Design Of Carbon-based Nanocomposites And Their Photoelectricocemical Performance

With the fast development of society,the dependence of human beings on materials and energy is becoming more and more heavy.The exploration and development of powerful materials and clean and efficient new energy forms can meet more production and living damands of human beings,as well as achieve sustainable development.Carbon is widely distributed on earth in a variety of forms,so far,people have developed various carbon nanomaterials,and more newly found carbon nanomaterials are still emerging.In addition,due to the excellent photoelectrochernical properties,carbon-based nanomaterials have become a global research focus and been widely used in many fields,especially polymer nanocomposites and photocatalysis.In this paper,a series of carbon-based nanocomposites were designed and prepared by facile and inexpensive methods,and their photoluminescence,electrochemical corrosion protection and photocatalytic properties were studied in detail.This thesis has introduced the establishment of(1)doped carbon dots/polymethyl methacrylate(CDs/PMMA)polymer nanocomposites,(2)nitrogen doped carbon dots/polyurethane(NCDs/PU)composite coating,negatively charged carbon dots/polyurethane(CDs/PU)composite coating,(3)carbon dots/cadmium sulfide(CDs/CDs)composite photocatalysts with different morphologies,(4)carbon dots/cadmium sulfide nanosheets(CDs-CdS)composite photocatalyst,(5)non-metallic photocatalyst carbon/carbon dots(NCN/CDs)composite photocatalytic system and(6)cobalt dopd mesoporous carbon nitride(Co-mCN)photocatalyst and the research of their applications in self-healing,antibacterial materials and photocatalysis.The self-healing,antibacterial.electrochemical corrosion resistance and photocatalytic mechanisms of the composite materials and the photoelectrochemical features and roles of carbon-based nanomaterials are studied by a series of characterization and experiments.The specific work includes the following parts:1.Self-healing is the way by which Nature repairs damage in bio and botanic entities and prolongs their life.A variety of practical applications require the design and processing of self-healing materials in general and self-healing polymers in particular Different(expensive and complex)methods provide the re-bonding of broken bonds,suppressing crack or local damage propagation.Here we report a versatile and cost-effective methodology for initiating healing in bulk hard polymers for fracture surface repair.Carbon dots(CDs)with a size of about 5 nm were blended into PMMA and other common polymer monomers by free radical polymerization to form polymer nanocomposites.Profiting from the photoluminescence properties of CDs.the polymer composites also possess fine photoluminescence properties,emitting bright blue light under ultraviolet light irradiation.The healing process is initiated by interfacial bonding(covalent,hydrogen and Van der Waals bonding)between the CDs and the polymer matrix and can be optimized by modifying the functional(amino,carboxyl and hydroxyl)groups which terminate the CDs.The healing properties of the bulk polymer-CDs composites are evaluated by comparing the tensile strength of pristine(bulk and coatings)composites to those of fractured composites that were healed.This work is a milestone in the application of CDs in the field of polymer remediation and provides reference and inspiration for future research2.In previous study,we found that carbon dots(CDs)have the property of triggering the repair of fracture surfaces of polymer.In this study,we use a facile method to achieve self-healing and electrochemical corrosion resistance properties of polymer coatings.We introduce a nitrogen-doped carbon dots(NCDs)into polymer coatings by simple physical mixing method,and healing/self-healing of the fracture surfaces and cracks are initiated by the bonding of functional groups between NCDs or by the interaction between NCD-polymer chains.The modified polyurethane coating(NCDs-PU)shows excellent self-healing ability of both fractured surface and cracks at room temperature.In addition,the composite coatings have excellent electrochemical corrosion resistance properties,because the addition of NCDs not only inhibits the formation and expansion of pores,but also can reduce oxygen through a four-electron pathway into water so that weaken the corrosion of the metal substrate.This work expands the application scope of carbon dots in polymer nanocomposites and opens a new door for the discovery of potential applications of carbon dots in industrial and engineering fields in the future.3.Multifunctional coatings,especially those with simultaneous anti-biofilm formation and electrochemical anti-corrosion properties are of great significance for the marine industry.Inspired by the function of fish mucus of blackhead fish,a biological epidermal secretion with negative surface potential that protects blackhead fish from colonization of microorganisms,we firstly introduce a concept of using negatively charged carbon nanodots(CDs)as a secure and economical dual-functional additive to prepare protective coatings by mechanical agitation.The prepared CDs with strong negative surface potential initiate robust anti-biofilm formation(anti-adhesion and anti-bacteria)and electrochemical anti-corrosion properties(about 60 days’ durability in seawater)of polymeric coatings.The incorporated CDs with negative surface potential take effect in the following ways:(1)suppressing bacterial adhesion by virtue of strong electrostatic repulsion;(2)sterilizing anchored bacteria via destroying bacteria cell walls;(3)impeding electron ejection from metal surface;(4)blocking aggressive species(H2O and O2)by narrowing the microchannels.This work provides a new train of thought propelling the development of potential materials for industrial and engineering applications.4.Photocatalytic hydrogen production on semiconductors has been intensively researched in the past years,yet it still lacks research on efficient and stable photocatalysis in seawater.In this paper,with cadmium sulfide(CdS)as a sample photocatalvst and carbon dots(CDs)as cocatalyst,we designed and fabricated a series of CDs/CdS nanocomposites with different morphologies serving as efficient hydrogen production photocatalysts in both water and seawater.In this case.the optimal CDs/CdS-S nanocomposites(nanosheet with cocatalyst content of 0.01 gCDs/gcatalyst)exhibit prominent H2 production rate of 4.64 mmol h-1 g-1(6.70 mmol h-1 g-1,water)and AQE of 11.8%(19.3%,water)under 420 nm light irradiation in seawater,which are about 265 and 169(78 and 77,water)times of those of the non-modified irregular CdS in seawater,respectively.It is also the best result of the currently reported photocatalytic hydrogen production from seawater splitting.The excellent activity and stability are attributed to the competency of CDs that not only dramatically improves the charge separation efficiency,but also plays indispensable role in resisting the distraction from various ionic components in seawater.We hope our work can provide a feasible perspective for constructing high-efficient photocatalyst towards practical applications.5.Cadmium sulfide(CdS)has long time been one of the most promising inorganic photocatalysts for hydrogen production driven by visible light.However,the photocorrosion of CdS is the most serious problem which constrains its development.Here,we report the design and fabrication of a carbon dots-cadmium sulfide(CDs-CdS)nanocomposite,showing significant photocatalytic water splitting properties with impressive stability without requirement for any sacrificial agents.The highest hydrogen production rate was obtained for about 2.55 μmol h-1 with an oxygen evolution rate about 0.52 μmol h-1 when the concentration of CDs in the sample is 0.03 gCDs/gcatalyst.Although the produced H2 and O2 are not equal to the stoichiometric ratio of 2:1(H2:O2),the CDs-CdS nanocomposite shows greater stability(8-time repetitive catalytic experiments)than the CdS catalysts reported up to now(without sacrificial agents).It is also inspiring that when we increased the concentration of CDs in the catalysts,the produced H2 and O2 were gradually adjusted to meet the stoichiometric ratio of 2:1 in spite of low hydrogen production rate(0.13 μmol h-1).The excellent stability of CDs-CdS composite photocatalyst is attributed to the tight bond between C Ds and CdS.enhancing the ability of S anions to resist the oxidation of produced hole.6.Hydrogen production from overall water splitting by photocatalyst is an ultimate clean and renewable energy strategy.Recent developments show that carbon based materials are considerable photocatalysts for overall water splitting under visible light because of their high activity,high stability,low-cost,easy fabrication and structural diversity.However,it still lacks a systematic study and deep understanding on the working mechanism of the carbon based photocatalysts.Herein,we show the design and fabrication of a carbon-based photocatalyst with abundant defects created by removing the nitrogen atoms from a N-doped precursor.The active defects b ond with water molecules during the photocatalytic reaction,which then work as oxidation sites for O2 generation.We also demonstrate an accessible strategy to produce more defects to observably enhance the photocatalytic activity(around 10 times)as well as to select between the 2-electron/2-electron and the 4-electron pathway water splitting.The synthesized photocatalyst is efficient in photocatalytic visible-light overall water splitting with an optimum H2 and O2 production of 2.54 and 1.25 μmol h-1.respectively Moreover,the quantum efficiencies and solar to hydrogen(STH)efficiency were measured to be 2.0%for wavelength λ=420±20 nm and 0.1%using AM 1.5G.respectively.This work has broadened the application of carbon based nanomaterials in the field of photocatalysis and inspired the design of more carbon based photocatalysts7.Solar water splitting via the famous metal-free photocatalyst,namely graphitic carbon nitride(g-C3N4)have achieved extensive attention in recent years due to its adjustable band gap,nontoxicity,abundance and low budget.However,it remains a big challenge for large-scale applications because efficient and stable overall water splitting for g-C3N4 is still difficult to accomplish.In this work,we design a one-pot synthetic method to fabricate a series of cobalt(Co)-doped mesoporous g-C3N4(Co-mCN)photocatalysts for efficient and stable overall water splitting.Compared with pristine g-C3N4,the adjustable Co doping content not only improves charge separation efficiency for prolonged lifetime of charge carrier,but also greatly enhances the tolerance of g-C3N4 against H2O2 poison for stable production of H2 and H2O2.The optimal performance for this catalyst is gained to be 1.21 μmol h-1 for H2 production and apparent quantum efficiency(AQE)of 2.2%at 420 nm with a working life for more than 216 h.Moreover,it is demonstrated that the produced H2O2 can be easily collected and directly applied in catalyzing cyclohexane oxidation into cyclohexanone(100%selectivity and 0.06%conversion)with titanium silicalite molecular sieve(TS-1)as the carrier.This work not only solves the main problems lying behind the photocatalytic process of carbon nitride materials,but also provides a new thinking for the application of photocatalytic water splitting technique and product treatment.