CVD Synthesis Of Metal Selenideand CdS Co-Sensitized TiO₂Nanorod Arrays Photoelectrode

Researchers focus eyes and have got many significant results on clean and renewableenergy facing the limit of fossil fuel energy and the environmental concerns. Among ofthese research results, solar energy conversion and application stands out. The sunirradiation energy on earth is3.78×1024J （about one hundred and twenty thousandterawatts） 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 resolvethe energy crisis. Photoelectrochemical （PEC） water splitting or PEC H₂/O₂generation isone of these environmental friendly techniques by converting solar energy to chemicalenergy 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 assemblingsensitizers （a relative narrower band gap semiconductor nanoparticles （NPs）, such asCdS，CdSe，CdTe，Bi₂S₃，PbS，InP or organic dye: P3HT，N719） with the common wideband gap semiconductor nanostructure materials, as the skeleton, to sensitized the devicesfrom UV into visible region to harvest more solar energy. Even so, about half of solarenergy, from the point of view of spectrum, was unused. For this reason, we want to utilizethe 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 IRenergy, a little larger grain size of NPs is required.2. When different kinds of materials areassembled together, thermodynamic equilibrium achieved and the Fermi levels alignedand to be identical everywhere. It leads to the shift and bending of conduction and valenceband edge levels at the junction sites, formation of space charge layer where thephotogenerated carriers produced. Furthermore, when an electrode possesses a type IIcascade structure of band edge levels, the photogenerated carriers will be facilitated across the boundary of nanomaterials flowing into external current circuit. Consequently, solarenergy is converted to chemical energy via H₂generation. But the narrow band gap metalselenide NPs, PbSe, Bi2Se3and CdSe, attach to TiO₂intimately, the type II band edgelevels structure cannot form between them and hinder the photocarriers transfer. Lee etal.’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 metalselenide NPs and TiO₂, together with the Nitrogen atoms doping into TiO₂lattice, tocoordinate the band edge levels structure of an electrode （PbSe/CdS/N doped TiO₂,Bi2Se3/CdS/TiO₂, CdS_xSe_y/TiO₂）. Here, we chose the Chemical Vapour Deposition （CVD）route, deposited CdS and Bi2Se3NPs in sequence onto TiO₂nanorod （NR） arrays toassemble 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 gotthe N doped TiO₂NR arrays （NT） electrode. And CdS and PbSe NPs were deposited insequence onto TiO₂nanorod （NR） arrays to assemble CdS and PbSeco-sensitized N dopedTiO₂NR arrays （PbSe/CdS/NT） photoelectrode used NT sample as the substrate by CVDstrategy.3. By CVD strategy, the reaction products, CdS_xSe_yNPs, were deposited N-dopedTiO₂NR arrays surfaces to fabricate the CdS_xSe_ysensitized N doped TiO₂（CdS_xSe_y/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-armelectrochemical cell, Na2S/0.25M and Na2SO3/0.35M aqueous solution electrolyte,Ag/AgCl （saturated KCl） reference electrode, Pt wire counter electrode. And theelectrochemical analysis method was surveyed to investigate the band edge levelsstructure of a photoelectrode. Here are the novelties of our work:（1）. According to the hydrothermal method provide by literatures, with a littlemodification, we used TiCl4as precursor, concentrated hydrochloric acid （35-38%） anddeionized water as solution, Fluorine doped Tin Oxide （FTO） conductive glass as substrate,to assemble large scale uniform, length controlled TiO₂NR arrays to assemblephotoelectrode. （2）. N doped TiO₂NR arrays film was fabricated by annealing TiO₂NR arrays inNH3/Ar gases in horizontal tube furnace and assembled to electrodes.（3）. Inspired by the character of CdS molecule’s sublimation in N2directly, we usedCdS powder as raw material and high pure N2as carrier gas, deposited sublimated CdSNPs onto the surfaces of TiO₂/N-doped TiO₂NR arrays in horizontal tube furnace toassemble CdS NPs sensitized TiO₂/N-doped TiO₂NR arrays （CdS/TiO₂and CdS/N-dopedTiO₂） photoelectrodes.（4）. CdS_xSe_yNPs were synthesized and deposited onto substrates to assembleCdS_xSe_yNPs sensitized TiO₂/N-doped TiO₂NR arrays （CdS_xSe_y/TiO₂andCdS_xSe_y/N-doped TiO₂） photoelectrodes, H₂（10%in Argon gas in volume） as the reactionand carrier gas, CdS and Se powder as precursors by CVD in horizontal tube furnace.（5）. Bi2Se3NPs were synthesized and deposited onto substrates, by CVD inhorizontal 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 andcarrier gas, BiI3and Se powder as precursors grounded thoroughly together instoichiometric proportions.（6）. PbSe NPs were synthesized and deposited onto substrates, by CVD in horizontaltube 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 andcarrier gas, PbI2and Se powder as precursors grounded thoroughly together instoichiometric proportions.（We have also assembled PbSe and CdS NPs co-sensitizedTiO₂NR arrays （CdS/PbSe/TiO₂） photoelectrode, which had inferiorer PEC performancethan PbSe/TiO₂photoelectrode. We surggested the reason of it is the damage to theconductivity of FTO which leaded to the low photocurrent density. Of course, there mustbe some factors we had not noticed which we are studying.）The last two photoelectrodes were made to harvest solar energy panchromaticly, withTiO₂harvesting UV, CdS NPs harvesting visible and narrow band gap metal selenide NPsharvesting 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 theelectrochemical analysis revealed that these two electrodes possess a cascade type IIstructure of band edge levels, with the help of intermediate layer of CdS NPs and the Natoms doped into TiO₂lattice to coordinate the band edge levels structure of electrodes. Ithas laid the foundation for the coming works.（We have not assembled CdS/Bi2Se3/TiO₂and CdS/PbSe/N-doped TiO₂electrodes, i.e. putting narrow band gap metal selenide intothe intermediate layer, for the apparent type I structure of band edge levels in them both onthe results of theoretical calculation and experimental experience when metal selenide NPsadhering to TiO₂directly in a large grain size.）...