Facet-Dependent Interfacial Charge Transfer in TiO2/Nitrogen-Doped Graphene Quantum Dots Heterojunctions for Visible-Light Driven Photocatalysis

Boosting the Efficiency of Photoactive Rod-Shaped Nanomotors via Magnetic Field-Induced Charge Separation

Photocatalytic nanomotors have attracted a lot of attention because of their unique capacity to simultaneously convert light and chemical energy into mechanical motion with a fast photoresponse. Recent discoveries demonstrate that the integration of optical and magnetic components within a single nanomotor platform offers novel advantages for precise motion control and enhanced photocatalytic performance. Despite these advancements, the impact of magnetic fields on energy transfer dynamics in photocatalytic nanomotors remains unexplored. Here, we introduce dual-responsive rod-like nanomotors, made of a TiO2/NiFe heterojunction, able to (i) self-propel upon irradiation, (ii) align with the direction of an external magnetic field, and (iii) exhibit enhanced photocatalytic performance. Consequently, when combining light irradiation with a homogeneous magnetic field, these nanomotors exhibit increased velocities attributed to their improved photoactivity. As a proof-of-concept, we investigated the ability of these nanomotors to generate phenol, a valuable chemical feedstock, from benzene under combined optical and magnetic fields. Remarkably, the application of an external magnetic field led to a 100% increase in the photocatalytic phenol generation in comparison with light activation alone. By using various state-of-the-art techniques such as photoelectrochemistry, electrochemical impedance spectroscopy, photoluminescence, and electron paramagnetic resonance, we characterized the charge transfer between the semiconductor and the alloy component, revealing that the magnetic field significantly improved charge pair separation and enhanced hydroxyl radical generation. Consequently, our work provides valuable insights into the role of magnetic fields in the mechanisms of light-driven photocatalytic nanomotors for designing more effective light-driven nanodevices for selective oxidations.

Carbon Dot-Modified Branched TiO2 Photoelectrochemical Glucose Sensors with Visible Light Response

The development of a photoelectrochemical (PEC) sensor for the sensitive and rapid detection of glucose is highly desirable. In PEC enzyme sensors, inhibition of the charge recombination of electrode materials is an efficient technique, and detection in visible light can prevent enzyme inactivation due to ultraviolet irradiation. In this study, a visible light-driven PEC enzyme biosensor was proposed, using CDs/branched TiO₂ (B-TiO₂) as the photoactive material and glucose oxidase (GOx) as the identification element. The CDs/B-TiO₂ composites were produced via a facile hydrothermal method. Carbon dots (CDs) can not only act as photosensitizers but also inhibit photogenerated electron and hole recombination of B-TiO₂. Under visible light, electrons in the carbon dots flowed to B-TiO₂ and further to the counter electrode through the external circuit. In the presence of glucose and dissolved oxygen, H₂O₂ generated through the catalysis of GOx could consume electrons in B-TiO₂, causing a decrease in photocurrent intensity. Ascorbic acid was added to ensure the stability of the CDs during the test. Based on the variation of the photocurrent response, the CDs/B-TiO₂/GOx biosensor presented a good sensing performance of glucose in visible light, its detection range was from 0 to 9.00 mM, and the detection limit was 0.0430 mM.

Surface-Terminated Hydroxyl Groups for Deciphering the Facet-Dependent Photocatalysis of Anatase TiO2

Understanding the relation between a crystal facet and photocatalytic performance is of great importance for the development of effective catalysts. In this work, we focus on anatase TiO₂ with controllable exposed facets toward photocatalytic hydrogen evolution by water splitting. By combining temperature-programmed desorption (TPD) and diffuse reflectance infrared spectroscopy (DRIFTS), we obtain that the adsorption of hydroxyl groups and the photo-driven breaking of hydroxyl groups depend strongly on the exposed facets. As a result, the higher catalytic hydrogen evolution activity of TiO₂ enclosed with (101) facets than that of (001) facets should be ascribed to the more favorable depletion of hydroxyl groups. Moreover, graphene quantum dots (GQDs) with rich surface functional groups are deliberately deposited on the TiO₂ surface. The determination of the states and dynamics of surface hydroxyl groups suggests that GQDs facilitate the reaction of hydroxyl groups on (001)TiO₂, thus leading to the activity enhancement. By contrast, the already active (101)TiO₂ become apparently less efficient after GQD deposition due to the restricted reaction of hydroxyl groups. Overall, our findings not only provide a unique guidance for understanding the crystal-plane-dependent photocatalysis but also present a powerful approach by which to tailor the photocatalytic performance.

N-Doped Graphene Quantum Dots/Titanium Dioxide Nanocomposites: A Study of ROS-Forming Mechanisms, Cytotoxicity and Photodynamic Therapy

Titanium dioxide nanoparticles (TiO2 NPs) have been proven to be potential candidates in cancer therapy, particularly photodynamic therapy (PDT). However, the application of TiO2 NPs is limited due to the fast recombination rate of the electron (e−)/hole (h+) pairs attributed to their broader bandgap energy. Thus, surface modification has been explored to shift the absorption edge to a longer wavelength with lower e−/h+ recombination rates, thereby allowing penetration into deep-seated tumors. In this study, TiO2 NPs and N-doped graphene quantum dots (QDs)/titanium dioxide nanocomposites (N-GQDs/TiO2 NCs) were synthesized via microwave-assisted synthesis and the two-pot hydrothermal method, respectively. The synthesized anatase TiO2 NPs were self-doped TiO2 (Ti3+ ions), have a small crystallite size (12.2 nm) and low bandgap energy (2.93 eV). As for the N-GQDs/TiO2 NCs, the shift to a bandgap energy of 1.53 eV was prominent as the titanium (IV) tetraisopropoxide (TTIP) loading increased, while maintaining the anatase tetragonal crystal structure with a crystallite size of 11.2 nm. Besides, the cytotoxicity assay showed that the safe concentrations of the nanomaterials were from 0.01 to 0.5 mg mL−1. Upon the photo-activation of N-GQDs/TiO2 NCs with near-infrared (NIR) light, the nanocomposites generated reactive oxygen species (ROS), mainly singlet oxygen (1O2), which caused more significant cell death in MDA-MB-231 (an epithelial, human breast cancer cells) than in HS27 (human foreskin fibroblast). An increase in the N-GQDs/TiO2 NCs concentrations elevates ROS levels, which triggered mitochondria-associated apoptotic cell death in MDA-MB-231 cells. As such, titanium dioxide-based nanocomposite upon photoactivation has a good potential as a photosensitizer in PDT for breast cancer treatment.

A drastic improvement in photocatalytic H2 production by TiO2 nanosheets grown directly on Ta2O5 substrates

Titanium dioxide (TiO₂) is the most frequently studied semiconducting material for photocatalytic water splitting. One of the favored forms of TiO₂ for photocatalytic applications is layers of erected single-crystalline anatase nanosheets (NSs), while the most frequently reported substrate used for its synthesis is a fluorine-doped tin oxide (FTO). Herein we demonstrate that anatase TiO₂ NS layers can be similarly grown on a Ta₂O₅ substrate. We found that a Ta₂O₅ back contact provides a remarkable improvement of the photocatalytic activity of the TiO₂ NSs in comparison to a FTO back contact. The TiO₂ NSs on Ta₂O₅ exhibit a 170-fold increase in photocatalytic H₂ production rate than that obtained by TiO₂ NSs on FTO substrate. The proposed mechanism reveals that such a drastic enhancement of optimized TiO₂ NS arrays on the Ta₂O₅ substrate is attributed to the blocking nature of Ta₂O₅ for photo-generated electrons in the TiO₂ NSs.

TiO2-Graphene Quantum Dots Nanocomposites for Photocatalysis in Energy and Biomedical Applications

The focus of current research in material science has shifted from “less efficient” single-component nanomaterials to the superior-performance, next-generation, multifunctional nanocomposites. TiO2 is a widely used benchmark photocatalyst with unique physicochemical properties. However, the large bandgap and massive recombination of photogenerated charge carriers limit its overall photocatalytic efficiency. When TiO2 nanoparticles are modified with graphene quantum dots (GQDs), some significant improvements can be achieved in terms of (i) broadening the light absorption wavelengths, (ii) design of active reaction sites, and (iii) control of the electron-hole (e−-h+) recombination. Accordingly, TiO2-GQDs nanocomposites exhibit promising multifunctionalities in a wide range of fields including, but not limited to, energy, biomedical aids, electronics, and flexible wearable sensors. This review presents some important aspects of TiO2-GQDs nanocomposites as photocatalysts in energy and biomedical applications. These include: (1) structural formulations and synthesis methods of TiO2-GQDs nanocomposites; (2) discourse about the mechanism behind the overall higher photoactivities of these nanocomposites; (3) various characterization techniques which can be used to judge the photocatalytic performance of these nanocomposites, and (4) the application of these nanocomposites in biomedical and energy conversion devices. Although some objectives have been achieved, new challenges still exist and hinder the widespread application of these nanocomposites. These challenges are briefly discussed in the Future Scope section of this review.

Editorial: Special Issue on “Emerging Trends in TiO2 Photocatalysis and Applications”

It is not an exaggerated fact that the semiconductor titanium dioxide (TiO2) has been evolved as a prototypical material to understand the photocatalytic process and has been demonstrated for various photocatalytic applications such as pollutants degradation, water splitting, heavy metal reduction, CO2 conversion, N2 fixation, bacterial disinfection, etc [...]