Novel Nanocrystals Enable Electroreduction of Carbon Dioxide and Upconversion Luminescence

Author:Liang Zhi Qin

Supervisor:xu xu zhao zuo ling


Degree Year:2019





The thesis is focused on the applications of electrocatalytic CO2 reduction and upconversion luminescence based on a series of novel nanocrystals.On the electrocatalysis of CO2 reduction,with the fast-increasing number of the global population and rapid development of the economy,the energy demand of human society has also improved with an unprecedented speed.The excessive consumption of fossil fuels,which is the main energy source,leads to more emission of CO2 gas-the main kind of greenhouse gas,and eventually causes the global warming.Therefore,the energy crisis and environment pollution have become the most serious challenges we are facing in the 21st century.Electrocatalytic CO2 reduction to value-added multi-carbon products can be an efficient approach to solve both of problems at the same time,achiving a powerful carbon cycle in nature.Meanwhile,the renewable energy-powered electricity can come from solar,wind and tide,making the electrochemical CO2 converison environmental-friendly,efficient and safe.Cu-based electrocatalysts are most promising for CO2 reduction,attracting a lot of interest.Rational design of the catalyst and reaction system with superior selectivity,activity and stability towards a single product from CO2 reduction is highly desired,but remains challenge.We focus on engineering a series of Cu-based catalysts and conducting electrocatalytic CO2 reduction.We synthesize Cu-based compounds with different composite,characterize the structure and morphology,and calculate the reaction barrier energy for the specific pathway.We then conduct CO2 reduction in both H-cell and Flow-cell systems,and achive efficient CO2 conversion.The results are summarized as follows:1.We provide a Flow-cell system for CO reduction to multi-carbon products using Cu2O derived Cu catalyst.*CO is known as a key rate-limiting step in CO2 reduction to C2+ products.We employ a gas diffusion layer(GDL)with porous structure and hydrophobic property as the working electrode.CO flows to the back side of GDL and the electrolyte flows to the other side loaded on the catalysts using a reaction pump.We build a gas-liquid-solid tri-boundary reaction interface and solve therefore the mass transfer problem.We achieve a selectivity of 85%of multi-carbon products using Cu2O derived Cu catalyst under CO reduction.2.We design a core/shell vacancy engineering catalyst to tune the selectivity.Through high-temperature solvothermal method and in-situ electroreduciton,we utilize sulfur atoms in the nanoparticle core and intentional copper vacancy in the shell to form a copper sulfide/copper-vacancy(Cu2S/Cu-V)catalyst.DFT calculations indicate that C2H4 and C2H5OH share a penultimate reaction intermediate(*C2H3O)until the last three steps.The catalyst with a vacancy on a copper shell and a Cu2S core increases barrier in C2H4 pathway,while leaving C2H5OH pathway most unaffected,thus shifting the selectivity towards C2H5OH.We achieve a C2+ alcohol production rate of 7.3 and 126 mA cm-2 with a selectivity of 23%and 32%in H-cell and Flow-cell systems,respectively.3.We build a composite with metal nitride supported surface Cu catalyst.We synthesize Cu-on-Cu3N catalyst through partial oxidation and the following in-situ reduction.In order to better investigate the role of the Cu3N support,we also synthesize Cu-on-Cu20 and pure Cu.In-situ XAS indicates the stable presence of CuO and Cu+ under CO2 reduction for Cu-on-Cu3N,while Cu-on-Cu2O is derived to Cu after 40 min.HRTEM-EELS mapping and ARXPS further suggest that part of Cu3N resides in the subsurface.We then achieve 64%of C2+ production during the 32 hours of reduction.We believe that the interaction between the subsurface Cu3N and surface Cu decreases the barrier energy of C-C coupling for Cu-on-Cu3N catalyst.Together with the suppressed reduction of Cu+ during reaction,we are able to achive the superior stability.On the upconversion luminescence,upconversion nanocrystals absorb low-energy photons and emit high-energy photons,which takes the advantage of low injury,penetration depth and no fluorescent background on biological cell.Therefore,ratioanl design and modification of upconversion nanocrystals with high luminescent intensity is highly meaningful and desired for the bioapplication.We investigate the upconversion luminescence of novel NaYF4 nanocrystals.We first dope a low concentration of K+ into the host material.It changes the local environment of Er3+ and increases the probability of 4f forbidden transition to enhance the upconversion fluorescence intensity.Meanwhile,we codope Li+ and K+ into NaYF4 with an enhancement of 7 and 10 times of green and red emission,respectively,compared with that without codoped.The result of in-vitro flow assay detection performs a high sensitivity of dual-target detection of two bacterias using the optimized upconversion nanocrystals.In addition,we explore Sc based upconversion host materials-LiScF4 shows a strong upconversion ability under the same condition with that of NaYF4,with an improvement in blue and red emission.