FUNCTIONALIZATION OF EXFOLIATED BLACK PHOSPHORUS FOR CATALYSIS AND ENERGY APPLICATIONS

In view of critical global issues such as fossil fuel depletion, global warming and high energy demand, photocatalysis is gaining an increasing attention by the scientific and industrial community as a highly promising candidate in the process of solar energy harvesting and conversion into fuels, mimicking the natural photosynthetic process. Nobel metal-free and stable hybrid photocatalysts are highly desirable to achieve a solar-driven hydrogen production which satisfies at the same time the criteria of low production cost and environmental sustainability. In principle, as suitable candidates for photocatalytic HER, there are a plethora of 2D semiconductors, among them the interest in black phosphorus has grown dramatically since its first exfoliation in 2014. The direct and narrow band gap, the high carrier mobility and the large specific surface area of BP open up great opportunities in different sectors of physics and chemistry including (photo)catalysis as described in Chapter I, which gives an overall introduction to the research project. In Chapter II, a new three-component TiO2/BP/CoP heterostructure has been developed, where BP nanosheets play efficiently as a photosensitizer and accelerator of electrons, by means of a strong interaction with TiO2 as shown by XPS, thus promoting the transfer of photogenerated carriers. Once BP is added to commercial TiO2 (P25) as only 1% wt., the H2 evolution rate increases up to 4 times reaching a value of 830 µmol/g·h under UV-Vis light irradiation. Integrating CoP nanoparticles as a cocatalyst up to 2% wt., the H2 production is furtherly promoted going to 7400 µmol/g·h. Photoluminescence and electrochemical impedance measurements provide a deep insight into charge carrier separation and transfer respectively, and demonstrated that addition of a very low amount of BP and CoP to TiO2, achieves a much more efficient charge separation and a reduction of the internal resistance, thus speeding up the hydrogen evolution rate. Combining UV-Diffuse Reflectance and Mott-Schottky data, a plausible mechanism for HER process was proposed. In Chapter III, following a similar approach and keeping in mind the idea to avoid the use of noble metals, cobalt co-catalyst was replaced by a cost-effective and ubiquitous metal as copper. The colloidal synthesis of Cu2O NPs without using BP as a support, gave nanostructures with a much broader size distribution, going from tenth of nm to several hundreds of nm, the advantage of in situ growth is a narrower size distribution with an outer diameter going from 100 nm to 200 nm roughly. By careful choice of suitable reducing agent and mild reaction conditions that assure at the same time the unaltered integrity of BP nanosheets and prevent the over-oxidation to Cu(II) or the reduction to Cu(0), the nanohybrid BP-Cu2O could be selectively synthesized. Furthermore, BP-Cu2O was assembled with TiO2 NPs using the latter as main component (≥ 98% wt) and the resulting nanocomposite was tested in the photocatalytic production of hydrogen. Interestingly, it was observed a boosted H2 evolution rate of 12.9 mmol/g h, which is 65 times higher than pristine TiO2 once the light irradiation was extended to IR region.