| Researchers focus eyes and have got many significant results on clean and renewable energy facing the limit of fossil fuel energy and the environmental concerns. Among of these research results, solar energy conversion and application stands out. The sun irradiation energy on earth is3.78×1024J (about one hundred and twenty thousand terawatts) yearly about ten thousand times than the world’s annual consumptive energy (about15TW). So, solar energy conversion is one of the most promising ways to resolve the energy crisis. Photoelectrochemical (PEC) water splitting or PEC H2/O2generation is one of these environmental friendly techniques by converting solar energy to chemical energy to get clean chemical fuel. Our work focuses on the PEC H2generation. In addition,5%of solar energy distribute in Ultraviolet (UV),43%in visible light,52%in Infrared (IR). The common wide band gap semiconductor, such as TiO2, ZnO, SnO2, WO3, α-Fe2O3, only harvests UV solar energy. Most researchers focus on the work of assembling sensitizers (a relative narrower band gap semiconductor nanoparticles (NPs), such as CdS, CdSe, CdTe, Bi2S3, PbS, InP or organic dye:P3HT, N719) with the common wide band gap semiconductor nanostructure materials, as the skeleton, to sensitized the devices from UV into visible region to harvest more solar energy. Even so, about half of solar energy, from the point of view of spectrum, was unused. For this reason, we want to utilize the IR part solar energy via the narrow band gap semiconductor (PbSe, Bi2Se3and CdSe) NPs sensitized TiO2.As is well known:1. The band gap of a nanoparticle changes with its grain size. Smaller grain size leads to higher surface effect and wider band gap. For utilizing more IR energy, a little larger grain size of NPs is required.2. When different kinds of materials are assembled together, thermodynamic equilibrium achieved and the Fermi levels aligned and to be identical everywhere. It leads to the shift and bending of conduction and valence band edge levels at the junction sites, formation of space charge layer where the photogenerated carriers produced. Furthermore, when an electrode possesses a type Ⅱ cascade structure of band edge levels, the photogenerated carriers will be facilitated across the boundary of nanomaterials flowing into external current circuit. Consequently, solar energy is converted to chemical energy via H2generation. But the narrow band gap metal selenide NPs, PbSe, Bi2Se3and CdSe, attach to TiO2intimately, the type II band edge levels structure cannot form between them and hinder the photocarriers transfer. Lee et al.’s work confirmed that an intermediated layer of CdS NPs lying between CdSe and TiO2, the type II structure of band edge levels formed in CdSe/CdS/TiO2photoelectrode. Inspired by this work, we also introduced an intermediated CdS layer between metal selenide NPs and TiO2, together with the Nitrogen atoms doping into TiO2lattice, to coordinate the band edge levels structure of an electrode (PbSe/CdS/N doped TiO2, Bi2Se3/CdS/Ti02, CdSxSey/TiO2). Here, we chose the Chemical Vapour Deposition (CVD) route, deposited CdS and Bi2Se3NPs in sequence onto TiO2nanorod (NR) arrays to assemble CdS and Bi2Se3co-sensitized TiO2NR arrays (Bi2Se3/CdS/TiO2) photoelectrode.2. TiO2NR arrays were annealed in NH3gas to dope the N atoms into TiO2lattice and got the N doped TiO2NR arrays (NT) electrode. And CdS and PbSe NPs were deposited in sequence onto TiO2nanorod (NR) arrays to assemble CdS and PbSeco-sensitized N doped TiO2NR arrays (PbSe/CdS/NT) photoelectrode used NT sample as the substrate by CVD strategy.3. By CVD strategy, the reaction products, CdSxSeyNPs, were deposited N-doped TiO2NR arrays surfaces to fabricate the CdSxSey sensitized N doped TiO2(CdSxSey/NT) photoelectrode. For compare, CdS NPs sensitized TiO2/N-doped TiO2NR arrays (CdS/TiO2and CdS/NT) photoelectrode.PEC performance of electrodes was characterized in a traditional standard three-arm electrochemical cell, Na2S/0.25M and Na2SO3/0.35M aqueous solution electrolyte, Ag/AgCl (saturated KCl) reference electrode, Pt wire counter electrode. And the electrochemical analysis method was surveyed to investigate the band edge levels structure of a photoelectrode. Here are the novelties of our work:(1). According to the hydrothermal method provide by literatures, with a little modification, we used TiCl4as precursor, concentrated hydrochloric acid (35-38%) and deionized water as solution, Fluorine doped Tin Oxide (FTO) conductive glass as substrate, to assemble large scale uniform, length controlled TiO2NR arrays to assemble photoelectrode. (2). N doped TiO2NR arrays film was fabricated by annealing TiO2NR arrays in NH3/Ar gases in horizontal tube furnace and assembled to electrodes.(3). Inspired by the character of CdS molecule’s sublimation in N2directly, we used CdS powder as raw material and high pure N2as carrier gas, deposited sublimated CdS NPs onto the surfaces of TiO2/N-doped TiO2NR arrays in horizontal tube furnace to assemble CdS NPs sensitized TiO2/N-doped TiO2NR arrays (CdS/TiO2and CdS/N-doped TiO2) photoelectrodes.(4). CdSxSeyNPs were synthesized and deposited onto substrates to assemble CdSxSeyNPs sensitized Ti02/N-doped TiO2NR arrays (CdSxSey/TiO2and CdSxSey/N-doped TiO2) photoelectrodes, H2(10%in Argon gas in volume) as the reaction and carrier gas, CdS and Se powder as precursors by CVD in horizontal tube furnace.(5). Bi2Se3NPs were synthesized and deposited onto substrates, by CVD in horizontal tube furnace, to assemble①Bi2Se3NPs sensitized TiO2NR arrays (Bi2Se3/TiO2) photoelectrode, and②Bi2Se3and CdS NPs co-sensitized TiO2NR arrays (CdS/Bi2Se3/TiO2) photoelectrode, H2(10%in Argon gas in volume) as the reaction and carrier gas, BiI3and Se powder as precursors grounded thoroughly together in stoichiometric proportions.(6). PbSe NPs were synthesized and deposited onto substrates, by CVD in horizontal tube furnace, to assemble①PbSeNPs sensitized TiO2NR arrays (PbSe/TiO2) photoelectrode, and②PbSe and CdS NPs co-sensitized N doped TiO2NR arrays (CdS/PbSe/NT) photoelectrode, H2(10%in Argon gas in volume) as the reaction and carrier gas, PbI2and Se powder as precursors grounded thoroughly together in stoichiometric proportions.(We have also assembled PbSe and CdS NPs co-sensitized TiO2NR arrays (CdS/PbSe/TiO2) photoelectrode, which had inferiorer PEC performance than PbSe/TiO2photoelectrode. We surggested the reason of it is the damage to the conductivity of FTO which leaded to the low photocurrent density. Of course, there must be some factors we had not noticed which we are studying.)The last two photoelectrodes were made to harvest solar energy panchromaticly, with TiO2harvesting UV, CdS NPs harvesting visible and narrow band gap metal selenide NPs harvesting Near-IR solar energy. This is an ideal model and the real ones were needed to be improved the energy conversion efficiency of electrodes to a higher level. And the electrochemical analysis revealed that these two electrodes possess a cascade type II structure of band edge levels, with the help of intermediate layer of CdS NPs and the N atoms doped into TiO2lattice to coordinate the band edge levels structure of electrodes. It has laid the foundation for the coming works.(We have not assembled CdS/Bi2Se3/Ti02and CdS/PbSe/N-doped TiO2electrodes, i.e. putting narrow band gap metal selenide into the intermediate layer, for the apparent type I structure of band edge levels in them both on the results of theoretical calculation and experimental experience when metal selenide NPs adhering to TiO2directly in a large grain size.)... |
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