Hydrogen production with Ga2BiSbO7, Fe2BiSbO7 and Gd2BiSbO7 as photocatalysts under visible light irradiation

M₂BiSbO₇ (M = Ga, Fe, Gd) were synthesized by solid state reaction process for the first time as novel photocatalysts. The structural and photocatalytic properties of M₂BiSbO₇ (M = Ga, Fe, Gd) were reported. The results showed that Ga₂BiSbO₇, Fe₂BiSbO₇ and Gd₂BiSbO₇ crystallized with the pyrochlore-type structure, cubic crystal system and space group Fd3m. The lattice parameter for Ga₂BiSbO_7, Fe₂BiSbO₇ or Gd₂BiSbO₇ was 10.34804 Å, 10.41030 Å or 10.70353 Å respectively. In addition, the band gaps of Ga₂BiSbO₇ and Gd₂BiSbO₇ were separately estimated to be 1.955 eV and 2.081 eV. For the photocatalytic water splitting reaction, H₂ or O₂ evolved abundantly with Ga₂BiSbO₇, Fe₂BiSbO₇ or Gd₂BiSbO₇ as photocatalyst from CH₃OH/H₂O or AgNO₃/H₂O solution respectively under visible light irradiation (Wavelength > 420 nm). The results indicated that the activities of three photocatalysts followed the order of Ga₂BiSbO₇ > Fe₂BiSbO₇ > Gd₂BiSbO₇. Moreover, photocatalytic H₂ evolution from aqueous methanol solution was observed with Ga₂BiSbO₇, Fe₂BiSbO₇ or Gd₂BiSbO₇ as photocatalyst under different light irradiation (λ > 390 nm and λ > 0 nm). The results showed that the photocatalytic activity decreased with increasing incident wavelength (λ). The photocatalytic activity was further enhanced under visible light irradiation with Ga₂BiSbO₇, Fe₂BiSbO₇ or Gd₂BiSbO₇ loaded by Pt, NiO or RuO₂ (λ > 420 nm). The effect of Pt was better than that of NiO or RuO₂ for improving the photocatalytic activity of Ga₂BiSbO₇, Fe₂BiSbO₇ or Gd₂BiSbO₇.     

Stability of an H2-producing photocatalyst (Ru/(CuAg)0.15In0.3Zn1.4S2) in aqueous solution under visible light irradiation

Efficient photocatalytic water-splitting systems require stable photocatalysts that have photocatalytic activity with repeated consecutive use. This study investigated H₂ production under visible light irradiation with an Ru/(CuAg)_0.15In_0.3Zn_1.4S₂ photocatalyst and KI as an electron donor. In addition, the stability and reusability of the catalyst were evaluated over multiple cycles of H₂ production and catalyst regeneration. The results show that sintering temperature influenced the crystallinity and photocatalytic activity, as indicated by the X-ray diffraction analyses and H₂ production rates. In particular, the catalyst sintered at 873 K yielded the highest quantum yield of 4.6% at 420 ± 5 nm of wavelength. After seven consecutive reaction cycles, the quantum yield decreased from 4.6% to 3.0% at the end of the seventh cycle. The decrease probably occurred because (1) particles of the catalyst underwent pronounced aggregation, which led to the increase in particle size; and (2) a release of significant metal ions was observed during H₂ production, which led to a loss of the catalyst mass and potential changes in the photocatalytic activity. This study will help facilitate a search of stable photocatalysts for water splitting.     

Ag3PO4 photocatalyst: Hydrothermal preparation and enhanced O2 evolution under visible-light irradiation

Visible-light-driven semiconducting photocatalysts of Ag₃PO₄ were prepared by a hydrothermal method, and were optimized by adjusting reaction conditions, i.e., temperature, pH of reaction solution, concentration of feedstock, and time of hydrothermal process. The obtained photocatalysts were then systematically characterized by different instruments, such as XRD, UV–vis, FESEM, and BET, to reveal the physicochemical properties. Furthermore, activities of photocatalysts for visible-light-driven O₂ evolution were evaluated, demonstrating that the photocatalytic activity of Ag₃PO₄ prepared by hydrothermal reaction (initial rate of O₂ evolution, 1156 μmol g⁻¹ h⁻¹) was more than two times as that of sample prepared by room-temperature reaction (initial rate of O₂ evolution, 533 μmol g⁻¹ h⁻¹), which could be attributed to its better ability to utilize visible light and more regulated morphology.     

Hydrogen production from water splitting under UV light irradiation over Ag-loaded mesoporous-assembled TiO2-ZrO2 mixed oxide nanocrystal photocatalysts

Hydrogen production from the photocatalytic water splitting reaction is very attractive because it is an environmentally friendly process, where hydrogen is produced from two abundantly renewable sources, i.e. water and solar energy, with the aid of photocatalysts. TiO₂ is the most widely investigated photocatalyst; however, it alone still exhibits low performance to photocatalytically produce hydrogen. Hence, the aim of this work focused on the enhanced photocatalytic hydrogen production over Ag-loaded mesoporous-assembled TiO₂-ZrO₂ mixed oxide nanocrystal photocatalysts under UV light irradiation. The TiO₂-ZrO₂ mixed oxides with various TiO₂-to-ZrO₂ molar ratios were synthesized by a sol-gel process with the aid of a structure-directing surfactant, followed by Ag loading via a photochemical deposition method. The influences of photocatalyst preparation parameters, i.e. calcination temperature, phase composition, and Ag loading, were studied. The results revealed that the mesoporous-assembled TiO₂-ZrO₂ mixed oxide nanocrystal photocatalyst with a TiO₂-to-ZrO₂ molar ratio of 93:7 calcined at 500 °C exhibited the highest photocatalytic hydrogen production activity, and the Ag loading of 0.5 wt.% further greatly enhanced the photocatalytic activity of such TiO₂-ZrO₂ mixed oxide photocatalyst.     

Solar photocatalytic degradation of methylene blue in carbon-doped TiO2 nanoparticles suspension

Carbon-doped TiO₂ nanoparticles were prepared by sol–gel auto-combustion method and characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Brunauer–Emmett–Teller method (BET), UV–vis diffuses reflectance spectroscopy (DRS). UV–vis diffuse reflectance spectra showed that carbon-doped TiO₂ exhibited obvious absorption in the visible light range. The visible light photocatalytic activity of carbon-doped TiO₂ was ascribed to the presence of oxygen vacancy state between the valence and the conduction bands because of the formation of Ti³⁺ species in the as-synthesized carbon-doped TiO₂. The sample calcined at 873 K showed the highest photocatalytic activity under solar irradiation. The effects of photocatalyst concentration, initial concentration of methylene blue, and pH value in aqueous solution were also presented.     

Cu2O/Cu/TiO2 nanotube Ohmic heterojunction arrays with enhanced photocatalytic hydrogen production activity

Cu₂O/Cu/TiO₂ nanotube heterojunction arrays were prepared by assembling Cu@Cu₂O core-shell nanoparticles on TiO₂ nanotube arrays (NTAs) using a facile impregnation-reduction method. SEM and TEM results show that Cu@Cu₂O plate-like nanoparticles with tens of nanometers in size are confined inside TiO₂ NTAs. Only the outmost several nanometers of the nanoparticles are Cu₂O and the predominant inner of the nanoparticles are Cu metals. Cu L₃VV Auger spectra of Cu₂O/Cu/TiO₂ NTAs suggest that Cu metals are enveloped by at least several nanometers Cu₂O on the surface, which further confirms the Cu@Cu₂O core shell structure of Cu nanoparticles. The ability of light absorption of Cu₂O/Cu/TiO₂ NTAs is enhanced. The range of absorption wavelengths changes from 400 to 700 nm due to the surface plasmon response of Cu metals core and Cu₂O nanoparticles shell. The photocatalytic hydrogen production rate of Cu₂O/Cu/TiO₂ heterojunction arrays is enhanced when compared with those of Cu₂O/TiO₂ NTAs and TiO₂ NTAs under UV light. Moreover, a stable H₂ generation property was obtained under visible light (λ gt; 400 nm). The Cu metal core is believed to play a key role in the enhancement of photocatalytic properties of Cu₂O/Cu/TiO₂ nanotube heterojunction arrays.     

Visible-light-driven ZnIn2S4/CdIn2S4 composite photocatalyst with enhanced performance for photocatalytic H2 evolution

ZnIn₂S₄/CdIn₂S₄ composite photocatalysts (x = 0–1) were successfully synthesized via a hydrothermal route. Compositions of ZnIn₂S₄/CdIn₂S₄ composite photocatalysts were optimized according to the photocatalytic H₂ evolution rate. XRD patterns indicate the as-prepared samples are mixtures of hexagonal and cubic structures. FESEM and TEM images show that the as-prepared samples are composed of flower-like microspheres with wide distribution of diameter. There is obviously distinguishing distribution of Zn, Cd elements among the composite architectures. UV–vis absorption spectra of different compositions exhibit that absorption edges of ZnIn₂S₄/CdIn₂S₄ composites slightly move towards longer wavelengths with the increment of CdIn₂S₄ component. A typical time course of photocatalytic H₂ evolution from an aqueous Na₂SO₃ and Na₂S solution over unloaded and PdS-loaded ZnIn₂S₄/CdIn₂S₄ composite photocatalyst is carried out. The initial activity for H₂ evolution over 0.75 wt% PdS-loaded sample is up to 780 μmol h⁻¹. And the activity of unloaded sample also reaches 490 μmol h⁻¹ with consistent stability.     

Artificial chloroplast: Au/chloroplast-morph-TiO2 with fast electron transfer and enhanced photocatalytic activity

The unique structures and functional features of chloroplasts in green plants provide a promising blueprint for greatly improving solar energy utilization efficiency. In this paper, a prototype of artificial chloroplast, Au/chloroplast-morph-TiO₂ with natural chloroplasts' nanostructure and analogous functional features, is provided. The nano-layered structures of chloroplast template inherited in the chloroplast-morph-TiO₂ lead to a large reaction area and fast photo-induced electron transfer; cocatalyst Au nanoparticles which work as reaction centers promote photo-induced charge separation and improve the overall photocatalytic activity in hydrogen production; Nitrogen and phosphorus self-doped from the bio-template increase visible light absorption, similar to the antenna pigment. With this new inorganic artificial photosynthetic system, we achieve effective light utilization, fast photo-induced charge separation, high electron transfer, enhanced photocatalytic activity for dye degradation rate and improved H₂ evolution efficiency. This concept provides the inspiration for constructing efficient photocatalysts by imitating the photosynthesis process from both structures and functions.     

Pd-Gardenia-TiO2 as a photocatalyst for H2 evolution from pure water

The photocatalytic activity for H₂ evolution from pure water over Pd loaded TiO₂ prepared by gardenia extract (Pd-Gardenia-TiO₂) is systematically investigated. The as-prepared photocatalysts are characterized by X-ray diffraction, high resolution transmission electron microscopy, Fourier transform infrared spectra, and X-ray photoelectron spectroscopy. Gardenia extract functions as reducing and stabilizing agents simultaneously. The mean size of the as-prepared Pd nanoparticles is in the range of 2.3 ± 0.5 nm based on TEM images. The Pd-Gardenia-TiO₂ catalyst exhibits good photocatalytic activity for H₂ evolution (93 μmol · h⁻¹ · g⁻¹), which is much higher than that of Pd photodeposited on TiO₂. Possible factors for its photocatalytic activity from pure water are also investigated.     

Electrospun Pt/TiO2 hybrid nanofibers for visible-light-driven H2 evolution

One-dimensional (1D) Pt/TiO₂ hybrid nanofibers (HNFs) with different concentrations of Pt were fabricated by a facile two-step synthesis route combining an electrospinning technique and calcination process. X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) results showed that the Pt nanoparticles (NPs) with the size of 5–10 nm were well dispersed in the TiO₂ nanofibers (NFs). Further investigations from the UV–Vis diffuse reflectance (DR) and X-ray photoelectron spectroscopy (XPS) analysis revealed that some Pt ions were incorporated into the TiO₂ lattice as Pt⁴⁺ state, which contributed to the visible light absorption of TiO₂ NFs. Meanwhile, the Pt²⁺ ions existing on the surface of Pt NPs resulted in the formation of Pt–O–Ti bond at Pt NPs/TiO₂ NFs interfaces that might serve as an effective channel for improving the charge transfer. The as-electrospun Pt/TiO₂ HNFs exhibited remarkable activities for photocatalytic H₂ evolution under visible light irradiation in the presence of l-ascorbic acid as the sacrificial agent. In particular, the optimal HNFs containing 1.0 at% Pt showed the H₂ evolution rate of 2.91 μmol h⁻¹ and apparent quantum efficiency of 0.04% at 420 nm by using only 5 mg of photocatalysts. The higher photocatalytic activity could be ascribed to the appropriate amount of Pt ions doping and excellent electron-sink effect of Pt NPs co-catalysts.     

The preparation of TiO2–nitrogen doped by calcination of TiO2·xH2O under ammonia atmosphere for visible light photocatalysis

The investigation on incorporating nitrogen group into titanium dioxide in order to obtain powdered visible light-active photocatalysts is presented. The industrial hydrated amorphous titanium dioxide (TiO₂·xH₂O) obtained directly from sulphate technology installation was modified by heat treatment at temperatures of 100–800 °C for 4 h in an ammonia atmosphere. The photocatalysts were characterized by UV–VIS–DR and XRD techniques. The UV–VIS–DR spectra of the modified catalysts exhibited an additional maximum in the VIS region (, ) which may be due to the presence of nitrogen in TiO₂ structure. On the basis of XRD analysis it can be supposed that the presence of nitrogen does not have any influence on the transformation temperature of anatase to rutile. The photocatalytic activity of the modified photocatalysts was determined on the basis of decomposition rate of phenol and azo-dye (Reactive Red 198) under visible light irradiation. The highest rate of phenol degradation was obtained for catalysts calcinated at 700 °C (6.55%), and the highest rate of dye decomposition was found for catalysts calcinated at 500 and 600 °C (ca. 40–45%). The nitrogen doping during calcination under ammonia atmosphere is a very promising way of preparation of photocatalysts which could have a practical application in water treatment system under broader solar light spectrum.     

CNs−Bi12O17Cl2复合体系制备及光催化降解性能研究

采用简易水解法制备了一系列CNs−Bi₁₂O₁₇Cl₂复合半导体材料并对其物相、光学性质和光催化降解性能进行了分析表征。X射线衍射光谱表明复合体系衍射峰与四方晶相Bi₁₂O₁₇Cl₂一致,紫外可见漫反射光谱证明复合材料在可见区域具有较强的光吸收能力,由此可提高光催化活性。在可见光照射下,复合体系相对于纯Bi₁₂O₁₇Cl₂对亚甲基蓝具有更高的降解效率,特别是具有合适组分的样品CB50可以在180 min后完全去除20 mg·L⁻¹的亚甲基蓝分子,这主要是由于CNs的引入抑制了光生载流子的复合,使复合体系表现出更高的光催化降解性能。最后,提出了可能的光催化机理。     

The pH effects on H2 evolution kinetics for visible light water splitting over the Ru/(CuAg)0.15In0.3Zn1.4S2 photocatalyst

Much progress has been made in the development of novel visible light photocatalysts that split water into hydrogen (H₂) and oxygen (O₂). In this study, we examine the impact of initial solution pH on H₂ production using an Ru/(CuAg)_0.15In_0.3Zn_1.4S₂ photocatalyst under visible light irradiation. In addition, the reaction mechanism was analyzed by examining the oxidation products of the electron donor (I‾) at different solution pH values. The results show that the initial pH significantly influenced the rate of H₂ production and quantum yield (QY). In particular, the photocatalyst yielded the highest apparent QY (∼12.8%) at 420 ± 5 nm and highest H₂ production rate (∼525 μmol h⁻¹) at pH 2; with increasing pH, the H₂ production and QY decreased significantly. The oxidation product of I‾ at pH < 6 was mainly I₃‾, whereas at pH > 6 water splitting did not occur at all, so no IO₃‾ or I₂ were observed.     

Cetyltrimethylammoniumbromide (CTAB)-assisted hydrothermal synthesis of ZnIn2S4 as an efficient visible-light-driven photocatalyst for hydrogen production

A series of ZnIn₂S₄ photocatalysts was synthesized via a cetyltrimethylammoniumbromide (CTAB)-assisted hydrothermal method. These ZnIn₂S₄ products were characterized by X-ray diffraction (XRD), UV–visible absorption spectra (UV–vis) and scanning electron microscopy (FESEM). The effects of hydrothermal time and CTAB on the crystal structures, morphologies and optical properties of ZnIn₂S₄ products were discussed in detail. The photocatalytic activities of the as-prepared samples were evaluated by photocatalytic hydrogen production from water under visible-light irradiation. It was found that the photocatalytic activities of these ZnIn₂S₄ products decreased with the hydrothermal time prolonging while increased with the amount of CTAB increasing. The highest quantum yield at 420 nm of ZnIn₂S₄ photocatalyst, which was prepared through the CTAB (9.6 mmol)-assisted hydrothermal procedure for 1 h, was determined to be 18.4%. The optimum amount of Pt loaded for the ZnIn₂S₄ photocatalyst was about 1.0 wt%, under the present photocatalytic system.     

Photocatalytic hydrogen production using Fe2O3-based core shell nano particles with ZnS and CdS

Stability and efficiency of photocatalysts are important to realize the practical applications of them for photocatalytic hydrogen production from industrial sulfide effluent. Novel, magnetically separable core–shell nano photocatalysts viz., CdS/Fe₂O₃, ZnS/Fe₂O₃ and (CdS + ZnS)/Fe₂O₃ were prepared and their hydrogen evolution activity under visible light was examined. The XRD result shows that CdS and ZnS were very well coated on the surface of the iron oxide core shell particles. The HR-TEM result also confirms the core shell formation. (CdS + ZnS)/Fe₂O₃ evolved higher volume of hydrogen than the other catalysts. It is ascribed to rapid migration of excited electrons from (CdS + ZnS) toward Fe₂O₃ suppressing electron hole annihilation compared to other catalysts. The catalysts can be easily recovered from the reaction medium using external magnetic bar and so the photocatalyst can be reused without any mass loss. Hence, it can be a potential catalyst for recovery of hydrogen from industrial sulfide containing waste streams.     

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.     

Photocatalytic H2 evolution on a novel CaIn2S4 photocatalyst under visible light irradiation

A novel visible-light-driven photocatalyst CaIn₂S₄ was synthesized using a facile hydrothermal method followed by a post-calcination process. The influence of the calcination temperature and time on the activities of the photocatalyst was investigated. CaIn₂S₄ exhibits optical absorption predominantly in visible region with an optical band gap of 1.76 eV. Considerable activity for hydrogen evolution from pure water was observed without any sacrificial agents or cocatalysts under visible light irradiation. The maximum hydrogen evolution rate achieved was 30.92 μmol g⁻¹ h⁻¹ without obvious deactivation of the photocatalytic activity for four consecutive runs of 32 h.     

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.     

ZnmIn2S3+m (m = 1–5, integer): A new series of visible-light-driven photocatalysts for splitting water to hydrogen

A new series of Zn_mIn₂S_3+m (m = 1–5, integer) photocatalysts was synthesized via a simple hydrothermal method. X-ray diffraction (XRD), Raman spectra, UV–vis–near-IR diffuse reflectance spectra (UV–vis), X-ray fluorescence (XRF) and scanning electron microscope (FESEM) were used to characterize these photocatalysts. These Zn_mIn₂S_3+m photocatalysts had a similar layered crystal structure. The absorption edge of Zn_mIn₂S_3+m shifted to shorter wavelength as the atomic ratio of Zn/In in the synthetic solution was increased (i.e. m increased from 1 to 5). Additionally, the morphology of Zn_mIn₂S_3+m greatly depended on the atomic ratio of Zn/In. The photocatalytic activity of Zn_mIn₂S_3+m was evaluated by photocatalytic hydrogen production from water under visible light. The Zn₂In₂S₅ product, with quantum yield at 420 nm determined to be 11.1%, had the highest photocatalytic activity among these Zn_mIn₂S_3+m (m = 1–5, integer) photocatalysts.     

Fabrication of nanocrystalline LaFeO3: An efficient sol–gel auto-combustion assisted visible light responsive photocatalyst for water decomposition

Synthesis of nano photocatalysts, LaFeO₃ with orthorhombic perovskite structure by sol–gel auto-combustion method was demonstrated. The samples were characterized by PXRD, SEM, HRTEM, XPS and optical absorption studies. Photocatalytic water decomposition over LaFeO₃ nanoparticles activated at various temperatures without any co-catalyst were investigated under visible light irradiation (λ >> 420 nm). Highest amount of H₂ and O₂ evolved in 180 min over the LaFeO₃ activated at 500 °C was recorded to be 1290 μmol and 640 μmol, respectively having apparent quantum efficiency (AQE) 8.07%. The pronounced activity of nano LaFeO₃ samples towards water decomposition was consistent with BET-surface area and particle size analyses.