Synthesis and characterization of metal doped titanium dioxide, transition metal phosphides, sulfides and thiophosphates for photocatalysis and energy applications

• Main Author: Coleman, Nathaniel, Jr.
• Other Authors: Gillan, Edward Gary, 1967-
• Format: Others
• Language: English
• Published: University of Iowa 2015
• Subjects: Chemistry
• Online Access: https://ir.uiowa.edu/etd/2060; https://ir.uiowa.edu/cgi/viewcontent.cgi?article=6705&context=etd

This thesis covers synthetic investigations, characterization, and applications of transition metal doped titanium dioxide materials and transition metal phosphide/sulfide structures. Both areas are useful in heterogeneous catalysis, battery energy storage, and in semiconductor light to energy conversion. Two main synthesis routes have been investigated: 1) rapid solid state metathesis (SSM) of transition metal oxides, phosphides, sulfides, and thiophosphates, and 2) sealed ampoule routes of transition metal phosphides and thiophosphates. SSM reactions tend to yield kinetically controlled multiphase products while sealed ampoule routes gave more thermodynamically favorable single phase materials. Approximately 10 mol% of many first row transition metals (M = Cr, Mn, Fe, Co, Ni, Cu) were targeted for doping into TiO₂, using MClx, and sodium peroxide in SSM reactions, targeting an ideal mixed phase of M₀.₁Ti₀.₉O₂. X-ray diffraction showed rutile TiO₂ forms and no separate dopant metal phases were seen until subsequent 1000 °C annealing in air. EDS, ICP, and XPS analysis showed slightly lower than the targeted M:Ti ratios however, the manganese sample had more than the ideal 10 mol % of dopant. DRS data showed estimated bandgap energies of the doped samples within 1.33-2.55 eV. Magnetic susceptibility showed small paramagnetic responses from all samples that increase upon annealing. SEM showed that the doped SSM-TiO₂ samples were mixtures of aggregates and blocky particles. The synthesized doped titanias were tested for methylene blue and methyl orange photodegradation under UV and visible light and for H₂ generation from water reduction under UV light. The doped titania samples absorb significant amounts of methylene blue dye in the dark with the manganese doped TiO₂ sample being the most absorbent. Degradation of methylene blue under UV illumination was observed, however, only modest degradation under visible light was observed. The samples all performed better than Degussa P25 TiO₂ standard under visible light. The doped titanias did not degrade methyl orange well under UV light and they did not show detectable H₂ generation from water in UV light even with surface platinum additions. Transition metal phosphide, sulfide and thiophosphate materials were synthesized in two different ways. The metal halides FeCl₃, CoCl₂, NiCl₂, and CuCl₂, red phosphorus and elemental sulfur (or P₂S₅) were common to both SSM and ampoule reactions. Both SSM reactions and sealed glass ampoules are solvent-less direct solid state reactions to target FeP₂, CoP₃, NiP₂, CuP₂, FeS₂, CoS₂, NiS₂, CuS, FePS₃, CoPS₃, NiPS₃ and Cu₃PS₄ phases. SSM reactions utilized MClx/Li₃N mixtures to produce elemental metals to then react with P/S reagents leading to metal-rich phosphides, sulfur-rich phases or mixes of M-P-S and sulfur rich phases. Phosphorus-rich phases were seldom seen. Ampoule reactions in contrast, produced single phase phosphorus-rich phases and M-P-S products. Sulfide phases were not produced in ampoule systems. To encourage unique product morphologies, the metal phosphide and thiophosphates were directly synthesized on P25 TiO₂ powders, and pre-made molten fluxes (KCl/LiCl eutectic, tin, or bismuth) to encourage crystal growth of unique structures. M-P products were successfully synthesized in the eutectic and tin fluxes, except for FeP₂ in the halide eutectic flux. FeP₂ was grown in the tin flux at lower than normal reaction temperatures (500 °C vs 700 °C). M-P-S products were seen in the eutectic flux only, while SnS, M-P, or M –Sn-P products were observed in tin fluxes. All reactions were unsuccessful in bismuth flux. The deposition reactions yielded M-P and M-P-S products on P25 TiO₂ powder. The NiP₂ and CuP₂ products were seen while FeP₂ and CoP₃ were not observed. From these samples, only FeP and CoP was detected on P25 TiO₂. The M-P-S reactions formed the targeted phases on P25 TiO₂ successfully. These deposited materials were tested for their photo-reactivity towards water reduction. Preliminary tests for UV light induced and electrolytic hydrogen evolution were done using some ampoule synthesized M-P and M-P-S materials. None of the samples showed H₂ generation using UV light, however H₂ was detected from several MPx and MPxSy materials in this thesis under acidic electrochemical conditions at fairly low applied overpotentials of -40 mV to -240 mV.