Reactivity of Trapped and Accumulated Electrons in Titanium Dioxide Photocatalysis

Ultrafast Surface-Specific Spectroscopy of Water at a Photoexcited TiO2 Model Water-Splitting Photocatalyst

A critical step in photocatalytic water dissociation is the hole-mediated oxidation reaction. Molecular-level insights into the mechanism of this complex reaction under realistic conditions with high temporal resolution are highly desirable. Here, we use femtosecond time-resolved, surface-specific vibrational sum frequency generation spectroscopy to study the photo-induced reaction directly at the interface of the photocatalyst TiO2 in contact with liquid water at room temperature. Thanks to the inherent surface specificity of the spectroscopic method, we can follow the reaction of solely the interfacial water molecules directly at the interface at timescales on which the reaction takes place. Following the generation of holes at the surface immediately after photoexcitation of the catalyst with UV light, water dissociation occurs on a sub-20 ps timescale. The reaction mechanism is similar at pH 3 and 11. In both cases, we observe the conversion of H2 O into Ti-OH groups and the deprotonation of pre-existing Ti-OH groups. This study provides unique experimental insights into the early steps of the photo-induced dissociation processes at the photocatalyst-water interface, relevant to the design of improved photocatalysts.

Dopant-driven recombination delay and ROS enhancement in nanoporous Cd1-xCuxS heterogeneous photocatalyst for the degradation of DR-23 dye under visible light irradiation

Developing an efficient heterogeneous photocatalyst for environmental remediation and treatment strategies using visible light harvesting processes is promising but challenging. Herein, Cd1-xCuxS materials have been synthesized and characterized by precise analytical tools. Cd1-xCuxS materials exhibited excellent photocatalytic activity for direct Red 23 (DR-23) dye degradation in visible light irradiation. The operational parameters, like dopant concentration, photocatalyst dose, pH, and initial concentration of dye were investigated during the process. The photocatalytic degradation process follows pseudo-first-order kinetics. As compared to other tested materials, 5% Cu doped CdS material revealed superior photocatalytic performance for the degradation of DR-23 (k = 13.96 × 10-3 min-1). Transient absorption spectroscopy, EIS, PL, and transient photocurrent indicated that adding copper to the CdS matrix improved the separation of photo-generated charge carriers by lowering the recombination rate. Spin-trapping experiments recognized the photodegradation primarily based on secondary redox products, i.e., hydroxyl and superoxide radicals. According to by Mott-Schottky curves, photocatalytic mechanism and photo-generated charge carrier density were elucidated regarding dopant-induced valence and conduction bands shifting. Thermodynamic probability of radical formation in line with the altered redox potentials by Cu doping has been discussed in the mechanism. The identification of intermediates by mass spectrometry study also showed a plausible breakdown mechanism for DR-23. Moreover, samples treated with nanophotocatalyst displayed excellent results when tested for water quality metrics such as DO, TDS, BOD, and COD. Developed nanophotocatalyst shows high recyclability with superior heterogeneous nature. 5% Cu-doped CdS also exhibit strong photocatalytic activity for the degradation of colourless pollutant bisphenol A (BPA) under visible light (k = 8.45 × 10-3 min-1). The results of this study offer exciting opportunities to alter semiconductors' electronic band structures for visible-light-induced photocatalytic activity for wastewater treatment.

Photoinduced Dynamics at the Water/TiO_{2}(101) Interface

We present a femtosecond time-resolved optical pump-soft x-ray probe photoemission study in which we follow the dynamics of charge transfer at the interface of water and anatase TiO_{2}(101). By combining our observation of transient oxygen O 1s core level peak shifts at submonolayer water coverages with Ehrenfest molecular dynamics simulations we find that ultrafast interfacial hole transfer from TiO_{2} to molecularly adsorbed water is completed within the 285 fs time resolution of the experiment. This is facilitated by the formation of a new hydrogen bond between an O_{2c} site at the surface and a physisorbed water molecule. The calculations fully corroborate our experimental observations and further suggest that this process is preceded by the efficient trapping of the hole at the surface of TiO_{2} by hydroxyl species (-OH), that form following the dissociative adsorption of water. At a water coverage exceeding a monolayer, interfacial charge transfer is suppressed. Our findings are directly applicable to a wide range of photocatalytic systems in which water plays a critical role.

A Review on Nano Ti-Based Oxides for Dark and Photocatalysis: From Photoinduced Processes to Bioimplant Applications

Catalysis on TiO₂ nanomaterials in the presence of H₂O and oxygen plays a crucial role in the advancement of many different fields, such as clean energy technologies, catalysis, disinfection, and bioimplants. Photocatalysis on TiO₂ nanomaterials is well-established and has advanced in the last decades in terms of the understanding of its underlying principles and improvement of its efficiency. Meanwhile, the increasing complexity of modern scientific challenges in disinfection and bioimplants requires a profound mechanistic understanding of both residual and dark catalysis. Here, an overview of the progress made in TiO₂ catalysis is given both in the presence and absence of light. It begins with the mechanisms involving reactive oxygen species (ROS) in TiO₂ photocatalysis. This is followed by improvements in their photocatalytic efficiency due to their nanomorphology and states by enhancing charge separation and increasing light harvesting. A subsection on black TiO₂ nanomaterials and their interesting properties and physics is also included. Progress in residual catalysis and dark catalysis on TiO₂ are then presented. Safety, microbicidal effect, and studies on Ti-oxides for bioimplants are also presented. Finally, conclusions and future perspectives in light of disinfection and bioimplant application are given.

Sustainable Carbon Dioxide Reduction of the P3HT Polymer-Sensitized TiO2/Re(I) Photocatalyst

In this study, a p-type π-conjugated polymer chain, poly(3-hexylthiophene-2,5-diyl) (P3HT), was physically adsorbed onto n-type TiO₂ nanoparticles functionalized with a molecular CO₂ reduction catalyst, (4,4-Y₂-bpy)Re^I(CO)₃Cl (ReP, Y = CH₂PO(OH)₂), to generate a new type of P3HT-heterogenized hybrid system (P3HT/TiO₂/ReP), and its photosensitizing properties were assessed in a heteroternary system for photochemical CO₂ reduction. We found that P3HT immobilization on TiO₂ facilitated photoinduced electron transfer (PET) from photoactivated P3HT* to the n-type TiO₂ semiconductor via rapid interfacial electron injection (∼65 ps) at the P3HT and TiO₂ surface interface (P3HT* → TiO₂). With such effective charge separation, the heterogenization of P3HT onto TiO₂ resulted in a steady electron supply toward the co-adsorbed Re(I) catalyst, attaining durable catalytic activity with a turnover number (TON) of ∼5300 over an extended time period of 655 h over five consecutive photoreactions, without deformation of the adsorbed P3HT polymer. The long-period structural stability of TiO₂-adsorbed P3HT was verified based on a comparative analysis of its photophysical properties before and after 655 h of photolysis. To our knowledge, this conversion activity is the highest reported so far for polymer-sensitized photochemical CO₂ reduction systems. This investigation provides insights and design guidelines for photocatalytic systems that utilize organic photoactive polymers as photosensitizing units.

Aqueous TiO2 Nanoparticles React by Proton-Coupled Electron Transfer

Redox reactions of aqueous colloidal TiO₂ 4 nm nanoparticles (NPs) have been examined, including both citrate-capped and uncapped NPs (c-TiO₂ and uc-TiO₂). Photoreduction gave stable blue colloidal c-TiO₂^R NPs with 10-60 electrons per particle. Equilibration of these reduced NPs with soluble redox reagents such as methylviologen (MV²⁺) provided measurements of the colloid reduction potential as a function of pH. The potentials of c-TiO₂ from pH 2-9 varied linearly with pH, with a slope of -60 ± 5 mV/pH. Estimates of the potential at pH 12 were consistent with extrapolating that line to high pH. The reduction potentials did not correlate with the zeta potentials (ζ) or the surface charge of the NPs across this pH range. Similar reduction potentials were observed for c- and uc-TiO₂ at low pH even though they have quite different ζ potentials. These results show that the common surface-charging explanation of the pH dependence is not tenable in these systems. Oxidation of reduced c-TiO₂^R with the electron-transfer oxidant potassium triiodide (KI₃) occurred with a significant drop in pH, showing that protons were released when the electrons were removed from the NPs. Smaller pH drops were observed for the proton-coupled electron transfer (PCET) reagents O₂ (air) and 4-MeO-TEMPO (4-methoxy-2,2,6,6-tetramethylpiperine-1-oxy radical). The difference in the number of protons released with KI₃ vs O₂ and 4-MeO-TEMPO was roughly one proton per electron removed. Thus, the thermodynamically preferred reactivity of these colloidal TiO₂ NPs is PCET over the pH 2-13 range studied. The measured redox potentials refer to the chemical process TiO₂ + H⁺ + e^- → TiO₂·e^-,H⁺; and therefore they do not correspond with an electronic energy such as a conduction band edge or flat band potential. The 1e^-/1H⁺ stoichiometry means that the TiO₂ reduction potentials correspond to a TiO₂-H bond dissociation free energy (BDFE), determined to be 49 ± 2 kcal mol^-1. The PCET description is consistent with the pH dependence of E(TiO₂/TiO₂·e^-,H⁺), the release of protons upon oxidation, the lack of correlation with ζ potentials, the similarity of capped and uncapped NPs, and the small change in the potential and BDFE from the first to the last electron/proton pair (H atom) removed. This behavior is suggested to be the norm for redox-active oxide/water interfaces.

Synthesis, Characterization, and Photocatalytic, Bactericidal, and Molecular Docking Analysis of Cu-Fe/TiO2 Photocatalysts: Influence of Metallic Impurities and Calcination Temperature on Charge Recombination

This research evaluated the potential photocatalytic efficiency of synthesized Cu-Fe/TiO2 photocatalysts against organic contaminants and biocontaminants through various synthesis methods (Cu-to-Fe ratio, metal loading, and calcination temperature) and reaction parameters (photocatalyst dose, irradiation time, and different initial methyl orange (MO) concentrations). In addition, the best photocatalysts were characterized through Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), differential reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS) analysis techniques. The best metal loading was 1 wt % with 5:5 Cu/Fe ratio and 300 °C calcination temperature (5Cu-5Fe/TiO2-300) having 97% MO decolorization. Further analysis indicates that the metal presence does not generate new channels for de-excitation but clearly affects the intensity and decreases charge recombination. The behavior of the photoluminescence intensity is (inversely) proportional to the activity behavior through the series, indicating that the main catalytic effect of Fe and Cu relates to charge recombination and that the Cu-Fe bimetallic catalyst optimizes such function. Moreover, the best-engineered photocatalysts asserted impactful bacteriostatic efficacy toward the tested Escherichia coli strain (in 30 min), and therefore, molecular docking studies were used to predict the inhibition pathway against E. coli β-lactamase enzyme. The photocatalyst had a high negative docking score (-5.9 kcal mol-1) due to intense interactions within the active site of the enzyme. The molecular docking study revealed that the ligand could inhibit β-lactamase from producing its bactericidal activity.

Influence of TiO2 surface defects on the adsorption of N719 dye molecules

Surface defects influence the dye adsorption on TiO₂ used as a substrate in dye-sensitized solar cells (DSSCs). In this study, we have used different Ar⁺ sputtering doses to create a controlled density of defects on a TiO₂ surface exposed to different pre-heating temperatures in order to analyse the influence of defects on the N719 dye adsorption. TiO₂ was pre-treated using two different treatments. The first treatment involved heating to 200 °C with subsequent sputtering at different doses. The second treatment included heating only, but at four different temperatures starting at 200 °C. After the pre-treatments, the TiO₂ samples were immersed into an N719 dye solution for 24 hours at room temperature to dye the TiO₂ substrates. The amount of Ti³⁺ surface defects introduced by the different pre-treatments and their influence on dye adsorption onto the TiO₂ surface were examined by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and metastable induced electron spectroscopy (MIES). Neutral impact collision ion scattering spectroscopy (NICISS) was used to determine the coverage of the TiO₂ surface by adsorbed dye molecules. It was found that Ti³⁺ surface defects were formed by Ar⁺ sputtering but not by pre-treatment through heating alone. MIES analysis of the outer-most layer and density of states calculations show that the thiocyanate ligand of the N719 dye becomes directed away from the TiO₂ surface. Both XPS and NICISS results indicate that the amount of adsorbed N719 dye decreases with increasing density of Ti³⁺ surface defects. Thus, the generation of surface defects reduces the ability of the TiO₂ surface to adsorb the dye molecules. Heating alone as pre-treatment of the TiO₂ substrates instead increases the dye adsorption, without causing detectable defects on the TiO₂ surface.

Phase-selective active sites on ordered/disordered titanium dioxide enable exceptional photocatalytic ammonia synthesis

Photocatalytic N2 fixation to NH3 via defect creation on TiO2 to activate ultra-stable N[triple bond, length as m-dash]N has drawn enormous scientific attention, but poor selectivity and low yield rate are the major bottlenecks. Additionally, whether N2 preferentially adsorbs on phase-selective defect sites on TiO2 in correlation with appropriate band alignment has yet to be explored. Herein, theoretical predictions reveal that the defect sites on disordered anatase (Ad) preferentially exhibit higher N2 adsorption ability with a reduced energy barrier for a potential-determining-step (*N2 to NNH*) than the disordered rutile (Rd) phase of TiO2. Motivated by theoretical simulations, we synthesize a phase-selective disordered-anatase/ordered-rutile TiO2 photocatalyst (Na-Ad/Ro) by sodium-amine treatment of P25-TiO2 under ambient conditions, which exhibits an efficient NH3 formation rate of 432 μmol g-1 h-1, which is superior to that of any other defect-rich disordered TiO2 under solar illumination with a high apparent quantum efficiency of 13.6% at 340 nm. The multi-synergistic effects including selective N2 chemisorption on the defect sites of Na-Ad with enhanced visible-light absorption, suitable band alignment, and rapid interfacial charge separation with Ro enable substantially enhanced N2 fixation.

Combined Spectroscopic Methods of Determination of Density of Electronic States: Comparative Analysis of Diffuse Reflectance Spectroelectrochemistry and Reversed Double-Beam Photoacoustic Spectroscopy

The diffuse reflectance spectroelectrochemistry (SE-DRS) and reversed double-beam photoacoustic spectroscopy (RDB-PAS) provide unique, complementary information on the density of electronic states (DOS) in the vicinity of the conduction band bottom. The measurements are performed under quite different conditions, representing the solid/liquid and solid/gas interfaces in SE-DRS and RDB-PAS, respectively. DOS profiles obtained from both types of measurements can be considered as unique "fingerprints" of the tested materials. The analysis of DOS profiles recorded for 16 different TiO₂ samples confirms that both methods similarly describe the shapes of DOS profiles around the conduction band edges. The states characterized by energy higher than VBT (valence-band top) + E_g can be considered as electronic states within the conduction band. Recognition of the potential of the conduction band bottom allows one to classify the electronic states as deep or shallow electron traps or conduction band states, which play different roles in photocatalysis. The comparative analysis shows that both methods provide very useful information which can be used in understanding and predicting the photo(electro)catalytic reactivity of semiconductors.

TiO2 Photocatalysis for the Transformation of Aromatic Water Pollutants into Fuels

The growing world energy consumption, with reliance on conventional energy sources and the associated environmental pollution, are considered the most serious threats faced by mankind. Heterogeneous photocatalysis has become one of the most frequently investigated technologies, due to its dual functionality, i.e., environmental remediation and converting solar energy into chemical energy, especially molecular hydrogen. H2 burns cleanly and has the highest gravimetric gross calorific value among all fuels. However, the use of a suitable electron donor, in what so-called “photocatalytic reforming”, is required to achieve acceptable efficiency. This oxidation half-reaction can be exploited to oxidize the dissolved organic pollutants, thus, simultaneously improving the water quality. Such pollutants would replace other potentially costly electron donors, achieving the dual-functionality purpose. Since the aromatic compounds are widely spread in the environment, they are considered attractive targets to apply this technology. In this review, different aspects are highlighted, including the employing of different polymorphs of pristine titanium dioxide as photocatalysts in the photocatalytic processes, also improving the photocatalytic activity of TiO2 by loading different types of metal co-catalysts, especially platinum nanoparticles, and comparing the effect of various loading methods of such metal co-catalysts. Finally, the photocatalytic reforming of aromatic compounds employing TiO2-based semiconductors is presented.

A facile synthesis of Zn-doped TiO2 nanoparticles with highly exposed (001) facets for enhanced photocatalytic performance

It is a great challenge to simultaneously improve the visible light absorption capacity and enhance photon-generated carrier separation efficiency of photocatalysts. Herein, Zn-doped TiO2 nanoparticles with high exposure of the (001) crystal face were prepared via a one-step hydrothermal decomposition method. A detailed analysis reveals that the electronic structures were modulated by Zn doping; thus, the responsive wavelength was extended to 600 nm, which effectively improved the visible light absorption of TiO2. More importantly, the surface heterojunction of TiO2 was created because of the co-existing specific facets of (101) and (001). Therefore, the surface separation efficiency of photogenerated electron and hole pairs was greatly enhanced. So, the optimal TiO2 photocatalyst exhibited excellent photocatalytic activity, in which the Rhodamine B (RhB) degradation efficiency was 98.7% in 60 min, under the irradiation of visible light. This study is expected to provide guidance for the rational design of TiO2 photocatalysts.

Different Kinetic Reactivity of Electrons in Distinct TiO2 Nanoparticle Trap States

Electrons added to TiO₂ and other semiconductors often occupy trap states, whose reactivity can determine the catalytic and stoichiometric chemistry of the material. We previously showed that reduced aqueous colloidal TiO₂ nanoparticles have two distinct classes of thermally-equilibrated trapped electrons, termed Red/e ^- and Blue/e ^-. Presented here are parallel optical and electron paramagnetic resonance (EPR) kinetic studies of the reactivity of these electrons with solution-based oxidants. Optical stopped-flow measurements monitoring reactions of TiO₂/e ^- with sub-stoichiometric oxidants showed a surprising pattern: an initial fast (seconds) decrease in TiO₂/e ^- absorbance followed by a secondary, slow (minutes) increase in the broad TiO₂/e ^- optical feature. Analysis revealed that the fast decrease is due to the preferential oxidation of the Red/e ^- trap states, and the slow increase results from re-equilibration of electrons from Blue to Red states. This kinetic model was confirmed by freeze-quench EPR measurements. Quantitative analysis of the kinetic data demonstrated that Red/e ^- react ~5 times faster than Blue/e ^- with the nitroxyl radical oxidant, 4-MeO-TEMPO. Similar reactivity patterns were also observed in oxidations of TiO₂/e ^- by O₂, which like 4-MeO-TEMPO is a proton-coupled electron transfer (PCET) oxidant, and by the pure electron transfer (ET) oxidant KI₃. This suggests that the faster intrinsic reactivity of one trap state over another on the seconds-minutes timescale is likely a general feature of reduced TiO₂ reactivity. This differential trap state reactivity is likely to influence the performance of TiO₂ in photochemical/electrochemical devices, and it suggests an opportunity for tuning catalysis.

Optimization of Photogenerated Charge Carrier Lifetimes in ALD Grown TiO2 for Photonic Applications

Titanium dioxide (TiO₂) thin films are widely employed for photocatalytic and photovoltaic applications where the long lifetime of charge carriers is a paramount requirement for the device efficiency. To ensure the long lifetime, a high temperature treatment is used which restricts the applicability of TiO₂ in devices incorporating organic or polymer components. In this study, we exploited low temperature (100–150 °C) atomic layer deposition (ALD) of 30 nm TiO₂ thin films from tetrakis(dimethylamido)titanium. The deposition was followed by a heat treatment in air to find the minimum temperature requirements for the film fabrication without compromising the carrier lifetime. Femto-to nanosecond transient absorption spectroscopy was used to determine the lifetimes, and grazing incidence X-ray diffraction was employed for structural analysis. The optimal result was obtained for the TiO₂ thin films grown at 150 °C and heat-treated at as low as 300 °C. The deposited thin films were amorphous and crystallized into anatase phase upon heat treatment at 300–500 °C. The average carrier lifetime for amorphous TiO₂ is few picoseconds but increases to >400 ps upon crystallization at 500 °C. The samples deposited at 100 °C were also crystallized as anatase but the carrier lifetime was <100 ps.

Mobile Small Polarons Qualitatively Explain Conductivity in Lithium Titanium Oxide Battery Electrodes

Lithium titanium oxide Li₄Ti₅O₁₂ is an intriguing anode material promising particularly long-life batteries, due to its remarkable phase stability during (dis)charging of the cell. However, its usage is limited by its low intrinsic electronic conductivity. Introducing oxygen vacancies can be one method for overcoming this drawback, possibly by altering the charge carrier transport mechanism. We use Hubbard corrected density functional theory to show that polaronic states in combination with a possible hopping mechanism can play a crucial role in the experimentally observed increase in electronic conductivity. To gauge polaronic charge mobility, we compute the relative stabilities of different localization patterns and estimate polaron hopping barrier heights.

Correlation of the Photocatalytic Activities of Cu, Ce and/or Pt-Modified Titania Particles with their Bulk and Surface Structures Studied by Reversed Double-Beam Photoacoustic Spectroscopy

Modified titania photocatalyst powder samples were prepared using the sol-gel method for copper (Cu) and cerium (Ce) doping and impregnation for platinum (Pt) loading. Their bulk crystalline structures were investigated using X-ray diffractometry (XRD) with the Rietveld analysis. The surface/bulk structure, surface properties, and morphologies were observed using reversed double-beam photoacoustic spectroscopy (RDB-PAS), nitrogen adsorption, and scanning electron microscopy, respectively. The results from the XRD revealed that all samples were mainly anatase (ca. 80% or higher) with small amounts of rutile and non-crystalline components. The specific surface areas of all samples were in the range of 115–155 m2 g−1. Ce and Cu species were mainly distributed, while Pt was potentially loaded as a partially oxidized form on the titania surface. The results from the RDB-PAS indicated the changing of the energy-resolved distribution of electron traps (ERDT) from the original titania surface upon doping of the metals (Cu, Ce, and Pt), which altered their catalytic activities. The metals photocatalytic activities with UV irradiation were measured in two representative reactions; (a) CO2 evolution from acetic acid under the aerobic condition and (b) H2 evolution from deaerated aqueous methanol. In reaction (a), the Cu and/or Ce modification gave almost the same or slightly lower activity compared to the non-modified titania samples, while platinum loading yielded ca. 5–6 times higher activity. For reaction (b), the photocatalytic tests were divided into two sets; without (b1) and with (b2) Pt deposition during the reaction. Similar enhancements of activity from the Pt loading sample (and by Cu modification) were observed in reaction (b1) without in-situ platinum deposition, while the unmodified and Ce-doped samples were almost inactive. For the activities of reaction (b2) with in-situ platinum deposition, the unmodified samples showed the highest activity while the Cu-modified samples showed significantly lower activity.

Smart, Photocatalytic and Self-Cleaning Asphalt Mixtures: A Literature Review

Nowadays, there is increasing concern in transportation engineering about the use of techniques less harmful to the environment and also about road safety. Heterogeneous photocatalysis based on the application of semiconductor materials onto asphalt mixtures is a promising technology because it can mitigate air pollution and road accidents. The functionalized asphalt mixtures with photocatalytic capability can degrade pollutants, such as damaging gases and oil/grease adsorbed on their surface, from specific reactions triggered by sunlight photons, providing significant environmental and social benefits. In this article, a review of photocatalysis applied in asphalt mixtures is presented. The most important characteristics related to the functionalization of asphalt mixtures for photocatalytic applications and their corresponding characterization are presented, and the achieved main results are also discussed.

TiO₂ Photocatalysis for Transfer Hydrogenation

Catalytic transfer hydrogenation reactions, based on hydrogen sources other than gaseous H₂, are important processes that are preferential in both laboratories and factories. However, harsh conditions, such as high temperature, are usually required for most transition-metal catalytic and organocatalytic systems. Moreover, non-volatile hydrogen donors such as dihydropyridinedicarboxylate and formic acid are often required in these processes which increase the difficulty in separating products and lowered the whole atom economy. Recently, TiO₂ photocatalysis provides mild and facile access for transfer hydrogenation of C=C, C=O, N=O and C-X bonds by using volatile alcohols and amines as hydrogen sources. Upon light excitation, TiO₂ photo-induced holes have the ability to oxidatively take two hydrogen atoms off alcohols and amines under room temperature. Simultaneously, photo-induced conduction band electrons would combine with these two hydrogen atoms and smoothly hydrogenate multiple bonds and/or C-X bonds. It is heartening that practices and principles in the transfer hydrogenations of substrates containing C=C, C=O, N=O and C-X bond based on TiO₂ photocatalysis have overcome a lot of the traditional thermocatalysis' limitations and flaws which usually originate from high temperature operations. In this review, we will introduce the recent paragon examples of TiO₂ photocatalytic transfer hydrogenations used in (1) C=C and C≡C (2) C=O and C=N (3) N=O substrates and in-depth discuss basic principle, status, challenges and future directions of transfer hydrogenation mediated by TiO₂ photocatalysis.

Titanium Dioxide Photocatalysis

Dating from the seminal work of Fujishima et al. [...]

Chiral α-hydroxy acid-coadsorbed TiO2 photocatalysts for asymmetric induction in hydrogenation of aromatic ketones

In enantioselective photohydrogenation of aromatic ketones on TiO₂, the enantioselectivity is strongly affected by not only chiral reagents but also the crystalline phase, surface structure, and morphology of TiO₂.