Synthesis And Photothermal Desalination Properties Of Molybdenum-Based Nanomaterials

The grave shortage of freshwater has been an unavoidable difficulty in the development of the society and economy.Addressing this challenge based on sustainable energy has attracted widespread attention.In recent years,solar steam generation（SSG）,which employs photothermal conversion materials（PCMs）to transform solar energy into heat for evaporation of seawater to obtain freshwater,has shown a promising application prospect.However,the real life application of SSG has been largely compromised by two main issues:（1）the reported PCMs are limited by their high cost,complicated synthesis and weak water evaporation performance,so it is urgent to develop low-cost and high-performance PCMs.（2）Albeit the developed photothermal evaporator performs well in deionized water,significant salt precipitation occurs rapidly when it is applied to seawater evaporation,resulting in a drop in SSG efficiency and even becoming invalid.To solve these issues,this thesis focuses on the synthesis of molybdenum based PCMs and their applications in SSG,and proposes a strategy to break the salt barrier for evaporator,constructing advanced SSG systems with long-term operation life in high salinity seawater.The main work contents are as follows:Firstly,the synthesis of Mo-precursor with nanostructure at room temperature was studied,and febricated a polyhedral Mo₂N nanomaterial.The solar absorption,photothermal conversion and water evaporation properties of the material in the whole solar spectrum（200-2500 nm）were studied.The experimental results showed that Mo₂N possess strong light absorption over the whole solar spectrum and sensitive light-driven temperature rise properties.Under the light intensity of 1,2 and 3 k W m^-2,the water evaporation rates of Mo₂N were 1.49,2.73 and 3.83 kg m^-2 h^-1,respectively,showing good cycling stability.Secondly,in order to improve the photothermal conversion capability,the dual-phase Mo₂N/Mo N（Mo N_x,x≈0.73）nanorambutans were synthesized by optimizing the synthesis of Mo-precursor.The weight fraction of hexagonal Mo N and cubic Mo₂N was 55.1%and 44.9%,respectively.Compared with polyhedral Mo₂N,rambutan-like Mo N_x has a more porous structure and high solar absorption.The experimental results showed that the evaporation rate was boosted to 1.69 kg m^-2 h^-1under 1 k W m^-2,which is higher than that of previously reported noble metal and semiconductor photothermal conversion materials.In artificial seawater,the presence of salt had little influence on the water evaporation rate.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 and US Environmental Protection Agency.Finally,from the perspective of practical application,the three-dimensional molybdenum carbide@carbon bouquets(α-Mo C_1-x@CB)composite was synthesised.It not only presents strong light absorption over the whole solar spectrum and sensitive light-driven temperature rise properties similar to molybdenum nitride nanomaterials,but also showed obvious load efficiency,which can significantly reduce the dosage.For SSG applications,theα-Mo C_1-x@CB product was fabricated into flat thin film and three-dimensional cone-shaped film with different loads,and then evaluated their photothermal conversion properties and solar-to-water performance.The long-term cycling stability of theα-Mo C_1-x@CB was also studied.In order to solve the salt accumulation-induced issues,we propose a strategy:applying cotton meshes between the SSG system with high salt concentration and bulk seawater with lower salt concentration to build transport bridges,driven by the concentration gradient,a closed salt–water loop was formed,enabling powerful self-salt-discharge and self-water-pumping functions.Combining employing the highly porous,super-hydrophobic material with competent light-harvesting and photothermal conversion capability,the strategy breaks the salt barrier for sustainable solar desalination with high evaporation rate.The experimental results showed that the cone-shapedα-Mo C_1-x@CB evaporator works well with a steady and high evaporation rate not only upon the long cycling in regular seawater,but also upon continuous desalination in the highly concentrated salt solution,without any sign of salt blockage,and confirmed the feasibility of the self-salt-discharge strategy.