Nitrate-group-grafting-induced assembly of rutile TiO2 nanobundles for enhanced photocatalytic hydrogen evolution

Optically Active Oxygen Defects in Titanium Dioxide Doped with Inorganic Acid Ions

Doping inorganic acid ions represents a promising pathway to improving the photocatalytic activity of TiO2, and oxygen vacancy has been regarded as the determinant factor for photocatalytic activity. A series of samples doped with Cl-, NO3-, and SO42- was prepared via a simple sol-gel method. Two different oxygen vacancies in the crystal layer of NO3-/TiO2 and Cl-/TiO2 were found, and those are [Ti3+]-V0-[Ti3+] and [Ti3+]-Cl, respectively. The photocurrent of NO3-/TiO2 with [Ti3+]-V0-[Ti3+] is significantly greater than that of Cl-/TiO2 with [Ti3+]-Cl. The least oxygen vacancy is in the gel layer of SO42-/TiO2, and the negligible photocurrent is due to difficulty in forming a stable sol. Furthermore, the process conditions for the application of TiO2 were investigated in this work. The optimal process parameters are to adjust the solution to pH = 3 during sol-gel preparation, to adopt 550 °C as the calcination temperature, and to use an alkaline electrolyte, while the rest of the preparation conditions remain unchanged. This work reveals a new avenue for designing efficient photocatalysts for air pollutant degradation.

Surface Modification of TiO2 by Hyper-Cross-Linked Polymers for Efficient Visible-Light-Driven Photocatalytic NO Oxidation

Solar-driven photocatalysis offers an environmentally friendly and sustainable approach for the removal of air pollutants such as nitric oxides without chemical addition. However, the low specific surface area and adsorption capacity of common photocatalysts restrict the surface reactions with NO at the ppb-level. In this study, imidazolium-based hyper-cross-linked polymer (IHP) was introduced to modify the surface of TiO₂ to construct a porous TiO₂/IHP composite photocatalyst. The as-prepared composite with hierarchical porous structure achieves a larger specific surface area as 309 m²/g than that of TiO₂ (119 m²/g). Meanwhile, the wide light absorption range of the polymer has brought about the strong visible-light absorption of the TiO₂/IHP composite. In consequence, the composite photocatalyst exhibits excellent performance toward NO oxidation at a low concentration of 600 ppb under visible-light irradiation, reaching a removal efficiency of 51.7%, while the generation of the toxic NO₂ intermediate was suppressed to less than 1 ppb. The enhanced NO adsorption and the suppressed NO₂ generation on the TiO₂/IHP surface were confirmed by in situ monitoring technology. This work demonstrates that the construction of a porous structure is an effective approach for efficient NO adsorption and photocatalytic oxidation.

How organic switches grafting on TiO2 modifies the surface potentials: theoretical insights

Hybrid organic switch-inorganic semiconductor systems have important applications in both photo-responsive intelligent surfaces and microfluidic devices. In this context, herein, we performed first-principles calculations to investigate a series of organic switches of trans/cis-azobenzene fluoride and pristine/oxidized trimethoxysilane adsorbed on low-index anatase slabs. The trends in the surface-adsorbate interplay were examined in terms of the electronic structures and potential distributions. Consequently, it was found that the cis-azobenzene fluoride (oxidized trimethoxysilane)-terminated anatase surface attains a lower ionization potential than the trans-azobenzene fluoride (pristine trimethoxysilane)-terminated anatase surface due to its smaller induced (larger intrinsic) dipole moment, whose direction points inwards (outwards) from the substrate, which originates from the electron charge redistribution at the interface (polarity of attached hydroxyl groups). By combining the induced polar interaction analysis and the experimental measurements in the literature, we demonstrate that the ionization potential is an important predictor of the surface wetting properties of adsorbed systems. The anisotropic absorbance spectra of anatase grafted with azobenzene fluoride and trimethoxysilane are also related to the photoisomerization and oxidization process under UV irradiation, respectively.

Exploring the Effects of Crystal Facet Orientation at the Heterojunction Interface on Charge Separation for Photoanodes

As one of the most effective strategies to promote the spatial separation of charges, constructing heterojunction has received extensive attention in recent years. However, it remains unclear whether the crystal facet orientation (CFO) at the heterojunction interface is contributory to charge separation. Herein, three types of TiO₂/CdS heterojunction films with different CFOs at the heterojunction interface were produced by adjusting the CdS CFO through in situ conversion. Among them, the TiO₂/CdS film with a mixed CdS CFO showed the maximum photocurrent density and charge separation efficiency. In contrast, the TiO₂/CdS film with a uniform CdS (100) (CdS-100) performed worst. According to the results of experimentation and DFT calculation, these three types of TiO₂/CdS films varied significantly in electron transport time. This is attributable to the different Fermi levels of CdS CFO and the formation of different built-in electric fields upon coupling with TiO₂. The rise in the Fermi level of CdS can increase the driving force required for charge migration at the heterojunction interface. Additionally, a stronger built-in electric field is conducive to charge separation. To sum up, these results highlight the significant impact of CFO at the heterojunction interface on charge separation.

A dual S-scheme TiO2@In2Se3@Ag3PO4 heterojunction for efficient photocatalytic CO2 reduction

The excellent photoresponse of semiconductors enables them to be promising photocatalysts for CO₂ reduction, but practical application is hampered by fast recombination of photogenerated carriers, low CO₂ capture capacity and poor stability. Herein, mesoporous hollow nanospheres of a dual S-scheme titanium dioxide@indium selenide@silver phosphate (TiO₂@In₂Se₃@Ag₃PO₄) heterojunction with a large specific surface area are designed and synthesized. The products of photocatalytic CO₂ reduction are CH₄, CH₃OH and CO with yields of 3.98, 4.32 and 8.2 μmol g^-1 h^-1, respectively, and the photocatalysts exhibit excellent cycle performance. The excellent photocatalytic performance is attributed to the large specific surface area of the samples and the construction of dual S-scheme heterojunctions. The large specific surface area can provide sufficient active sites for photocatalytic activity. Simultaneously, the built-in electric field (IEF) in the dual S-scheme exposed to light can facilitate the migration of photogenerated electrons from the CB of the oxidation photocatalyst (OP) to the VB of the reduction semiconductor (RP), where they recombine with the photogenerated holes on the VB of the RP, leaving behind photogenerated carriers with high redox ability for photocatalytic activity. This work provides new insights into the mechanism and design of highly efficient heterojunction photocatalysts for CO₂ reduction.

Integrated p-n/Schottky junctions for efficient photocatalytic hydrogen evolution upon Cu@TiO2-Cu2O ternary hybrids with steering charge transfer

Solar-driven photocatalytic H₂ evolution could tackle the issue of fossil fuels-triggered greenhouse gas emission with sustainable clean energy. However, splitting water into hydrogen with high performance by a single semiconductor is challenging because of the poor charge separation efficiency. Herein, a novel ternary Cu@TiO₂-Cu₂O hybrid photocatalyst with multiple charge transfer channels has been designed for efficient solar-to-hydrogen evolution. Indeed, the ternary Cu@TiO₂-Cu₂O hybrid by coupling Cu@TiO₂ with Cu₂O nanoparticles shows highly-efficient photocatalytic hydrogen generation with rate of 12000.6 μmol·g^-1·h^-1, which is 4.4, 2.1, and 1.9 times higher than the pure TiO₂ (2728.8 μmol·g^-1·h^-1), binary Cu@TiO₂ (5595.5 μmol·g^-1·h^-1), and TiO₂-Cu₂O (6076.8 μmol·g^-1·h^-1) composite, respectively. In such a Cu@TiO₂-Cu₂O hybrid, the formed internal electric field in the TiO₂-Cu₂O p-n junction allows the electrons in Cu₂O to migrate to TiO₂, while the electrons in the CB of TiO₂ could flow into Cu via the Schottky junction at the Cu@TiO₂ interface. In this regard, a multiple charge transfer is achieved between the Cu@TiO₂ and Cu₂O, which facilitates promoted charge separation and results in the construction of electron-accumulated center (Cu) and hole-enriched surface (Cu₂O). This p-n/Schottky junctions with steered charge transfer assists the hydrogen production upon the Cu@TiO₂-Cu₂O ternary photocatalyst.

Efficient Charge Transfer Channels in Reduced Graphene Oxide/Mesoporous TiO2 Nanotube Heterojunction Assemblies toward Optimized Photocatalytic Hydrogen Evolution

Interface engineering is usually considered to be an efficient strategy to promote the separation and migration of photoexcited electron-hole pairs and improve photocatalytic performance. Herein, reduced graphene oxide/mesoporous titanium dioxide nanotube heterojunction assemblies (rGO/TiO₂) are fabricated via a facile hydrothermal method. The rGO is anchored on the surface of TiO₂ nanosheet assembled nanotubes in a tightly manner due to the laminated effect, in which the formed heterojunction interface becomes efficient charge transfer channels to boost the photocatalytic performance. The resultant rGO/TiO₂ heterojunction assemblies extend the photoresponse to the visible light region and exhibit an excellent photocatalytic hydrogen production rate of 932.9 μmol h^-1 g^-1 under simulated sunlight (AM 1.5G), which is much higher than that of pristine TiO₂ nanotubes (768.4 μmol h^-1 g^-1). The enhancement can be ascribed to the formation of a heterojunction assembly, establishing effective charge transfer channels and favoring spatial charge separation, the introduced rGO acting as an electron acceptor and the two-dimensional mesoporous nanosheets structure supplying a large surface area and adequate surface active sites. This heterojunction assembly will have potential applications in energy fields.

C. SS, Bajiri MA, Drmosh QAQ, Krishnappagowda LN, Ananda S. Rational design of 2D TiO2-MoO3 Step-scheme heterostructure for boosted photocatalytic overall water splitting

Designing of step-scheme (S-scheme) heterostructure photocatalyst is a promising strategy for the high utilization of photogenerated charge carriers. Herein, a novel S-scheme two-dimensional (2D) TiO2-MoO3 heterojunction photocatalyst is fabricated by...

Ration design of 0D/3D Sn3O4/NiS nanocomposite for enhanced photocatalytic hydrogen generation

Developing economic and high-performance noble-metal-free photocatalysts is the key to realize efficient photocatalysis. In this work, NiS nanoparticles and hierarchical Sn3O4 nanostructures are tightly bonded by a two-step method combining...

Ag NPs decorated C-TiO2/Cd0.5Zn0.5S Z-scheme heterojunction for simultaneous RhB degradation and Cr(VI) reduction

In this study, heterojunction photocatalysts, XAg@C-TCZ, based on MOF-derived C-TiO₂ and Cd_0.5Zn_0.5S decorated with Ag nanoparticles (Ag NPs) were successfully synthesized through hydrothermal and calcination methods. The catalytic effectiveness of XAg@C-TCZ was evaluated by simultaneous photocatalytic degradation of rhodamine B (RhB) and reduction of Cr(VI) under simulated sunlight irradiation. The presence of the Z-scheme heterojunction was demonstrated through trapping experiments, X-ray photoelectron spectroscopy (XPS), time-resolved photoluminescence (PL) investigations, and electron spin resonance (ESR) spectroscopy. With an initial RhB and Cr(VI) concentration of 7 mg L^-1 and 5 mg L^-1, the catalyst 10Ag@C-TCZ achieved a simultaneous removal of 95.2% and 95.5% within 120 min, respectively. With the same catalyst, the degradation rate of RhB was 2.75 times higher and the reduction rate of Cr(VI) was 9.3 times higher compared to pure Cd_0.5Zn_0.5S. Total organic carbon (TOC) analysis confirmed the extent of mineralization of RhB, while the reduction of Cr(VI) was corroborated by XPS. Compared to pure RhB and Cr(VI) solutions, the reaction rates are smaller in the solution containing both contaminants, which is attributed to the competition for ·O₂^-. 10Ag@C-TCZ also exhibited a stable catalytic performance in tap water and lake water. This work provides a new perspective on the construction of heterojunctions with doped MOF derivatives for the purification of complex pollutant systems.

O, S-Dual-Vacancy Defects Mediated Efficient Charge Separation in ZnIn2S4/Black TiO2 Heterojunction Hollow Spheres for Boosting Photocatalytic Hydrogen Production

Defective ZnIn₂S₄ nanosheets/mesoporous black TiO₂ heterojunction hollow spheres (H-ZIS/b-TiO₂) are prepared through hydrothermal and surface low-temperature hydrogenation strategies, which show broad-spectrum response and excellent charge separation efficiency. This H-ZIS/b-TiO₂ flower-like heterojunction hollow spheres with a narrow band gap of ∼1.88 eV expand the light response to visible light and show excellent photocatalytic hydrogen evolution rate (278 μmol h^-1 50 mg^-1) under visible-light irradiation, which is 1.5 times as high as that of ZnIn₂S₄/black TiO₂ heterojunction hollow spheres (ZIS/b-TiO₂) (181 μmol h^-1 50 mg^-1). The excellent photocatalytic performance is due to the formation of O, S dual vacancies in b-TiO₂ and H-ZIS providing more active sites for photocatalytic reaction and improving the charge separation efficiency, heterojunctions promoting transport of photogenerated carriers, and the hollow structure increasing light utilization by reflecting light. The novel heterojunction hollow sphere with high performance has broad application prospects in the field of energy.

ZIF-67 derived Co@NC/g-C3N4 as a photocatalyst for enhanced water splitting H2 evolution

Graphitic carbon nitride (g-C₃N₄), as the one of the most promising photocatalysts, usually relies on noble metal co-catalysts in the photocatalytic water splitting H₂ evolution process, which greatly increases the use cost. Here, a zeolite imidazole framework (ZIF-67) derived Co@NC/g-C₃N₄ composite was constructed through facile thermal condensation of ZIF-67 and melamine. The obtained Co@NC/g-C₃N₄ composites can drive water splitting H₂ evolution without any noble metal co-catalyst under simulated sunlight. The optimal sample exhibits the highest H₂ evolution rate of 161 μmol g^-1·h^-1, which is 6 times of pure g-C₃N₄. The N doped carbon in carbonized ZIF-67 can not only quickly capture separated electrons from g-C₃N_4, but also serve as the co-catalyst. The well dispersed cobalt intermediate on carbonized ZIF-67 also play a role in promoting electron conversion. The formation of junction between carbonized ZIF-67 and g-C₃N₄ could promote quick charge carrier separation and transfer. This work provides a new idea for photocatalytic H₂ evolution without noble metal co-catalysis.