Fabrication Of Silicon-based Semiconductor Heterojunctions And Their Performance In Photoelectrocatalytic Reduction Of CO₂

Along with the rapid development of global economy,serious energy crisis and environmental problems have been resulted by the excessive consumption of fossil fuels and the releasing of a large amount of CO₂ gas.The strategy convert CO₂ from the atmosphere into high value-added green fuels using photoelectrocatalytic technology is the best way of killing two birds with one stone to manage energy crisis and environmental problems.The photoelectrocatalytic reduction of CO₂ is a great promising technology to convert CO₂ and H₂O into valuable chemicals like natural photosynthesis in plant.At present,semiconductor is widely used in photoelectrocatalytic system.Compared with many semiconductor materials,silicon semiconductor is the most promising photocathode materials due to its narrow band gap（1.12 e V）,suitable conduction band for reduction reactions and commercial maturity.However,many problems remain to be solved in the application of photoelectrocatalytic system,such as high surface reflectivity,low photogeneration voltage,poor stability and reactivity.To address the problems with semiconductor silicon,additional strategies need to be developed to improve its catalytic performance.In this thesis,the silicon semiconductor was used as the substrate for the photocathode,and diverse semiconductor heterojunction materials were designed and prepared by regulating surface microstructure,morphology control,covalent triazine frameworks modification and other methods.It is not only reduced the reflection of planar silicon,but also injected abundant active sites into the surface of silicon electrode,thus enhancing its photoelectrocatalytic CO₂ reduction activity.The mechanism of photoelectrocatalytic reduction of CO₂ to carbon-based chemicals by using the as-prepared heterojunction photocathode was investigated in detail.The main contents of this thesis include three parts as follows:（1）Si NW@Ti O₂ heterojunction with three-dimensional dendritic branching structure:Heterojunction electrodes with Ti O₂ nanorods grown in situ on different silicon substrate structures were designed and prepared.The effect of the macrostructure of silicon electrode heterojunction on its photoelectrocatalytic reduction of CO₂ was systematically investigated.The results showed that the obvious advantages of silicon substrate electrodes with nanowire array structure（Si NW）have been observed in terms of reduced optical reflectance and CO₂ reduction conversion,compared with planar Si and nanosheet structure（Si NS）.The Si NW@Ti O₂ heterojunction photocathode with a three-dimensional dendritic branching structure exhibited efficient photoelectrocatalytic CO₂reduction performance.At an applied potential of-0.9 V（vs.SCE）,the photocathode Si NW@Ti O₂generated mainly C₂₊products such as ethanol,acetone and acetic acid with an apparent faradaiac efficiency of 713%for the carbon-based products and a corresponding apparent quantum efficiency of 0.63%.This is attributed to its optimized heterojunction interface generating a stronger built-in electric field,which improves the separation efficiency of photogenerated charges.Moreover,the three-dimensional branching structure provided more reactive sites for the adsorption of CO₂molecules and intermediates during the reaction,which is more conducive to the realization of C-C coupling.The key intermediates of*COOH and*CHO during CO₂ reduction was confirmed by using operando FTIR tests,and the possible mechanism of this system was further proposed.（2）Si@WO₃-NS heterojunction mimicking plant thylakoid structure:Silicon nanowires heterojunction photocathode（Si@WO₃-x）modified with different morphologies of WO₃,including nanosheet（NS）,nanoblock（NB）and nanoneedle（NN）,was designed and prepared.The effect of morphology control on the photoelectrocatalytic performance of the CO₂ reduction was studied in Si NW@WO₃-x|SCE|Bi VO₄ system.The results indicated that the C-C coupling process was facilitated and the selectivity of C₂₊products reached 62.7%via the Si@WO₃-NS heterojunction electrode with nanosheet morphology,which attributed to the better simulation of layered structure of the thylakoid.On the other hand,the apparent quantum efficiency was up to 0.49%（comparable to 0.4%in plants）,which was nearly 25 times higher than that of single Si NW photocathode.This benifited from the synergistic effect of heterojunction formation and morphological control,thus effectively promoting the separation of photogenerated electron-hole pairs and enhancing the photoelectrocatalytic performance.Futhermore,the carbon and oxygen sources in the reduction products were proved to be CO₂ and H₂O by ¹³CO₂ and H₂¹⁸O isotope labeling experiments,respectively.And the possible mechanism of photoelectrocatalytic CO₂ reduction was verified by operando FTIR and DFT calculations.（3）Covalent triazine framework（CTFs）modified Si@Cu CTFs heterojunction:Covalent triazine frame with different pore structures modified silicon nanowires heterojunction electrode（Si@Cu CTFs）was firstly fabricated by organic grafting and impregnation methods.And the effects of pore structures and the introduction of copper ions on the photoelectrocatalytic CO₂ reduction performance were systematically investigated.The results proved that ordered porous structures and rich N-sites on the triazine ring of covalent triazine frameworks materials facilitated the adsorption of CO₂ molecules and provided a limited space for CO₂ reduction reaction,which effectively promoted the C-C coupling.Among them,Si@CTF6 electrode containing bipyridine structure unit exhibited a more excellent CO₂ reduction efficiency.,the introduction of metallic copper ions led to the formation of abundant Cu-N coordination sites in Si@Cu CTF6 was confirmed by synchrotron radiation tests,which greatly inhibited the hydrogen precipitation reaction and further enhanced the efficiency of CO2 reduction.Meanwhile,the formation of abundant Cu-N coordination in Si@Cu CTF6 owing to the introduction of copper ions confirmed through synchrotron radiation test,which greatly inhibited the hydrogen evolution reaction and further improved the efficiency of CO₂reduction.At applied potential of-1.4 V（vs.SCE）,the C₂₊product selectivity of Si@Cu CTF6 was as high as 95.6%,and the corresponding apparent faradaiac efficiency and apparent quantum efficiency of carbon-based products reached 510%and 0.89%,respectively.In addition,the surface electrostatic potential,frontier orbitals and energy levels of different structures were analyzed by using DFT calculations,which explained the important role of bipyridine structure in Si@Cu CTF6.Combined with the results of operando FTIR and ¹³CO₂ isotope labeling experiments,a possible photoelectrocatalytic mechanism of the organic-inorganic hybrid heterojunction system was proposed.The possible photoelectrocatalytic mechanism of this organic-inorganic heterojunction system was proposed by combining the results of operando FTIR and ¹³CO₂ isotope labeling experiments.The above research work provides a good scientific idea and basis for the modification of silicon-based semiconductors,the design of heterojunction materials and their applications.