Facile and green synthesis of titanate nanotube/graphene nanocomposites for photocatalytic H2 generation from water

A facile and green one-step method was used to prepare titanate nanotube/graphene (TNT/GR) photocatalysts via an alkaline hydrothermal process. The as-prepared samples were characterized by X-ray diffraction, transmission electron microscopy, Raman spectroscopy, ultraviolet–visible diffuse reflectance spectroscopy and photoluminescence emission spectroscopy. The photocatalytic performance was evaluated by H₂ generation from water splitting under Xe-lamp illumination. A significantly enhanced photocatalytic activity for H₂ evolution (12.1 μmol/h) was obtained over the compostion-optimized TNT/GR composite (with 1.0 wt% GR), two times higher than that of pure TNT (4.0 μmol/h). During hydrothermal reaction, the reduction of graphene oxide (GO) into GR without using any reducing agents and the formation of 1-D TNT were achieved simultaneously, which resulted in the direct growth of well-defined TNTs uniformly distributed on GR substrates.     

Facile preparation and size-dependent photocatalytic activity of Cu2O nanocrystals modified titania for hydrogen evolution

Degussa TiO₂ powder (P25) with Cu₂O nanocrystals of different sizes deposited on the surface were prepared by a facile ethanol-induced deposition followed with a calcination process at 350 °C and characterized by XRD, XPS, HRTEM and UV–Vis diffuse reflectance spectra, respectively. In our protocol, Cu₂O nanocrystals is formed by reducing Cu(II) with the ethanol adsorbed on the surface of P25. The absorption edge as well as photocatalytic activity for H₂ evolution are closely related to the size of the Cu₂O nanocrystals depending on the Cu content in the as-prepared composites. The 0.9 mol% Cu₂O/TiO₂ composite with ca. 4 nm Cu₂O nanocrystals exhibits the highest photocatalytic H₂ evolution rate of 318 μmol h⁻¹ under the illumination of simulant solar light and an apparent quantum efficiency of 28.6% at 365 nm, 397 times higher than that of P25. This enhancement is mainly attributed to the quantum confinement effect of quantum-sized Cu₂O nanocrystals (q-Cu₂O), which can upgrade the conduction band bottom of q-Cu₂O, and thus facilitates the effective separation of photogenerated charge carriers to enhance the photocatalytic activity. The results provided here not only offers a green and simple way for depositing q-Cu₂O on the surfaces of P25 to get the photocatalyst with an excellent photocatalytic activity for H₂ evolution, but also sheds a new insight for the fabrication of efficient p–n heterojunctions for the separation of electron-hole pairs by quantum confinement effect.     

A green and facile synthesis of TiO2/graphene nanocomposites and their photocatalytic activity for hydrogen evolution

This work reports a green and facile approach to synthesize chemically bonded TiO₂/graphene sheets (GS) nanocomposites using a one-step hydrothermal method. The as-prepared composites were characterized by X-ray diffraction, transmission electron microscopy, Raman spectroscopy and ultraviolet visible (UV-Vis) diffuse reflectance spectra. The photocatalytic activity was evaluated by hydrogen evolution from water splitting under UV-Vis light illumination. An enhancement of photocatalytic hydrogen evolution was observed over the TiO₂/GS composite photocatalysts, as 1.6 times larger for TiO₂/2.0 wt%GS than that of Degussa P25. This fabrication process features the reduction of graphene oxide and formation of TiO₂ simultaneously leading to the well dispersion of generated TiO₂ nanoparticles on the surface of GS.     

Carbon-incorporated TiO2 microspheres: Facile flame assisted hydrolysis of tetrabutyl orthotitanate and photocatalytic hydrogen production

We report a novel and facile energy-saving method to prepare microspherical carbon-incorporated titania powders by flame assisted hydrolysis of tetrabutyl orthotitanate. The as-prepared samples were fully characterized by XRD, SEM, XPS, UV-Vis absorption spectra and photocatalytic hydrogen production. Anatase TiO₂ can be obtained directly without any post heat treatment. The as-prepared TiO₂ is formed of microspheres with sizes in a range of 0.5∼2.0 μm. XPS measurement shows the presence of carbon species which come from the incomplete combustion of organic compounds. Enhanced photocatalytic hydrogen production rates were observed for the as-prepared samples. A maximum hydrogen production rate was 8.1 μmol h⁻¹, which was 1.8 times larger than that of Degussa P25. The improved photocatalytic activity is attributed to enhanced light absorption behavior, which is caused by carbon incorporation and microspherical structure. This work demonstrates a novel and efficient strategy to synthesize microspherical anatase TiO₂ photocatalyst without any special equipment or setup.     

Solvothermal synthesis of SnNb2O6 nanoplates and enhanced photocatalytic H2 evolution under visible light

Well-defined SnNb₂O₆ nanoplates are synthesized here by a facile template-free solvothermal route in a mixed solvent of water and ethanol without an organic surfactant. The synthesized nanoplates have widths ranging from 200 to 400 nm and thicknesses in a range of 20–30 nm. The nanoplates were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), UV–Vis spectroscopy, Raman spectrometry, and by the Brunauer–Emmett–Teller method. The variation of the lattice parameters and the optical properties of the nanoplates were discussed in detail based on the crystal and electronic structure. The SnNb₂O₆ nanoplates exhibited greatly enhanced photocatalytic activity in terms of the reduction of water for H₂ generation under visible light irradiation as compared to the same compound prepared by a solid–state reaction method. This was mainly attributed to its higher surface area and extremely high two-dimensional anisotropy, which provided a short migration distance along the thickness direction.     

Fabrication of porous titanium dioxide fibers and their photocatalytic activity for hydrogen evolution

TiO₂ semiconductor is one of the important photocatalysts for solar light conversion. The challenge is how to improve their efficiency. Creation of porous structures on/in the fibers could favor them a higher surface area as compared to the conventional solid counterparts, which thus could make the achievement for the desired high efficiency. In present work, we report the fabrication of porous TiO₂ fibers with high purity via electrospinning of butyl titanate (TBOT) and polyvinylpyrrolidone (PVP) combined with the subsequent calcination in air. It is found that the TBOT content in the spinning solution plays a profound effect on the growth of the fibers, enabling the synthesis of porous TiO₂ fibers with tunable structures and high purity. The photocatalytic activity for hydrogen evolution of the as-fabricated TiO₂ nanostrcutres has been investigated, suggesting that porous TiO₂ nanomaterials with a high purity and well-defined one-dimensional fiber shape could be an excellent candidate to be utilized as the photocatalyst for hydrogen evolution.     

Simple synthesis and characterization of SrSnO3 nanoparticles with enhanced photocatalytic activity

SrSnO₃ nanoparticles with peanut-like morphologies were synthesized by a simple wet chemical reaction. These peanut-shaped SrSnO₃ were formed by the fusion of two or more nanoparticles with an average size of 45 nm. The resulting powders were characterized in detail using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. Moreover, the photocatalytic activity for hydrogen evolution from pure water was investigated under UV light irradiation. The peanut-shaped nanoparticles exhibited a much higher photocatalytic activity compared to SrSnO₃ powder synthesized by a solid-state reaction. This was attributed to their higher structural order, caused by the formation of a carbonate-free pure phase, as well as their higher surface area resulting from the decrease in the particle size.     

Noble-metal-free CuS/CdS composites for photocatalytic H2 evolution and its photogenerated charge transfer properties

CuS/CdS composites have been successfully prepared by a simple hydrothermal and cation exchange method. Even without noble-metal cocatalyst, the prepared CuS/CdS composites exhibited enhanced photocatalytic H₂ evolution activity. CuS content had a great influence on photocatalytic activity and an optimum amount of CuS was determined to be ca. 3 mol%, at which the CuS/CdS displayed the highest photocatalytic activity, giving an H₂ evolution rate of 332 μmol g⁻¹ h⁻¹, exceeding that of pure CdS by 3.5 times. The results of SPV (surface photovoltage) and SPC (surface photocurrent) revealed that photogenerated electrons were captured by CuS loaded. TPV (transient photovoltage techniques) indicated that photogenerated charges lifetime in CdS, was prolonged with CuS loaded. Those are the main reasons for the improvement of photocatalytic H₂ evolution.     

Low temperature sonochemical synthesis of boron doped TiO2 nanoparticles with enhanced visible light induced photocatalytic activity

Abstract

A simple one-step sonochemical synthesis method for the preparation of boron doped TiO₂ (B-TiO₂) nanoparticles at low temperatures has been developed. The B-TiO₂ nanoparticles were synthesised through sonication of a solution of tetraisopropyl titanium and boric acid in ethanol at 70°C for 150 min. The as prepared B-TiO₂ sample was characterised using X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller surface area and ultraviolet–visible absorption spectroscopy. The average crystallite size of anatase B-TiO₂ is 6·2 nm. B doping of TiO₂ shifted the absorption edge towards higher wavelength. The band energy of B-TiO₂ was estimated as 2·74 eV. The photocatalytic activity of the as prepared photocatalyst was evaluated via the photodegradation of methylene blue dye. The results show that the B-TiO₂ nanoparticles prepared via the sonochemical method exhibit an excellent photocatalytic activity under simulated sunlight.     

Controlled synthesis and photocatalytic investigation of different-shaped one-dimensional titanic acid nanomaterials

Different-shaped one-dimensional (1D) titanic acid nanomaterials (TANs) were prepared by hydrothermal synthesis. By changing the reaction temperature (120, 170 and 200 °C), three kinds of 1D TAN, short-nanotubes (SNT), long-nanotubes (LNT), and nanorods (NR), were obtained. The obtained TANs were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), and solid-stated diffuse reflectance UV–vis spectra (UV–vis DRS) techniques. Based on these 1D TAN, Eosin Y-sensitized Pt-loaded TAN were prepared by the in situ impregnation and photo-reduction method. Their photocatalytic activity for hydrogen generation was evaluated in triethanolamine (TEOA) aqueous solution under visible light irradiation (λ ≥ 420 nm). The results indicated that the morphology difference led to a significant variation of photocatalytic performance for hydrogen generation, with the activity order as follows: Eosin Y-sensitized Pt-loaded LNT > Eosin Y-sensitized Pt-loaded NR > Eosin Y-sensitized Pt-loaded SNT. The experimental conditions for photocatalytic hydrogen generation such as Pt loading content, the mass ratio of Eosin Y to TAN, and so on, were optimized. As a result, the highest apparent quantum yields of hydrogen generation for Eosin Y-sensitized Pt-loaded SNT, LNT, and NR were 6.65, 17.36, and 15.04%, respectively. The stability of these photocatalysts and the reaction mechanism of the photocatalytic hydrogen generation are also discussed in detail.     

Preparation of ZnIn2S4/fluoropolymer fiber composites and its photocatalytic H2 evolution from splitting of water using Xe lamp irradiation

This paper focuses on the preparation of ZnIn₂S₄/fluoropolymer fiber composites and their performance for H₂ evolution from splitting of water using Xe lamp irradiation. Hexafluorobutyl acrylate-co-methacrylic acid (poly(HFBA-co-MAA)) is synthesized by a solution polymerization. Next, the fluoropolymer fibers, which have around 100 nm in average diameter, of poly(HFBA-co-MAA) and polyvinylidene fluoride (PVDF) mixtures are obtained by electrospinning. Then, zinc and indium ions are introduced onto the fiber surface by coordinating with carboxyls of MAA. After that, sulfide ions are incorporated to react with zinc and indium ions by a hydrothermal synthesis. Thus, ZnIn₂S₄ particles of around 800 nm in average size, are obtained and well loaded on the fiber surface. The absorption edge of ZnIn₂S₄/fluoropolymer fiber composites is at 510 nm within the visible-light region. Photocatalytic H₂ evolution from water was investigated using Xe lamp. It was found that the average rate of H₂ evolution of ZnIn₂S₄ powders gradually decreased, while the average rate of H₂ evolution of ZnIn₂S₄/fluoropolymer fiber composites increased from the first to the third run. The average rate of H₂ evolution using the ZnIn₂S₄/fluoropolymer fiber composites as the catalyst achieved 9.1 mL/h in the third run.     

Synergistic effect of Cu2O/TiO2 heterostructure nanoparticle and its high H2 evolution activity

Cu₂O/TiO₂ nanoparticles were prepared by solvothermal method, which formed the heterostructure of Cu₂O/TiO₂. Due to the heterostructure, the H₂ evolution rate under simulated solar irradiation was increasingly promoted. Meanwhile a certain amount of Cu particles which were confirmed by Transmission Electro Microscopy (TEM) and X-Ray Photoelectron Spectroscopy (XPS), formed on the surface of Cu₂O/TiO₂, and the photoactivity was accordingly further enhanced. The stabilized activity was maintained after many times irradiation. It is interesting that after a few hours irradiation the amount of Cu particles on the surface kept unchanged in the presence of Cu₂O and TiO₂. The Cu particles that formed during hydrogen generation reaction play a key role in the further enhancement of the hydrogen production activity. In this study, it is the first time to study the details on the formation of the stable ternary structure under simulated solar irradiation and their synergistic effect on the photoactivity of the water splitting.     

The influence of the electrodeposition potential on the morphology of Cu2O/TiO2 nanotube arrays and their visible-light-driven photocatalytic activity for hydrogen evolution

The influence of the electrodeposition potential on the morphology of Cu₂O/TiO₂ nanotube arrays (Cu₂O/TNA) and their visible-light-driven photocatalytic activity for hydrogen evolution have been investigated for the first time in this work. The photocatalytic hydrogen evolution rate of the as-prepared Cu₂O/TNA at the deposition potential of −0.8 V was about 42.4 times that of the pure TNA under visible light irradiation. This work demonstrated a feasible and simple electrodeposition method to fabricate an effective and recyclable visible-light-driven photocatalyst for hydrogen evolution.     

Effects of crystal and electronic structures of ANb2O6 (A=Ca,Sr, Ba) metaniobate compounds on their photocatalytic H2 evolution from pure water

Alkali-earth metaniobate compounds, ANb₂O₆ (A = Ca, Sr, Ba), were prepared by the conventional solid-state reaction route and their electronic band structures and photocatalytic activities were investigated. The prepared powders were characterized using X-ray diffraction (XRD), field-emission electron microscopy (FE-SEM), UV–vis diffuse reflectance spectroscopy, and fluorescence spectroscopy. It was found that the particle sizes (∼1 μm) and BET surface areas (∼1 m²/g) of the metaniobate compounds were nearly identical. From the electronic band structure calculations, however, the band-gap energies of these metaniobate compounds were found to be in the order of CaNb₂O₆ > SrNb₂O₆ > BaNb₂O₆. These calculated band-gap energies were consistent with those estimated from the UV–vis diffuse reflectance spectra. Moreover, the conduction-band edge (reduction potential) of SrNb₂O₆ calculated from the electronegativity data was higher than those of CaNb₂O₆ and BaNb₂O₆. The photoluminescence spectra revealed that CaNb₂O₆ exhibited a strong blue luminescence emission (at 300K), while no obvious emissions were observed in either SrNb₂O₆ or BaNb₂O₆. The luminescence behaviors of these metaniobate compounds and their band structure variations originating from their crystal structures play an important role in their photocatalytic activity for the evolution of H₂ from pure water. SrNb₂O₆, which has a higher conduction-band edge potential than the other compounds, exhibited higher photocatalytic activity.     

Synthesis of mesoporous-assembled TiO2 nanocrystals by a modified urea-aided sol-gel process and their outstanding photocatalytic H2 production activity

Mesoporous-assembled TiO₂ nanocrystals with very high photocatalytic H₂ production activity were synthesized through a modified sol-gel process with the aid of urea as mesopore-directing agent, heat-treated under various calcination temperatures, and assessed for their photocatalytic H₂ production activity via water splitting reaction. The resulting mesoporous-assembled TiO₂ nanocrystals were systematically characterized by N₂ adsorption-desorption analysis, surface area and pore size distribution analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The experimental results showed that the photocatalytic H₂ production activity of the synthesized mesoporous-assembled TiO₂ nanocrystal calcined at 500 °C, which possessed very narrow pore size distribution, was extraordinarily higher than that of the commercially available P-25 TiO₂ and ST-01 TiO₂ powders.     

A new photosensitive coordination compound [RuL(bpy)2](PF6)2 and its application in photocatalytic H2 production under the irradiation of visible light

A new organic–inorganic photosensitive coordination compound [RuL(bpy)₂](PF₆)₂ (to represent by TM1) had been synthesized by reaction of L (L = 2-hydroxyl-5-(imidazo-[4,5-f]-1,10-phenanthrolin) benzoic acid) with bipyridyl ruthenium, and further characterized by UV–vis, IR, NMR MS and CV. The target photocatalyst 6 wt% TM1-0.5 wt% Pt-TiO₂ (Ⅰ) was obtained by sensitization of Pt-loaded TiO₂ with TM1. The H₂ production activity of target photocatalyst Ⅰ was systematically evaluated by the reaction of photocatalytic H₂ production from water under visible light irradiation. The maximum H₂ evolution of 386.7 μmol in irradiation 3 h and H₂ production rate of 2578 μmol · h⁻¹ · g⁻¹ was detected under the optimal conditions with pH 5, target photocatalyst Ⅰ 50 mg and 5% sacrificial reagent TEOA (v/v).     

Monodispersed p(2-VP) and p(2-VP-co-4-VP) particle preparation and their use as template for metal nanoparticle and as catalyst for H2 production from NaBH4 and NH3BH3 hydrolysis

The monodispersed poly(2-vinyl pyridine) (p(2-VP)) and poly(2-vinyl pyridine-co-4-vinyl pyridine) (p(2-VP-co-4-VP)) particles of different compositions were synthesized by a surfactant-free emulsion polymerization system using divinyl benzene (DVB) as cross-linker. The diameter of p(2-VP) and p(2-VP-co-4-VP) particles were measured between 370 and 530 nm. Co, Ni and Cu metal nanoparticles were prepared inside these microgels after quaternization with HCl and loading of metal salts, such as CoCl₂, NiCl₂, and CuCl₂, in ethyl alcohol followed by reduction with NaBH₄. The prepared metal nanoparticles within these particles were used as catalyst for H₂ production via hydrolysis of NaBH₄ and NH₃BH₃. Various parameters of the polymeric microgels such as template, metal types, reuse, the amount of NaOH, and temperature were investigated. From hydrolysis reactions the activation energy (E_a), enthalpy (ΔH), and entropy (ΔS) were calculated for Co metal nanoparticles as catalyst for the NaBH₄ hydrolysis reaction in the temperature range of 0–50 °C. The activation parameters of NaBH₄ hydrolysis catalyzed by Co nanoparticle composite systems were calculated as 46.44 ± 1.1 kJ mol⁻¹ for E_a, 36.39 ± 6.5 kJ mol⁻¹ for ΔH and −170.56 ± 20.1 kJ mol⁻¹ K⁻¹ for ΔS.     

Synthesis of (CuIn)xCd2(1−x)S2 photocatalysts for H2 evolution under visible light by using a low-temperature hydrothermal method

A new series visible-light driven photocatalysts (CuIn)_xCd_2(1−x)S₂ was successfully synthesized by a simple and facile, low-temperature hydrothermal method. The synthesized materials were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area measurement, X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible spectroscopy (UV–Vis DRS). The results show that the morphology of the photocatalysts changes with the increase of x from 0.01 to 0.3 and their band gap can be correspondingly tuned from 2.37 eV to 2.30 eV. The (CuIn)_xCd_2(1−x)S₂ nanocomposite show highly photocatalytic activities for H₂ evolution from aqueous solutions containing sacrificial reagents, SO₃²⁻ and S²⁻ under visible light. Substantially, (CuIn)_0.05Cd_1.9S₂ with the band gap of 2.36 eV exhibits the highest photocatalytic activity even without a Pt cocatalyst (649.9 μmol/(g h)). Theoretical calculations about electronic property of the (CuIn)_xCd_2(1−x)S₂ indicate that Cu 3d and In 5s5p states should be responsible for the photocatalytic activity. Moreover, the deposition of Pt on the doping sample results in a substantial improvement in H₂ evolution than the Pt-loaded pure CdS and the amount of H₂ produced (2456 μmol/(g h)) in the Pt-loaded doping system is much higher than that of the latter (40.2 μmol/(g h)). The (CuIn)_0.05Cd_1.9S₂ nanocomposite can keep the activity for a long time due to its stability in the photocatalytic process. Therefore, the doping of CuInS₂ not only facilitates the photocatalytic activity of CdS for H₂ evolution, but also improves its stability in photocatalytic process.     

Hydrogen production by thermal partial oxidation of ethanol: Thermodynamics and kinetics study

In this study thermodynamics and kinetics analysis of the thermal partial oxidation (TPOX) of ethanol for producing hydrogen is performed. Equilibrium and kinetics calculations are performed in order to find the limiting parameters for the thermal partial oxidation. The effects of air ratio λ (the ratio of the oxidizer -to- fuel ratio to the stoichiometric oxidizer -to- fuel ratio) and mixture inlet temperatures (T_mix-in) on the reforming efficiency, the H₂ mole number, the reaction progress, the equilibrium time and the ignition delay time are investigated. Furthermore, the analysis is performed using different kinetics schemes and the results are compared. The optimum practical operating conditions of the partial oxidation process of ethanol are identified. In this way, the results of this work can be useful as a guideline in experimental work.     

Decomposition of hydrogen sulfide (H2S) on Ni(100) and Ni3Al(100) surfaces from first-principles

Spin-polarized density functional theory studies of hydrogen sulfide (H₂S) adsorption and decomposition on Ni(100) and Ni₃Al(100) surfaces were conducted to understand the aluminum (Al) alloying effect on H₂S dissociation. For such purpose, we first determined the near surface structure of fully ordered Ni₃Al alloy along the [100] direction by calculating the Al segregation energy to the surface and then examined the adsorption energies of the adsorbates (H₂S, HS, S, and H) and the activation barriers for the H₂S and HS decomposition by using Climbing Image-Nudged Elastic Band method. We found that regardless of the way to terminate the surface, Al atom in bimetallic Ni₃Al(100) tends to exist in the first surface layer, rather than in the second or third layer, and the Ni₃Al(100) surface can substantially retard the H₂S decomposition by reducing the adsorption energy of sulfur compounds compared to the pure Ni(100) case. Finally, we presented how the Al in Ni₃Al modifies the activity of surface Ni atoms toward the sulfur compounds by calculating the local density of states and charge distribution in alloying components. This work hints the importance of knowing how to properly tailor the reactivity of Ni based materials to enhance the resistance for sulfur poisoning.