A light-heat synergism in the sub-bandgap photocatalytic response of pristine TiO2: a study of in situ diffusion reflectance and conductance

Commonly Existing Hole-Capturer Organics Adsorption-Induced Recombination over Metal/Semiconductor Perimeters: A Possible Important Factor Affecting Photocatalytic Efficiencies

Photocatalysis is a physiochemical effect arising from the relaxation of photoinduced electrons from the conduction band to the valence band. Controlling the electron relaxation to occur through photocatalytic pathways and prohibiting other relaxations is the main scientific thought for photocatalytic studies. It is needed to know the parallel relaxation pathways that can compete with photocatalytic reactions. By means of in situ photoconductances (PCs) and photoinduced absorptions (PAs), the current research studied the photoinduced electron relaxations of the Au/TiO2 in different atmospheres and at different temperatures. The PC and PA relaxations became different and fast when methanol, ethanol, isopropanol, and acetone were introduced; they also tend to decrease as temperature increases, while that of the undecorated TiO2 in all atmospheres and the Au/TiO2 in pure N2 increased. The results indicated that the organic adsorptions over the Au/TO2 perimeters change the relaxation pathway, and a hole-capturing organics adsorption-induced recombination over the Au/TiO2 perimeter was proposed to explain the relaxations. We found that this relaxation also exists for Ag/TiO2, Pt/TiO2, and Au/ZnO, so it is a commonly existing physical course for the metal/semiconductor (M/S) materials. The effect of the organics and M/S structures on the relaxation was discussed, and the relationship with photocatalytic reactions was also analyzed. Our finding means that blocking this relaxation pathway is an effective way to increase photocatalytic activities, which might open a door for highly active photocatalyst developments.

Kinetic pathways of sub-bandgap induced electron transfer in Ag/TiO2 and the effect on isopropanol dehydrogenation under gaseous conditions

Electron transfer and its kinetics play a major role in the photocatalysis of metal/semiconductor systems. Using in situ photoconductances, in situ photoabsorption, and photoinduced spectroscopic techniques, the present research aimed to gain a deep insight into electron transfer pathways and their kinetics for Ag/TiO2 systems under sub-bandgap light illumination and gaseous conditions. The results revealed that electrons generated in TiO2 can transfer to Ag nanoparticles at fast rates, and plasmon-generated electrons in Ag nanoparticles can also transfer to TiO2. However, it was found that plasmon-assisted hot electron transfer efficiency is much lower than the electron transition from the valence band to the conduction band of TiO2. Rather than plasmonic active spots, the results showed that Ag nanoparticles acted as co-catalyst sites bridging electron transfer to recombination in a methanol-containing N2 atmosphere. As a result, photocatalytic isopropanol dehydrogenation was decreased. Independent of Ag decorations, it was also indicated that isopropanol dehydrogenation mainly occurred over TiO2 surfaces; therefore, Ag nanoparticles did not increase photocatalytic activities. Our results may provide a different viewpoint on sub-bandgap light-induced Ag/TiO2 photocatalysis under gaseous conditions; this may also facilitate the understanding of the photocatalytic mechanism of metal/semiconductor systems.

Fermi-level shift, electron separation, and plasmon resonance change in Ag nanoparticle-decorated TiO2 under UV light illumination

Noble metal nanoparticles are widely used as co-catalysts for storing and separating electrons in semiconductor photocatalysis. Thus, evaluating this ability is important and meaningful to understand the photocatalytic mechanism. Employing Ag nanoparticles, the present study combined in situ photoconductance and theoretical analysis to evaluate the Fermi-level (E_F) shift in a UV-illuminated Ag/TiO₂ system under gaseous conditions. Based on this, the role of the Ag nanoparticles in storing and separating electrons was discussed. It was found that the E_F of Ag/TiO₂ is located deeper in the gap and a variation in temperature has less effect on the E_F of Ag/TiO₂ compared to the undecorated TiO₂. The analysis showed that ∼46 electrons can be stored in 10 nm Ag nanoparticles under our experimental conditions, which does not change with temperature. The electron traps in TiO₂ can affect the electron distribution in the TiO₂ and Ag nanoparticles. It was observed that the localized surface plasmon resonance (LSPR) of the Ag nanoparticles exhibited a blue-shift under UV light illumination, which is generally ascribed to the electron storage in the Ag nanoparticles. However, we showed that the blue-shift is not related to the electron storage in the Ag nanoparticles, and thus it cannot be used as an indicator for evaluating their electron-storage ability. The in situ XPS analysis also does not support that the LSPR blue shift is associated with the reduction in the Ag₂O layer and TiO₂.

The Effect of Cu(II) Nanoparticle Decoration on the Electron Relaxations and Gaseous Photocatalytic Oxidations of Nanocrystalline TiO2

A photocatalytic effect arises from the electron relaxation of semiconductors. Directing the electron relaxation toward photocatalytic reactions is the focus of photocatalytic studies. Co-catalyst decoration is a main way to modulate the electron relaxation, and the Cu(II) nanoparticles have been widely studied as an important co-catalyst. However, the detailed mechanism is still not well known. The current study is devoted to investigating the effect of the Cu(II) nanoparticle decoration on the electron relaxations for TiO2 through in situ photochromism and photoconductances, based on which the relation to the photocatalytic properties was discussed. The result shows that the Cu(II)/Cu(0) redox couple assists the double electron transfer from TiO2 to O2, while the Cu(I)/Cu(0) redox couple assists the single electron transfer to O2. Although the Cu(II) decoration changes the mechanism and increases the rate of the electron relaxations, the electron relaxation does not occur via the Cu redox couple assistance. It was found that the electron relaxation kinetics depends on the reduced Cu species, which can be greatly increased when the Cu(II) was reduced to Cu(0). It is also revealed that the electron relaxation corresponds to the electron transfer from TiO2 to O2, but it does not occur through the Cu redox couple assistance. The result also shows that the increase in the electron relaxation is mainly directed toward the recombination rather than photocatalytic reactions. The present research gains some insights on the role of the co-catalysts in the electron relaxations and its relation to photocatalysis; this should be meaningful for designing novel photocatalytic materials.

Catalytic Metasurfaces Empowered by Bound States in the Continuum

Photocatalytic platforms based on ultrathin reactive materials facilitate carrier transport and extraction but are typically restricted to a narrow set of materials and spectral operating ranges due to limited absorption and poor energy-tuning possibilities. Metasurfaces, a class of 2D artificial materials based on the electromagnetic design of nanophotonic resonators, allow optical absorption engineering for a wide range of materials. Moreover, tailored resonances in nanostructured materials enable strong absorption enhancement and thus carrier multiplication. Here, we develop an ultrathin catalytic metasurface platform that leverages the combination of loss-engineered substoichiometric titanium oxide (TiO_2-x) and the emerging physical concept of optical bound states in the continuum (BICs) to boost photocatalytic activity and provide broad spectral tunability. We demonstrate that our platform reaches the condition of critical light coupling in a TiO_2-x BIC metasurface, thus providing a general framework for maximizing light-matter interactions in diverse photocatalytic materials. This approach can avoid the long-standing drawbacks of many naturally occurring semiconductor-based ultrathin films applied in photocatalysis, such as poor spectral tunability and limited absorption manipulation. Our results are broadly applicable to fields beyond photocatalysis, including photovoltaics and photodetectors.