Hierarchical spheres assembled from large ultrathin anatase TiO2 nanosheets for photocatalytic hydrogen evolution from water splitting

We report the synthesis of TiO₂ hierarchical spheres (THS) with large specific surface area via a facile one-pot solvothermal method. The as-prepared THS are self-assembled by ultrathin TiO₂ nanosheets with thickness of several nanometers and they show a uniform spherical morphology with an average size of 500–700 nm. However, the as-prepared light yellow THS exhibit inferior photocatalytic activity for hydrogen evolution from water splitting due to the poor crystallization of TiO₂ and the existence of oxygen vacancies. Significantly, a subsequent thermal treatment improves the crystallinity of THS, reduces the oxygen vacancies, and thereby enhances the photocatalytic performance. It demonstrates that the sample annealed at 550 °C (THS550) exhibits the highest photocatalytic activity, about 5 times higher than that of commercial TiO₂ nanoparticles (CTiO₂). Moreover, the THS550 sample loaded with 1 wt% Pt exhibits an hydrogen evolution rate as high as 17.9 mmol h^?1g^?1, and the corresponding apparent quantum efficiency has been determined to be 28.46% under 350 nm light irradiation.     

Band structure and photocatalytic activities for H2 production of ABi2Nb2O9 (A = Ca,Sr, Ba)

A new series of layered perovskite photocatalysts, ABi₂Nb₂O₉ (A = Ca, Sr, Ba), were synthesized by the conventional solid-state reaction method and characterized by X-ray diffraction (XRD) and UV-visible spectrometer. The results showed that the structure of ABi₂Nb₂O₉ (A = Ca, Sr) is orthorhombic, while that of BaBi₂Nb₂O₉ is tetragonal. The band gaps of CaBi₂Nb₂O₉, SrBi₂Nb₂O₉, and BaBi₂Nb₂O₉ were estimated to be 3.46, 3.43, and 3.30 eV, respectively. It was found from the electronic band structure study, using the density functional theory (DFT) with planewave basis, that the conduction bands of these photocatalysts mainly consist of Nb 4d + Bi 6p + O 2p orbitals and the valence bands are composed of hybridization with O 2p + Nb 4d + Bi 6s orbitals. The photocatalytic activities for water splitting were investigated under UV-light irradiation and indicated that these photocatalysts showed photocatalytic activity for H₂ and O₂ evolution from aqueous solutions containing sacrificial reagents (methanol and Ag⁺).     

Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A = Ca,Sr and Ba)

The crystal structure of a photocatalyst generally plays a pivotal role in its electronic structure and catalytic properties. In this work, we synthesized a series of La/Cr co-doped perovskite compounds ATiO₃ (M = Ca, Sr and Ba) via a hydrothermal method. Their optical properties and photocatalytic activities were systematically explored from the viewpoint of their dependence on structural variations, i.e. impact of bond length and bond angles. Our results show that although La/Cr co-doping helps to improve the visible light absorption and photocatalytic activity of these wide band gap semiconductors, their light absorbance and catalytic performance are strongly governed by the TiO bond length and TiOTi bond angle. A long TiO bond and deviation of TiOTi bond angle away from 180° deteriorate the visible light absorption and photocatalytic activity. The best photocatalytic activity belongs to Sr_0.9La_0.1Ti_0.9Cr_0.1O₃ with an average hydrogen production rate ～2.88 μmol/h under visible light illumination (λ ≥ 400 nm), corresponding to apparent quantum efficiency ～ 0.07%. This study highlights an effective way in tailoring the light absorption and photocatalytic properties of perovskite compounds by modifying cations in the A site.     

Synthesis and photochemical performance of morphology-controlled CdS photocatalysts for hydrogen evolution under visible light

Novel CdS nanomaterials were synthesized by a simple “one-pot” hydrothermal biomolecule-assisted method using glutathione (GSH) as the sulfur source and structure-directing reagent. Various morphologies of CdS photocatalysts, such as solid nanospheres (s-CdS), hollow nanospheres (h-CdS) and nanorods (r-CdS), were obtained by controlling only the hydrothermal temperatures. The X-ray diffraction patterns confirmed that all of the samples were typical hexagonal wurtzite CdS. It was found that the absorption edge of s-CdS was at 465 nm with a greater blue shift compared to that of h-CdS and r-CdS. The photocatalytic activity of s-CdS was superior to that of h-CdS and r-CdS under visible light. Photoluminescence measurements revealed their different photogenerated electron/hole recombination ability, which was in accordance with the order of s-CdS < h-CdS < r-CdS. The excellent photocatalytic activity of s-CdS was ascribed to the small sizes of sub-nanocrystallites, which make it easy for photoinduced electrons and holes on the solid sphere to migrate to the surface and react with water and the sacrificial agent quickly. It was crucial to control the temperature for preparing CdS photocatalysts via hydrothermal methods. The formation mechanism of different morphology might be due to complexation, S-C bond rupture, spherical aggregation and Ostwald ripening processes.     

Synthesis and high photocatalytic hydrogen production of SrTiO3 nanoparticles from water splitting under UV irradiation

Cubic SrTiO₃ powders were synthesized by three methods: the polymerized complex (PC) method, the solid state reaction, and the milling assistant method. The samples obtained were characterized by X-ray diffraction (XRD), UV–vis spectroscopy (UV–vis), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The mean diameters of the as-synthesized SrTiO₃ particles were 30 nm by the polymerized complex method, 140 nm by the solid state reaction, and 30 nm by the milling assistant method. The photocatalytic activity of hydrogen evolution from water splitting over SrTiO₃ powders by the polymerized complex method is higher than that by the solid state reaction and the milling assistant method. Particle size, uniformity of components, and particle aggregation extent affect the photocatalytic activity of SrTiO₃ for hydrogen evolution. The best rate of photocatalytic hydrogen evolution over SrTiO₃ by the polymerized complex method under UV illumination is as high as 3.2 mmol h⁻¹ g⁻¹.     

Photocatalytic hydrogen production using TiO2 coated iron-oxide core shell particles

Photocatalytic water splitting plays a challenging role as it is one of the most important reactions for solving energy, environmental problems and sustainability. Photocatalytic water splitting was improved by using a novel kind of magnetically separable core shell nano photocatalyst TiO₂/Fe₂O₃, prepared by co-precipitation method. It was characterised for particle size (XRD), band gap (UV-DRS), morphology (SEM), particle size (HRTEM), elemental composition (EDS) and electrochemical studies. Photocatalytic splitting of water was examined in tubular reactor of 500 mL capacity with various sacrificial agents viz., methanol, ethanol, acetic acid, lactic acid, EDTA and triethanolamine. To enhance the hydrogen production, various operating parameters viz., effect of sacrificial agents, catalytic dosage, light irradiation and recycle flow rate were optimized. With the optimized operating parameters (0.2 g catalyst dosage, 60 mL/min recycle flow rate, 96 W/m² light irradiation and EDTA as sacrificial agent) the maximum hydrogen achieved was 2700 μmol/h for the quantum yield of 3.86% at 550 nm. The reusability studies were conducted and the TiO₂ coated Fe₂O₃ core shell particles were found to be stable than the plain TiO₂ nano particles. Effective charge transfer from TiO₂ to Fe₂O₃ and the suppression of e^?/h⁺ pair recombination attributed significant enhancement in photoactivity, thereby increasing the hydrogen production.     

Photocatalytic water splitting on protonated form of layered perovskites K0.5La0.5Bi2M2O9 (M = Ta; Nb) by ion-exchange

Protonated layered perovskite oxides H_1.9K_0.3La_0.5Bi_0.1Ta₂O₇ (HKLBT) and H_1.6K_0.2La_0.3Bi_0.1Nb₂O_6.5 (HKLBN), which were prepared from K_0.5La_0.5Bi₂M₂O₉(M = Ta; Nb)(KLBT(N)) by H ion-exchange in 3M HCl solution, were found as good photocatalysts for water splitting under UV light irradiation. The characterization by XRD, ICP and TG revealed that HKLBT(N) retained layered structure of their parent materials KLBT(N) after HCl treatment. An amount of exfoliation of Bi during the protonated process caused the decrease of contribution of Bi 6p in conduction band (CB) and thus resulted in more negative CB potential. HKLBT(N) showed considerable higher photocatalytic activity for H₂ and/or O₂ evolution than KLBT(N) in the absence of sacrificial reagents, which was attributed to the higher position of conduction band and the layered structure after acid treatment. It was concluded that the interlayer modification via ion-exchange for layered K_0.5La_0.5Bi₂M₂O₉ (M = Ta; Nb) is a potential way to construct novel photocatalysts with high activity for water splitting.     

Enhanced photocatalytic hydrogen production over In-rich (Ag–In–Zn)S particles

The ratio of ZnS to AgInS₂ is usually adjusted to tune the band gaps of this quaternary (Ag–In–Zn)S semiconductor to increase photocatalytic activity. In this study, the [Zn]/[Ag] ratio was kept constant. The hydrogen production rate was enhanced by increasing the content of indium sulfide. Compared to the steady H₂ evolution rate obtained with equal moles of indium and silver ([In]/[Ag] = 1, 0.64 L/m² h), that obtained with In-rich photocatalyst ([In]/[Ag] = 2, 3.75 L/m² h) is over 5.86 times higher. The number of nanostep structures, on which the Pt cocatalysts were loaded by photodeposition, increased with the content of indium. The indium-rich samples did not induce phase separation between Ag_xIn_xZn_yS_2x+y and AgIn₅S₈, instead forming a single-phase solid solution. Although the photocatalytic activity decreased slightly for bare In-rich photocatalysts, Pt loading played a critical role in the hydrogen production rate. This study demonstrates the significant effect of In₂S₃ on this unique (Ag–In–Zn)S photocatalyst.