Nitrogen-doped graphene quantum dots hydrogels for highly efficient solar steam generation
Graphical abstract
Introduction
Solar evaporation is an effective strategy to use renewable and sustainable solar energy to address the global clean freshwater shortage crisis [1], [2]. In this strategy, a solar absorber platform is used to harvest solar energy, and the harvested thermal energy can then be used to generate steam via technologies such as heat localization [2]. In the heat localization mechanism, the harvested thermal energy is localized in a spot to minimize the loss of energy [3]. The accumulation of harvested heat increases the local temperature of water molecules in a thin layer of water and leads to steam generation [3]. Consequently, porous nanostructures with high solar absorption, high porosity, and low thermal conductivity can be ideal platforms for solar water evaporation applications; as these nanostructures can present not only high solar energy harvesting capabilities and water entrapment in their porous structure but also energy localization and high water evaporation efficiencies [4], [5], [6].
The evaporation temperature and the energy removal rate in a porous water evaporation system strongly depend on the microstructure of the porous medium, as the volume of the water contained in the pores of the porous medium controls the evaporation process [7]. Different porous structures such as polymer-absorber/wood composites [8], metal-foam-based structures [9], carbon fiber-based porous systems [10], [11], and nanostructures based on carbonous nanomaterials [12], [13] can be used to develop solar steam generation systems. Among these structures, carbonous porous materials have recently attracted great attention for solar evaporation applications, due to their high solar absorbance, open-cell porosity, tunable structure, and designable surface chemistry [12].
Graphene-based porous nanostructures are ultralightweight materials with a carbonous skeleton and interconnected pores and high light absorbance capabilities [1]. Moreover, graphene-based porous nanostructures can present a wide range of hydrophilicity and thermal conductivity, depending on the precursor type, chemical structure, and fabrication conditions [14], [15]. Such a tunable structure has made graphene-based porous nanostructures perfect platforms for solar water evaporation applications [16], [17]. In addition, solar-to-thermal conversion efficiency and thermal conductivity of graphene-based porous nanostructures can be designed by controlling the structure of these three-dimensional materials [18] and by doping the graphene-based skeleton of these materials with heteroatoms like nitrogen atoms [19].
Graphene quantum dots (GQDs) are disk-like graphene-based nanomaterials with a lateral size range that is low enough to present the quantum confinement and edge effects [20]. It has been shown that the contribution of the quantum confinement in GQDs increases significantly when the lateral size of GQDs is less than 100 nm [20]. The optical properties of GQDs are one of the main advantages of these nanomaterials, which are known to be directly related to their size and structure [21], [22]. GQDs and nitrogen-doped GQDs (N-GQDs) are known to have interesting photo-to-thermal energy conversion capabilities, particularly in ultraviolet, near-infrared, and infrared electromagnetic wave regions [23], [24]. Up to now, this important characteristic of GQDs has been extensively employed in photothermal therapy applications [24], and GQDs have hardly been considered as candidates to improve the photo-to-thermal energy conversion performance of solar evaporation systems [25], [26].
Here, N-GQDs hydrogels are introduced as a new class of porous solar steam generation platforms with high solar water evaporation efficiencies. Different N-GQDs hydrogels are prepared from N-GQDs precursor solutions with diverse N-GQDs lateral size ranges through the hydrothermal aggregation process. The low thermal conductivity of these hydrogels leads to the heat localization and the temperature increase of water molecules pumped inside the hierarchical interconnected pore network of prepared N-GQDs hydrogels. Results presented here suggest that N-GQDs hydrogels are effective solar steam generation platforms and great candidates to develop solar-based water purification devices.
Section snippets
Materials
l-ascorbic acid (crystalline powder, LAA) was bought from Acros Organics, and branched polyethylenimine (Mw ~800, PEI) was purchased from Sigma-Aldrich.
Synthesis of PEI-derived N-GQDs
A new modified hydrothermal synthesis method was used in this study for the down-to-top synthesis of N-GQDs from PEI precursor aqueous solutions. The lateral dimension range of N-GQDs was controlled by changing the time of the hydrothermal process. In a typical synthesis process of PEI-derived N-GQDs, a 50 mg/ml solution of PEI in deionized
Impacts of the synthesis duration on the structure of N-GQDs
In this study, N-GQDs with different lateral size ranges were fabricated by controlling the interval of the carbonization process. The duration of the carbonization process had a direct impact on the lateral size of N-GQDs, as shown in Fig. 1. The average size of N-GQDs synthesized during 3 h of the hydrothermal process (3h-N-GQDs) was about 6 nm (Fig. 1a). As the duration of the synthesis process increased to 12 h (12h-N-GQDs), the average lateral size of synthesized N-GQDs increased to around
Conclusions
In this work, N-GQDs hydrogels were prepared from a series of N-GQDs precursor solutions with different lateral size ranges of N-GQDs. Prepared N-GQDs hydrogels presented the colloidal morphology with a hierarchical interconnected network of pores with pore sizes in micro, meso, and macropore ranges. The structure of hydrogels consisted of spherical primary nanoparticles that formed large nanoparticles with micropores. Large nanoparticles merged as the lateral size range of N-GQDs in the
CRediT authorship contribution statement
Ahmad Allahbakhsh: Conceptualization; Data curation; Formal analysis; Funding acquisition; Investigation; Methodology; Project administration; Resources; Software; Supervision; Validation; Visualization; Roles/Writing - original draft; Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The author would like to thank Iran National Science Foundation (INSF) for its supports.
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