(513dt) Manipulating Spin Polarization of Titanium Dioxide for Efficient Photocatalytic Hydrogen Production and Pollutant Degradation

Photocatalysis has been regarded as one of the best strategies for hydrogen energy production (from water), environmental remediation (degradation) and synthesis of high value-added chemicals, for which titanium dioxide (TiO₂) serves as the primary photocatalyst owing to its low cost, inertness, nontoxicity, and strong reducing/oxidizing capabilities. The generation of abundant photo-induced electron-hole pairs and their rapid transfer/separation are crucial to maximize the photocatalytic efficiency. To achieve this goal, various strategies have been explored, such as doping with impurity atoms, manipulating exposed facet, introducing vacancies and controlling the morphology and crystal phase. By these means, the electronic structure of TiO₂ is tuned to either extend the light absorption range or accelerate the charge separation. However, in many cases, the intrinsic mechanism behind these results is still unclear, and especially the spin degree of electronic freedom is rarely considered.

The intrinsic characteristics of electrons, such as electron spin properties, could dominate the property of photocatalyst, so the electronic configuration with different spin states may greatly affect the photocatalytic behaviors. Actually, recent work has shown that the performance of some catalysts can be improved by modulating the spin states. For example, the structural distortion in atomically thin nanosheets of Co₃S₄, NiSe₂, and NiS results in delocalized spin states that provides not only a high electrical conductivity but also a low adsorption energy of reaction intermediates in oxygen evolution reaction (OER). Also, it has been consolidated that the OER kinetics photocatalyzed by Mn-NC motifs is dependent on the eg occupancy of Mn³⁺ in volcano type shape with a summit at ca. 0.95. Notably, the induced spin polarization by chiral molecules or chiral films (on TiO₂, Fe₃O₄, CuO, etc.) can improve the performances of electrocatalytic and photoelectrochemical (PEC) water splitting, by means of suppressing the formation of hydrogen peroxide (H₂O₂) and favoring the high yield of paramagnetic triplet molecule oxygen (via the parallel spin alignment of oxygen atoms) due to the chiral induced spin selectivity (CISS) effect. Very recently, the external magnetic field has been applied to strengthen the spin restricted water oxidation process by accelerating the parallel alignment of oxygen radicals during the formation of O–O bond. Generally, both electrocatalysis and photocatalysis require rapid charge transfer and long lifetime of intermediated species for redox reactions, so the electron spin property is expected to be an intrinsic factor affecting the performance of photocatalyst.

TiO₂ commonly is a nonmagnetic semiconductor due to the lack of unpaired electrons. However, recent researches show that semiconductors like TiO₂ and ZnO with abundant metal vacancies exhibit obvious room-temperature ferromagnetism, suggesting the appearance of asymmetric spin-up and spin-down channels in these metal-defected oxides. Also, these photocatalysts show considerably improved activity, but the intrinsic mechanism is still unclear. Fortunately, the above results hint a possible way to modulate the electron spin polarization of metal oxides.

Therefore, in this work, we regulate the spin polarization of electrons by controlling the content of metal vacancies and clarify the relationship between the spin polarization and photocatalytic performance. The characterizations confirm the emergence of spatial spin polarization among Ti-defected TiO₂, which promotes the efficiency of charge separation and surface reaction via the parallel alignment of electron spin orientation. Specifically, Ti_0.936O₂, possessing intensive spin polarization, performs 20-fold increased photocatalytic hydrogen evolution and 8-fold increased phenol photodegradation rates, compared with stoichiometric TiO₂. Notably, we further observed the positive effect of external magnetic fields on photocatalytic activity of spin-polarized TiO₂, attributed to the enhanced electron-spin parallel alignment. This reliable spin polarization-modulation strategy via defects tuning provides practical insights into the design of the spin-dependent electronic structures for highly effective heterogeneous catalysts.