Fabrication Of Wide Spectral Responsive Dye-Sensitized Semiconductor And Its Photocatalytic Hydrogen Production Property

Fabrication of highly efficient, stable and wide spectral responsive system for photocatalytic hydrogen production is an extremely defiant issue in the field of new energy. Due to the multiformity, tunable molecular structure as well as light responsive ability and range of dye molecules, dye sensitization becomes an important strategy to broaden the light responsive range and improve the photocatalytic hydrogen production activity of wide band-gap semiconductors (such as widely used TiO2 and g-C3N4). However, most conventional metal complexes, organic dyes and natural pigment (such as xanthenes dyes) can just absorb visible light region of 400-600 nm, and cannot response the red as well as IR light region of solar light. In this case, we carried out the investigations into photocatalytic hydrogen production properties and corresponding mechanism over dye-sensitized semiconductors based on molecular designing of dye. co-sensitization and exploiting panchromatic visible/near-IR light responsive dye. The main contents and conclusions are as follows:1. Series of mono/di-nuclear ruthenium bipyridyl complexes with/without terminal carboxyl group are adopted to improve the visible-light-induced photocatalytic hydrogen production activity of TiO2 (P25), and it is found that the molecular structure of dye as well as the linkage mode between dye and T1iO2 can significantly affect the photosensitization effect of the catalyst:1) Di-nuclear ruthenium bipyridyl complexes possess multichannel intramolecular charge transfer (such as MLCT and MMCT) under visible light irradiation due to their "antenna-sensitizer" structure, which will be favorable to improve the light absorption efficiency; 2) The tightly linked dye with TiO2 through carboxyl group is favorable for not only the photogenerated electron injection but also the back-reaction (recombination) process between them; On the other hand, some loosely linkage can also happen between TiO2 and dyes without carboxyl group through 0 and N et al., which can act as the photogenerated electron transfer channel, but the recombination process can be efficiently restrained due to the loosely attached dye can build "dynamic equilibrium between the linkage of the ground state dye with TiO2 and the divorce of the oxidization state dye from the surfaces". Therefore, TiO2, sensitized with di-nuclear ruthenium bipyridyl complex without carboxyl group, exhibits the best photocatalytic hydrogen production activity and stability among the tested dyes, and the corresponding apparent quantum yield (AQY) under 420 nm monochromatic light irradiation is 16.8%, which is about ten times higher than that of dyes with carboxyl group. This investigation can provide some guiding significance on the molecular designing and band-gap controlling of dye, and the fabrication of highly efficient, stable and wide visible light responsive catalyst for photocatalytic hydrogen production.2. Highly asymmetric zinc na-/phthalocyanine derivatives (Zn-tri-PcNc-1, Zn-tri-PcNc-2 and Zn-tri-PcNc-3) based on electronic push-pull effect are utilized to improve the red/NIR light responsive ability of g-C3N4, and it is found that:1) The spectral response range of g-C3N4 can be expanded to red/NIR region of 600-800 nm through phthalocyanine sensitization; 2) The synergistic effect of asymmetric molecular structure, direction of electron transfer and electronic push-pull effect is the key factor for improving the photocatalytic hydrogen production property of phthalocyanine sensitized semiconductor. A high AQY of≥ 1.0% at 700 nm monochromatic light irradiation is obtained over these asymmetric zinc na-/phthalocyanine derivatives sensitized g-C3N4, which is much higher than that of literature values (such as MgPc:～0.07%at 660 nm; MnPc:～0.06%at 670 nm). This investigation will promote the breakthrough of red/NIR-light-hydrogen conversion efficiency.3. CDCA, the most commonly used coadsorbent in phthalocyanine-based solar cells, is introduced into Zn-tri-PcNc-1-Pt/g-C3N4 catalyst during preparation process, and it is found that the coadsorption of CDCA can play dual roles of diminishing the aggregation of Zn-tri-PcNc-1 on g-C3N4 and enhancing the photogenerated electron transfer efficiency of the system. After CDCA coadsorption, the photocatalytic hydrogen production activity of Zn-tri-PcNc-1-Pt/g-C3N4 under λ≥500 nm is improved by 50.5%, and the AQY under 700 nm monochromatic light irradiation further reaches to a new record of 1.85% in the field of phthalocyanine-sensitized semiconductors for hydrogen production. This investigation gives a new strategy for further improving the photosensitization hydrogen production efficiency under red/NIR light based on phthalocyanine sensitization.4. A new photocatalyst, which can response the wide range of 400-800 nm, is successfully fabricated by co-sensitization of g-C3N4 with asymmetric zinc na- /phthalocyanine (Zn-tri-PcNc-1) and organic dye (LI-4) with D-π-A structure, which possess complementary absorption ability and range. It is found that the photocatalytic hydrogen activity of co-sensitized LI-4/g-C3N4/Zn-tri-PcNc-1 catalyst under λ≥420 nm is almost the summation of the two single dye-sensitized system (LI-4/g-C3N4 and Zn-tri-PcNc-1/g-C3N/4), and exhibits high AQY values of 16.3,7.7 and 1.75%under 420,500 and 700 nm monochromatic light irradiation respectively. This investigation gives a new direction for preparation highly efficient, stable and wide spectral responsive hydrogen production system, which is favorable to promote the breakthrough and practical application of light-hydrogen energy conversion efficiency.5. Based on a surface LMCT complex, which is in situ formed between low-cost, environment-friendly ascorbic acid (AA) and TiO2, and an asymmetric zinc na-/phthalocyanine (Zn-tri-PcNc-1), a new co-sensitized hydrogen production catalyst (Zn-tri-PcNc-1-TiO2-AA) which can work under wide range of 400-800 nm is successfully fabricated. It is found that:1) The LMCT catalyst (AA-TiO2) can be in situ and forthwith formed when TiO2 is added into AA aqueous solution, which can response the light region of 400-600 nm and exhibits high photocatalytic hydrogen production activity underλ≥420 nm; 2) With cosensitization of phthalocyanine, Zn-tri-PcNc-1-TiO2-AA exhibits higher photoactivity, and possesses high AQY values under the whole 400-800 nm light region. Especially, high AQY values of 16.9 and 0.97%are obtained under 420 and 700 nm monochromatic lights irradiation respectively. This investigation breaks the conventional cosensitization modes in the field of DSSCs, and gives a new strategy for further expanding the visible and even NIR light responsive ability of wide band-gap semiconductors.6. A bulk heterojunction photocatalyst (P3HT/g-C3N4), which can response the wide light region of 400-700 nm, is constructed and shows robust photocatalytic hydrogen production activity. It is found that:1) The activity of the catalyst can be significantly affected by the type of sacrificial reagents and the corresponding oxidation half-reaction; 2) With AA as sacrificial reagent, the catalyst exhibits extremely high and new record of AQY values of 77.4,59.4,3.2 and 1.7%under 420,500,700 and 760 nm monochromatic lights irradiation respectively. The polymer/polymer heterojunction exhibits not only wider visible/near-IR light responsive but also hydrogen production ability than common dye-sensitized catalyst, which is promising in improving the visible/near-IR. panchromatic light induced hydrogen production activity and stability.