Fabrication And Photoelectrochemical Overall Water-splitting Of MoS₂/g-C₃N₄ Heterojunction Films

Semiconductor-based photoelectrocatalysis for H₂ evolution is an attractive way to solve the environmental pollution and energy shortage crisis. Graphitic carbon nitride?g-C₃N₄? is a novel metal- free semiconductor with an analogous layered structure to graphene. With an energy gap of 2.7 eV, the band edge positions of g-C₃N₄ straddles the redox potential window of water, and g-C₃N₄ shows obvious absorption in the visible-light region. Due to the unique energy structure and excellent thermal and chemical stability, g-C₃N₄ has been attracting a world-wide attention in the photocatalysis filed for water-splitting and degradation of organic pollutants. Like most of semiconductor materials, however, the low photo-quantum efficiency due to the high recombination rate of photogenerated electrons and holes serio usly inhibits the practical applications. Also, as a visible- light response semiconductor, the visible- light utilization of g-C₃N₄ is still not high enough. The recent studies suggest that coupling with other functional semiconductor materials can facilita te the efficient separation and transfer of photogenerated carriers. The composite of two dimension layered materials, with much quicker transfer of photogenerated carriers at the interface due to the larger contact area, can show more ideal photocatalytic activity than the composites from other dimension materials. Among these functional materials, MoS₂, with a narrow band gap of 1.2¹.9 eV, shows good visible- light absorption and photoelectrocatalysis potential, and has an analogous layered structure. Moreover, the intrinsic band positions of MoS₂ and g-C₃N₄ construct a type-II heterojunction energy band structure. Thus, constructing the MoS₂/g-C₃N₄ heterostructures can significantly enhance the photocatalytic performance. In addition, an external voltage can be applied to compensate for the potential deficiency of materials for driving the redox reactions by a photoelectrochemical stategy. Therefore, this doctoral thesis focuses on the fabrication of the MoS₂/g-C₃N₄ heterojunction films and the improvement of their photoelectrochemical performance, aiming at realizing the photoelectrochemical overall water-splitting. The main work includes the followings:?1? The g-C₃N₄ films were successfully prepared on the ITO substrates by the copolymerization of melamine and thiourea via a C VD way. Results suggest thiourea apparently affects the crystal qualities, the surface morphologies and the energy band structures of g-C₃N₄ films by modulating the polymerization process of the precursors, and simultaneously introduces S dopants into the g-C₃N₄ films. The obtained g-C₃N₄ film by adding moderate dosage of thiourea appears uniform, smooth and transparent and can be reproduced. The optical absorption edge locates around 435 nm and the optical band gap is estimated to 2.85 eV. Moreover, the g-C₃N₄ film show very good photoelectrochemical stability and the generated photocurrent keeps at ¹0-5 A/cm2 under visible- light irradiation, which is much higher than the film?¹0-8-10-7 A/cm2? prepared without of thiourea. The successful fabrication of g-C₃N₄ film provides a footstone for constructing the MoS₂/g-C₃N₄ heterojunction films.?2? Ultrathin MoS₂ nanosheets were prepared by a simple hydrothermal route. The obtained MoS₂ nanosheets with a few S-Mo-S structures show a prominent photoluminescence peak and high adsorption capacity?90 mg/g? to methyl orange due to the large specific surface area. Furthermore, the electrical transport properties of MoS₂ nanosheets were successfully tuned by P doping via the intercalating way. Both experimental results and theoretical calculations indicate that P atoms are much easier to intercalate into the interlayers of MoS₂, compared with substituting the lattice Mo and S sites. Due to the intercalation of P atoms, the ordered stacking of?002? plane in MoS₂ nanosheets suffers obvious inhibition, accompanied with the enlargement of local interlayer spacing and the influences on the typic al Raman peaks, which is analyized resonbably by the theoretical calculation result. Besides, from the electronic charge density diff erence profile and Hall data, the extra electrons are introduced into the MoS₂ system by intercalating P atoms. Therefore, the conductivity of MoS₂ can be gradually modulated from p-type to n-type by increasing the concentration of intercalated P atoms. This work provides a factual basis for the formation of the MoS₂/g-C₃N₄ p-n heterojunction film in the subsequent study.?3? MoS₂/g-C₃N₄ p-n heterojunction films were fabricated. A hydrothermal route is used to grow MoS₂ film layer on the g-C₃N₄ film deposited on ITO substrates via a CVD way. The XRD, Raman, XPS and SEM results confirm the existence of heterojunction between MoS₂ and g-C₃N₄. The obtained MoS₂/g-C₃N₄ heterojunction film improves the visible- light absorption of g-C₃N₄ film, facilitates the separation of photogenerated carriers and efficiently suppresses the recombination of photogenerated electrons and holes. Under the visible-light irradiation, the MoS₂/g-C₃N₄ film shows enhanced anodic photocurrent response and with an applied voltage of +0.5V vs Ag/AgC l, the generated photocurrent reaches to ¹.2×10-4 A/cm2, which is about two times of S doped g-C₃N₄ film. The enhanced photoelectrochemical performance benefits from the improved light absorption and the efficient charge separation of photogenerated carriers due to the formation of p-n heterojunction between MoS₂ and g-C₃N₄ films.?4? High quality and uniform MoS₂/g-C₃N₄ n-n heterojunction films were prepared by PLD technique. The characterizations on the structure, component, energy band position and electrical properties suggest the fabricated MoS₂/g-C₃N₄ films are the n-n type heterojunctions, which belong to the type-II band alignments. The n- n type MoS₂/g-C₃N₄ heterojunction films not only broaden the visible-light absorption of single g-C₃N₄ films but also show an enhanced conductivity. During the photoelectrochemical measurements, the MoS₂/g-C₃N₄ films produce an enlarged polarization current?¹0-3A/cm2? and visible- light cathodic photocurrent?¹0-4A/cm2? with an applied voltage of-0.8 V vs SCE. Both in the dark and light conditions, the overall water-splitting phenomena are observed, and the visible- light driven H₂ evolution rate?252 ?mol/h? of MoS₂/g-C₃N₄ film is about 1.3 times of that in the dark case.