Preparation Of Molybdenum-based Semiconductor Heterojunction And Their Application For Photoelectrocatalytic Reduction Of CO₂

Over-reliance on fossil fuels has led to the serious energy crisis and greenhouse effect facing the world today.Resultantly,global CO₂ levels in the atmosphere was 280ppm in the pre-industrial era;it has now increased to 418 ppm at October 2022according to the Global Monitoring Laboratory（daily global CO₂ trend）,and it is projected to reach up to 570 ppm by the end of the 21st century.Pursuing renewable sources of energy and reducing carbon dioxide in the atmosphere has become a global consensus.Carbon dioxide recycling could reduce global warming and fossil fuel consumption,which is an effective way to alleviate energy and environmental pollution.The photoelectrochemical（PEC）reduction of CO₂ is considered to be a potential strategy for CO₂ recycling and utilization.Metallic organic skeletons（MOFs）derivatives are a new type of CO₂ reduction catalyst emerging in recent years due to their designable porosity,modified skeletons,flexible active site structure,adjustable charge transfer pathways and controllable morphology.At present,there are reports on the photocatalytic CO₂ reduction of MOFs-derived materials,and the reduction products are mainly C1 products,while the efficiency for multi-carbon（C2+）products is still low.Therefore,it is still a challenge to design and develop catalysts with high selectivity for C2+products.So,we designed and synthesized different MOFs derived materials,studied the photoelectrocatalytic CO₂ properties of the materials.The active intermediates in photoelectrocatalytic CO₂ reduction were investigated by Operando infrared spectroscopy（FTIR）and a reasonable reaction path from CO₂ to C2 products was proposed.The main research contents of this dissertation include the following three parts:（1）NiMoO₄/Zn O-x semiconductor heterojunction materials were designed and synthesized,and their performance in photoelectrocatalytic CO₂ reduction was investigated.The CN-containing heterojunction materials with dodecahedra were prepared by MOFs precursors,and were comprehensively characterized in terms of morphological,physicochemical,optical,electrical and semiconductor properties,such as SEM,TEM,XRD,XPS,UV-vis,Raman,PL,LSV,EIS,etc.It was shown that NiMoO₄/Zn O-x can capture visible light and generate photogenerated electrons for CO₂reduction,and the residual CN in the material with graphene-like structure can produce a large number of thermal electrons that contribute to the reduction of CO₂.The optimal catalytic materials NiMoO₄/Zn O-3 heterojunction produced C2 product in a rate of 29.2μM h^-1 cm^-2 under 0.1 M Na₂SO₃ aqueous solution and-0.56 V vs.NHE bias potential,which was 3 times higher than the sum of photocatalytic（PC）and electrocatalytic（EC）CO₂ reduction with 72.6%selectivity for C2 products.The ¹³CO₂ isotope labeling experiments proved that the product is derived from CO₂ gas.The*OCHO and*OCH₂intermediates were observed by Operando FTIR.A new mechanism of bimetallic-center catalytic C-C coupling was proposed in combination with DFT calculations,indicating the formation pathways of C2 products.In addition,the effect of thermal electron on the photoelectrocatalytic performance of NiMoO4/Zn O-3 was also investigated.（2）Cu₂S/MoS₂ octahedral heterojunction materials enriched sulfur vacancies were designed and fabricated and their performance in photoelectrocatalytic CO₂ reduction was investigated.The C-containing heterojunction materials were prepared by MOFs precursors,comprehensively characterized by SEM,TEM,XRD,XPS,UV-vis,Raman,PL,LSV,EIS,etc.Under-0.46 V vs.NHE conditions,the formation rate of C2 products in Cu₂S/MoS₂-Vs heterojunction materials is up to 52μM h^-1 cm^-2 and the total electron transfer rate reaches 541μM cm^-2 h^-1 with a selectivity 100%for C2 products.The introduction of sulfur vacancies directly improves the adsorption performance of CO₂,and the formation of heterojunction significantly reduces the surface charge transfer resistance and increases the charge transfer rate.The C material obtained after MOFs carbonization not only acts as an octahedron skeleton support,but also produces thermal effects under photoelectric conditions.Furthermore,the reduction products are derived from CO₂,proven by ¹³CO₂ isotope labeling experiment.Operando FTIR spectra indicate the existence of*CHO intermediate.（3）CuS/MoN octahedron Schottky junction materials assembled from MoN nanoparticles and CuS nanoparticles were designed and synthesized,and their performance in photoelectrocatalytic CO₂ reduction was investigated.The C-containing Schottky junction materials with dodecahedra were prepared by MOFs precursors,and were comprehensively characterized by SEM,TEM,XRD,XPS,UV-vis,Raman,LSV,EIS,etc.At-0.46 V vs.NHE,CO₂ reduction products were mainly C2 products in the optimal material of CuS/MoN-60 with a production rate of 25μM h^-1 cm^-2.It has been found that the CuS/MoN Schottky junction materials were dominated by C2 products at low bias potential and C1 products at high bias potential.Under a low bias potential,the intermediate might stay on the surface of the material for a long time that favors to C-C coupling into C2 product,while under a higher bias potential,the intermediate is easy to desorb from the material surface reducing the residence time,so that the C-C coupling is hard to archive.Pyridine nitrogen,Schottky junction,and interstitial metal MoN jointly enhance the photoelectrocatalytic CO₂ reduction efficiently.Moreover,the reaction path of CO₂ reduction is proposed based on the ¹³CO₂ isotope labeling and operando FTIR experiments.