun Yueming a
Dai Yunqian daiy@seu.edu.cn
a 2 https://orcid.org/0000-0002-4913-7492

Xiong Yujie yjxiong@ustc.edu.cn
b e 2
aSchool of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu 211189, China
bAnhui Engineering Research Center of Carbon Neutrality, School of Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui 241000, China
cSchool of Electronic Science and Engineering, Southeast University, Nanjing, Jiangsu 211189, China
dSchool of Civil Engineering, Southeast University, Nanjing, Jiangsu 211189, China
eSchool of Chemistry and Materials Science, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China
2To whom correspondence may be addressed. Email: daiy@seu.edu.cn or yjxiong@ustc.edu.cn.
Edited by Alexis Bell, University of California, Berkeley, CA; received October 4, 2023; accepted January 25, 2024

1X.M., X.W., and K.Y. contributed equally to this work.

20 3 2024
26 3 2024
20 9 2024
121 13 e231719212104 10 2023
25 1 2024
Copyright © 2024 the Author(s). Published by PNAS.
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).

Significance

Integrating photothermal conversion into photocatalysis has sparked substantial interest in producing survival necessities, synthetic fuels, and chemicals. Previous researches commonly ignore the negative effects of photothermal heating on photocatalysis and lack a method for thermal management at the photothermal−photocatalysis nanosite. Herein, we designed photothermal−photocatalysis sites to benefit from heat while also preventing detrimental heat effects using in situ nanofluid cooling. This design endowed the exothermic photocatalytic microplastic upcycling with high-valued chemicals production and complete in situ separation. This case on regulating the photothermal heat on the photocatalytic nanosite to prevent heat-induced adverse effects provided a perspective in photothermal−photocatalysis.

Photothermal heating and photocatalytic treatment are two solar-driven water processing approaches by harnessing NIR and UV-vis light, respectively, which can fully utilize solar energy if integrated. However, it remains a challenge to achieve high performance in both approaches when integrated in a material due to uncontrollable heat diffusion. Here, we report a demonstration of heat confinement on photothermal sites and fluid cooling on photocatalysis sites at the nanoscale, within a well-designed heat and fluid confinement nanofiber reactor. Photothermal and photocatalytic nanostructures were alternatively aligned in electrospun nanofibers for on-demand nanofluidic thermal management as well as easy folding into 3D structures with enhanced light utilization and mass transfer. Such a design showed simultaneously high photothermal evaporation rate (2.59 kg m−2 h−1, exceeding the limit rate) and efficient photocatalytic upcycling of microplastics pollutant into valued products. Enabled by controlled photothermal heating, the valued main product (i.e., methyl acetate) can be evaporated out with 100% selectivity by in situ separation.

p​hot​oca​tal​ysi​s
photothermal
nanofiber
electrospun
microplastics
MOST | NSFC | National Outstanding Youth Science Fund Project of National Natural Science Foundation of China (IUSS) 100014717 21725102 Yujie Xiong
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pmcIntegration of photothermal and photocatalytic processes can fully utilize solar energy for water processing, which harvests lower-energy near-infrared (NIR) photons to generate heat on photothermal materials and higher-energy ultraviolet (UV) or visible (vis) photons to drive chemical transformations on photocatalytic materials, respectively. Photothermal and photocatalytic technology has recently emerged as a highly promising platform for clean water evaporation, micropollutant removal, and many others (1–3). A vital and common problem in photothermal systems is the uncontrolled heat diffusion driven by a temperature gradient, which leads to unsatisfactory heat focusing on targeted locations and undesirable heating on the surroundings. The thermal effect originating from photothermal conversion has been recently demonstrated with beneficial results in activating chemical bonds and accelerating the mass transfer of reactants/products (4, 5). However, in many photocatalytic reactions, the excessive photothermal heating on catalytic sites is often viewed as an undesirable effect on reaction rates due to the promotion of charge recombination, especially in the case that efficient charge separation is highly demanded. In addition, the uncontrolled temperature rise can cause backward reactions in exothermic photocatalytic systems as well as a potential sintering effect on catalyst surface atoms (6, 7). As such, it remains a grand challenge to achieve high performance when photothermal and photocatalytic applications are integrated in a single material system. A strong demand for addressing this challenge is solar-driven water processing, in which photothermal heating and photocatalytic reactions can work for water evaporation and pollutant treatment, respectively. To overcome the limitation, ideally, a cooling field should be built at the photocatalytic sites which are surrounded by the photothermal hot field. Such an on-demand thermal management at the nanoscale will ensure photocatalytic reactions free of unfavorable disruption by photothermal conversion but calls for extremely high requirement for material design.

In this concept, the material should essentially serve as a nanoconfinement reactor, isolating or localizing mass and heat at target space in nanoscale (4, 8). One-dimensional (1D) nanofiber, as a perfect nanoconfinement reactor, possesses unique anisotropic heat/mass transport properties, enabling the confinement of photothermal heat along the diametrical direction and the orientational nanofluidic transfer along the axial direction inside each nanofiber (9). Inspired by the macroscale water-cooling cables for timely removing the harmful overheat among electronic element arrays, we can alternatively arrange photothermal sites and photocatalytic sites along the long axis of 1D nanofiber in order, constructing a nanoscale water-cooling system. This structure design will allow the directional nanofluidic flow through the confined channels between photothermal sites and photocatalytic sites and thus accurately transfer water/reactant fluid to the photothermal hot spots for efficient interfacial heating with reduced heat loss. More importantly, the flowing fluid with high specific heat capacity can timely carry out the excessive heat from photocatalyst sites and ably build the cooling field at the location surrounded by the photothermal hot field.

Despite the attractive potential in controlling both heat and fluid distribution at the nanoscale, it is a tough task for realizing the concept of photothermal−photocatalytic binary nanoscale domains enabling water cooling, with high controllability and large-scale expandability. Recent approaches to creating such binary nanoscale domains, which require complex templates and etching processes, cannot maintain their free-standing feature after removal of templates (10, 11). In parallel, nontemplate methods, such as evaporation assembly of binary nanocrystal superlattices and deposition of nanodot arrays, can only generate zero-dimensional and micrometer-scale alignment (12, 13). In contrast, electrospinning technology shows high potential in growing such binary domains inside 1D nanoconfinement structure under the unique alignment effect of electronic field, and the 1D structures (e.g., nanofibers) can be assembled into large-scale free-standing assemblies without binders or templates. As such, electrospinning is a promising approach to realizing our concept. The core concept is to maximize the anisotropic transfer behavior of 1D structure while ensuring highly directional cooling-fluid flow through the channel between photothermal and photocatalytic sites. To this end, the photothermal−photocatalytic domains should be compactly arranged inside ultrathin nanofibers. Nevertheless, it is a challenging task to control the organization mode of nanoscale domains given the collective interactions between numerous adjacent photothermal−photocatalytic subcomponents.

Here, we report a demonstration of concept on timely cooling of photocatalytic sites during photothermal heating at the nanoscale. By implementing a well-controlled electrospinning−calcination method, we fabricated the aligned fluid-cooling nanochannels between photothermal reduced graphene oxide (RGO) and photocatalytic TiO2 nanocrystals in 1D nanofiber. This design allowed directing water/reactant to RGO hot spots for efficient interfacial heating while cooling TiO2 sites to avoid aggravation of electron−hole recombination by excessive heat accumulation. To better demonstrate our concept, we selected simultaneous photothermal water evaporation and photocatalytic upcycling of microplastics waste in water as model applications. It is noteworthy that the brittleness of designed 1D nanochannels was overcome to meet practical demands, which enabled to readily fold the simulation-predesigned fibrous structures into water-floating reactors for promoting light utilization and mass transfer. This foldable fibrous reactor achieved a high solar evaporation rate of 2.59 kg m−2 h−1, as well as high photocatalytic activity with a 67.8% total yield of chemical production (i.e., methyl acetate and formic acid) from microplastics upcycling. More importantly, taking advantage of the controllable photothermal heating and nanofluidic flow, the main product of methyl acetate can be automatically separated from liquid products and water with a 100% product selectivity. This work sheds light on controlling photothermal heating and cooling at the nanoscale for on-demand water-related applications and will inspire more rational design of photothermal and photocatalytic water processing toward future human necessities production and valued chemical transformation in a sustainable way.

Results

Design of Aligned Graphene/TiO2 Nanochannels with Heat Confinement and Fluid Cooling.

To realize the localized photothermal heating at target interfaces while avoiding the excessive heat disruption on photocatalytic reaction areas (Fig. 1A), we designed nanochannels for heat confinement and fluid cooling using molecular dynamics (MD) and finite element analysis (FEA). When solar-driven heat is preinput in model, a thermal gradient can be generated in the nanochannel, which causes the thermo-osmotic nanofluid flow (14). MD results (SI Appendix, Fig. S1 A and B) show that the nanofluid flows against the temperature gradient (i.e., from lower to higher temperature) on hydrophilic TiO2 with a positive excess surface enthalpy. In contrast, nanofluid flows along the temperature gradient (i.e., from higher to lower temperature) on hydrophobic RGO with a negative excess surface enthalpy (15–17). As a result, low-temperature water can be continuously pumped to the TiO2 surface for circularly cooling the photocatalyst sites, and the heat generated on RGO can be taken away by nanofluid to avoid heat accumulation around TiO2. The heterogeneous nanochannel with different wetting behaviors (i.e., hydrophobic−hydrophilic) has a greater cooling effect on photocatalyst, as evidenced by a higher temperature dropping amplitude than the homogeneous nanochannel with the same wetting property (e.g., hydrophilic−hydrophilic) (Fig. 1B and SI Appendix, Fig. S2).

Fig. 1. Design concept of the building cool area in the photothermal hot area by nanofluidic cooling. (A) Schematic illustration for the design concept to solve the issues of photocatalytic performance decay arising from photothermal heating. (B) MD calculation showing the cooling effect on TiO2 induced by the thermo-osmosis nanofluidic flow in the nanochannel between hydrophobic graphene and hydrophilic TiO2 surfaces. The dark gray spheres denote carbon; light gray, titanium; red, oxygen; and white, hydrogen. (C) FEA showing the cooling effect on TiO2 by evaporation nanofluidic flow in the nanochannel. (D) Schematic design concept of aligned graphene/TiO2 nanochannels within the nanoconfinement fiber reactor, for maximal heating–cooling consolidation.

Except for the thermo-osmosis fluid cooling behavior, the rapid interfacial water evaporation in graphene/TiO2 nanochannel can also timely cool the photocatalytic TiO2 sites during photothermal heating, as demonstrated by MD and FEA simulations. The surface enthalpy and water interactions on graphene/TiO2 nanochannel result in significantly reduced vaporization enthalpy and improved water diffusion rate. In detail, the same water models including the adsorbed and free water layers were constructed on graphene/TiO2 nanochannel surface and bulk water in calculations at the same temperature. After evaporation, the vaporization enthalpy on graphene/TiO2 nanochannel surface was calculated as low as 1,730 kJ kg−1, which was substantially lower than that of pure water (2,401 kJ kg−1) (18). The polar hydrophilic groups within nanochannels can capture water molecules via stronger hydrogen bonds than those observed in bulk water, generating bound water. Water molecules adjacent to such bound water molecules, called free water, interact with fewer than the surrounding water molecule networks, requiring less energy to break hydrogen bonds and escape from the liquid surface (19). Radial distribution functions (RDFs) analysis, which examines the H−H and H−O bonds among the free water on graphene/TiO2 (SI Appendix, Fig. S3A), gives the physical understanding into the enthalpy reduction on graphene/TiO2 nanochannel. Compared to bulk water, the interactions among free water on graphene/TiO2 channel surface are rather weak, indicating the efficient activation of water molecules for significantly accelerating water evaporation under a given amount of energy (19), which leads to a higher calculated water diffusion coefficient of 3.44 in channel than in bulk water (3.17, SI Appendix, Fig. S3B). As shown in FEA results (Fig. 1C), this rapid endothermic evaporation process can effectively cool the TiO2 nanosites after 1,000-ps irradiation. The graphene locally concentrates thermal energy (95 °C), whereas TiO2 maintains a cool temperature close to room temperature (31 °C), without excessive thermal accumulation that could disturb photocatalytic reactions.

In addition to calculations and simulations, we investigated the cooling effect of such a nanofluid design on photocatalytic components. Considering the lack of an efficient method to directly imaging temperature profile at such a nanoscale space in the liquid environment, we designed a nanofluid chip with nanofluid channels (width: 800 nm, depth: 800 nm) adjacent to TiO2 components (SI Appendix, Fig. S4, details in Materials and Methods). Nanofluid was cycled to simulate the evaporation flow (details in Materials and Methods). Raman bands of metal oxide would shift to a lower frequency along as temperature increased, due to the thermal expansion of the lattice and changes in the vibration energy (6, 20). Hence, the microscopic temperature of TiO2 can be tested in our nanofluid chip based on the relationship between Raman shift and temperature. The Raman shift of Eg mode for anatase TiO2, which was measured at different temperatures ranging from 273 to 1,073 K by the heat holder, was prerecorded to establish the relationship between Raman shift and temperature (SI Appendix, Fig. S5). TiO2 was then irradiated by laser with increasing heating power. As the heating power increased from 0.14 to 0.53 mW, the temperature of TiO2 grew from 67.5 to 83.6 °C without nanofluid (SI Appendix, Fig. S6 A and B). In sharp contrast, the temperature only increased from 14.9 to 19.2 °C with nanofluid (SI Appendix, Fig. S6 C and D). The visual temperature distribution over TiO2 demonstrated the remarkable cooling effect of nanofluid more clearly (SI Appendix, Fig. S6).

Taken together, the high-speed water evaporation and thermo-osmosis nanofluidic flow through the nanochannel make each designed channel become a nanoscale heat-exchanger (SI Appendix, Fig. S7), which confines photothermal heat and builds cold areas in photothermal hot areas. Based on the unique anisotropic heat/fluid transfer behavior of 1D nanofiber structure, we can align such graphene/TiO2 heat-exchangers inside the nanofiber reactor, to achieve the serial heating–cooling units in nanochannels. The cooling by this design can be more efficient than that by conventional fibrous surface design (details can be found in SI Appendix, Note S1). This feature will enable interfacial heating on photothermal graphene and cooling on photocatalytic TiO2 at the same time (Fig. 1D).

Fabrication of RGO/TiO2 Nanofibers with Predesigned Heat/Fluid Nanochannels.

Guided by the simulations and calculations design, we developed an electrospinning−calcination approach to fabrication of RGO/TiO2 nanofibers with the targeted channel structures (Fig. 2A). During the fabrication of GO/PVP/TiO2 composite nanofibers, PVP/TiO2-precursor matrix was mixed for 12 h first before the RGO precursor (i.e., GO, SI Appendix, Fig. S8) was added into the matrix for 1-h mixing. Thin GO sheets with liquid crystal behaviors can be arranged for the minimum exclusion volume under the entropy effect (21), while poly(vinyl pyrrolidone) (PVP) and titanium tetraisopropoxide (TTIP) were simultaneously driven to orient in alignment in precursor. Consequently, the PVP polymer chains in the precursor were polarized and further stretched by electronic field in the 1D nanofiber during electrospinning. As a result, the nanofibers with aligned TTIP/GO/PVP were ably obtained, as confirmed by the well-aligned element distribution inside the typical nanofiber (Fig. 2 B and C). The hydrophilic−lipophilic PVP agent linked TiO2 and RGO precursor, whose removal by heat treatment will generate nanochannels between TiO2 and RGO.

Fig. 2. Structural analyses of the RGO/TiO2 nanofiber with the target nanochannel structures. (A) Schematic formation mechanism of aligned channels within the RGO/TiO2 nanofiber by electrospinning and heat treatment. (B and C) Electron energy loss spectroscopy (ELLS) mapping of Ti, C, and N of GO/PVP/TiO2 nanofiber. The scale bars in (C) represent 10 nm. (D) (Top) STEM image and (Bottom) TEM image of RGO/TiO2 sectional slice after FIB cutting. The inserted scheme shows the FIB cutting direction of the nanofiber. The blue frames in (D) highlight the TEM observation area. (E) TEM image with a higher magnification from D (highlighted by the red frame in D), showing the alternative distribution of RGO among TiO2. The red lines in pairs in (E) highlights RGO; yellow color, TiO2. (F) (Left) TEM image showing the continuous penetrating and aligned nanochannels inside the long RGO/TiO2 nanofiber and (Right) with targeted nanostructures inside the nanofiber.

Upon the fabrication of nanofibers, the aligned fluid nanochannels were finally formed in nanofibers after the thermal treatment. While PVP was removed, GO was reduced into RGO and TTIP was transformed into anatase TiO2 grains with exposed (101) planes inside each nanofiber, as confirmed by thermogravimetric analysis (TGA), Raman spectroscopy, C 1s X-ray photoelectron spectroscopy (XPS), and TEM results in SI Appendix, Figs. S9–S12. To clearly observe the structure inside RGO/TiO2 nanofiber, thin section slides of nanofiber were cut using focused ion beam scanning electron microscopy (FIB-SEM) technology and further observed by transmission electron microscopy (TEM). Fig. 2 D and E shows the RGO alternating with the TiO2 grains inside the nanofiber. As such, aligned and penetrating 1D nanochannels were successfully generated inside each RGO/TiO2 nanofiber (Fig. 2F). The chemical states of the RGO/TiO2 nanochannel structures were also analyzed in SI Appendix, Fig. S13 and Note S2.

Moreover, the RGO/TiO2 nanofibers with target nanochannels in order, exhibit the strong mechanical properties and can be easily assembled into 3D-folded structures for practical use. In situ TEM images recorded the rapid mechanical vibration (37° ms−1, Fig. 3 A, i and Movie S1), compression (25% s−1, Fig. 3 A, ii) and large-angle bending (61°, SI Appendix, Fig. S14) of a single nanofiber without any observed cracking. The inserted FEA results illustrated the concentrated shear force in nanofiber during rapid vibration, showing the harsh condition setting in tests. Besides the nanoscale investigation, microscale confocal imaging under tensile loads also revealed the structural stability and antitearing feature of fibrous mats (SI Appendix, Note S3 and Figs. S15 and 16). The tensile test confirmed that the nanofibers were mechanically strong due to the well-aligned refined TiO2 grains with RGO protection (SI Appendix, Fig. S17A). The tensile stress and Young’s modulus of aligned RGO/TiO2 nanofibers were almost 9.7 and 5.2 times of those achieved by conventional random TiO2 nanofibers, respectively. The alignment designs on both inside and outside RGO/TiO2 nanofibers increased the tensile stress and Young’s modulus from 0.9 MPa and 310 MPa to 2.5 MPa and 675 MPa, respectively (SI Appendix, Fig. S17B). Meanwhile, the fracture work of the multilevel aligned RGO/TiO2 was as high as 193 kJ m–3, even comparable to the polymer-based materials (22, 23).

Fig. 3. Strong and flexible RGO/TiO2 nanofibers with large-scale practicability. (A) In situ TEM observation of RGO/TiO2 under i, vibration and ii, compression mechanical tests, with inserted FEA on stress distribution in RGO/TiO2 under rapid vibration. (B) SEM images of the folded and knotted RGO/TiO2 fibrous mat without any observed damage. The folding curvature was highlighted in B. (C) The 3D-DIC setup and microstrain imaging of RGO/TiO2 mats under the real stress-loading conditions. (D) Optical images of the flexible and scalable RGO/TiO2 mats before and after elastic folding, with the inserted scheme for illustrating the folding process.

After interweaving the strong nanofibers into aligned mats, the mat kept stable on both micro or nanoscale structures under folding or knotting deformations with large curvatures (Fig. 3B), laying the basis for flexible folding of RGO/TiO2 into desired 3D macroreactors. Advanced noncontact 3D-digital image correlation (DIC) technology was used to dynamically capture the microstrain (resolution: 220 nm) of fibrous mat during the real deformations (Fig. 3C). Under the same applied tensile load (10 N), the multilevel aligned RGO/TiO2 fibrous mat exhibited uniform and smaller microstrain, demonstrating its structural stability during large deformations. Inspired by origami art, the aligned RGO/TiO2 fibrous mat can be folded into a stretchable spring-like structure, allowing reversible folding with 90° folding at least 35 times and easy stretching into the length over 1 m (Fig. 3D). These mechanical features endowed our designed RGO/TiO2 nanofibers with the great potential in large-scale extendibility and easy 3D-shape design for practical photothermal−photocatalytic applications.

Synchronous Photothermal Water Evaporation and Photocatalytic Microplastics Upcycling by 3D RGO/TiO2 Fibrous Reactor.

Our designed RGO/TiO2 fibrous mat has been featured with confined heating on photothermal interfaces and cooling at photocatalytic sites, enabling its use as a nanoconfinement photothermal−photocatalytic reactor with high efficiency on both photothermal water evaporation and photocatalytic water treatment. In terms of mass transfer and light utilization, FEA predictions help us identify the optimal structures of this photothermal−photocatalytic reactor at multiple scales. Simulations suggested that aligning RGO/TiO2 fluid channels from the nanoscale to microscale can result in high-speed mass transfer (Fig. 4A). Also, under a same light-flux input, the 3D-helical fibrous reactor can achieve the superior photothermal temperature, compared to the 2D planar mat and 3D-Z-like fibrous reactor (Fig. 4B). According to this targeted structure design, the flexible RGO/TiO2 nanofibers were interwoven into aligned fibrous mat (SI Appendix, Fig. S18A) and then folded into 3D-helical fibrous reactor (Fig. 4C and SI Appendix, Fig. S18B), with an ultralow density (6.61 mg cm–3). Fig. 4C depicts the aligned fluid channels among nanofibers within mat and among helical rings in 3D reactor, with the targeted short diffusion routes for rapid liquid water transfer and vapor release during evaporation. In this photothermal-photocatalytic RGO/TiO2 reactor design, each aligned nanofiber was oriented perpendicular to the underlying reactants or water. The bending strain generated by ring-by-ring rolling in the reactor was applied axially among aligned nanofibers rather than radially within each nanofiber, ensuring rapid nanofluid flow along these vertical nanochannels and avoiding potential impact on nanofluid cooling by channel blocking.

Fig. 4. Photothermal RGO/TiO2 fibrous reactor with aligned channels for rapid water evaporation. (A and B) FEA results and schematic illustration for designing the structures for optimal fluid transfer (A) and photothermal conversion (B). The white area in A is the geometric model. (C) SEM images show the target 3D-helical fibrous reactor structure with multilevel aligned channels, with an inserted scheme illustrating the folding process of the targeted 3D reactor from aligned fibrous mat. (D) The infrared image of 3D-helical RGO/TiO2 reactor under light irradiation. (E) Recording of light flux, outdoor temperature, and water production in outdoor test of RGO/TiO2. (F) Comparison of water mass change during evaporation by different RGO/TiO2 reactors. (G) Optical images showing the device in outdoor tests on Changjiang River in Nanjing, China.

As predicted, the experimental photothermal temperature of the resultant 3D helical RGO/TiO2 reactor reached 97 °C, approaching to the water boiling point (Fig. 4D). The photothermal conversion efficiency was calculated as high as 89.3%. It can be attributed to the enhanced light absorption by multiple light scattering and the superior thermal insulation among helical walls. In addition, the equivalent vaporization enthalpy of water constrained on 3D RGO/TiO2 helical reactor (1,781 kJ kg−1) was less than that of bulk water (2,437 kJ kg−1), 2D RGO sheet (2,296 kJ kg−1) and 2D RGO/TiO2 aligned mat (1,806 kJ kg−1) (SI Appendix, Fig. S19). This feature is due to the increased exposed evaporation surface in the helical reactor and the enhanced disturbance of the hydrogen bonding network caused by the hydrophilic groups on TiO2 domains (24). These comparisons indicated that the 3D helical RGO/TiO2 reactor design can activate the water bonding state faster than other structures during evaporation.

Practical outdoor water treatment tests by RGO/TiO2 fibrous reactors were conducted on Changjiang River (Nanjing, China, 4th September, 2022), with real-time monitoring of solar flux, ambient temperatures, and water evaporation rate (Fig. 4E). Under the natural sunlight (0.5 to 0.9 kW m−2), RGO/TiO2 yield a total water production of 7.03 kg m−2. The water evaporation rate of the 3D-helical aligned RGO/TiO2 reactor was as high as 2.59 kg m–2 h−1, which was 13 and 37% higher than the random and 2D structures, respectively (Fig. 4F). This high solar-to-vapor conversion efficiency over 100% was attributed by the rapid evaporation induced the energy gained from environment (energy analysis can be found in SI Appendix, Note S4; the IR image of wet RGO/TiO2 reactor during evaporation is shown in SI Appendix, Fig. S20). The evaporator also showed good stability even after 7-d cycling evaporation with a rate decrease of only 3.4% (SI Appendix, Fig. S21). Besides, in the simulated seawater (3.4 wt% of salinity, details in Materials and Methods), real seawater collected from Yellow Sea (120.6° E, 36.0° N, 3.2 wt% of salinity) and Bohai Sea (120.8° E, 40.6° N, 3.0 wt% of salinity), the evaporation rate of RGO/TiO2 is 2.53, 2.50 and 2.55 kg m2 h−1, respectively (SI Appendix, Fig. S22). These high antisalt evaporation performances are superior to the recent advanced works (25−27). The highly aligned nanochannels within nanofibers oriented perpendicular to the underlying bulk water can serve as the mass-transfer bridges for real-time transporting salt ions back to the bulk water. Also, the thermos-osmosis nanofluid moving from up to down along RGO walls can return salts to bulk water, further contributing to the antisalt evaporation. Except for the evaporation advantages in practical river and seawater, RGO/TiO2 evaporated water at a high rate of 2.37 kg m2 h−1 even under 0.5 Sun, showing unique metrics in applying in more weather conditions (e.g., cloudy) or areas (28). Meanwhile, the disperse-blue dye which is classified as an II-typed carcinogenic water pollutant (WHO International Agency for Research on Cancer, 2017) and the amoxicillin drug pollutant which can cause serious endocrine disorders and antibiotic resistance in humans (29) were both efficiently photocatalytically removed in underlying water during the outdoor evaporation by RGO/TiO2. SI Appendix, Figs. S23 and S24 demonstrated the purification of polluted water, showing high potential in simultaneous freshwater production and polluted water treatment in practical water processing applications. All the outdoor tests were conducted in a self-floated transparent pyramid solar prototype with condensate collection troughs and bottle, as shown in Fig. 4G (clear images without mist covering in SI Appendix, Fig. S25). As the temperature increased from room temperature (RT, 25 °C), 50, 80, to 100 °C, the intensity of the photoluminescence peaks for TiO2 centered at 459 and 586 nm was remarkably enhanced (SI Appendix, Fig. S26A). The intensity of the peaks was primarily determined by the recombination rate of photogenerated electrons and holes (30, 31). Similarly, increasing the temperature reduced the photocurrent response (SI Appendix, Fig. S26B). Taken together, the measurement results indicated that heating can significantly accelerate charge recombination. As the reaction temperature cooled from 80 to 25 °C, the photocatalytic degradation efficiency of dispersed blue and amoxicillin was continuously improved, after the saturated adsorption in dark (SI Appendix, Fig. S27). This result confirmed that cooling can facilitate the photocatalytic water pollutant degradation process over RGO/TiO2.

Aside from producing clean water from the common pollutants, the 3D RGO/TiO2 reactor can photocatalytically convert the recently emerging microplastic water pollutants into selective valued chemicals, aided by rapid photothermal evaporation. Microplastics as tiny water pollutants resulting from the breakdown of plastic wastes (e.g., synthetic clothing, bottles, and packages) are harmful to the environment and human health. The massive generation of microplastic wastes in seawater necessitates the development of effective treatment and recycling techniques (32). Polyethylene (PE) is the commonly used plastics in packaging food, cloths, and electronic devices, becoming the most abundant type of microplastic waste in seawater (33). Here, our designed RGO/TiO2 photothermal−photocatalytic reactor can efficiently convert PE microfibers into valued chemicals/fuels and selectively separate the desired product by photothermal evaporation (Fig. 5A).

Fig. 5. Photothermal−photocatalytic RGO/TiO2 fibrous reactor for solar-driven microplastics upcycling and autoselection of value-added products by evaporation. (A) Schematic illustration for photocatalytic upcycling of microplastics and photothermal evaporation-driven product separation by the RGO/TiO2 reactor. (B) In situ FTIR mapping for confirming the microplastics sample’s component. (C) Schematic illustration and optical image of the device for microplastics conversion and product collection. (D) The production of BaCO3 solid product. The inset is the collected BaCO3 product (purity>99%). (E) Infrared image of the RGO/TiO2 reactor under light irradiation, with a schematic illustration of the product selection according to the liquid product boiling points (b.p.) by photothermal evaporation. (F) 1H-NMR spectra showing the production and autoseparation of the methyl acetate product by the RGO/TiO2 reactor. (G) Schematic illustration concluding the photothermal−photocatalytic microplastics upcycling into fuels, chemicals, and materials by the RGO/TiO2 reactor.

During the rapid water wicking in solar evaporation, tiny-sized microplastics could be autocaptured on RGO/TiO2 channel surfaces (SI Appendix, Fig. S28A). After evaporation for 1 h under 1 Sun, the removal ratio of microplastics by 3D RGO/TiO2 was as high as 97.9% (SI Appendix, Fig. S28B). To understand the removal mechanism, PE microplastics were carefully collected after evaporation and observed by an in situ Fourier transform infrared spectrometry (FTIR) microscope for dynamically focusing on the tiny samples and accurately analyzing their components. The initial PE microplastic was in a cylinder microfiber shape with a diameter approaching 750 μm and a smooth surface (SI Appendix, Fig. S29A). After evaporation and photocatalysis, PE microfiber was seriously etched with indented cracks (SI Appendix, Fig. S29B), with component confirmation by in situ FTIR mapping (Fig. 5B). Moreover, the 1:2:2:1 and 1:1:1:1 quartet patterns, displayed in electron paramagnetic resonance (EPR) spectra (SI Appendix, Fig. S30), demonstrated that PE microplastics were etched and converted by the ·OH and ·O2− radicals.

Then, a well-designed sealed setup with reaction cells and sampling channels was used to test the products converted from microplastic by RGO/TiO2 (Fig. 5C). Under the light irradiation of 300 to 2,500 nm, CO2 gas products were produced (production rate: 190.5 μmol g−1 h−1, SI Appendix, Fig. S31) and then were converted into BaCO3 powders (a basic ceramic material in electronic industry) with a high purity over 99.9% by pouring gas into Ba(OH)2. The collected high-purity BaCO3 chemicals are shown in Fig. 5D. Moreover, methyl acetate (production rate: 27.4 μmol g−1 h−1) and formic acid (production rate: 24.9 μmol g−1 h−1) were detected in liquid products (SI Appendix, Fig. S32A). When the light source was changed to 365 nm UV irradiation, in addition to the methyl acetate (production rate: 15.6 μmol g−1 h−1) and formic acid (production rate: 11.6 μmol g−1 h−1) (SI Appendix, Fig. S32B), CH4 (production rate: 11.8 μmol g−1 h−1) and CO (production rate: 0.440 μmol g−1 h−1) fuel gases were detected in gas (SI Appendix, Fig. S33). The CH4 and CO were produced from the decomposition of methyl acetate under UV irradiation. The suppression of water oxidation on photocatalytic sites (as the competing reaction to microplastics oxidation) through the in situ water removal by photothermal evaporation, led to this efficient PE upcycling into value-added products, in contrast to the recent advanced reports that primarily producing H2 in photocatalytic microplastics conversion (34, 35).

To understand the unique working processes of solar evaporation and photocatalytic conversion, we monitored the time-dependent production of water and chemicals (SI Appendix, Fig. S34). Prolonging the reaction time, evaporated water was produced at a nearly constant rate of 73.0 mol g−1 h−1 (normalized to the TiO2/RGO, the same hereinafter). Meanwhile, the chemicals converted from microplastics had an increasing average production of 243 μmol g−1 within 1 h and 964 μmol g−1 within 4 h. To reveal the cooling impact of solar evaporation on photocatalysis, the evaporation rate was elegantly suppressed by increasing the relative humidity to 100% above bulk water (details in Materials and Methods). Upon sluggish solar evaporation, the chemical production decreased from 964 μmol g−1 to 712 μmol g−1 after 4-h irradiation (SI Appendix, Fig. S35). In contrast, photocatalysis has an ignorable effect on evaporation in the coupling process (SI Appendix, Fig. S36, details in SI Appendix, Note S5). These results revealed that the photocatalytic conversion was coupled with solar evaporation, with remarkable boosting by built-in cooling.

The temperature-dependent control tests for the photocatalytic microplastic (i.e., PE) conversion were also conducted (SI Appendix, Fig. S37). Heating from 25 to 50 °C, the total production rate and PE conversion were reduced from 242.8 to 107.2 μmol g−1 h−1 and from 24.7 to 14.2% h−1, respectively. In the meantime, the selectivity of the C2+ methyl acetate product decreased from 11.3 to 8.5%. As the temperature was elevated to 80 °C, the production rate and PE conversion were reduced to 69.5 μmol g−1 h−1 and 9.2% h−1, respectively. In addition, the C2+ methyl acetate vanished, replaced by H2 and O2 from water splitting.

The differential scanning calorimetry (DSC) test under light irradiation revealed that PE microplastic conversion was dominated by a constant exothermic process (90.8 to 95.1%) with an initial and fast endothermic process (SI Appendix, Fig. S38). On the thermodynamics, the heating thus suppressed the photocatalytic PE conversion. Moreover, the heat input can accelerate the water-splitting reaction (36), which is the competing reaction to microplastic oxidation (37), further inhibiting the target PE conversion. As a result, the production rate was remarkably reduced as the temperature rose. During the chemical conversion, generating one C=O bond (728 kJ mol−1) in formic acid is easier than forming one C=O bond and two C−O bonds (326 kJ mol−1) in methyl acetate (38). As a result, inhibiting PE upcycling reduced the selectivity for C2+ methyl acetate product. The suppressed charge separation kinetics, detrimental thermodynamics, and improved competing water splitting (SI Appendix, Fig. S39 and Note S6), all contributed to the notable decrease in the production rate and the selectivity of C2+ product in photocatalytic PE conversion during heating.

This behavior can also be found in the PE conversion by pure TiO2 (i.e., removing RGO from RGO/TiO2 through calcination in air). As the reaction temperature increased, the PE conversion products (14.5 μmol g−1 h−1 of CO2 at 25 °C) disappeared. Instead, we observed the increased products from the competing water splitting (7.3 μmol g−1 h−1 of H2 and 4.4 μmol g−1 h−1 of O2 at 25 °C; 17.7 μmol g−1 h−1 of H2 and 8.3 μmol g−1 h−1 of O2 at 50 °C; 23.7 μmol g−1 h−1 of H2 and 10.4 μmol g−1 h−1 of O2 at 80 °C) (SI Appendix, Fig. S40). Taken together, the evaporation cooling at the macroscale and the built-in nanofluid cooling at the microscale over the photocatalyst are significantly beneficial to the photocatalytic microplastic upcycling.

Besides, the microplastic conversion efficiency was reduced by only 3.6% after 7 d of cycling. The microplastic was upcycled into chemicals with 24.6 μmol g−1 h−1 of methyl acetate, 30.0 μmol g−1 h−1 of formic acid, and 182.3 μmol g−1 h−1 of CO2 after 7-d cycling, which was very close to that before cycling (SI Appendix, Figs. S41–S43). The satisfied stabilities of microplastic-to-chemical conversion during cycling ensures a long service lifetime in practice. Aside from PE, RGO/TiO2 can totally remove polyethylene terephthalate (PET, commonly used in bottles and frozen food packaging, SI Appendix, Fig. S44A) and polylactic acid (PLA, an emerging plastic derived from natural sources and used in textiles and healthcare, SI Appendix, Fig. S44B) microplastics in water within 20 h under light. With a conversion rate of 46.1% h−1, the PET can be converted into 20.3 μmol g−1 h−1 of 1,4-benzenedicarboxaldehyde, 208 μmol g−1 h−1 of 1,4-dimethylbenzene, 130.6 μmol g−1 h−1 of ethylene glycol, 22.6 μmol g−1 h−1 of acetic acid (SI Appendix, Fig. S45), and 120.4 μmol g−1 h−1 of CO2 (SI Appendix, Fig. S46). The PLA can be upcycled into 554 μmol g−1 h−1 of formic acid (SI Appendix, Fig. S47) and 179.1 μmol g−1 h−1 of CO2 (SI Appendix, Fig. S48), with a rapid conversion rate of 72.7% h−1. Furthermore, by enhancing the light intensity at 6 Sun, the water flux rate and PE conversion rate can be improved to 5.28 kg m−2 h−1 and 61.0% h−1, respectively. The PE in water can be totally removed within 2 h, with a significantly enhanced production rate (912 μmol g−1 h−1 of methyl acetate, 837 μmol g−1 h−1 of formic acid, and 261 μmol g−1 h−1 of CO2, SI Appendix, Fig. S49 and S50). Also, this efficient microplastics upcycling causes no clog of structure or contamination of water, which is safe for the environment (detailed in Materials and Methods).

More interestingly, the PE upcycling product (e.g., methyl acetate primary product) can be selectivity separated by the evaporation at controlled photothermal temperature. For example, the controlled photothermal temperature of 3D RGO/TiO2 fibrous reactor was 56.7 °C under 0.5 sun irradiation (Fig. 5E), which was close to the boiling point of methyl acetate (56 °C) and far below the boiling point of formic acid coproduct (100.8 °C). Therefore, methyl acetate can be easily collected in the form of vapor and then condensed into pure liquid chemical. 1H-NMR spectra in Fig. 5F confirmed the product selection by evaporation. In addition, this controlled photothermal heating and evaporation can promote forward equilibrium of the PE upcycling reactions, by timely removing liquid products from the system, thereby enhancing both activity and selectivity.

Overall, taking advantage of the efficiency maximization of both photocatalytic chemical conversion and photothermal liquid evaporation without interference on photochemical paths, our designed RGO/TiO2 reactor can efficiently convert PE microplastic wastes to gas fuels (i.e., CH4, CO, formic acid) of new energy vehicles, valued liquid products (i.e., methyl acetate, formic acid) in chemical industry, and solid materials (i.e., BaCO3) used in electronic devices (Fig. 5G). The PE 100%-conversion by RGO/TiO2 reactor showed a total yield of methyl acetate at 48.6% and a total yield of formic acid at 19.2%, along with a high production rate of 76.9 μmol g−1 h−1 (normalized on PE input), and a 100% production selectivity on methyl acetate, which outperformed recent advanced reports on plastics upcycling under a lower energy consumption (SI Appendix, Table S1). It opens a broad avenue for efficient microplastics upcycling during solar-driven seawater desalination, which can simultaneously solve the emerging “PM 2.5 in sea” (i.e., microplastics) global issues and meet the future demand of circular chemical economy in a sustainable way.

Discussion

To confine photothermal heat and cool photocatalytic sites for efficient solar-driven applications without unwanted interference, a photothermal−photocatalytic nanoconfinement fibrous reactor with alternative RGO and TiO2 aligned nanochannels was designed. This nanoconfinement fibrous reactor was mechanically strong and flexible, making it easy to fold into photothermal-conversion and mass-transfer-favorable 3D structures. The high controllability of heat and fluid inside fibrous nanochannels allows for remarkable process intensification in a photothermal−photocatalysis system. It demonstrated over 100% solar-to-vapor efficiency, rapid photocatalytic removal of liquid pollutants (dye: disperse-blue, drug: amoxicillin), and effective upcycling of solid microplastics (i.e., PE). Aside from outperforming recent advanced studies in activity, 100% product selectivity in microplastics conversion was achieved with the aid of controlled photothermal evaporation. This work represents an upcycling solution to the emerging microplastic water pollutant problem. Also, the findings will inspire further research in building cool fields in photothermal hot fields at the nanoscale within a single material system, as well as the rational design of photothermal/thermal catalytic systems with different demands and mechanisms, toward sustainable environmental treatment and chemical transformation.

Materials and Methods

MD and FEA Simulations.

MD simulations of thermo-osmosis behavior were conducted using the LAMMPS software package and the method previously reported in ref. 14. The units in the model are nondimensionalized, based on the three basic parameters of the fluid particle (σ, the length of fluid particle; ε, the interaction energy among fluid particles; m, the mass of fluid particle). In addition, τ ∝ (m × σ2 × ε−1)1/2 (15–17). Thermo-osmosis coefficients β12 for hydrophilic and hydrophobic surfaces were obtained from excess enthalpy profiles via the modified Derjaguin Eq. 1 (15):[1] β12=1η∫ys0y-ys+bδH(y)dy.

where ys (σ) is the position of hear plane, which is selected as the first peak of the excess enthalpy curve; η (ετ × σ−3) is the viscosity of bulk liquid; and b (σ) is the slip length, which were determined through Green–Kubo relations in other equilibrium MD simulations, respectively, referring to our previous work (14). The calculated thermo-osmosis coefficients were −0.41 σ2 τ−1 for hydrophobic surfaces and 0.08 σ2 τ−1 for hydrophilic surfaces. The negative sign means the flow direction along the temperature gradient, i.e., from high to low temperature, and vice versa. Using typical values of σ = 0.34 nm and τ = 2 ps, thermo-osmosis coefficients were −2.31 × 10−8 for hydrophobic surface and 0.46 × 10−8 for hydrophilic surface m2 s−1, respectively. These coefficients were on the order of 10−9 ~ 10−8 m2 s−1, which are comparable to the experiment value of 10−10 ~ 10−9 m2 s−1 reported by Bregulla et al. (39).

MD simulations of water evaporation were conducted with the Materials Studio and the Forcite tools. The evaporation mainly occurred on the photothermal graphene surfaces, so we conducted the RDF analysis of water molecules during evaporation on the graphene surfaces. According to the analysis of TEM images, the (101) plane of anatase TiO2 and graphene layer is exposed in the model. A thin layer of water (50 molecules) was strongly adsorbed on the model interface, and a free water layer (100 water molecules) was set on this absorbed water layer (density: 1.0 g cm−3). The heights of simulation cells were adjusted to 200 Å to allow for sufficient evaporation space. Evaporation was performed on two simulation cells at a typical elevated temperature (313 K) under an isobaric-isothermal ensemble for 200 ps. The temperature was chosen to ensure the proper water release rates for better observation. The water evaporation enthalpy was calculated based on the following Eq. 2:[2] H=U+PV,

where U is the total energy, P is the pressure value set in simulation, and V is the lattice volume at the end of simulation.

Simulations of heat and fluid distribution were conducted by the Solid Heat Transfer and Laminar Flow ports, Steady-state domain interface in COMSOL Multiphysics. The structure was modeled based on the predesign. The initial temperature of graphene and other areas were set as 95 and 20 °C, respectively. The flowing water fluid was positioned between graphene and TiO2 with a flow rate of 0.15 ms–1 (based on the diffusion rate of water molecules in evaporation in MD calculation). Photothermal conversion results were simulated by the Heat Transfer port, Steady-state interface in COMSOL Multiphysics. The source of heat was selected as solar irradiation, using the External Radiation Source port (1 kW m–2). For all exposed surfaces, the convective heat flux boundary condition utilized a bulk heat-transfer-coefficient of 20 W m–2 K–1. In all simulations, the effect of the mesh size was tested, and convergence was achieved.

The capillary attraction force is generated along the hydrophilic TiO2 surface, attracting cool water transport from the underlying bulk water to TiO2 for continuous cooling. Meanwhile, a capillary repulsion force is developed along the hydrophobic graphene surface, reducing the interaction between water molecules and graphene, and therefore facilitating water evaporation on photothermal graphene. The capillary force in the hydrophilic–hydrophobic nanochannel can increase cooling water transportation on photocatalytic TiO2 and accelerate water evaporation on photothermal graphene.

Chemical and Materials.

TTIP (Ti[OCH(CH3)2]4, 97%) and PVP (MW ≈ 1.3 × 106) were purchased from Sigma-Aldrich. Ethanol (C2H5OH, anhydrous, 94 to 96%) and graphite powder (99.95%, 325 mesh) were purchased from Alfa Aesar. Acetic acid (CH3COOH, glacial), sodium chloride (NaCl), potassium permanganate (KMnO4, 98%), sulfuric acid (H2SO4, 98%), hydrogen peroxide (H2O2, 30%), hydrochloride (HCl, 37%) and other chemicals were all obtained from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received. The water used in all experiments was filtered through a Millipore filtration system with resistivity above 18 MΩ cm. The simulated seawater contains 3.1 wt% of NaCl, 0.2 wt% of MgCl, and 0.1 wt% of KCl.

Electrospinning of RGO/TiO2 Nanofibers.

GO sheets were synthesized according to our previous report (40). A given volume of the purified GO colloid was vacuum-dried at 40 °C and precisely weighed. GO colloid was further diluted to 1 mg mL–1 with ethanol, followed by 2-h ultrasonication for further use. The total amount of ethanol was kept at 4.5 mL. Then, 3 mL of CH3COOH, 2.5 mL of TTIP, and 0.3 g PVP were added into the homogeneous solution. The yellow precursor was stirred at 280 rpm for 8 h and added with GO (0.2 mL) for mixing 1 h before being electrospun at a voltage of 17.5 V with a flow rate of 0.3 mL h–1. The as-spun nanofibers were exposed to air overnight and then treated in a constant N2 flow at 450 °C for 3 h. The TiO2 nanofibers were calcinated in air at 450 °C for 3 h to remove GO in as-spun nanofibers.

DIC Method for Visualization of Microstrain.

A binocular camera system was set up and stereo calibration was carried out for 3D-DIC measurement. In the binocular system, the focal length of the lens was 50 mm, the resolution of the camera was 2,048 × 2,048, and the field of view was approximately 45 mm. Tiny fluorescent ink droplets were evenly sprayed on the specimen surface with a spray gun (GP-1, FUSO SEIKI). The fluorescent droplets were illuminated by ultraviolet light sources at 365 nm and served as high-quality imaged speckles. The average diameter of the imaged speckle was approximately 6 pixels, which is sufficient for DIC calculations.

Estimation of Water Evaporation and Pollutant Treatment.

The RGO/TiO2 fibrous evaporator was floated on a water surface. The relative humidity was 75%, and the surrounding temperature was 23 °C. The evaporation was performed under the irradiation of a Xenon lamp (CEL-HXF300, Beijing Aulight Co., Ltd.) with a light density of 1 kW m−2. The power density was measured by an optical power meter (S314C, THORLABS) and adjusted by the current of the lamp. The water mass change was real-time tested using an electronic microbalance (XP26, Mettler Toledo). The water evaporation rates were calculated by subtracting the dark evaporation rate from the total water evaporation rate (41). The Sarea of the 3D helical reactor included both the top surface area and side surface area of its cylindrical shape, which was calculated by Eq. 3:[3] Sarea=r2+2r·h·π,

where r is the radius of the top surface of the reactor (m), and h is the height of the reactor (m).

The solar-to-vapor conversion efficiency (η) was calculated from the following Eq. 4 (41):[4] η=m·hLVρL,

where m is the water evaporation rate (kg m–2 h–1), hLV is the total enthalpy of water-vapor phase conversion, including sensible heat and phase conversion enthalpy (J kg–1), and ρL is the light power density (W m–2). The height of a 3D helix reactor was 3 cm, and the diameter was 1.5 cm. Each reactor was weighted as 21.2 mg. The weight of the evaporator inside the outdoor devices was 4.57 g. The concentration of disperse-blue and amoxicillin in the simulated pollution water was 0.5 g L–1 and 0.8 g L–1, respectively. The temperature-dependent photoluminescence tests were conducted on a fluorescence spectrophotometer (Fls−980, Edinburgh) with an excitation wavelength of 300 nm. The temperature-dependent photocurrent measurements were performed at an applied potential of 0.5 V (vs. RHE) under the light illumination (1 Sun, 300 to 2,500 nm).

Commercial PE microfibers (Qinglong plastic factory Co., Ltd., Shandong) with an average length of 3 mm and diameter of 750 μm were used as simulated microplastic pollutants. The microplastics were dispersed in water (number density: 2.4 microfibers per square centimeter) before evaporation and PE conversion. The PET and PLA microplastics were obtained from Suzhan plastic factory Co., Ltd. in Guangzhou, with number density of 2 microparticles per square centimeter. After evaporation and conversion, the PE, PET, and PLA residues were carefully filtered by a nitrocellulose filter membrane (pore size: 0.8 μm) with ethanol and water, followed by drying at 30 °C for 12 h. The morphology and component of the collected microplastic fibers were detected by FTIR microscopy system (Spotlight 200i, PerkinElmer Co., Ltd.). All of the PE, PET, and PLA conversions in sealed reaction cells were conducted under a light irradiation intensity of 2 Sun. 1H-NMR spectroscopy (1H-NMR) spectra were collected on the Bruker AVANCE III HD 600 MHz spectrometer in 10% D2O using the water suppression mode, with dimethyl sulfoxide as the internal standard. The produced hydrocarbons were quantified by a gas chromatograph (GC, 7890B, Ar carrier, Agilent) equipped with a thermal conductivity detector (TCD) and flame ionization detector (FID). Another GC (Techcomp GC-7900, China) equipped with a TDX-01 packed column was employed to determine the amounts of CO2. To analyze the content of CO2 products, excess Ba(OH)2 powders were added into the solution in collecting bottles (Fig. 5C), and the white precipitate could be obtained for weighting. The amount of CO2 in the air and dissolved in the water were deducted. The purification degree of BaCO3 product was tested by inductively coupled plasma−mass spectrometry (ICP−MS) and titration experiment according to GB/T-654-2011 Chemical Reagent-Barium Carbonate. PE conversions by 3D helical RGO/TiO2 reactors and 2D RGO/TiO2 mats were compared under the same reaction conditions in separate setup cells (Fig. 5C). As depicted in SI Appendix, Fig. S28B and Fig. 5D, the PE removal ratio and BaCO3 production of 3D RGO/TiO2 reactors were greater than those of 2D RGO/TiO2 mat. The heating of PE/water from 25 to 80 °C resulted in no mass change of PE or detection of products.

The TEMPO (0.5 mM, Sigma) scavenger was added into water samples before light irradiation, for quenching ·O2− ROS radicals. The evaporation water productions in SI Appendix, Figs. S34 and S35 were collected by condensation. The suppressing of evaporation was conducted by cycling water vapor flow with relative humidity at 100% into the air above TiO2/RGO in the reaction cell. A 100-mL and five-port sealed beaker is used as a humidity generator, containing 50 mL of deionized water and continuously boiling at 100 °C. The hygrometer (S21A, Pengyun Instrument and Meter Co., Ltd.) is plugged into the lid of the sealed beaker for monitoring the relative humidity of the produced water vapor. The water vapor is output from the sealed beaker, pumped by the peristaltic pump (CX100, Shanghai Chuxi Instrument Co., Ltd.) with a flow rate of 60 mL/min, injected to the reaction chamber, and returned from the reaction chamber to the humidity generator through the gas pinelines. The continuous cycling of humidity can avoid the pressure increase in reaction cell.

A transparent glass cuvette is made into a rectangular shape with a height of 0.9 cm (same as the distance between TiO2/RGO and the light window of reaction cell), a width of 5 cm and a length of 5 cm. The cuvette was filled with water vapor by the humidity generator. The light intensity is corrected by the optical power meter (S314C, THORLABS) with an integrated broadband sensor. The optical meter was operated under the vapor-filled cuvette for eliminating the influence of water vapor on the quantification of light intensity. Suppressing evaporation for 2 h and 4 h in SI Appendix, Fig. S35 was conducted by cycling water vapor into reaction cell after processing photocatalysis-evaporation reaction for 2 h and at the beginning of the reaction, respectively. The gas and liquid products in suppressing evaporation experiments were collected in both reaction cell and humidity generator beaker. The structural stability of RGO/TiO2 after the simultaneous PE conversion and water evaporation for 7 d can be found in SI Appendix, Fig. S51. Water contamination was investigated by monitoring water samples collected from underneath water and evaporated water following microplastic complete conversion. The water samples were carefully filtered by nitrocellulose filter membranes and then dried in an ultraclean chamber. When comparing the filter membrane after filtering pure water (SI Appendix, Fig. S52), no microplastic residue was seen on the filter membrane after filtering the microplastic conversion water sample (SI Appendix, Fig. S53). Also, the average element ratio of the membrane after filtering the microplastic conversion water sample (carbon: 85.52%, oxygen: 14.48%) is substantially identical to that after filtering pure water (carbon: 85.57%, oxygen: 14.43%). These results further confirmed that there is no contamination in water caused by microplastic breakdown after total conversion. Besides, the sectional slices of nanofibers at randomly selected positions from outer to inner within the RGO/TiO2 reactor after PE removal were cut for further TEM imaging. The absence of microplastic clogs in the nanochannel architectures was demonstrated by the clean channels within these randomly selected nanofibers (SI Appendix, Fig. S54).

Characterizations.

TEM images were collected using a Tecnai G2 T20 (FEI) operated at 200 kV. HRTEM images. HAADF-STEM images and the elemental maps were taken at TITAN 80-300 (FEI) operated at 300 kV affiliated with ELLS. SEM images were obtained by Inspect F50 (FEI). Atomic force microscopy (AFM) images were taken with SPA-300HV & SPI3800N at tapping mode in air. TGA was conducted using an SDT Q600 under N2 flow and heated at a rate of 10 °C min−1. XPS measurements were performed with ESCALAB MK II. Raman spectra were recorded on a micro-Raman spectrometer with a 514-nm laser (Alpha300RA), as shown in SI Appendix, Fig. S55 A–C. The temperature tests were conducted on the laser confocal Raman system with the temperature-control holder. The heating laser was at 532 nm, with a continuous power-adjusting function. Each heating step was maintained for the same time for 120 s. The fluid flow was controlled by a liquid cycling system containing a pump and pipes (SI Appendix, Fig. S55 B and D). The chip design sketch can be found in SI Appendix, Fig. S56. The mechanical properties of the mats were measured by Keysight UTM150 at 25 °C with a relative humidity of 45%. The width and thickness of each strip were measured using a low digital micrometer. All samples were tested for three times. The surface area (9.6 m2 g−1) and average pore size (13.9 nm) of RGO/TiO2 were measured by an automatic microporous physical adsorption analyzer (ASAP2020 Plus) (BET spectra in SI Appendix, Fig. S57). The vaporization enthalpy was tested by DSC measurement on DSC8000 (PerkinElmer), with a heating rate of 5 K min−1, a gas flow rate of 20 mL min−1, and a temperature range from 30 to 200 °C. The photocatalytic heat change of PE conversion was measured by UV-DSC (DSC3+, Mettler Toledo, Co., Ltd.). The net heat flow was subtracted from the heat flow of water, PE, and RGO/TiO2 under light irradiation. The DSC curves of water/PE and pure water can be found in SI Appendix, Fig. S58. The light irradiation was at 300-nm wavelength with intensity of 987 mW cm−2. The infrared images were recorded by the infrared imager (Ti 9, Fluke). The dye and drug removal were tested by a UV–vis spectrophotometer (Cary 60, Agilent). All the temperature-dependent experiments were conducted by external heating via thermostatic water bath. The details of the confocal imaging system can be found in SI Appendix, Fig. S59.

Supplementary Material

Appendix 01 (PDF)

Movie S1. TEM video recording of rapid vibration of nanofiber.

This work was financially supported by the National Key Research and Development Program of China (No.2022YFA1505700, 2020YFC1511902), the National Natural Science Foundation of China (21975042, 22232003, 12174050, 21725102), the Project of Six Talents Climax Foundation of Jiangsu (XCL-082), and the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Jiangsu Funding Program for Excellent Postdoctoral Talent.

Author contributions

Y.D. and Y.X. designed research; X.M., X.W., K.Y., Y.J., L.G., Z.T., X.R., M.T., X.S., L.S., and Y.S. performed research; X.M., X.W., K.Y., L.G., Y.D., and Y.X. analyzed data; X.M., X.W., Y.D., and Y.X. wrote the paper.

Competing interests

The authors declare no competing interest.

Data, Materials, and Software Availability

All study data are included in the article and/or supporting information. Source data file has been deposited in Figshare (42); the simulation input files have been deposited in GitHub (43).

Supporting Information

This article is a PNAS Direct Submission.
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