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Dual-phase molybdenum nitride nanorambutans for solar steam generation under one sun illumination
Author’s Accepted Manuscript Dual-Phase Molybdenum Nitride Nanorambutans for Solar Steam Generation under One Sun Illumination Lin Zhu, Lei Sun, Hong Zhang, Dengfeng Yu, Hüsnü Aslan, Jinggeng Zhao, Zhenglin Li, Ye Sun, Flemming Besenbacher, Miao Yu www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(18)30972-8 https://doi.org/10.1016/j.nanoen.2018.12.058 NANOEN3306
To appear in: Nano Energy Received date: 15 October 2018 Revised date: 4 December 2018 Accepted date: 18 December 2018 Cite this article as: Lin Zhu, Lei Sun, Hong Zhang, Dengfeng Yu, Hüsnü Aslan, Jinggeng Zhao, Zhenglin Li, Ye Sun, Flemming Besenbacher and Miao Yu, Dual-Phase Molybdenum Nitride Nanorambutans for Solar Steam Generation under One Sun Illumination, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.12.058 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Dual-Phase Molybdenum Nitride Nanorambutans for Solar Steam Generation under One Sun Illumination Lin Zhua,c, Lei Suna, Hong Zhangb, Dengfeng Yub, Hüsnü Aslanc, Jinggeng Zhaob, Zhenglin Lia, Ye Sunb,*, Flemming Besenbacherc,*, and Miao Yua,* a
State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical
Engineering, Harbin Institute of Technology, Harbin 150000, China b
Condensed Matter Science and Technology Institute and Department of Physics, Harbin Institute of Technology,
Harbin 150000, China c
Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Aarhus 8000, Denmark
E-mail: [email protected], [email protected], [email protected] *Corresponding author.
Abstract: Water evaporation and steam production have been recognized to be considerably crucial due to the vast applications, ranging from waste water treatment, water purification, to alternative green energy solutions by water splitting, catalysis, and in-door heating. Albeit the big variety of photothermal conversion materials (PCMs) developed for this purpose, certain drawbacks, e.g. high cost, complicated synthesis, weak/narrow absorbance, bulkiness, and low evaporation rate, have hindered the application potential. Herein, we report the dual-phase molybdenum nitride nanorambutans, synthesized by a facile method, for solar steam generation. Not only the inherent properties, 1 / 21
including strong full-spectrum absorbance, high-efficiency photothermal conversion, and superhydrophilicity, benefit their water evaporation performance, the interconnected open mesopores of the nanorambutans further boost their capability of light harvesting and water/vapor transportation. Solar energy conversion efficiency of 97% under one sun together with excellent cycling stability has been demonstrated. In the desalination systems, integrating with the high salt rejection rate, the nanorambutans film can produce a water evaporation rate as high as 1.70 kg m−2 h−1 with an efficiency of 98%. Besides its compact size, the record-breaking water evaporation performance of these nanorambutans has exceeded the previous best inorganic PCM. This work introduces molybdenum nitride as a new PCM for efficient solar steam generation and all applications that can benefit from highly localized heating from nano to macro scale.
Graphical Abstract
Keywords: molybdenum nitride, photothermal conversion, solar energy, water evaporation, desalination.
1. Introduction Development of adequate photothermal conversion materials (PCMs) upon solar energy harvest has aroused increasing level of interest to meet the rising demands on solar water evaporation applications [13], such as desalination of seawater, water purification and wastewater treatment, 2 / 21
heating of residential water, and so on. A big variety of PCMs, including metallic plasmonic nanoparticles [47], inorganic semiconductors [811], organic polymers [1214], and carbon-based materials (e.g. graphene, aerogel, carbon nanotubes, graphite) [1519], have been successfully fabricated and applied for this purpose. Albeit the promising potentials of these developed PCMs, certain drawbacks have been recognized. For instance, noble metal nanoparticles normally have the unique surface plasmon resonance effect which benefits the photothermal conversion, but are limited by their high cost and low absorbance over the full solar spectrum. In contrast, carbon-based nanomaterials, with controllable one- to three-dimensional structures, indeed possess intrinsic broadband absorption and considerable photothermal conversion capability [16,19]. However, besides the complicated synthesis procedures, carbon materials as well as organic polymers are rather bulky due to their low mass density, which is disadvantageous for water/vapor transportation [12,20]. In this context, compact inorganic PCMs derived from commercially-available raw materials, e.g. various TiOx (x 2) [8,9,11,21], WO2.9 [22], Bi2Se3 [2325], Bi2S3 [26], CoS [27], CoP [28], and Prussian blue [29,30] nanostructures, have been regarded as fascinating alternatives. However, in most cases, the photothermal effect was only employed for antitumor and antibacterial applications by utilizing near-infrared irradiation [31,32], whilst their performance on water evaporation remains unexplored. Few studies, which did investigate water evaporation with compact inorganic PCMs, show only low evaporation rates under one sun (0.1 W cm-2) or natural light (Table S1). In addition to the definitive properties of an ideal PCM, i.e. inherent strong/broadband optical absorption and high photothermal conversion efficiency, an appropriate morphology and a desirable configuration of the resultant heating film also play crucial roles for enhancement of water evaporation. In particular, highly porous PCMs with abundant meso- or macro-pores are preferred. Porous materials have larger light harvesting capacity due to the multiple scattering within the inner structures [8,11] as well as the shorter and more convenient transport path for water evaporation [33]. Molybdenum nitride (MoNx), known for its low cost, mechanical stability and electronic structure analogous to noble metals (Group VIII), has been extensively studied as electrocatalysts, 3 / 21
and electrode materials for energy storage [3438]. However, its potential for photothermal conversion remains unexplored. In this work, we report the first application of dual-phase MoN/Mo2N as an efficient PCM for solar steam generation under one sun (Scheme 1). The resultant particles, referred as nanorambutans, are composed of highly crystalline nanograins and interconnected mesopores, showing significant absorbance over the whole solar spectrum ranging from 200 to 2500 nm. It is revealed that the nanorambutan film possesses potent photothermal water evaporation capability, demonstrating excellent photothermal conversion stability and durability. Especially, in the desalination systems, a water evaporation rate as significant as 1.70 kg m−2 h−1 with high conversion efficiency (98%) and efficient salt rejection are accomplished by the nanorambutans film. Besides its compact size, the record-breaking water evaporation performance of these nanorambutans has exceeded the previous best inorganic PCM. Such dual-phase molybdenum nitride nanorambutans, produced by the cost-efficient and facile synthesis, deliver great promise as a powerful PCM for solar steam generation in real-life applications.
Scheme 1. Schematic illustration of the fabrication of dual-phase MoN/Mo2N nanorambutans and their application for solar steam generation system.
2. Results and discussion The facile fabrication of MoNx nanorambutans is schematically illustrated in Scheme 1. Mo powder was chosen as the starting material. Phase evolution during the synthesis was studied in detail by Xray Diffraction (XRD) patterns. Mo was firstly oxidized and subsequently reacted with ammonium 4 / 21
acetate. The XRD pattern of the as-prepared Mo based precursor was indexed to (NH4)6Mo7O24·4H2O (JCPDS card No. 27-1013), as shown in Fig. S1a. The morphological characterization of the precursor showed uniformly distributed nanoparticles (Fig. S1b). The sample was then nitrified. Successful synthesis of MoNx was confirmed by the XRD pattern (Fig. 1a), showing high crystallization, and two groups of peaks assigned to two MoNx phases, i.e. hexagonal MoN (JCPDS card No. 77-1999) and cubic Mo2N (JCPDS card No. 25-1366), respectively. Refinement analysis on the XRD data in Fig. 1a revealed that the weight fraction of MoN and Mo2N was 55.1% and 44.9%, respectively. The corresponding lattice parameters and atomic coordinates are listed in Table S2.
Fig. 1. Composition and structure of the MoNx sample. (a) XRD pattern, (b) Mo 3d and (c) N 1s XP spectra, (d) N2 adsorptiondesorption isotherms (inset: pore-size distribution) of the MoNx.
X-ray Photoelectron Spectroscopy (XPS) analysis was also performed to explore the chemical structure of the MoNx. The survey spectrum showed obvious signals of Mo, N, O, and C elements (Fig. S2). The high-resolution Mo 3d XP spectrum (Fig. 1b) can be deconvoluted into three doublets. 5 / 21
The peaks at 229.1 and 232.4 eV are corresponding to the MoN bonding in the molybdenum nitride; those at 229.8/233.1 and 236.0 eV are ascribed to Mo4+ and Mo6+ oxidation state, respectively. These oxidation states of Mo are attributed to the slight oxidation of the MoNx surface during the sample preparation for XPS measurement. In the high-resolution N 1s spectrum (Fig. 1c), two individual peaks at 397.9 and 395.0 eV were observed, which are ascribed to MoN. The average particle size of the dual-phase MoNx nanoparticles was of ∼200 nm, as shown in the Scanning Electron Microscopy (SEM) images (Fig. S3). The detailed structure was further determined by Transmission Electron Microscopy (TEM). As shown in Fig. 2, the MoNx particle adopted a rambutan-like morphology, assembled by slightly-curved and well-crystallized nanograins with a width of ca. 1015 nm and length of ca. 3050 nm, respectively (Fig. 2ab). The formation of such delicate structure may lower the system energy. It was obvious that interconnected and open pores were constructed spontaneously during the growth of MoNx nanorambutans. The specific surface area (SSA) and pore volume were measured to be 23.3 m2 g−1 and 0.06 cm3 g−1, respectively (Fig. 1d). Benefiting from the nanograins and rambutan-like structure, the MoNx is rich of 1030 nm mesopores (inset of Fig. 1d). Such a structure provides (1) a high SSA for light harvesting because of abundant interconnected opened structures by which the light is easy to reach the interior, and (2) high-efficiency transportation channels for water provision and vapor release in the solar steam generation.
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Fig. 2. Morphology and structure of the MoNx nanorambutans. (a, b) TEM images of the MoNx, composed by nanograins. HRTEM images of the MoNx nanograins and the corresponding FFT images: (c) MoN phase, (d) Mo2N phase.
Meanwhile, the hydrophilic property was characterized by testing the surface contact angle. The substrate was blank glass with a contact angle of 87 (Fig. S4). The sample was prepared by coating the MoNx powder onto to the blank glass. Initial contact angle of 8.4o was measured at t = 0 s (Fig S5a), i.e. the first moment of the water droplet to touch the MoNx sample surface. This result conforms that the MoNx sample was super-hydrophilic. This is primarily attributed to the intrinsic nature of MoNx material itself [39,40]. Meanwhile, the specific microstructure of the nanorambutans, i.e. the mesoscopic porosity afforded by the nanograins and rambutan-like structure, may result in capillarity to further enhance the hydrophilicity. The water droplet was completely absorbed in the MoNx sample within 3 s (Fig. S5b), indicating that the MoNx may act like a water pump in water evaporation process. The dual phases of the MoNx sample were also identified by high-resolution TEM (HRTEM) images. Both hexagonal MoN as indicated by the (200) plane of 2.48 Å and cubic Mo2N as indicated by the (111) plane of 2.40 Å were observed (Fig. 2cd). The Fast Fourier Transformation 7 / 21
(FFT) images of the diffraction patterns (inset of Fig. 2cd) revealed the single crystal structures of both hexagonal MoN and cubic Mo2N. According to the literature, such well-crystallized nanograins may afford significant heat transfer capability for water evaporation [8]. The MoNx thin film was fabricated by dispersing MoNx powder (10.0 mg) in N-methyl-2pyrrolidone solvent upon ultrasonic treatment for 10 min, then mixing with the poly(vinylidene fluoride) (PVDF) binder at a mass ratio of MoNx:PVDF of 14.3 upon ultrasound for another 10 min, and finally being filtrated and supported via a commercial Nylon66 film (NF, 50 mm in size) and dried at 60 C for 2 h. The thickness of the obtained film was determined by the side-view SEM imaging. As illustrated in Fig. S6a–b, the film surface was smooth and compact, with the MoNx and NF layer thickness of 10 μm and 130 μm, respectively. There were abundant interconnected macro-/meso-pores in the film, which may serve as water and vapor channels for rapid solar steam generation. Both the MoNx film and pure NF were hydrophilic, as water droplet completely flattened and spread out when falling on the MoNx film or pure NF in the contact angle measurements. The Ultraviolet–visible–Near-Infrared (UV–vis–NIR) reflection spectrum of the MoNx film was measured with a Carry5000 UV–vis–NIR spectrophotometer including an integrating sphere (diffusive and specular reflectance). The film presented strong light absorption over the whole solar spectrum ranging from 200 to 2500 nm (Fig. 3a). The diffuse reflectance spectra indicated its broadband and strong adsorption property with an extremely low average reflectance of 4%. The solar absorption of MoNx/NF was calculated to be 96%. Meanwhile, the photothermal conversion capability of the MoNx film was carefully evaluated by an infrared camera (FLUKE Ti25). The results showed sharp temperature response mediated by the MoNx upon the solar light (Fig. 3b). Under solar irradiation of one sun, the temperature of the MoNx film was quickly increased up to 83 °C within 10 s and achieved an equilibrium temperature 90 °C after 60 s (Fig. S7), whereas a pure NF film was only 33 °C under a largely extended irradiation for 470 s. The MoNx showed more rapid and efficient photothermal conversion capability than previously reported PCMs, e.g. oxygen plasma treated graphene aerogel (one sun for 10 s, ~75 8 / 21
o
C) [20], narrow-bandgap Ti2O3 nanoparticles (one sun for 700 s, ~52 °C) [9], hierarchically
nanostructured gel (one sun for 500 s, ~45 °C) [41], and even higher than the photo-electro-thermal effect pet-PGS/GF architecture (one sun for 300 s, ~83.8 °C) [42]. Needless to say, excellent light harvesting and rapid photothermal conversion capability are paramount for efficient solar steam generation. Thus, dual-phase MoNx possess outstanding advantages: (1) the intrinsic property of MoNx endows it an outstanding light absorption property over the whole solar spectrum, (2) wellcrystallized nanograins accelerate the solid-to-liquid interface heat transfer properties, (3) it shows rapid photothermal conversion capability under one sun, (4) MoNx has super-hydrophilic nature and (5) nanosized hierarchical structure with opened and abundant interconnected pores, providing higheffective channels for light, water as well as vapor in the advanced solar steam generation systems. We then explored the performance of the MoNx on water evaporation. Since the localized heat effect can significantly improve the solar vapor generation by concentrating the energy at the solidto-liquid interface for water evaporation instead of heating of the bulk water [43], the experiments were then set up as shown in Fig. S8, where the MoNx/PF film (the MoNx film atop of the PF film) was floating at the water surface. The mass loading of all the MoNx films was fixed at a low amount of 0.85 mg cm−2. All the measurement of the MoNx films was performed under one sun (0.1 W cm-2). In contrast, fresh NF and polystyrene foam (PF) were selected as the control samples. As depicted in Fig. 3c, in a sharp comparison and as expected, the MoNx film delivered a water evaporation rate of 1.42 kg m−2 h−1, which is 2.9 folds higher than that with NF alone (0.49 kg m−2 h−1). Importantly, water evaporation capacity was further enhanced by inducing the PF heat barrier. As shown in Fig. S9, the PF was a thin foam with a thickness of 3 mm and a diameter of 4 cm. Thanks to the hydrophilicity hence capillary force afforded by MoNx and NF, water can be successfully absorbed and supply to the MoNx film. In the presence of PF, the evaporation rate was boosted to 1.69 kg m−2 h−1, which was 3.8 times higher than the case of PF alone (0.44 kg m−2 h−1). The evaporation rate of MoNx is evidently higher than previously reported materials under one sun irradiation, e.g. 9 / 21
1.31 kg m−2 h−1 for reduced graphene oxide and carbon nanotubes [44], 1.32 kg m−2 h−1 for Ti2O3 [9], 0.80 kg m–2 h–1 for TiOx [8], 1.475 kg m−2 h−1 for carbonized mushrooms [45], and the highest, i.e. 1.62 kg m−2 h−1 for vertically aligned graphene sheets membrane [16]. As shown in Fig. S10, the solar-to-water efficiency of MoNx was of 83% (without PF) and 97% (with PF), respectively. In contrast, for the control group of PF or NF only, the efficiency was as low as 12% and 14%. All these results support the high photothermal conversion capability of the MoNx nanorambutans.
Fig. 3. Optical properties and water evaporation performance of the MoNx film under one sun. (a) Solar spectrum and UV−vis−NIR diffuse reflectance spectra of the MoNx film in the range of 200−2500 nm. (b) Time-dependent surface temperature of the MoNx film upon the irradiation, with a pure NF film as comparison. (c) Time-dependent water evaporation rate of different films, including pure NF (pale blue), pure PF (light brown), MoNx (blue), MoNx/PF (red). (d) Cycling stability of the MoNx/PF under one sun. The solar-to-water efficiencies is calculated by subtracting the natural water evaporation efficiency in dark.
It is worthy to mention that, to get more accurate evaluation on the evaporation capability of the samples, all solar-to-water efficiencies presented here were calculated by subtracting the evaporation efficiency (6.8%) of pure water in the dark condition, nevertheless most results in the literature did not exclude the natural evaporation of water. Following such commonly adopted calculation method without excluding this factor, the solar-to-water efficiency of our MoNx nanorambutans can be even 10 / 21
higher than 100% (103.8%). As the stability and reusability of a PCM are vital properties for real-life applications, the cycling stability performance of the MoNx for water evaporation was evaluated. Each cycle was carried out under the same conditions for at least 3 h. As shown in Fig. 3d and S11, the MoNx film can steadily produce water vapor at an average rate of 1.66 kg m−2 h−1 under the solar irradiation of one sun upon the repeated usage cycles for one week. The efficiency can be maintained to 91% after one-week usage. It was found that the MoNx can preserve its activity even after illumination for more than 66 h under one sun. The temperature of the MoNx upon irradiation can be directly visualized and recorded by an infrared camera. As demonstrated in Fig. 4, floating on the water surface of a beaker and irradiated at solar power of one sun, the MoNx generated a localized hot region, with the temperature quickly elevated to 51 C within 1 min and remained stable. In contrast, the temperature of the pure NF with heat barrier was only increased by 3 C in 3 h upon the same irradiation (Fig. S12). The high performance of the MoNx originates from its intrinsic photothermal conversion capability and highly porous morphology for effective heat generation and appropriate heat transport.
Fig. 4. Infrared thermal images of a beaker of water with the MoNx/PF floated on the surface, irradiated with the simulated solar power of one sun for (a) 0 min, (b) 1 min, (c) 30 min.
The MoNx film was also applied in solar desalination systems. As anticipated, the average water evaporation rate of the MoNx was rather high (as high as 1.70 kg m−2 h−1) with a conversion efficiency of 98% (Fig. 5a). The results implied that the presence of salt had little influence on the evaporation efficiency. And the water evaporation rate can be further improved by adjusting the 11 / 21
structure of MoNx particles. Moreover, used MoNx which had been exposed to light thus reached to the high temperature of 90 C as above mentioned in Fig. 3b, was also able deliver a high evaporation rate of 1.63 kg m−2 h−1 with an efficiency of 93%. These results indicate the great stability and reusability of the MoNx. The concentration of original artificial seawater and condensed water was tested by inductively-coupled plasma mass spectrometry (Fig. 5b). Ion concentrations of the condensed water were remarkably reduced by three orders of magnitude as compared with the original artificial seawater. It has fully met the standard for healthy drinking water defined by World Health Organization (WHO) and US Environmental Protection Agency (EPA). Compared with the big variety of reported PCMs, e.g. carbon materials [45,46], single-walled nanotube-MoS2 hybrid [33], noble metal [47], TiOx [8,9], the dual-phase MoNx nanorambutans have a much higher photothermal conversion efficiency for solar steam generation.
Fig. 5. (a) Time-dependent water evaporation rate of artificial seawater: fresh MoNx film (the red line) and the film after illumination for 15 min under one sun (the green line), in the presence of heat barrier. (b) The measured concentration of four primary metal ions in an artificial seawater sample (red) and the clean water which was collected by cooling the water vapor (blue).
3. Conclusions Dual-phase molybdenum nitride nanorambutans with high crystallinity, high thermal stability, and abundant mesopores, have been fabricated through a facile method and applied as a new PCM under one sun. The nanorambutans possess a light harvesting capability as high as 97% over the whole solar spectrum, together with super-hydrophilic nature, rapid and efficient photothermal conversion 12 / 21
capability, and structural compactness while providing kinetically favorable mass transportation channels, superior to most of reported PCMs. Owing to the above-mentioned merits, the MoNx film can deliver a staggering water evaporation rate of 1.69 kg m−2 h−1 under one sun, together with outstanding cycling stability and reusability. Additionally, in solar desalination systems, the water evaporation rate of MoNx film is as high as 1.70 kg m−2 h−1 with high thermal stability as well as high salt rejection performance. There is no doubt that the MoNx acts as one of the most promising candidates for the state of art photothermal evaporation system. In future, in addition to steam generation related applications, such as alternative green energy production, nano/micro steam engines, we expect the MoNx to find use in a wide range of photon mediated local-/interfacialheating functions like antibacterial practices, antifouling, cancer therapy, sterilization, and photocatalysis.
4. Experimental section 4.1. Synthesis of MoNx The MoNx nanorambutans were synthesized by annealing a Mo-based precursor under a 5% ammonia atmosphere. Typically, 0.2 g Mo powder was dispersed in 15 mL ethanol under magnetic stirring for 30 min. Afterwards, 0.7 mL H2O2 (30%) solution was added dropwise to the Mo suspension. After constant stirring for 48 h, the solution turned blue. Then, the precursor solution was mixed with 0.6 g CH3COONH4 for 24 h. The obtained precipitate was collected by centrifuging at 13000 rpm for 10 min, and then dried at 40 oC. Finally, the solid precipitate was annealed at 650 C (heating rate 10 C min−1) under an ammonia atmosphere (5% NH3 and 95% Ar) for 6 h to obtain the molybdenum nitride powder. 4.2. Characterization XRD analysis of the MoNx was carried out on an X-ray diffractometer (Empyrean, Panalytical, The Netherlands) using Cu Kα source and PIXcel3D detector. The experimental data were collected in the range of 30° to 120° and analyzed with Rietveld method by using the General Structure Analysis 13 / 21
System (GSAS) software [4850]. The morphology was characterized by a high-resolution TEM (TECNAI G2 F30 S-Twin (FEI, USA)) at 120.0 kV and a field emission high resolution SEM (Hitachi Limited, Japan) at 1.0 kV. The optical absorption was investigated with a Carry5000 UV– vis–NIR spectrophotometer including an integrating sphere (diffusive and specular reflectance) in the wavelength range of 200-2500 nm. An N2 adsorption analyzer (Autosorb-IQ2-MP-C system) was employed to collect the N2 adsorption-desorption isotherms of the sample at 82 K. The SSA and pore size distribution were calculated using the multipoint Brunauer–Emmett–Teller method and quenched-solid density functional theory. XPS analysis was performed on an ESCALAB-250Xi spectrometer (Thermo Fisher Scientific, USA) with an Al Kα source. The temperature response of the sample was measured by an infrared camera (FLUKE TiX640). Ion concentrations were measured using the inductively coupled plasma method. The steam generation experiments were conducted using a solar simulator (PLS-SXE300C) with an optical filter for the standard AM 1.5 G spectrum (0.1 W cm-2, Table S3). Light intensity was measured by a light intensity detector (ML-01 Si-Pyranometer, Japan, Serial no. S16064435, Sensitivity 48.3 μV W-1m2). In order to test the hydrophobicity of the samples, small amounts of powder were coated onto the blank glass to measure the contact angle using a contact angle device (JC2000D3). 4.3. Water evaporation performance under one sun The MoNx was prepared by simple vacuum filtration. The circular samples (39.2 mm in diameter) with/without heat barrier were floated on water in a glass beaker. The samples were irradiated by a solar simulator under one sun, and the evaporation rate was measured for at least 3 h each cycle at room temperature. The weight loss and surface temperature were recorded per 30 min using an electronic mass balance with an accuracy of 0.1 mg and an infrared camera, respectively. To investigate the clean water producing capability, artificial seawater was prepared by dissolving NaCl (27.2 g), MgCl2 (3.8 g), MgSO4 (1.7 g), CaSO4 (1.4 g), K2SO4 (0.6 g), and KBr (0.1 g) in deionized water (1.0 L).
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4.4. Photothermal conversion efficiency measurement The corresponding solar energy conversion efficiencies () can be calculated using the following classic equation:
where
is the solar-thermal conversion efficiency, dm/dt is the water evaporation rate, He is the total
enthalpy of sensible heat and the phase change of liquid to water (≈2266 kJ kg−1), and I is the power density of light (0.1 W cm-2). Considering the influence of natural water evaporation, all the solar energy conversion efficiencies were calculated by subtracting the evaporation rate (0.11 kg m−2 h−1) of pure water in the dark condition (Fig. S13). The photothermal conversion property of dry MoNx film was tested by infrared camera. The MoNx film was irradiated with a solar simulator (one sun) until the temperature was steady. The sample temperature was measured every 10 s. When solar simulator was turned off, the sample surface temperature was measured every 20 s until the system temperature was cooled to the ambient temperature. Fresh NF was used as a control. Appendix A. Supplementary material Additional information as noted in the text. This material is available free of charge via the internet at https://www.journals.elsevier.com/nano-energy.
Acknowledgement This work is financially supported by the National Natural Science Foundation of China (21473045, 51772066), and State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (2018DX04). 15 / 21
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Lin Zhu is a PhD candidate in Professor Miao Yu's group at School of Chemistry and Chemical Engineering, Harbin Institute of Technology, China. She received her B.E. and M.S. degree from Qufu Normal University in 2012 and 2015 respectively. From 2013 to 2015, she was a visiting student in Department of Chemical Engineering, Tsinghua University, China. Her current research focuses on transition metal nitride for energy conversion and storage.
Lei Sun received his B.E. and M.S. degree from Changchun University of Technology, China, in 2008 and 2012, respectively. He is pursuing PhD at School of Chemistry and Chemical Engineering, Harbin Institute of Technology, China. His research interests focus on the synthesis and modification of nanostructured metal oxides and their applications in photocatalysis and antitumor photothermal therapy.
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Hong Zhang received her M.S. degree from Harbin Institute of Technology in 2017 and currently is a PhD candidate in Professor Ye Sun’s group at School of Science, Harbin Institute of Technology, China. Her current research focuses on high sensitivity and selectivity of single cell detection based on surface-enhanced Raman scattering using nanostructured platforms of inorganic hybrids .
Dengfeng Yu received his B. E. and M. E. degree from Harbin Institute of Technology in 2014 and 2016, and now is a PhD candidate in Professor Ye Sun’s group at School of Science, Harbin Institute of Technology. His current research interests focus on the carbon materials derived from biomass and their applications in high-performance supercapacitors and transition metal compounds for energy storage.
Hüsnü Aslan received his PhD degree from Aarhus University, Denmark, in 2016. His research interest is general about nanoscience and nanotechnology, especially biological self-assembly and the use of advanced scanning probe microscopy. To this end, he studies the interactions at nanoscale that form and affect biological systems of various dimensions. It is also in his scope to develop or improve scientific instruments to aid his investigations.
Jinggeng Zhao received his B.S. and Ph. D. degrees from Jilin University and Institute of Physics, Chinese Academy of Sciences in 1998 and 2008, respectively. He is now an Associate Professor at Physics Department, Harbin Institute of Technology. His research focuses on the crystal structure and physical properties of materials under high pressure by using a diamond anvil cell technique and high-pressure and high-temperature synthesis by using a cubic-anvil type highpressure facility.
Zhenglin Li received his bachelor degree (2011), master degree (2013) and doctor degree (2018) from Harbin Institute of Technology, China. He is currently a research fellow in Singapore Institute for Neurotechnology (SINAPSE), Center for Life Sciences, National University of Singapore. His research interests focuses on nanostructured materials with ingenious designs for biological applications. Ye Sun received his PhD degree in chemistry from Bristol University (UK) in 2007, and then worked at Cardiff University (UK) and Aarhus University (Denmark) as a postdoctoral fellow from 2007-2012. Since 2012, he is a full professor at School of Science, Harbin Institute of Technology, China. His research interests cover photoluminescence, photocatalysis, highperformance carbon materials for energy storage, temperature/gas sensing, bioimaging, and multimodal antitumor therapies.
Flemming Besenbacher received his PhD degree from Aarhus University (AU), Denmark. He is a full professor at the Department of Physics of AU since 1996 and was the Director of the Interdisciplinary Nanoscience Center (iNANO) from 2002 to 2012. His research interests cover scanning tunneling microscopy, atomic force microscopy, fabrication and function of nanoclusters, hydrogen storage, photocatalyitc nanomaterials and sustainable energy materials.
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Miao Yu received her PhD degree from Warwick University in 2007, and then worked at Aarhus University, Harvard University and Massachusetts Institute of Technology as a postdoctoral fellow. Since 2011, she is a full professor at School of Chemistry and Chemical Engineering, Harbin Institute of Technology, China. Her research interests cover energy storage, in situ reaction on surfaces, multifunctional antitumor nanoagents, and photocatalytic nanomaterials.
Highlights
First application of molybdenum nitride as a powerful photothermal conversion material.
The MoN/Mo2N nanorambutans have significant absorbance over the whole solar spectrum.
The nanorambutans possesse potent photothermal water evaporation capability and durability.
A water evaporation rate (1.70 kg m−2 h−1) and conversion efficiency (98%) are accomplished.
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PII: DOI: Reference:
S2211-2855(18)30972-8 https://doi.org/10.1016/j.nanoen.2018.12.058 NANOEN3306
To appear in: Nano Energy Received date: 15 October 2018 Revised date: 4 December 2018 Accepted date: 18 December 2018 Cite this article as: Lin Zhu, Lei Sun, Hong Zhang, Dengfeng Yu, Hüsnü Aslan, Jinggeng Zhao, Zhenglin Li, Ye Sun, Flemming Besenbacher and Miao Yu, Dual-Phase Molybdenum Nitride Nanorambutans for Solar Steam Generation under One Sun Illumination, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.12.058 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Dual-Phase Molybdenum Nitride Nanorambutans for Solar Steam Generation under One Sun Illumination Lin Zhua,c, Lei Suna, Hong Zhangb, Dengfeng Yub, Hüsnü Aslanc, Jinggeng Zhaob, Zhenglin Lia, Ye Sunb,*, Flemming Besenbacherc,*, and Miao Yua,* a
State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical
Engineering, Harbin Institute of Technology, Harbin 150000, China b
Condensed Matter Science and Technology Institute and Department of Physics, Harbin Institute of Technology,
Harbin 150000, China c
Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Aarhus 8000, Denmark
E-mail: [email protected], [email protected], [email protected] *Corresponding author.
Abstract: Water evaporation and steam production have been recognized to be considerably crucial due to the vast applications, ranging from waste water treatment, water purification, to alternative green energy solutions by water splitting, catalysis, and in-door heating. Albeit the big variety of photothermal conversion materials (PCMs) developed for this purpose, certain drawbacks, e.g. high cost, complicated synthesis, weak/narrow absorbance, bulkiness, and low evaporation rate, have hindered the application potential. Herein, we report the dual-phase molybdenum nitride nanorambutans, synthesized by a facile method, for solar steam generation. Not only the inherent properties, 1 / 21
including strong full-spectrum absorbance, high-efficiency photothermal conversion, and superhydrophilicity, benefit their water evaporation performance, the interconnected open mesopores of the nanorambutans further boost their capability of light harvesting and water/vapor transportation. Solar energy conversion efficiency of 97% under one sun together with excellent cycling stability has been demonstrated. In the desalination systems, integrating with the high salt rejection rate, the nanorambutans film can produce a water evaporation rate as high as 1.70 kg m−2 h−1 with an efficiency of 98%. Besides its compact size, the record-breaking water evaporation performance of these nanorambutans has exceeded the previous best inorganic PCM. This work introduces molybdenum nitride as a new PCM for efficient solar steam generation and all applications that can benefit from highly localized heating from nano to macro scale.
Graphical Abstract
Keywords: molybdenum nitride, photothermal conversion, solar energy, water evaporation, desalination.
1. Introduction Development of adequate photothermal conversion materials (PCMs) upon solar energy harvest has aroused increasing level of interest to meet the rising demands on solar water evaporation applications [13], such as desalination of seawater, water purification and wastewater treatment, 2 / 21
heating of residential water, and so on. A big variety of PCMs, including metallic plasmonic nanoparticles [47], inorganic semiconductors [811], organic polymers [1214], and carbon-based materials (e.g. graphene, aerogel, carbon nanotubes, graphite) [1519], have been successfully fabricated and applied for this purpose. Albeit the promising potentials of these developed PCMs, certain drawbacks have been recognized. For instance, noble metal nanoparticles normally have the unique surface plasmon resonance effect which benefits the photothermal conversion, but are limited by their high cost and low absorbance over the full solar spectrum. In contrast, carbon-based nanomaterials, with controllable one- to three-dimensional structures, indeed possess intrinsic broadband absorption and considerable photothermal conversion capability [16,19]. However, besides the complicated synthesis procedures, carbon materials as well as organic polymers are rather bulky due to their low mass density, which is disadvantageous for water/vapor transportation [12,20]. In this context, compact inorganic PCMs derived from commercially-available raw materials, e.g. various TiOx (x 2) [8,9,11,21], WO2.9 [22], Bi2Se3 [2325], Bi2S3 [26], CoS [27], CoP [28], and Prussian blue [29,30] nanostructures, have been regarded as fascinating alternatives. However, in most cases, the photothermal effect was only employed for antitumor and antibacterial applications by utilizing near-infrared irradiation [31,32], whilst their performance on water evaporation remains unexplored. Few studies, which did investigate water evaporation with compact inorganic PCMs, show only low evaporation rates under one sun (0.1 W cm-2) or natural light (Table S1). In addition to the definitive properties of an ideal PCM, i.e. inherent strong/broadband optical absorption and high photothermal conversion efficiency, an appropriate morphology and a desirable configuration of the resultant heating film also play crucial roles for enhancement of water evaporation. In particular, highly porous PCMs with abundant meso- or macro-pores are preferred. Porous materials have larger light harvesting capacity due to the multiple scattering within the inner structures [8,11] as well as the shorter and more convenient transport path for water evaporation [33]. Molybdenum nitride (MoNx), known for its low cost, mechanical stability and electronic structure analogous to noble metals (Group VIII), has been extensively studied as electrocatalysts, 3 / 21
and electrode materials for energy storage [3438]. However, its potential for photothermal conversion remains unexplored. In this work, we report the first application of dual-phase MoN/Mo2N as an efficient PCM for solar steam generation under one sun (Scheme 1). The resultant particles, referred as nanorambutans, are composed of highly crystalline nanograins and interconnected mesopores, showing significant absorbance over the whole solar spectrum ranging from 200 to 2500 nm. It is revealed that the nanorambutan film possesses potent photothermal water evaporation capability, demonstrating excellent photothermal conversion stability and durability. Especially, in the desalination systems, a water evaporation rate as significant as 1.70 kg m−2 h−1 with high conversion efficiency (98%) and efficient salt rejection are accomplished by the nanorambutans film. Besides its compact size, the record-breaking water evaporation performance of these nanorambutans has exceeded the previous best inorganic PCM. Such dual-phase molybdenum nitride nanorambutans, produced by the cost-efficient and facile synthesis, deliver great promise as a powerful PCM for solar steam generation in real-life applications.
Scheme 1. Schematic illustration of the fabrication of dual-phase MoN/Mo2N nanorambutans and their application for solar steam generation system.
2. Results and discussion The facile fabrication of MoNx nanorambutans is schematically illustrated in Scheme 1. Mo powder was chosen as the starting material. Phase evolution during the synthesis was studied in detail by Xray Diffraction (XRD) patterns. Mo was firstly oxidized and subsequently reacted with ammonium 4 / 21
acetate. The XRD pattern of the as-prepared Mo based precursor was indexed to (NH4)6Mo7O24·4H2O (JCPDS card No. 27-1013), as shown in Fig. S1a. The morphological characterization of the precursor showed uniformly distributed nanoparticles (Fig. S1b). The sample was then nitrified. Successful synthesis of MoNx was confirmed by the XRD pattern (Fig. 1a), showing high crystallization, and two groups of peaks assigned to two MoNx phases, i.e. hexagonal MoN (JCPDS card No. 77-1999) and cubic Mo2N (JCPDS card No. 25-1366), respectively. Refinement analysis on the XRD data in Fig. 1a revealed that the weight fraction of MoN and Mo2N was 55.1% and 44.9%, respectively. The corresponding lattice parameters and atomic coordinates are listed in Table S2.
Fig. 1. Composition and structure of the MoNx sample. (a) XRD pattern, (b) Mo 3d and (c) N 1s XP spectra, (d) N2 adsorptiondesorption isotherms (inset: pore-size distribution) of the MoNx.
X-ray Photoelectron Spectroscopy (XPS) analysis was also performed to explore the chemical structure of the MoNx. The survey spectrum showed obvious signals of Mo, N, O, and C elements (Fig. S2). The high-resolution Mo 3d XP spectrum (Fig. 1b) can be deconvoluted into three doublets. 5 / 21
The peaks at 229.1 and 232.4 eV are corresponding to the MoN bonding in the molybdenum nitride; those at 229.8/233.1 and 236.0 eV are ascribed to Mo4+ and Mo6+ oxidation state, respectively. These oxidation states of Mo are attributed to the slight oxidation of the MoNx surface during the sample preparation for XPS measurement. In the high-resolution N 1s spectrum (Fig. 1c), two individual peaks at 397.9 and 395.0 eV were observed, which are ascribed to MoN. The average particle size of the dual-phase MoNx nanoparticles was of ∼200 nm, as shown in the Scanning Electron Microscopy (SEM) images (Fig. S3). The detailed structure was further determined by Transmission Electron Microscopy (TEM). As shown in Fig. 2, the MoNx particle adopted a rambutan-like morphology, assembled by slightly-curved and well-crystallized nanograins with a width of ca. 1015 nm and length of ca. 3050 nm, respectively (Fig. 2ab). The formation of such delicate structure may lower the system energy. It was obvious that interconnected and open pores were constructed spontaneously during the growth of MoNx nanorambutans. The specific surface area (SSA) and pore volume were measured to be 23.3 m2 g−1 and 0.06 cm3 g−1, respectively (Fig. 1d). Benefiting from the nanograins and rambutan-like structure, the MoNx is rich of 1030 nm mesopores (inset of Fig. 1d). Such a structure provides (1) a high SSA for light harvesting because of abundant interconnected opened structures by which the light is easy to reach the interior, and (2) high-efficiency transportation channels for water provision and vapor release in the solar steam generation.
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Fig. 2. Morphology and structure of the MoNx nanorambutans. (a, b) TEM images of the MoNx, composed by nanograins. HRTEM images of the MoNx nanograins and the corresponding FFT images: (c) MoN phase, (d) Mo2N phase.
Meanwhile, the hydrophilic property was characterized by testing the surface contact angle. The substrate was blank glass with a contact angle of 87 (Fig. S4). The sample was prepared by coating the MoNx powder onto to the blank glass. Initial contact angle of 8.4o was measured at t = 0 s (Fig S5a), i.e. the first moment of the water droplet to touch the MoNx sample surface. This result conforms that the MoNx sample was super-hydrophilic. This is primarily attributed to the intrinsic nature of MoNx material itself [39,40]. Meanwhile, the specific microstructure of the nanorambutans, i.e. the mesoscopic porosity afforded by the nanograins and rambutan-like structure, may result in capillarity to further enhance the hydrophilicity. The water droplet was completely absorbed in the MoNx sample within 3 s (Fig. S5b), indicating that the MoNx may act like a water pump in water evaporation process. The dual phases of the MoNx sample were also identified by high-resolution TEM (HRTEM) images. Both hexagonal MoN as indicated by the (200) plane of 2.48 Å and cubic Mo2N as indicated by the (111) plane of 2.40 Å were observed (Fig. 2cd). The Fast Fourier Transformation 7 / 21
(FFT) images of the diffraction patterns (inset of Fig. 2cd) revealed the single crystal structures of both hexagonal MoN and cubic Mo2N. According to the literature, such well-crystallized nanograins may afford significant heat transfer capability for water evaporation [8]. The MoNx thin film was fabricated by dispersing MoNx powder (10.0 mg) in N-methyl-2pyrrolidone solvent upon ultrasonic treatment for 10 min, then mixing with the poly(vinylidene fluoride) (PVDF) binder at a mass ratio of MoNx:PVDF of 14.3 upon ultrasound for another 10 min, and finally being filtrated and supported via a commercial Nylon66 film (NF, 50 mm in size) and dried at 60 C for 2 h. The thickness of the obtained film was determined by the side-view SEM imaging. As illustrated in Fig. S6a–b, the film surface was smooth and compact, with the MoNx and NF layer thickness of 10 μm and 130 μm, respectively. There were abundant interconnected macro-/meso-pores in the film, which may serve as water and vapor channels for rapid solar steam generation. Both the MoNx film and pure NF were hydrophilic, as water droplet completely flattened and spread out when falling on the MoNx film or pure NF in the contact angle measurements. The Ultraviolet–visible–Near-Infrared (UV–vis–NIR) reflection spectrum of the MoNx film was measured with a Carry5000 UV–vis–NIR spectrophotometer including an integrating sphere (diffusive and specular reflectance). The film presented strong light absorption over the whole solar spectrum ranging from 200 to 2500 nm (Fig. 3a). The diffuse reflectance spectra indicated its broadband and strong adsorption property with an extremely low average reflectance of 4%. The solar absorption of MoNx/NF was calculated to be 96%. Meanwhile, the photothermal conversion capability of the MoNx film was carefully evaluated by an infrared camera (FLUKE Ti25). The results showed sharp temperature response mediated by the MoNx upon the solar light (Fig. 3b). Under solar irradiation of one sun, the temperature of the MoNx film was quickly increased up to 83 °C within 10 s and achieved an equilibrium temperature 90 °C after 60 s (Fig. S7), whereas a pure NF film was only 33 °C under a largely extended irradiation for 470 s. The MoNx showed more rapid and efficient photothermal conversion capability than previously reported PCMs, e.g. oxygen plasma treated graphene aerogel (one sun for 10 s, ~75 8 / 21
o
C) [20], narrow-bandgap Ti2O3 nanoparticles (one sun for 700 s, ~52 °C) [9], hierarchically
nanostructured gel (one sun for 500 s, ~45 °C) [41], and even higher than the photo-electro-thermal effect pet-PGS/GF architecture (one sun for 300 s, ~83.8 °C) [42]. Needless to say, excellent light harvesting and rapid photothermal conversion capability are paramount for efficient solar steam generation. Thus, dual-phase MoNx possess outstanding advantages: (1) the intrinsic property of MoNx endows it an outstanding light absorption property over the whole solar spectrum, (2) wellcrystallized nanograins accelerate the solid-to-liquid interface heat transfer properties, (3) it shows rapid photothermal conversion capability under one sun, (4) MoNx has super-hydrophilic nature and (5) nanosized hierarchical structure with opened and abundant interconnected pores, providing higheffective channels for light, water as well as vapor in the advanced solar steam generation systems. We then explored the performance of the MoNx on water evaporation. Since the localized heat effect can significantly improve the solar vapor generation by concentrating the energy at the solidto-liquid interface for water evaporation instead of heating of the bulk water [43], the experiments were then set up as shown in Fig. S8, where the MoNx/PF film (the MoNx film atop of the PF film) was floating at the water surface. The mass loading of all the MoNx films was fixed at a low amount of 0.85 mg cm−2. All the measurement of the MoNx films was performed under one sun (0.1 W cm-2). In contrast, fresh NF and polystyrene foam (PF) were selected as the control samples. As depicted in Fig. 3c, in a sharp comparison and as expected, the MoNx film delivered a water evaporation rate of 1.42 kg m−2 h−1, which is 2.9 folds higher than that with NF alone (0.49 kg m−2 h−1). Importantly, water evaporation capacity was further enhanced by inducing the PF heat barrier. As shown in Fig. S9, the PF was a thin foam with a thickness of 3 mm and a diameter of 4 cm. Thanks to the hydrophilicity hence capillary force afforded by MoNx and NF, water can be successfully absorbed and supply to the MoNx film. In the presence of PF, the evaporation rate was boosted to 1.69 kg m−2 h−1, which was 3.8 times higher than the case of PF alone (0.44 kg m−2 h−1). The evaporation rate of MoNx is evidently higher than previously reported materials under one sun irradiation, e.g. 9 / 21
1.31 kg m−2 h−1 for reduced graphene oxide and carbon nanotubes [44], 1.32 kg m−2 h−1 for Ti2O3 [9], 0.80 kg m–2 h–1 for TiOx [8], 1.475 kg m−2 h−1 for carbonized mushrooms [45], and the highest, i.e. 1.62 kg m−2 h−1 for vertically aligned graphene sheets membrane [16]. As shown in Fig. S10, the solar-to-water efficiency of MoNx was of 83% (without PF) and 97% (with PF), respectively. In contrast, for the control group of PF or NF only, the efficiency was as low as 12% and 14%. All these results support the high photothermal conversion capability of the MoNx nanorambutans.
Fig. 3. Optical properties and water evaporation performance of the MoNx film under one sun. (a) Solar spectrum and UV−vis−NIR diffuse reflectance spectra of the MoNx film in the range of 200−2500 nm. (b) Time-dependent surface temperature of the MoNx film upon the irradiation, with a pure NF film as comparison. (c) Time-dependent water evaporation rate of different films, including pure NF (pale blue), pure PF (light brown), MoNx (blue), MoNx/PF (red). (d) Cycling stability of the MoNx/PF under one sun. The solar-to-water efficiencies is calculated by subtracting the natural water evaporation efficiency in dark.
It is worthy to mention that, to get more accurate evaluation on the evaporation capability of the samples, all solar-to-water efficiencies presented here were calculated by subtracting the evaporation efficiency (6.8%) of pure water in the dark condition, nevertheless most results in the literature did not exclude the natural evaporation of water. Following such commonly adopted calculation method without excluding this factor, the solar-to-water efficiency of our MoNx nanorambutans can be even 10 / 21
higher than 100% (103.8%). As the stability and reusability of a PCM are vital properties for real-life applications, the cycling stability performance of the MoNx for water evaporation was evaluated. Each cycle was carried out under the same conditions for at least 3 h. As shown in Fig. 3d and S11, the MoNx film can steadily produce water vapor at an average rate of 1.66 kg m−2 h−1 under the solar irradiation of one sun upon the repeated usage cycles for one week. The efficiency can be maintained to 91% after one-week usage. It was found that the MoNx can preserve its activity even after illumination for more than 66 h under one sun. The temperature of the MoNx upon irradiation can be directly visualized and recorded by an infrared camera. As demonstrated in Fig. 4, floating on the water surface of a beaker and irradiated at solar power of one sun, the MoNx generated a localized hot region, with the temperature quickly elevated to 51 C within 1 min and remained stable. In contrast, the temperature of the pure NF with heat barrier was only increased by 3 C in 3 h upon the same irradiation (Fig. S12). The high performance of the MoNx originates from its intrinsic photothermal conversion capability and highly porous morphology for effective heat generation and appropriate heat transport.
Fig. 4. Infrared thermal images of a beaker of water with the MoNx/PF floated on the surface, irradiated with the simulated solar power of one sun for (a) 0 min, (b) 1 min, (c) 30 min.
The MoNx film was also applied in solar desalination systems. As anticipated, the average water evaporation rate of the MoNx was rather high (as high as 1.70 kg m−2 h−1) with a conversion efficiency of 98% (Fig. 5a). The results implied that the presence of salt had little influence on the evaporation efficiency. And the water evaporation rate can be further improved by adjusting the 11 / 21
structure of MoNx particles. Moreover, used MoNx which had been exposed to light thus reached to the high temperature of 90 C as above mentioned in Fig. 3b, was also able deliver a high evaporation rate of 1.63 kg m−2 h−1 with an efficiency of 93%. These results indicate the great stability and reusability of the MoNx. The concentration of original artificial seawater and condensed water was tested by inductively-coupled plasma mass spectrometry (Fig. 5b). Ion concentrations of the condensed water were remarkably reduced by three orders of magnitude as compared with the original artificial seawater. It has fully met the standard for healthy drinking water defined by World Health Organization (WHO) and US Environmental Protection Agency (EPA). Compared with the big variety of reported PCMs, e.g. carbon materials [45,46], single-walled nanotube-MoS2 hybrid [33], noble metal [47], TiOx [8,9], the dual-phase MoNx nanorambutans have a much higher photothermal conversion efficiency for solar steam generation.
Fig. 5. (a) Time-dependent water evaporation rate of artificial seawater: fresh MoNx film (the red line) and the film after illumination for 15 min under one sun (the green line), in the presence of heat barrier. (b) The measured concentration of four primary metal ions in an artificial seawater sample (red) and the clean water which was collected by cooling the water vapor (blue).
3. Conclusions Dual-phase molybdenum nitride nanorambutans with high crystallinity, high thermal stability, and abundant mesopores, have been fabricated through a facile method and applied as a new PCM under one sun. The nanorambutans possess a light harvesting capability as high as 97% over the whole solar spectrum, together with super-hydrophilic nature, rapid and efficient photothermal conversion 12 / 21
capability, and structural compactness while providing kinetically favorable mass transportation channels, superior to most of reported PCMs. Owing to the above-mentioned merits, the MoNx film can deliver a staggering water evaporation rate of 1.69 kg m−2 h−1 under one sun, together with outstanding cycling stability and reusability. Additionally, in solar desalination systems, the water evaporation rate of MoNx film is as high as 1.70 kg m−2 h−1 with high thermal stability as well as high salt rejection performance. There is no doubt that the MoNx acts as one of the most promising candidates for the state of art photothermal evaporation system. In future, in addition to steam generation related applications, such as alternative green energy production, nano/micro steam engines, we expect the MoNx to find use in a wide range of photon mediated local-/interfacialheating functions like antibacterial practices, antifouling, cancer therapy, sterilization, and photocatalysis.
4. Experimental section 4.1. Synthesis of MoNx The MoNx nanorambutans were synthesized by annealing a Mo-based precursor under a 5% ammonia atmosphere. Typically, 0.2 g Mo powder was dispersed in 15 mL ethanol under magnetic stirring for 30 min. Afterwards, 0.7 mL H2O2 (30%) solution was added dropwise to the Mo suspension. After constant stirring for 48 h, the solution turned blue. Then, the precursor solution was mixed with 0.6 g CH3COONH4 for 24 h. The obtained precipitate was collected by centrifuging at 13000 rpm for 10 min, and then dried at 40 oC. Finally, the solid precipitate was annealed at 650 C (heating rate 10 C min−1) under an ammonia atmosphere (5% NH3 and 95% Ar) for 6 h to obtain the molybdenum nitride powder. 4.2. Characterization XRD analysis of the MoNx was carried out on an X-ray diffractometer (Empyrean, Panalytical, The Netherlands) using Cu Kα source and PIXcel3D detector. The experimental data were collected in the range of 30° to 120° and analyzed with Rietveld method by using the General Structure Analysis 13 / 21
System (GSAS) software [4850]. The morphology was characterized by a high-resolution TEM (TECNAI G2 F30 S-Twin (FEI, USA)) at 120.0 kV and a field emission high resolution SEM (Hitachi Limited, Japan) at 1.0 kV. The optical absorption was investigated with a Carry5000 UV– vis–NIR spectrophotometer including an integrating sphere (diffusive and specular reflectance) in the wavelength range of 200-2500 nm. An N2 adsorption analyzer (Autosorb-IQ2-MP-C system) was employed to collect the N2 adsorption-desorption isotherms of the sample at 82 K. The SSA and pore size distribution were calculated using the multipoint Brunauer–Emmett–Teller method and quenched-solid density functional theory. XPS analysis was performed on an ESCALAB-250Xi spectrometer (Thermo Fisher Scientific, USA) with an Al Kα source. The temperature response of the sample was measured by an infrared camera (FLUKE TiX640). Ion concentrations were measured using the inductively coupled plasma method. The steam generation experiments were conducted using a solar simulator (PLS-SXE300C) with an optical filter for the standard AM 1.5 G spectrum (0.1 W cm-2, Table S3). Light intensity was measured by a light intensity detector (ML-01 Si-Pyranometer, Japan, Serial no. S16064435, Sensitivity 48.3 μV W-1m2). In order to test the hydrophobicity of the samples, small amounts of powder were coated onto the blank glass to measure the contact angle using a contact angle device (JC2000D3). 4.3. Water evaporation performance under one sun The MoNx was prepared by simple vacuum filtration. The circular samples (39.2 mm in diameter) with/without heat barrier were floated on water in a glass beaker. The samples were irradiated by a solar simulator under one sun, and the evaporation rate was measured for at least 3 h each cycle at room temperature. The weight loss and surface temperature were recorded per 30 min using an electronic mass balance with an accuracy of 0.1 mg and an infrared camera, respectively. To investigate the clean water producing capability, artificial seawater was prepared by dissolving NaCl (27.2 g), MgCl2 (3.8 g), MgSO4 (1.7 g), CaSO4 (1.4 g), K2SO4 (0.6 g), and KBr (0.1 g) in deionized water (1.0 L).
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4.4. Photothermal conversion efficiency measurement The corresponding solar energy conversion efficiencies () can be calculated using the following classic equation:
where
is the solar-thermal conversion efficiency, dm/dt is the water evaporation rate, He is the total
enthalpy of sensible heat and the phase change of liquid to water (≈2266 kJ kg−1), and I is the power density of light (0.1 W cm-2). Considering the influence of natural water evaporation, all the solar energy conversion efficiencies were calculated by subtracting the evaporation rate (0.11 kg m−2 h−1) of pure water in the dark condition (Fig. S13). The photothermal conversion property of dry MoNx film was tested by infrared camera. The MoNx film was irradiated with a solar simulator (one sun) until the temperature was steady. The sample temperature was measured every 10 s. When solar simulator was turned off, the sample surface temperature was measured every 20 s until the system temperature was cooled to the ambient temperature. Fresh NF was used as a control. Appendix A. Supplementary material Additional information as noted in the text. This material is available free of charge via the internet at https://www.journals.elsevier.com/nano-energy.
Acknowledgement This work is financially supported by the National Natural Science Foundation of China (21473045, 51772066), and State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (2018DX04). 15 / 21
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Lin Zhu is a PhD candidate in Professor Miao Yu's group at School of Chemistry and Chemical Engineering, Harbin Institute of Technology, China. She received her B.E. and M.S. degree from Qufu Normal University in 2012 and 2015 respectively. From 2013 to 2015, she was a visiting student in Department of Chemical Engineering, Tsinghua University, China. Her current research focuses on transition metal nitride for energy conversion and storage.
Lei Sun received his B.E. and M.S. degree from Changchun University of Technology, China, in 2008 and 2012, respectively. He is pursuing PhD at School of Chemistry and Chemical Engineering, Harbin Institute of Technology, China. His research interests focus on the synthesis and modification of nanostructured metal oxides and their applications in photocatalysis and antitumor photothermal therapy.
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Hong Zhang received her M.S. degree from Harbin Institute of Technology in 2017 and currently is a PhD candidate in Professor Ye Sun’s group at School of Science, Harbin Institute of Technology, China. Her current research focuses on high sensitivity and selectivity of single cell detection based on surface-enhanced Raman scattering using nanostructured platforms of inorganic hybrids .
Dengfeng Yu received his B. E. and M. E. degree from Harbin Institute of Technology in 2014 and 2016, and now is a PhD candidate in Professor Ye Sun’s group at School of Science, Harbin Institute of Technology. His current research interests focus on the carbon materials derived from biomass and their applications in high-performance supercapacitors and transition metal compounds for energy storage.
Hüsnü Aslan received his PhD degree from Aarhus University, Denmark, in 2016. His research interest is general about nanoscience and nanotechnology, especially biological self-assembly and the use of advanced scanning probe microscopy. To this end, he studies the interactions at nanoscale that form and affect biological systems of various dimensions. It is also in his scope to develop or improve scientific instruments to aid his investigations.
Jinggeng Zhao received his B.S. and Ph. D. degrees from Jilin University and Institute of Physics, Chinese Academy of Sciences in 1998 and 2008, respectively. He is now an Associate Professor at Physics Department, Harbin Institute of Technology. His research focuses on the crystal structure and physical properties of materials under high pressure by using a diamond anvil cell technique and high-pressure and high-temperature synthesis by using a cubic-anvil type highpressure facility.
Zhenglin Li received his bachelor degree (2011), master degree (2013) and doctor degree (2018) from Harbin Institute of Technology, China. He is currently a research fellow in Singapore Institute for Neurotechnology (SINAPSE), Center for Life Sciences, National University of Singapore. His research interests focuses on nanostructured materials with ingenious designs for biological applications. Ye Sun received his PhD degree in chemistry from Bristol University (UK) in 2007, and then worked at Cardiff University (UK) and Aarhus University (Denmark) as a postdoctoral fellow from 2007-2012. Since 2012, he is a full professor at School of Science, Harbin Institute of Technology, China. His research interests cover photoluminescence, photocatalysis, highperformance carbon materials for energy storage, temperature/gas sensing, bioimaging, and multimodal antitumor therapies.
Flemming Besenbacher received his PhD degree from Aarhus University (AU), Denmark. He is a full professor at the Department of Physics of AU since 1996 and was the Director of the Interdisciplinary Nanoscience Center (iNANO) from 2002 to 2012. His research interests cover scanning tunneling microscopy, atomic force microscopy, fabrication and function of nanoclusters, hydrogen storage, photocatalyitc nanomaterials and sustainable energy materials.
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Miao Yu received her PhD degree from Warwick University in 2007, and then worked at Aarhus University, Harvard University and Massachusetts Institute of Technology as a postdoctoral fellow. Since 2011, she is a full professor at School of Chemistry and Chemical Engineering, Harbin Institute of Technology, China. Her research interests cover energy storage, in situ reaction on surfaces, multifunctional antitumor nanoagents, and photocatalytic nanomaterials.
Highlights
First application of molybdenum nitride as a powerful photothermal conversion material.
The MoN/Mo2N nanorambutans have significant absorbance over the whole solar spectrum.
The nanorambutans possesse potent photothermal water evaporation capability and durability.
A water evaporation rate (1.70 kg m−2 h−1) and conversion efficiency (98%) are accomplished.
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