High-absorption solar steam device comprising [email protected]2MoO6-CDs: Extraordinary desalination and electricity generation

Journal Pre-proof High-absorption solar steam device comprising Au@Bi2MoO6-CDs: extraordinary desalination and electricity generation Zemin Zheng, Huiyong Li, Xudong Zhang, Hao Jiang, Xuemin Geng, Simin Li, Hongyu Tu, Xinran Cheng, Peng Yang, Yanfen Wan PII:

S2211-2855(19)31005-5

DOI:

https://doi.org/10.1016/j.nanoen.2019.104298

Reference:

NANOEN 104298

To appear in:

Nano Energy

Received Date: 18 October 2019 Revised Date:

12 November 2019

Accepted Date: 13 November 2019

Please cite this article as: Z. Zheng, H. Li, X. Zhang, H. Jiang, X. Geng, S. Li, H. Tu, X. Cheng, P. Yang, Y. Wan, High-absorption solar steam device comprising Au@Bi2MoO6-CDs: extraordinary desalination and electricity generation, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.104298. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

Dr. Peng YANG obtained his Ph.D. (2010) from Pierre and Marie Curie University and joined Yunnan University since 2016 as an associate professor. His research is focused on the noble metal related 2D materials and their applications in photoelectronics and energy conversion.

Dr. Yanfen WAN obtained her Ph.D. (2013) from the Pierre and Marie Curie University in France. She is currently an associate professor at the School of Materials Science and Engineering, Yunnan University, China. She received the award of “Thousand Youth Talents Plan of Yunnan Province” in 2017. Her research focuses on noble metal-based/carbon-based nanocomposites and their multifunctional application in solar desalination and energy conversion for generator.

Zemin Zheng received his B.S. degree from University of Jinan (Jinan, China) in 2016, and now he is a M.S. student in Yunnan University (Kunming, China). His current research mainly focuses on semiconductor/carbon-based composites and their application in heat management and photothermal power generation.

Huiyong Li obtained his B.S. in Liaoning University of Technology in 2016. As a master candidate, he takes research in Yunnan University now. His current research focuses on the synthesis of noble metal nanoparticles, and the photocatalytic and

photothermal properties of the semiconductor heterojunction materials.

Xudong Zhang obtained his B.S. in East China Jiaotong University in 2017, and now he is currently a M.S. student in Yunnan University. His research is centered on the synthesis and application of advanced materials, and using atomic force microscopy.

Hao jiang is currently a M.S. student in Yunnan University, China. His research focus on the construction of solar thermal evaporation systems, the preparation and characterization of biomass-carbon photothermal materials.

Xuemin Geng is currently a second-grade M.S. student in Yunnan University. Her research focuses on the fabrication of biomass carbon material and their composites for applying in seawater desalination and energy conversion.

Simin Li is currently a M.S. student in Yunnan University. His major research focused on solar steam generation and biomass-derived carbon dots.

Hongyu Tu is currently a M.S. student in Yunnan University. His main research focuses on carbon dots and photothermal materials.

Xinran Cheng is currently an undergraduate in Yunnan University. Her research focuses on green chemistry and FDTD simulation of the optical properties of nanoparticles.

High-absorption

solar

steam

device

comprising

Au@Bi2MoO6-CDs:

extraordinary desalination and electricity generation

Zemin Zheng1, Huiyong Li1, Xudong Zhang, Hao Jiang, Xuemin Geng, Simin Li, Hongyu Tu, Xinran Cheng, Peng Yang* and Yanfen Wan*

School of Materials Science and Engineering, Yunnan Key Laboratory for Micro/Nano Materials & Technology, Yunnan University, Kunming 650091, China

Keywords: 3D nanocomposites, photothermal conversion, solar steam, seawater desalination, electricity generation

Solar steam generation as a sustainable water purification technology offers remarkable potential to address global drinking-water shortage. In this work, inspired by the marine benthos coral with dendritic structure, an innovative 3D nanocomposite Au@Bi2MoO6-carbon dots (CDs) solar steamer has been prepared: it exhibits outstanding light absorption of 70%, a photothermal conversion efficiency of 97.1% and an evaporation rate of 1.69 kg m-2 h-1 under one sun illumination. This solar steam generator can be used for seawater desalination and has shown an ion removal rate of ~100% in the purification of metal ions, which effectively reduces the ion

concentration in seawater. In addition, a solar thermoelectric generator (STEG) has been prepared. Compared with the blank thermoelectric generator, the solar thermoelectric generator coated with Au@Bi2MoO6-CDs shows better performance. The maximum recorded electrical output of the STEG is 97.4 µW cm-2, which is a promising potential solution to the world energy crisis. The unique design of biomimetic nanocomposite materials can help achieve better performance for the solar steamer and electricity generator compared to that of a single-component material. This kind of hybrid material can thus offer pathways to the development of solar thermal technologies applicable in various energy-related applications. 1. Introduction In recent decades, sanitary water resources and sustainable energy sources have emerged as two urgent issues of primary importance. The former is more severe in nations suffering from water shortage, and it is predicted that nearly two-thirds of all countries will fall into the predicament of lack of fresh water by 2025.[1, 2] Water scarcity will inevitably bring about global issues such as conflict over water supply between countries, economic depression and energy shortage. The global energy demand is also of concern, because fossil energy depletion and environmental pollution owing to the use of fossil fuels plague humans. At present, the total annual energy consumption of the world is about 13.4 billion tons of standard coal, of which 85% comes from fossil sources such as oil, natural gas and coal. Currently, most of the electricity is also dependent on fossil fuels. Thus, the need for another source of energy with potential for abundant long-term storage is critical. Known as blue energy,

the ocean could be the best alternative as it provides inexhaustible, clean and renewable energy for humans,[3] including microwave energy and tidal energy.[4] In order to alleviate fresh water scarcity and the impending energy crisis, researchers have paid significant attention to using blue resources for clean water and power generation,[5-15] substantial progress has already been made in this regard.[16-19]

Most hydrologic regimes on the earth are not freshwater resources. Hence, efficient, large-scale applicable technologies need to be developed for water purification. However, owing to the poor optical absorption of seawater and acute heat loss, the natural light-to-vapor conversion efficiency is too low to generate fresh water supply to meet the demand. Compared to traditional desalination technologies including reverse osmosis[2, 20] and multi-stage flash,[21] solar steam generation for replenishing terrestrial clean-water supply is emerging as a promising environment-friendly solution, owing to the clean, inexhaustible and abundant nature of solar energy for a sustainable hydrologic cycle. Photothermal materials could realize sequential energy conversion by harvesting solar energy. These materials absorb solar irradiation and convert it into heat. The solar steam hence produced can generate fresh water and thermoelectric power. The key to efficient solar energy conversion is to make optimal use of sunlight, for which the absorption needs to be maximized and while simultaneously minimizing the reflectance of the light. To realize this goal, selecting appropriate materials and constructing a smart evaporation setup are essential to achieve the desirable optical, thermal and wetting properties for efficient

solar-to-vapor conversion. Thus, researchers have attempted to design and fabricate special structures, both artificial as well as biomimicking ones.

Many natural species have evolved and optimized their internal structure and unique external geometric architecture to manage their energy demand for sustenance.[22] For example, the rough black skin of polar bears under their white hair can absorb sunlight efficiently and convert it to thermal energy, which is crucial to maintaining their internal body temperature at a steady 37 ℃ in severely cold climates.[23] Coral-like nanostructures undertake significant light scattering, enhancing the light-harvesting efficiency.[24] The West African Gaboon viper (Bitis rhinoceros) has black spots on its dorsal scales and geometric patterns on its skin, which can impede light reflection and improve the absorption of sunlight.[25] Inspired by these biostructures, hierarchical MXene nanocoatings have been fabricated, which have shown broadband light absorption of up to 93.2% and realized improved photothermal performance at equilibrium temperatures of up to 65.4 ℃ under one sun illumination, guaranteeing a high steam-generation performance of 1.33 kg m-2 h-1.[22]

Photothermal materials can usually be roughly classified into three categories in terms of the various photothermal conversion mechanisms involved. The plasmonic metal is a representative material that exhibits the localized surface Plasmon resonance (LSPR) effect. The three sequential phenomena induced by the LSPR effect are

near-field coupling enhancement, hot electron generation and photothermal conversion. Many metals have been used for effective solar steam generation, such as Au, Ag, Al, and Pt.[1, 26-29] An unique material with confined assemblages of silver nanoparticles in rod-shaped tubular spaces was reported to show broadband absorption in the visible and near-IR spectrum for efficient solar steam generation; when the power of the incident light increased from one to five sun, the evaporation rate of water increased from 1.38 to 5.21 kg m-2 h-1.[1] Lei Miao et al.[27] synthesized Ag NPs by the liquid phase reduction method, dispersed them in water to test the water evaporation rate, and achieved a photothermal conversion efficiency of 82.45%. Secondly, the photothermal effect of semiconductor materials is displayed by the process wherein excited electrons return to their low-level states, which is accompanied by local heating of the lattice, wherein non-radiative relaxation-based energy release occurs in the form of phonons. Many metal oxides and chalcogenides such as MoS2,[30] Cu7S4,[31] black TiOx,[11] narrow-band gap Ti2O3[32] and MoO3-x have been adopted for photothermal vaporization.[33] Young-Shin Jun et al.[30] used MoS2 nanosheets as a photothermal material, and its photothermal conversion efficiency reached 82% at a light intensity of 5.35 kW m−2. Tom Wu et al.[32] used nano-scale Ti2O3 as a photothermal material to obtain a photothermal conversion efficiency of 92%. The third type comprises carbon-based photothermal materials dependent on the thermal vibration of molecules. The matchable incident light excites an electronic transition within the molecule from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Owing to this transition, an

electron–phonon coupling could induce the transfer of energy from the excited electrons to the atomic lattice vibration, resulting in light-absorber material ramp-up. Various carbon-based materials have exhibited excellent light absorption and light-to-heat conversion effect, such as graphene, graphene oxide, carbon quantum dots, carbon nanotubes and carbonized biomass.[34-36] Huang et al.[37] carbonized boxwood to obtain carbon particles, which were loaded on boxwood to obtain a two-layer system. The system achieved 65% photothermal conversion efficiency in one sun light. Zhu et al.[38] used graphene oxide (GO) as a precursor to prepare a hydrogel by compounding with sodium alginate. The work used a composite hydrogel as a photothermal material and achieved a photothermal conversion efficiency of 83%.

Despite extensive research on different materials, single-component materials have not proven sufficient to satisfy the requirements due to their limited photothermal conversion properties. It is thus worth investigating hybrid materials endowed with synergetic optical properties and higher conversion efficiency, fabricated by smarter design and tailoring. Quan et al.[39] combined the MoS2 nanosheet with single-walled carbon nanotubes to obtain a SWNT–MoS2 film. In comparison to the SWNT film that showed a photothermal conversion efficiency of 58.9%, the hybrid film exhibited an improved efficiency of 91.5%. Zhou et al.[40] reported Au-graphene oxide hybrid structures exhibiting increased absorption in a broadband close to the infrared region. Owing to coupling effects between Au nanorods and GO nanosheets, the solar steam

generation efficiency reached 84.1% under one sun illumination, corresponding to an approx 20% enhancement compared to the performance of a GO dispersion alone. However, for the composite formed by coupling of Ag and graphene,[41] the heat conversion efficiency was still very low. Thus, it remains a big challenge to explore novel hybrid nanomaterials and composites for efficient photothermal energy conversion.

In this work, to achieve efficient exciton separation and improved photothermic conversion, a heterogeneous composite was designed and prepared using an optimal protocol

to

fabricate

a

high-efficiency

solar

steam

system.

A

3D

mesoscopic-nanometer (meso-nano) solar absorber material embracing three types of nanocomponents of plasmonic NPs, semiconductor and biomass-derived CDs is reported as a result. Coral-like Au@Bi2MoO6-CDs heterostructures were synthesized for the first time by wrapping plasmonic Au nanobipyramids (NBPs) inside 3D Bi2MoO6 spheres with numerous 2D nanosheets stacked outside. Au NBPs were sparsely scattered on the surface of Bi2MoO6 coral-shaped bulk simultaneously with fragmentary CDs attached to Bi2MoO6 (Figure 1). This 3D superstructure offers synergetic high efficiency in both broadband light-absorption and solar-to-heat conversion, which can be attributed to multiple internal reflections of light, benefiting from the 3D cavities formed across the 2D nanosheets on the outer shell of Bi2MoO6 spheres and the banded coral structure. For this design, the high surface areas and mesoporous structures of the meso-nano heterostructures supported multiple

reflections. Compared with pure Au, Bi2MoO6 and C, this ternary composite provided better avenues for charge transfer to facilitate photo generated electron–hole pairs to improve the ability of energy conversion and surficial geometry that offered numerous active cavities to promote solar light harvesting, and the 3D heterogeneous material made full use of the various advantages of the three typical photothermic components. The main reason behind the enhanced photothermal conversion was the heterojunction formed between Au, Bi2MoO6 and C, which effectively facilitated the separation of excitons and transfer of electrons. Our results have demonstrated that Au@Bi2MoO6-CDs composites attained a photothermal conversion efficiency of 97.1% under one sun, accompanied by a water evaporation rate of 1.69 kg m-2 h-1. ICP results showed that the desalinated seawater underwent ~100% ion removal, which effectively reduces the ion concentration in seawater. In addition, we deposited the integral materials on a commercial thermoelectric generator to make a solar thermoelectric generator, which displayed an improved electric output power of 97.4 µW cm-2. This ternary meso-nano composite provided a synergistic concept of constructing a new type of multifunctional material to harvest renewable solar energy for desalination and energy generation that could offer clean water and energy.

Figure 1. Schematic of the synergistic photothermal water evaporation process based on Au@Bi2MoO6-CDs composite. 2. Results and discussion 2.1 Hierarchical dendritic structure of corals Light is one of the main factors determining coral growth. Corals prefer to grow in shallow waters. As the shallow water area is where the sunlight penetrates, it is beneficial to the photosynthesis of the coral symbiotic algae, thus providing the oxygen and nutrients needed for coral growth. However, as the depth of the seawater increases, the intensity of the light decreases, which inhibits the growth of the coral. The corals on the seabed have evolved to exhibit a hierarchal dendritic structure in order to make better use of sunlight. At the same time, the branches of the coral are covered with corrugated stripes. These structures increase the area for absorption of sunlight, while the corrugated structure enhances the multi-level reflection of light, resulting in higher solar absorption.

2.2 Biomimetic composites with broadband light absorption Inspired by the hierarchal dendritic structure of corals, in this work, we chose three typical photothermic components, such as plasmonic metal, semiconductor and carbonized biomass to construct a heterogeneous composite material. In this study, coral-like Au@Bi2MoO6-CDs heterostructures were prepared for the first time by wrapping plasmonic Au NBPs inside Bi2MoO6 spheres where many 2D nanosheets were stacked outside, as well as sparsely scattering CDs on the surface of the coral-shaped Bi2MoO6 bulk. The overall geometric shape involved two main constituents: the coral-like Bi2MoO6 block combined with hierarchical 3D spheres of 2D Bi2MoO6 nanosheets (Figure 2a). The highlight of this unique coral-like integrated structure lies in the efficient harvesting of solar light owing to the branches covered with streak-like protrusions. At the same time, there were spheres about 100 nm at the external rim of the banded block and lamellar nanomaterials self-grown on the surfaces of the spheres fabricated on the cavities, which increased the multi-level reflection of sunlight, helping achieve a higher light absorption rate.

TEM and SEM images (Figure 2 and 3) exhibit the coral-like Au@Bi2MoO6-CDs block. The morphology consisted of staggered branches and microspheres attached on the top of the branches. Figure 2b,c show the SEM image and element mapping of the branches. As can be seen from the figure, the surface of the branches had many folds and was not smooth. This kind of 3D irregular structure could achieve multiple

reflections under solar irradiation, which is the aim for broadband light harvesting. Figure 2d,e show the SEM images and element mapping of the top part. The top had a spherical structure with uneven size. Further, the surface of the microspheres was full of 2D sheets stacked closely together. The wrinkles on the surface of the branches as well as the nanosheets on the surface of the microspheres increased the surface area of the structure, resulting in higher light absorption. Figure 2f-j shows the elemental mapping for Au, Bi, Mo, O and C, corresponding to Figure 2d, e. The figure indicates that the sample contained five elements: Au, Bi, Mo, O and C, which is consistent with the results of XPS. In order to obtain a more refined morphology of the material, we subjected Au@Bi2MoO6-CDsto TEM (Figure 3). Figure 3a shows the TEM image of the hybrid nanospheres. For the spheroidal Bi2MoO6, the elemental mapping by HAADF is shown in Figure 3b. Au, Bi, Mo, O and C were distributed in the sample uniformly, which is consistent with the XPS results. Figure 3c shows the TEM image of Au NBPs (47.6±0.29×21.1±0.08 nm), with sharp tips on the two ends, which are essential to generate inter tip hot-spots for an enhanced LSPR effect. From these images, it can be concluded that Au NBPs were present in the hybrid sample. In order to determine the location of Au NBPs, we amplified the sample 400,000 times and analyzed its micro morphology (Figure 3d). We did not observe Au NBPs on the cross-sheet surface of the Au@Bi2MoO6-CDs composite, even inside the cavities across these 2D nanosheets. Therefore, we concluded that Au NBPs were wrapped inside the Bi2MoO6 microspheres. Figure 3e shows the HRTEM image of CDs. The SAED diagram further demonstrates the synthesis of CDs.

Figure 2. (a) SEM image of Au@Bi2MoO6-CDs composites. SEM (b) and corresponding mapping images (c) of staggered branches. SEM (d) and corresponding mapping images (e) of microspheres. Elemental mapping (f-j) of Au, Bi, Mo, O, C of microspheres.

Figure 3. (a) TEM image of Au@Bi2MoO6-CDs spheres and (b) corresponding elemental mapping of Au, Bi, Mo, O and C in the selected region. (c) TEM image of

Au NBPs. (d) SEM image of Au@Bi2MoO6. (e) HRTEM and corresponding SAED pattern of CDs. Au@Bi2MoO6-CDs were obtained by the seed growth method combined with the solvothermal method. To demonstrate the chemical composition and oxidation state of the sample, we performed XPS analysis (Figure 4a–f). By roughly scanning the sample, we obtained four elements: O, C, Mo and Bi (Figure 4a). Though it was assumed that Au would be easily discernible in Figure 4a, several scans were needed to obtain traces of Au because of its low content (Figure 4b). The binding energy of Au 4f7/2 is 84.0 eV,[42] which corresponds to zero-valent Au atoms in the Au NBPs. At the same time, the binding energy of Au 4f5/2 is 87.2 eV, which corresponds to Au ion with a valence of +3, which may be due to the presence of HAuCl4 in the precursor. The XPS profile of Bi 4f shows two peaks at the binding energies of 164.5 eV and 159.2 eV, corresponding to Bi 4f5/2 and Bi 4f7/2 of [Bi2O2]2+, respectively (Figure 4c). At the same time, there were four peaks at the binding energies of 160.1 eV, 165.2 eV, 157.9 eV and 163.3 eV, indicating that the Bi-O-C bond was formed between Bi2MoO6 and carbon.[43] Figure 4d shows the XPS profile of Mo 3d. There are two peaks at the binding energies of 235.5 eV and 232.4 eV, which correspond to MoO42-. At the same time, there were two peaks at 234.5 eV and 231.4 eV, corresponding to Mo6+, which proved that the Mo-O-C bond was formed between Bi2MoO6 and carbon. As shown in Figure 4e, the XPS profile of O 1s showed two peaks at 529.5 eV and 532.1 eV, corresponding to C=O and Mo-O bonds, respectively. Meanwhile, at binding energies of 530.8 eV and 532.4 eV, there are two peaks corresponding to

Mo-O-C and Bi-O-C bonds, respectively. The XPS profile of C 1s can be divided into four peaks (Figure 4f). The peak at 289.8 eV corresponds to the C=O bond. The peak at 288.1 eV corresponds to Mo-O-C and Bi-O-C bonds. At the binding energies of 286.1 eV and 284.8 eV, there are two peaks corresponding to C=C and C-C bonds, respectively.

The crystal structure of Au@Bi2MoO6-CDs was characterized by XRD, as shown in Figure 4g. It can be seen from the XRD pattern that we obtained highly crystalline Au and Bi2MoO6, as well as carbon. The peaks of the Au NBPs appear at 38.2°, 44.3° and 64.6°, corresponding to the (111), (200) and (220) crystal faces of Au (PDF#00-004-0784), respectively. The peaks of Bi2MoO6 appear at 28.3°, 32.5°, 46.7°, 55.4° and 58.5°, corresponding to the (131), (200), (202), (331) and (262) crystal faces, respectively (PDF#00-021-0102). At the same time, a bulge can be seen around 20°, which proves that carbon had formed.[44] Figure 4h shows the FTIR spectrum of Au@Bi2MoO6-CDs. A strong infrared peak is visible at 737 cm-1, which corresponds to the asymmetric bending vibration of MoO6. At the same time, the peaks at 561 cm-1, 1111 cm-1 and 1385 cm-1 correspond to the stretching vibration of Bi-O, the stretching vibration of C-O and the stretching vibration of C-C, respectively. A peak appears at 1602 cm-1, owing to C-C vibration. The UV–Vis absorption spectra of CDs, Au@Bi2MoO6 and Au@Bi2MoO6-CDs are shown in Figure 4i. As can be seen from the figure, Au shows good absorbance in the near-infrared region, and its absorption peak appears at 680 nm. The carbon absorption peak appears at 285 nm.

Au@Bi2MoO6-CDs shows no absorption peak, but it exhibits good absorbance throughout the visible region.

Figure 4. XPS profiles obtained for Au@Bi2MoO6-CDs: (a) survey; (b) Au 4f; (c) Bi 4f; (d) Mo 3d; (e) O 1s and (f) C 1s. (g) XRD and (h) FTIR spectra of Au@Bi2MoO6-CDs. (i) UV–Vis absorption spectra of CDs, Au@Bi2MoO6 and Au@Bi2MoO6-CDs. 2.3 Meso-nano composites with outstanding photothermic performance For the purpose of minimum heat loss, an interfacial solar-driven evaporation system was employed wherein the photothermal materials (so-called solar absorber) were separated from the bulk working fluid. The solar absorber received and absorbed solar energy, which it converted into thermal energy that was used to heat up superficial liquid water to generate solar vapor (Figure S1). The scheme for the adoptive solar

steam system is shown in Figure 5a. Au@Bi2MoO6-CDs was transferred to the surface of the filter paper by suction filtration as the light absorber. The filter paper was then loaded on top of gauze-wrapped polystyrene (PS) foam. The foam acted as an insulating layer to separate the absorber paper from the water bulk, which helped reduce the conduction heat loss from the absorber to the water. Under the irradiation of simulated sunlight (1 kW m−2), Au@Bi2MoO6-CDs converted light energy into heat, thereby evaporating the water on the interface. The mass change of water was monitored in real time by an electronic balance, and readings were recorded every 2 min. Temperature changes on the surface of the absorber were monitored by an infrared camera. The test time for each sample was 1 h. Figure 4b and 4c show plots for the mass change and surface temperature change for pure CDs, respectively. Mass change gradually increased as the sample volume increased (Figure 5b). When the amount of CDs was 80 mL, the water evaporation rate of the sample reached 1.53 kg m-2 h-1, with a light-to-heat conversion efficiency of 80.8%. When polyvinyl alcohol (PVA) was used instead of PS, we obtained similar results. When using PS as the heat-insulating layer, the water evaporation rate was a little higher. Figure 5c shows the same tendency: as the amount of CDs increased, the rate of temperature rise on the surface of the absorber also increased. After 1 h of illumination, the temperature of the sample surface reached 38.4 °C in the case of 80 mL CDs. Compared to single-component CDs, the binary hybrid Au@Bi2MoO6 was synthesized to analyze the role of Au NBPs addition. The mass change and surface temperature change for the Au@Bi2MoO6 composite are shown in Figure 5d and 5e. Here, a clear trend is

displayed in that the mass change and temperature rise for two evaporation systems differed from that of the insulating layer PS or PVA. Generally, a system with insulating PS foam exhibits better photothermal performance. Through comparison, it was demonstrated that when the volume of Au@Bi2MoO6 was 20 mL, the water evaporation rate of the sample was 1.58 kg m-2 h-1, with a photothermal conversion efficiency of 84.8%. When PVA was used instead of PS, we obtained the same trend, with the exception of water evaporation rate, which was slightly lower. Similar to CDs, the surface temperature of Au@Bi2MoO6 composite increased more quickly as the amount of sample increased. After 1 h of illumination, the surface temperature of the absorber reached 38.4

(Figure 5e). To further investigate the ternary composite,

we fabricated Au@Bi2MoO6-CDs. Figure 5f and 5g show plots of mass change and surface temperature change for Au@Bi2MoO6-CDs, respectively. In the case of 20 mL Au@Bi2MoO6-CDs, with PS as the thermal insulation layer, the water evaporation rate of the sample was the best, reaching 1.69 kg m-2 h-1, with a photothermal conversion efficiency of 97.1% (Figure 5f). On comparing the water evaporation rate and photothermal conversion efficiency of single-component CDs, binary Au@Bi2MoO6 and ternary Au@Bi2MoO6-CDs (Figure 5h), we concluded that the ternary composite showed the highest photothermal efficiency, which is consistent with the previous mechanism.

Figure 5. (a) Schematic illustration of solar steam generation with thermally insulating layer. Mass changes (b) and corresponding surface temperature changes (c) of CDs under one sun. Mass changes (d) and corresponding surface temperature changes (e) of Au@Bi2MoO6. Mass changes (f) and corresponding surface temperature changes (g) of Au@Bi2MoO6-CDs. (h) Comparisons of water evaporation rate and photothermal conversion efficiency of CDs, Au@Bi2MoO6 and Au@Bi2MoO6-CDs.

The aforementioned results show that the water evaporation rate increased as the sample volume increased. However, when there is a critical value of volume, and the amount of sample is greater than the critical volume, the water evaporation rate

decreased. PVA was used as the heat insulation layer. When the amount of Au@Bi2MoO6-CDs was 20 mL, the maximum water evaporation rate was 1.45 kg m-2 h-1. On the other hand, if the amount continued to increase to 40 mL, the rate decreased to 1.27 kg m-2 h-1 (Figure 5f). Thus, 20 mL was regarded as the most suitable volume at which the NPs completely covered the filter paper to achieve a high water evaporation rate. When the amount of Au@Bi2MoO6-CDs increased continuously, the excess NPs inevitably thickened the solar absorber layer, which hindered the water transport, resulting in decrease in the water evaporation rate. Therefore, 20 mL was the optimum amount of Au@Bi2MoO6-CDs. In addition, we studied the effect of different insulation layers on the water evaporation rate. The corresponding infrared photo is shown in Figure S2. It can be concluded from Figure 6a that PS foam was the best insulation material in this evaporation system.

The stability of the material is a key factor in practical applications. We tested the same sample six times and the results showed that the ternary material offers good cycle stability (Figure 6b). Figure 6c shows the reflectance of Au@Bi2MoO6-CDs. The corresponding transmittance is shown in Figure S3. From the reflectance spectra , we can draw the conclusion that Au@Bi2MoO6-CDs can achieve high light absorption of 70%. In terms of real-life applications, it is imperative to demonstrate good seawater desalination ability compared to that of deionized water. Using homemade seawater (Table S1), the water evaporation rate of the same sample reached 1.64 kg

m-2 h-1 (Figure 6d), accompanied by a 96.3% photothermal conversion efficiency. To meet real-world demands, we tested the solar steam setup outdoors to examine its water evaporation rate, as shown in Figure 6e. The system was placed outside for 12 h in the daytime, and 50 mL of purified water was collected to achieve a photothermal conversion efficiency of 0.87 L m-2 h-1. The concentrations of Na+, K+, Ca2+ and Mg2+ in seawater and purified water were further tested by ICP (Figure 6f). The image illustrates that the ternary composite material exhibited outstanding seawater desalination ability, as the ion removal rate reached up to ~100%.

Figure 6. (a) Mass changes of Au@Bi2MoO6-CDs for different insulation layers under one sun illumination. (b) Cycling performance of Au@Bi2MoO6-CDs. (c) Reflectance spectra of Au@Bi2MoO6-CDs. (d) Mass changes of Au@Bi2MoO6-CDs for seawater and deionized water. (e) Photograph of outdoor test setup. (f) Salinity comparison before and after solar-to-heat seawater desalination. 2.4 Photothermal mechanism of the ternary hybrid

A schematic diagram of the photothermal mechanism of the Au@Bi2MoO6-CDs ternary hybrid is shown in Figure 7. Bi2MoO6 is a layered N-type semiconductor, and the [Bi2O2]2+ layer and the MoO42− layer are alternately arranged. Due to its remarkable intrinsic properties, Bi2MoO6 shows unique photothermal properties. When Bi2MoO6 is excited by photons larger than the band gap energy, electron–hole pairs are generated. However, lower electron mobility and rapid recombination of carriers still hinder their photothermal performance. We hope to find ways to reduce the electron–hole pair recombination to improve the photothermal properties of Bi2MoO6. The band gap of Bi2MoO6 could be adjusted using different modification strategies to improve its photothermal conversion efficiency. Plasmonic metals are effective materials for adjusting the band gap of Bi2MoO6. The Schottky barrier is formed between plasmonic metal NPs and the semiconductor, which can effectively promote interface electron transfer, thereby significantly preventing carrier recombination.[45] At the same time, plasmonic metal NPs can respond to visible light due to LSPR effects. Among these plasmonic metals, Au has been the focus of research due to its low toxicity and good environmental compatibility. Moreover, biomass carbon materials show good applicability in seawater desalination, bioimaging and photothermal therapy due to their low cost, good biocompatibility and environmental friendliness. Carbon material is also an effective material for adjusting the band gap of Bi2MoO6. On the one hand, the presence of Mo-O-C bonds and Bi-O-C bonds promotes electron transfer from Bi2MoO6 to the carbon layer and suppresses recombination of electron-hole pairs.[43] On the other hand, the carbon

adsorbed on Bi2MoO6 enhances the absorption in the visible region, whereas the absorption range of pure Bi2MoO6 is less than 460 nm. As photothermal materials, Au, Bi2MoO6 and CDs have their own advantages and disadvantages. In this work, we combined Au, CDs and Bi2MoO6 to obtain a meso-nano composite with higher photothermal conversion efficiency. The addition of Au and CDs effectively inhibited the recombination of electron–hole pairs, thereby enhancing the photothermal properties of Bi2MoO6. The experimental results show that the photothermal conversion efficiency of Au@Bi2MoO6-CDs was far higher than that of the single components, and the photothermal conversion efficiency reached 97.1%.

Figure 7. Energy band diagram of Au@Bi2MoO6-CDs (a) before and (b) after contact. 2.5 Thermoelectric power generation

In addition to solar steam generation for replenishing clean water supply, thermoelectric power generation is also of paramount importance. Parallel to the light-to-thermal energy conversion by the photothermal materials, the generated heat could realize immediate sequential power generation. The photothermal materials were used to coat a thermoelectric heating sheet to obtain a solar thermoelectric device

for

better

thermoelectric

performance.

Figure

8a

shows

the

Au@Bi2MoO6-CDs thermoelectric device. The upper surface of the thermoelectric heating sheet was coated with the composite material. The bottom surface of the thermoelectric heating sheet was connected to the water circulation, which is advantageous for enlarging the temperature difference to produce a higher electric power. Based on the Seebeck effect, the conversion efficiency was theoretically proportional to the temperature difference across the device. The temperature difference between the Au@Bi2MoO6-CDs thermoelectric devices produced a voltage output of 203 mV with a voltage output of 88 mV for the blank device (Figure 8b). Figure 8c shows an infrared image of the thermoelectric device, exhibiting the surface temperature distribution of the device sheet without light and with light, respectively. The system eventually produced a temperature difference of 9.4 °C. In addition, in order to verify the durability and stability of the thermoelectric device, we performed a repetitive experiment on the system (Figure 8d). The test results showed that the Au@Bi2MoO6-CDs thermoelectric device has good cycle stability. By controlling the illumination, we could control the output and shutdown of the electrical energy, where the light acts as a switch for the electrical energy output. The corresponding current

output changed with time. As shown in Figure 8e, the temperature difference of the Au@Bi2MoO6-CDs thermoelectric device produced a current output of 31 mA, compared with a current output of 12.5 mA for the blank device. At the same time, we calculated the maximum electrical output power of the device under one sun illumination (Figure 8f). The maximum electric output power of the device reached 97.4 µW cm-2, which is higher than the reported power of 60 µW cm-2,[46] offering a promising avenue to solving the energy crisis.

Figure 8. (a) Photograph of the integrated thermoelectric device. (b) Voc of the bare generator

and

Au@Bi2MoO6-CDs

generator.

(c)

Infrared

image

of

the

Au@Bi2MoO6-CDs generator. (d) Cycling performance of the Au@Bi2MoO6-CDs generator. (e) Ioc of the bare generator and Au@Bi2MoO6-CDs generator. (f) I–V and P–V curve obtained for the Au@Bi2MoO6-CDs generator. 3. Conclusion In summary, inspired by corals with multi-branched structure which has good light absorption properties, the biomimetic branched Au@Bi2MoO6-CDs was successfully

synthesized by a seed growth–solvent thermal method. The addition of Au and CDs inhibited the recombination of electron–hole pairs, which enhanced the photothermal properties of Bi2MoO6. The ternary composite achieved a water evaporation rate of 1.69 kg m-2 h-1 with a photothermal conversion efficiency of 97.1%. Au@Bi2MoO6-CDs showed good stability and the sample maintained a high water evaporation rate even after repeated tests. Furthermore, the purified water underwent ~100% ion removal, which effectively reduces the ion concentration in seawater. In summary, the use of ternary materials in thermoelectric power generation devices enables efficient thermoelectric enhancement. This work thus provides a new idea for the preparation of multiple photothermal materials. It also addresses the two urgent problems of freshwater shortage and energy crisis and provides a new pathway toward energy conversion. 4. Experimental section 4.1 Synthesis of Au nanobipyramids (NBPs) Au NBPs were prepared by a previously reported method.[47] First, CTAC (0.1 mol L−1, 5 mL), HAuCl4 (5 mmol L−1, 0.5 mL) and Na3Ct (0.1 mol L−1, 0.5 mL) were mixed with 4.5 mL of deionized water and stirred well. Then, NaBH4 (25 mmol L−1, 0.25 mL) was added and the mixture was vigorously stirred for 2 min. Finally, the mixture was incubated at 80

for 4 h. The Au seeds were considered synthesized

when the solution changed from bright yellow to red. The growth solution was a mixed solution comprising CTAB (0.1 mol L−1, 10 mL), HAuCl4 (0.01 mol L−1, 0.5

mL), AgNO3 (0.01 mol L−1, 0.1 mL), HCl (1 mol L−1, 0.2 mL) and AA (0.1 mol L−1, 80 µL). 0.1 ml of the seed solution was added to the growth solution under vigorous stirring for 10 min. Subsequent aging for 2 h helped obtain Au NBPs, which were dispersed in water. 4.2 Preparation of Au@Bi2MoO6 Au@Bi2MoO6 was synthesized via epitaxial growth. First, 0.21 g of Bi(NO3)3•5H2O and 0.05 g of Na2MoO4•2H2O were dissolved in 5 mL of ethylene glycol, which were labeled as Solution A and Solution B, respectively. Solution B and 30 mL of ethanol were slowly added to Solution A under vigorous stirring for 15 min. Following this, an appropriate amount of Au NBPs was added to the mixed solution and stirred well, and transferred to a Teflon-lined stainless steel autoclave. The mixed solution was reacted at 160

for 10 h, followed by cooling to room temperature to obtain

Au@Bi2MoO6. The NPs were centrifuged twice with ethanol and then dispersed in water. 4.3 Synthesis of CDs and preparation of Au@Bi2MoO6-CDs system CDs were synthesized by a one-step hydrothermal method using coffee grounds as raw materials, provided by the Coffee House of Yunnan University. First, the coffee grounds were pulverized and screened through a 100 mesh. Then, 3 g of coffee grounds was dispersed in 60 mL of deionized water under vigorous stirring for 30 min. Subsequently, the mixed solution was transferred to a Teflon-lined autoclave. The temperature was raised to 180

and held for 20 h, followed by cooling to room

temperature. The mixed solution was centrifuged at 12,000 rpm for 10 min. The supernatant comprised the CDs, which were collected for further use.

In order to obtain the aqueous phase system containing Au@Bi2MoO6-CDs, 20 mL of CDs and 30 mL of Au@Bi2MoO6 were mixed. The mixture was stirred at room temperature for 12 h and then allowed to stand for 2 h to obtain Au@Bi2MoO6-CDs NPs. 4.4 Characterization SEM images were taken on a field emission scanning electron microscopy instrument (FEI Quanta-200) operated at 5 kV. TEM and HRTEM images were obtained using a JEM-2100 microscope with an accelerating voltage of 200 KV. Powder X-ray diffraction patterns were obtained by a Rigaku UltimaIV X-ray diffractometer with Cu Kα radiation. XPS analysis was performed on a photoelectron spectrometer (K-Alpha+). FTIR spectra were obtained on a Nicolet iS10 FT-IR spectrometer. Absorption spectra of the samples were obtained using an ultraviolet Vis-NIR spectrometer UV-6300 (200–1100 nm). Transmittance and reflection was analyzed using ARM160. Electrical tests were conducted using a KEITHLEY 2400. The concentrations of salt ions were determined by a Perkin Elmer Nexion 300 inductively coupled plasma mass spectrometry (ICP-MS) instrument. 4.5 Water evaporation tests

The water evaporation experiment was carried out in a petri dish having a diameter of 3.75 cm, at room temperature (about 25

) and humidity of about 60%. When

preparing the solar evaporator, an appropriate amount of water was first added to the vessel, and then a gauze-wrapped foam was floated on the surface of the water, and finally the sample having a diameter of 3.75 nm was spread on the surface of the foam. During the experiment, simulated light with a radiation intensity of 1 kW m−2 was used as the light source, and the quality of the system was monitored by an electronic balance, which was counted every two minutes. Temperature changes were monitored in real time by an infrared camera. The experiment time was 1 h. Acknowledgment The tests described herein were carried out at the Analytical Test Center of Yunnan University. The authors acknowledge financial support from the National Science Foundation of China (51871196 and 51771170), Yunnan Applied Basic Research Project (2017FB080 and 2018FB090), and joint fund of Yunnan University and Science & Technology Department of Yunnan Province (2019FY003013). References [1] J. Chen, J. Feng, Z. Li, P. Xu, X. Wang, W. Yin, M. Wang, X. Ge, Y. Yin, Nano Lett, 19 (2019) 400-407. [2] M. Elimelech, W.A. Phillip, Science, 333 (2011) 712-717. [3] Z. Shiyou, F. Fang, Z. Guangyou, C. Guorong, O. Liuzhang, Z. Min, S. Dalin, Chemistry of Materials, 20 (2008). [4] X. Cao, Y. Jie, N. Wang, Z.L. Wang, Advanced Energy Materials, 6 (2016). [5] M. Fang, G. Dong, R. Wei, J.C. Ho, Advanced Energy Materials, 7 (2017). [6] M.H. Lee, Y.H. Kang, J. Kim, Y.K. Lee, S.Y. Cho, Advanced Energy Materials, 9 (2019). [7] H. Liu, Z. Huang, K. Liu, X. Hu, J. Zhou, Advanced Energy Materials, 9 (2019). [8] S.D. Tilley, Advanced Energy Materials, 9 (2019).

[9] K. Wang, Y. Hou, B. Poudel, D. Yang, Y. Jiang, M.G. Kang, K. Wang, C. Wu, S. Priya, Advanced Energy Materials, (2019). [10] S. Wu, G. Xiong, H. Yang, B. Gong, Y. Tian, C. Xu, Y. Wang, T. Fisher, J. Yan, K. Cen, T. Luo, X. Tu, Z. Bo, K. Ostrikov, Advanced Energy Materials, 9 (2019). [11] M. Ye, J. Jia, Z. Wu, C. Qian, R. Chen, P.G. O'Brien, W. Sun, Y. Dong, G.A. Ozin, Advanced Energy Materials, 7 (2017). [12] J. Zeng, Q. Wang, Y. Shi, P. Liu, R. Chen, Advanced Energy Materials, (2019). [13] J. An, Z.M. Wang, T. Jiang, X. Liang, Z.L. Wang, Advanced Functional Materials, 29 (2019). [14] Y. Bai, L. Xu, C. He, L. Zhu, X. Yang, T. Jiang, J. Nie, W. Zhong, Z.L. Wang, Nano Energy, 66 (2019). [15] G. Liu, H. Guo, S. Xu, C. Hu, Z.L. Wang, Advanced Energy Materials, (2019). [16] X. Li, J. Tao, X. Wang, J. Zhu, C. Pan, Z.L. Wang, Advanced Energy Materials, 8 (2018). [17] T.X. Xiao, X. Liang, T. Jiang, L. Xu, J.J. Shao, J.H. Nie, Y. Bai, W. Zhong, Z.L. Wang, Advanced Functional Materials, 28 (2018). [18] M. Xu, S. Wang, S.L. Zhang, W. Ding, P.T. Kien, C. Wang, Z. Li, X. Pan, Z.L. Wang, Nano Energy, 57 (2019) 574-580. [19] S.L. Zhang, M. Xu, C. Zhang, Y.-C. Wang, H. Zou, X. He, Z. Wang, Z.L. Wang, Nano Energy, 48 (2018) 421-429. [20] R. Verbeke, V. Gómez, I.F.J. Vankelecom, Progress in Polymer Science, 72 (2017) 1-15. [21] A.D. Khawaji, I.K. Kutubkhanah, J.-M. Wie, Desalination, 221 (2008) 47-69. [22] K. Li, T.H. Chang, Z. Li, H. Yang, F. Fu, T. Li, J.S. Ho, P.Y. Chen, Advanced Energy Materials, (2019). [23] T. Stegmaier, M. Linke, H. Planck, Philos Trans A Math Phys Eng Sci, 367 (2009) 1749-1758. [24] Y.-A. Lu, T.-H. Chang, S.-H. Wu, C.-C. Liu, K.-W. Lai, Y.-C. Chang, Y.-C. Chang, H.-C. Lu, C.-W. Chu, K.-C. Ho, Nano Energy, 58 (2019) 138-146. [25] M. Spinner, A. Kovalev, S.N. Gorb, G. Westhoff, Sci Rep, 3 (2013) 1846. [26] M. Gao, C.K. Peh, H.T. Phan, L. Zhu, G.W. Ho, Advanced Energy Materials, 8 (2018). [27] H. Wang, L. Miao, S. Tanemura, Solar RRL, 1 (2017). [28] L. Zhou, Y. Tan, J. Wang, W. Xu, Y. Yuan, W. Cai, S. Zhu, J. Zhu, Nature Photonics, 10 (2016) 393-398. [29] Y.L. Mingwei Zhu, Fengjuan Chen, Xueyi Zhu, Jiaqi Dai, Yongfeng Li, Zhi Yang, Xuejun Yan, Jianwei Song, Yanbin Wang, Emily Hitz, Wei Luo, Minhui Lu, Bao Yang, and Liangbing Hu*, Advanced Energy Materials, (2017). [30] D. Ghim, Q. Jiang, S. Cao, S. Singamaneni, Y.-S. Jun, Nano Energy, 53 (2018) 949-957. [31] X. Huang, W. Zhang, G. Guan, G. Song, R. Zou, J. Hu, Accounts of Chemical Research, 50 (2017) 2529-2538. [32] J. Wang, Y. Li, L. Deng, N. Wei, Y. Weng, S. Dong, D. Qi, J. Qiu, X. Chen, T. Wu, Adv Mater, 29 (2017). [33] D. Ding, W. Huang, C. Song, M. Yan, C. Guo, S. Liu, Chem Commun (Camb), 53 (2017) 6744-6747. [34] P. Mu, Z. Zhang, W. Bai, J. He, H. Sun, Z. Zhu, W. Liang, A. Li, Advanced Energy Materials, 9 (2019). [35] J. Xu, F. Xu, M. Qian, Z. Li, P. Sun, Z. Hong, F. Huang, Nano Energy, 53 (2018) 425-431. [36] H. Wang, X. Mi, Y. Li, S. Zhan, Adv Mater, (2019) e1806843. [37] X. Luo, C. Huang, S. Liu, J. Zhong, International Journal of Energy Research, 42 (2018) 4830-4839. [38] X. Hu, W. Xu, L. Zhou, Y. Tan, Y. Wang, S. Zhu, J. Zhu, Advanced Materials, 29 (2017). [39] X. Yang, Y. Yang, L. Fu, M. Zou, Z. Li, A. Cao, Q. Yuan, Advanced Functional Materials, 28 (2018).

[40] J. Zhou, Y. Gu, Z. Deng, L. Miao, H. Su, P. Wang, J. Shi, Sustainable Materials and Technologies, 19 (2019). [41] K. He, Z. Zeng, A. Chen, G. Zeng, R. Xiao, P. Xu, Z. Huang, J. Shi, L. Hu, G. Chen, Small, 14 (2018) e1800871. [42] M.T. Camci, B. Ulgut, C. Kocabas, S. Suzer, ACS Omega, 2 (2017) 478-486. [43] Y. Sun, J. Wu, T. Ma, P. Wang, C. Cui, D. Ma, Applied Surface Science, 403 (2017) 141-150. [44] A. Eiad-ua, B. Jomhataikool, W. Gunpum, N. Viriya-empikul, K. Faungnawakij, Materials Today: Proceedings, 4 (2017) 6153-6158. [45] L. Guo, Q. Zhao, H. Shen, X. Han, K. Zhang, D. Wang, F. Fu, B. Xu, Catalysis Science & Technology, 9 (2019) 3193-3202. [46] X. Zhang, W. Gao, X. Su, F. Wang, B. Liu, J.-J. Wang, H. Liu, Y. Sang, Nano Energy, 48 (2018) 481-488. [47] J. Chang, A. Zhang, Z. Huang, Y. Chen, Q. Zhang, D. Cui, Talanta, 198 (2019) 45-54.

Highlights · Au@Bi2MoO6-CDs were used as a powerful photothermal conversion material for the first time. · Coral-like Au@Bi2MoO6-CDs showed a solar evaporation rate of 1.69 kg m-2 h-1 with photothermal efficiency of 97.1% under one sun. · The solar evaporator achieved cycling and stability for deionized water and seawater. · The system showed remarkable desalination ability with an ion removal rate of ~100%. · The Au@Bi2MoO6-CDs-based solar thermoelectric generator showed enhanced performance with electric output power of 97.4 µW cm-2.

Declaration of interests The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

This manuscript has not been published or presented elsewhere in part or in entirety and is not under consideration by another journal. We have read and understood your journal’s policies, and we believe that neither the manuscript nor the study violates any of these. There are no conflicts of interest to declare.