| The development foundation of human civilization mainly depends on the consumption of energy.But the decreasing trend of traditional energy(coal,oil,etc.)and environmental problems facilitate the research on sustainable clean energy.As the most promising renewable energy,solar energy has become one of the key research topics in the 21st century.In recent years,the solar energy conversion and storage based on semiconductor photocatalytic process have become a promising advanced technology.Thereinto,the development of efficient and stable photocatalytic materials has become the key part of semiconductor photocatalytic technology.Cadmium sulfide(CdS)is a kind of typical transition metal sulfide(II-VI)semiconductor with an appropriate bandgap(2.4 eV)and position.In addition,as a promising photocatalyst,it possesses good visible light response intensity and reactivity.However,the disadvantages of fast photogenerated carriers recombination and severe photo-corrosion issues hinder the improvement of its photocatalytic efficiency.Based on the energy band engineering theory,this dissertation has studied the structural regulation of CdS and optimized the spatial behavior of photo generated carriers through designing different energy band systems,further investigating the corresponding photocatalytic mechanism.The specific research works include the following four parts:1.Based on the crystal phase engineering,the phase junction CdS photocatalytic nanomaterials with different phases(cubic phase and hexagonal phase)were prepared,which efficiently promoted the separation of photogenerated electrons and holes,thereby improving the photocatalytic hydrogen production performance.The transition from cubic phase to hexagonal phase of CdS was realized via adjusting the acid-base condition,thus establishing the phase junction successfully.Through regulating the bonding region width between these two phases under different growing times,the optimization of energy band structure of CdS was realized.Compared with cubic phase or hexagonal phase CdS,this phase junction CdS with suitable bonding region width exhibited longer lifetime of photogenerated carriers,which endowed it with excellent photocatalytic hydrogen production performance(4.9 mmol·h-1·g-1)and good stability(100 h).Meanwhile,the quantum efficiency at the wavelength of 420 nm was 41.5%.The in-depth study revealed that the recombination of photogenerated carriers in CdS was effectively inhibited by introducing suitable bonding region width,further improving its photocatalytic efficiency and stability.2.Based on the energy band engineering theory,a novel hierarchical tandem p-n heterojunction CdS/Ti3C2/CoO composite photocatalytic material was prepared.The lifetime of photogenerated carriers was prolonged effectively via the multi-internal electric field,realizing the photocatalytic hydrogen production in pure water.The establishment of hierarchical tandem p-n heterojunction was achieved step by step.The good conductivity and affinity of Ti3C2 nanosheets not only solved the compatibility problem between CdS(n-type semiconductor)and CoO(p-type semiconductor)but also built a multi-internal electric field,thereby inhibiting the recombination of photogenerated electron and holes.Thus a photocatalytic H2 evolution activity of 134.46 ?mol·h-1·g-1 was reached in pure water without any sacrificial agents.The in-depth research revealed that the photogenerated electrons and holes were effectively separated by the multi-internal electric field in this special energy band structure,thus avoiding the accumulation of photogenerated holes on CdS.Therefore,the photo-corrosion process of CdS was effectively inhibited,thereby maintaining the good stability(50 h).3.A multi-energy level separation system of Ti3C2(TiO2)/CdS/MoS2 composite was designed and prepared.And the spatial behavior of photogenerated carriers was optimized through the multi-level separation process,further improving the photocatalytic hydrogen production in pure water.The special band matching relationship between MoS2 and CdS promoted the migration of photogenerated electrons to MoS2.Meanwhile,Ti3C2 served as the mediator for attracting holes because of its oxidation feature.Thus,a new Z-scheme between CdS and TiO2 transformed from Ti3C2 oxidation was gradually formed,further strengthening the above process.Therefore,the regulation of electrons and holes separation direction was realized through the multi-energy level separation system,which significantly improved the lifetime of photogenerated carriers and inhibited the photo-corrosion process of CdS.This composite photocatalyst showed excellent photocatalytic H2 evolution of 344.74?mol·h-1·g-1 in pure water and maintained good stability.This design concept provides a new avenue for optimizing photocatalytic hydrogen production in pure water.4.The concept of band-matching transformation was proposed,which was realized in a novel CdS/BCNNTs composite photocatalytic material.The photocatalytic pure water splitting ability was boosted dramatically through the transformation of band-matching relationship from type I to type II.Through the first-principle calculation,the nature of p-n homojunction in BCNNTs was investigated,the formation mechanism of which mainly stemmed from the variation of charge distribution due to the electronegativity difference between heterogeneous atoms.The regulation of bandgap and optimization of p-n homojunction in BCNNTs were manipulated via tuning the carbon content,thereby realizing the transformation of band-matching relationship.Based on the photo-deposition experiment,it proved that the photogenerated electrons would migrate to the CdS surface efficiently and photogenerated holes would move into BCNNTs during the band matching transformation process.The variation in relative positions of band and the introduction of built-in field effectively guaranteed the opposite migration direction of photogenerated carriers,thus greatly boosting their separation efficiency and survival time.The composite photocatalyst exhibited superior photocatalytic hydrogen evolution(526.02 ?mol·h-1·g-1)in pure water and high quantum efficiency(4.01%at 420 nm).The initial performance could still maintain 98%after 50 h of cycling.This work opens up a new design idea for exploring advanced photocatalytic solar energy conversion system. |
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