|Fig. 1: Titanium dioxide powder. (Source: Wikimedia Commons)|
In the current hydrocarbon economy today, transportation is powered mainly by burning petroleum fuel, which raises concerns over their limited availability and the environmental pollution caused. A future based on a hydrogen economy would enable us to use hydrogen as a potential source of power for various applications including sustainable transport. One of the many ways to generate hydrogen is to use water splitting with the help of photocatalysts. Semiconductor photocatalysis is known to be an effective method to harness the energy of natural sunlight to split water into hydrogen and oxygen. During this process, the energy of photons is absorbed to excite electrons to the conduction band, leaving holes in the valance band. These electron-hole pairs can then travel to the surface of the semiconductor to participate in water splitting. Pioneering work was done by Fujishima and Honda in 1972 regarding the discovery of water splitting using a titanium dioxide electrode.  Ever since, the field of semiconductor photocatalysis has advanced greatly and extensive work has been done in increasing the photocatalytic efficiency.
The photocatalytic efficiency of titanium dioxide is often limited by fast recombination of electron-hole pairs. In order to tackle this problem, one approach is to synergistically couple titanium dioxide with noble metal nanoparticles such as silver or gold. To this end, Kamat and co-workers synthesized centrosymmetric silver-titanium dioxide core-shell nanostructures to use as photocatalysts under UV irradiation.  They found that the silver core acted as an electron sink, helping to separate the electron-hole pairs generated in titanium dioxide and reducing their recombination rate for more effective photocatalysis. Subsequently, Chen and co-workers also demonstrated the use of non-centrosymmetric gold-titanium dioxide nanostructures with a Janus morphology as photocatalysts under UV light.  The unique advantage of the Janus morphology is that it positions the gold and titanium dioxide on opposite ends of the structure to keep the electron-hole pairs far apart, resulting in further increase in photocatalytic efficiency.
The abundance of sunlight has led to increased emphasis on visible-light photocatalysis. Because of its large band gap of 3.2 eV, titanium dioxide mainly absorbs in the UV region and has very limited absorption in the visible region. However, it was found that by coupling titanium dioxide with plasmonic metal nanoparticles, the visible-light photoactivity of titanium dioxide can potentially be increased. One of the mechanisms used to explain this visible-light photocatalysis is based on the localized surface plasmon resonance (LSPR) effect of metal nanoparticles. Garcia and co-workers reported photocatalytic hydrogen generation using gold-titanium dioxide nanocomposites under visible-light irradiation.  Moreover, hydrogen gas was also evolved using a 532 nm visible light laser, which corresponds to the LSPR wavelength of the gold-titanium dioxide nanocomposites. On the other hand, bare gold and titanium dioxide itself did not exhibit any photocatalytic activity under the same conditions. Using these results, the authors proposed a mechanism based on photoexcitation of the LSPR band of gold under visible-light irradiation leading to charge transfer.  In this process, electrons from gold are injected into the conduction band of titanium dioxide, where they participate in redox reactions and reduce hydrogen ions in solution to form hydrogen gas. Meanwhile, the resulting holes in gold will be quenched by the sacrificial electron donor.
In another work, Cronin and co-workers explained the visible-light photocatalytic process without using the concept of charge transfer.  Instead, they proposed the plasmonic near-field enhancement effect of gold nanoparticles under visible-light irradiation in the LSPR region. They found that using a 633 nm visible light laser (the LSPR wavelength), there was a 66-fold increase in the rate of water splitting of gold-titanium dioxide nanocomposites compared to titanium dioxide itself. The results of electromagnetic simulations indicate the presence of plasmonic near-field enhancement of the gold nanoparticles, which increases the electric field intensity at the titanium dioxide surface around the LSPR wavelength. In the hot spot region, the electric field intensity can reach about 1000 times that of the incident intensity, which greatly increases the rate of electron- hole pair generation.  Furthermore, because the plasmonic near-fields are localized at the titanium dioxide surface, the majority of the electron-hole pairs are generated close to the surface and have a shorter diffusion distance, thus reducing the probability of recombination. Overall, these works provide new avenues towards the development of high-performance titanium dioxide-based photocatalysts.
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