Resumen de Tio2-based heterostructure photocatalysts for enhanced hydrogen production
The photocatalytic production of hydrogen from water and biomass derivatives such as ethanol, glycerol and sugars is a highly attractive strategy to generate environmentally benign hydrogen. Ethanol is more easily oxidized than water by holes in the valence band (VB) of photoexcited semiconductors, which also helps to suppress the recombination of electron-hole pairs, thus increasing the usage of electrons in the conduction band (CB) of photoexcited semiconductors to yield hydrogen.
Titanium dioxide (TiO2) has been widely investigated in the field of photocatalysis due to its photosensitivity, low cost, natural abundance, non-toxicity, and good chemical and thermal stability. However, the solar energy conversion efficiency of TiO2 is hindered by its large bandgap (3.2 eV). Here, we demonstrate that the combination of TiO2 with Ni, Co and Cu can substantially promote local spatial charge separation and proton activation in TiO2, achieving high-efficiency for H2 photoproduction.
In chapter 2, we present a strategy to produce porous NiTiO3/TiO2 nanostructures with excellent photocatalytic activity toward hydrogen generation. Nickel-doped TiO2 needle bundles were synthesized by a hydrothermal procedure. Through the sintering in air of these nanostructures, porous NiTiO3/TiO2 heterostructured rods were obtained. Alternatively, the annealing in argon of the nickel-doped TiO2 needle bundles resulted in NiOx/TiO2 elongated nanostructures. Porous NiTiO3/TiO2 structures were tested for hydrogen evolution in the presence of ethanol. Such porous heterostructures exhibited superior photocatalytic activity toward hydrogen generation, with hydrogen production rates up to 11.5 mmol h-1 g-1 at room temperature. In chapter 3, CoTiO3/TiO2 composite catalysts with controlled amounts of highly distributed CoTiO3 nanodomains for photocatalytic ethanol dehydrogenation are developed and studied. To take advantage of solar light, the CoTiO3 nanoparticles with a band gap around 2.3 eV were synthesized by a hydrothermal procedure. We demonstrate these materials to provide outstanding hydrogen evolution rates under UV and visible illumination. The origin of this enhanced activity is extensively analysed. In contrast to previous assumptions, UV-vis absorption spectra and ultraviolet photoelectron spectroscopy (UPS) prove CoTiO3/TiO2 heterostructures to have a type II band alignment, with the CB minimum of CoTiO3 below the H+/H2 energy level. Additional steady-state photoluminescence (PL) spectra, time-resolved PL spectra (TRPLS), and electrochemical characterization prove such heterostructures to result in enlarged lifetimes of the photogenerated charge carriers. These experimental evidences point toward a direct Z-scheme as the mechanism enabling the high photocatalytic activity of CoTiO3/TiO2 composites toward ethanol dehydrogenation.
The optimization of photodehydrogenation of ethanol requires the use of highly active, stable and selective photocatalytic materials based on abundant elements, and the proper adjustment of the reaction conditions, including temperature. In chapter 3, Cu2O-TiO2 type-II heterojunctions with different Cu2O amounts are obtained by a one-pot hydrothermal method. The structural and chemical properties of the produced materials and their activity toward ethanol photodehydrogenation under UV and visible light illumination are evaluated. The structural and chemical properties of the produced materials and their activity toward ethanol photodehydrogenation under UV and visible light illumination are evaluated. The Cu2O-TiO2 photocatalysts exhibit high selectivity toward acetaldehyde production and up to tenfold higher hydrogen evolution rates compared to bare TiO2. We further discern here the influence of temperature and visible light absorption on photocatalytic performance.
