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.     

Visible-light sensitive hydrogen evolution photocatalyst ZnRh2O4

We investigated the ability of the oxide ZnRh₂O₄ to serve as a solar H₂-evolution photocatalyst due to the predicted potential of its conduction band bottom, which may allow thermodynamically favorable H₂ evolution in spite of its small band-gap of 1.2 eV. ZnRh₂O₄ produced H₂ in the presence of HCHO, but only scarcely in the presence of CH₃OH, indicating that the potential of the valence band top of ZnRh₂O₄ lies at ∼0.1 V (vs. SHE). Thus, the conduction band bottom potential (∼−1.1 V) lies much more negative than the potential of H⁺/H₂, allowing thermodynamically favorable H₂ evolution. In addition, the irradiated-light-wavelength dependence of the quantum efficiency (QE) for H₂ evolution was consistent with the solar spectrum, and the QE was quite high (∼27%), even at a wavelength of 770 ± 25 nm. Taken together, our findings indicate that ZnRh₂O₄ can utilize solar light effectively, not only the entire range of UV and visible light, but is also sensitive to infrared light.     

Nanocomposite of graphene oxide with nitrogen-doped TiO2 exhibiting enhanced photocatalytic efficiency for hydrogen evolution

The application of hydrogen energy potentially addresses energy and environmental problems. In order to improve the photocatalytic efficiency, nanocomposite of N-doped TiO₂ with graphene oxide (NTG) is prepared and characterized with Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectra, X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), photoluminescent spectra. The application of NTG to hydrogen evolution exhibits high photocatalytic efficiency of 716.0 or 112.0 μmol h⁻¹ g⁻¹ under high-pressure Hg or Xenon lamp, which is about 9.2 or 13.6 times higher than P25 photocatalyst. This is mainly attributed to the N-doping of TiO₂ and the incorporation of graphene oxide resulting in narrow band gap, together with the synergistic effect of fast electron-transporting of photogenerated electrons and the efficient electron-collecting of graphene oxide retarding charge recombination. These results provide a significant theoretical foundation for the potential application of N-doping photocatalysts to hydrogen evolution.     

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.     

Controllable O2 oxidization graphene in TiO2/graphene composite and its effect on photocatalytic hydrogen evolution

TiO₂/reduced graphene oxide composite (T-rGO) was synthesized and its performance was evaluated with photocatalytic hydrogen evolution. It was found that the hydrogen evolution rate of T-rGO increased significantly after injecting small amount of air into the vacuum pumped and UV irradiated sealed reaction cell. The IR, XPS, Raman and ESR spectra analysis indicated that the O₂^•−, which generated from the reaction of photoinduced electrons and the injected O₂ can moderately and controllably increase the oxygen groups on graphene planar of T-rGO at ambient condition. The amount of oxygen groups on graphene planar of T-rGO will affect the p-doping concentration of graphene, thus affect the p–n junction and the performance of T-rGO for photocatalytic hydrogen evolution.     

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.     

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.     

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.     

TiO2 nanosheets loaded with Cu: A low-cost efficient photocatalytic system for hydrogen evolution from water

Rutile TiO₂ nanosheets were prepared by a simple solvothermal process, and Cu was loaded on the surface of TiO₂ nanosheets using the in situ photo-deposition method. Meanwhile, photocatalytic H₂ evolution from water over the as-prepared TiO₂ nanosheets loaded with Cu was explored using methanol as a sacrificial reagent. The results indicate that the TiO₂ nanosheets loaded with Cu is an efficient photocatalyst under UV irradiation. During the first 5 h, a rate of H₂ evolution of approximately 22.1 mmol g⁻¹ h⁻¹ was achieved under optimal conditions. Furthermore, for practical purposes, the photocatalytic hydrogen evolution was studied as a function of content of Cu, pH of solution, concentration of methanol and dosage of photocatalyst, respectively. At last, the photocatalytic mechanism was preliminarily discussed.     

Cobalt sulfide quantum dots modified TiO2 nanoparticles for efficient photocatalytic hydrogen evolution

Cobalt sulfide quantum dots (CoSx QDs) modified TiO₂ nanoparticles are prepared with a precipitation-deposition method using TiO₂, cobalt acetate and sodium sulfide as the precursors. CoSx QD acts as an effective cocatalyst, which accelerates the transfer of the photo-generated electrons and serves as the active site for the reaction between electrons and H₂O, thus enhancing the separation of the e⁻/h⁺ pairs and the photocatalytic H₂ production activity of TiO₂. The amount of CoSx exhibits an optimum value at about 5% (mole ratio to TiO₂), at which the H₂ production rate achieves 838 μmol h⁻¹ g⁻¹ using ethanol as the sacrificial reagent. This exceeds that of the pure TiO₂ by more than 35 times.     

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.     

Polyoxomolybdate-derived MoS2/nitrogen-doped reduced graphene oxide hybrids for efficient hydrogen evolution

Integrating MoS₂ with carbon-based materials, especially graphene, is an effective strategy for preparing highly active non-noble-metal electrocatalysts in the hydrogen evolution reaction (HER). This work demonstrates a convenient hydrothermal method to fabricate molybdenum disulfide nanosheets/nitrogen-doped reduced graphene oxide (MoS₂/NGO) hybrids using polyoxomolybdate as the Mo precursor. Introducing more defects and expanding interlayer spacing of MoS₂ can be achieved through decreasing the pH value of the reactive system due to the existed high-nuclear polyoxometalate clusters. MoS₂/NGO hybrids prepared at low pH exhibit superior HER activity to those obtained at high pH. MoS₂/NGO-pH1.5 exhibits an ultralow overpotential of 81 mV at 10 mA cm⁻², a low Tafel slope of 60 mV·dec⁻¹ and good stability in alkaline electrolyte. Such excellent electrocatalytic activity is contributed by the abundant HER catalytic active sites, the increased electrochemically-accessible area and the synergetic effects between the active MoS₂ catalyst and NGO support.     

Highly efficient and durable phosphine reduced iron-doped tungsten oxide/reduced graphene oxide nanocomposites for the hydrogen evolution reaction

The design and development of inexpensive and highly efficient electrocatalysts for hydrogen production from water splitting are highly crucial for green energy and the hydrogen economy. Herein, we report phosphine reduced an iron-doped tungsten oxide nanoplate/reduced graphene oxide nanocomposite (Fe-WOxP/rGO) as an excellent electrocatalyst for the hydrogen evolution reaction. This electrocatalyst was synthesized using a hydrothermal method, followed by reduction with phosphine (PH₃), which was generated from sodium hypophosphite. The catalyst onset potential, Tafel slope, and stability were investigated. Accordingly, Fe-WOxP/rGO exhibited impressively high electrocatalytic activity with a low overpotential of 54.60 mV, which is required to achieve a current density of 10 mAcm^?2. The Tafel slope of 41.99 mV dec^?1and the linear sweep voltammetry curve is almost the same as 2000 cycles and electrolysis under static overpotential (54.60 mV) is remain for more than 24 h in 0.5 M H₂SO₄. The catalytic activity and conductivity of Fe-WOxP/rGO were higher than WO_XP, Fe-WOxP and WOxP/rGO. Such an outstanding performance of the Fe-WOxP/rGO nanocomposite is attributed to the coupled synergic effect between high oxygen vacancies formation on tungsten oxide in the nanoplate-like structure of Fe-WOxP and rGO nanosheet, making it as an excellent electrocatalyst for hydrogen evolution reaction.     

Two dimensional MAX supported copper oxide/nickel Oxide/MAX as an efficient and novel photocatalyst for hydrogen evolution

An innovative 2D MAX structure comprising of Ti₃AlC₂ multilayers and copper oxide (CuO)/nickel oxide (NiO) (CN) composite was fabricated via a facile chemical route for improving photocatalytic hydrogen evolution activity. The physicochemical properties of the synthesized nanocomposites were analyzed through various structural, morphological, and elemental techniques. The 2D Ti₃AlC₂/CuO/NiO composite showed a maximum H₂ generation rate of 20.7 mmol g^?1 h^?1, which is greater than that of the CN nanocomposite. This improved activity can be attributed to the presence of Ti₃AlC₂ multilayers on CuO/NiO, which showed excellent photoinduced charge carrier separation via the s-scheme mechanism. As an electron-bridge, CN NC can support the photoelectrons to transfer from the CB of CN to the CB of MAX, from where the photoelectrons respond with hydrogen ion to release hydrogen. The time-resolved photoluminescence measurement results showed that the CuO/NiO/MAX (CNM) composite had a charge carrier lifetime of 3 ns. The outcomes of this study will be beneficial in realizing the industrial applications of Ti₃AlC₂ MAX-based structured catalysts for hydrogen evolution and other ecological energy systems.     

Metalloid Ni2P and its behavior for boosting the photocatalytic hydrogen evolution of CaIn2S4

The development of high-efficiency and low-cost photocatalysts in photocatalytic H₂ evolution systems from water remains challenging. The substitution of a noble metal as the co-catalyst is still one of the important and meaningful issues in this field. Herein, we report a series of CaIn₂S₄ catalysts combined with Ni₂P, which acts as the co-catalyst, for boosting photocatalytic hydrogen evolution under visible light. The integrated system of the Ni₂P/CaIn₂S₄ composite exhibited high efficiency and durability, which were even higher than those of Pt decorated catalysts. The promoting effect of Ni₂P can be ascribed to its excellent reductive ability and analogous metallic character, which can accelerate the transfer and consumption of the photo-generated electrons. Moreover, based on the surface photo-voltage technique and electrochemical tests, the unique mechanism of Ni₂P for the movement of photo-generated charges during the photocatalysis process is proposed for the first time.     

Understanding the effect of TiO2, VCl3, and HfCl4 on hydrogen desorption/absorption of NaAlH4

The main objective of this work was to investigate the different effects of transition metals (TiO₂, VCl₃, HfCl₄) on the hydrogen desorption/absorption of NaAlH₄. The HfCl₄ doped NaAlH₄ showed the lowest temperature of the first desorption at 85 °C, while the one doped with VCl₃ or TiO₂ desorbed at 135 °C and 155 °C, respectively. Interestingly, the temperature of desorption in subsequent cycles of the NaAlH₄ doped with TiO₂ reduced to 140 °C. On the contrary, in the case of NaAlH₄ doped with HfCl₄ or VCl₃, the temperature of desorption increased to 150 °C and 175 °C, respectively. This may be because Ti can disperse in NaAlH₄ better than Hf and V; therefore, this affected segregation of the sample after the desorption. The maximum hydrogen absorption capacity can be restored up to 3.5 wt% by doping with TiO₂, while the amount of restored hydrogen was lower for HfCl₄ and VCl₃ doped samples. XRD analysis demonstrated that no Ti-compound was observed for the TiO₂ doped samples. In contrast, there was evidence of Al–V alloy in the VCl₃ doped sample and Al–Hf alloy in the HfCl₄ doped sample after subsequent desorption/absorption. As a result, the V- or Hf-doped NaAlH₄ showed the lower ability to reabsorb hydrogen and required higher temperature in the subsequent desorptions.     

Highly efficient CuO incorporated TiO2 nanotube photocatalyst for hydrogen production from water

Highly dispersed CuO was introduced into TiO₂ nanotube (TNT) made by hydrothermal method via adsorption-calcination process or wet impregnation process, to fabricate CuO incorporated TNT photocatalysts (CuO-TNT) for hydrogen production. It was found that CuO-TNT possessed excellent hydrogen generation activity, which was constantly vigorous throughout 5 h reaction. Depending on the preparation method, hydrogen evolution rates over CuO-TNT were founded in the range of 64.2-71.6 mmol h⁻¹ g⁻¹_catalyst, which was much higher than the benchmark P25 based photocatalysts, and even superior to some Pt/Ni incorporated TNT. This high photocatalytic activity of CuO-TNT was mainly attributed to the unique 1-D tubular structure, large BET surface area and high dispersion of copper component. Compared to wet impregnation, adsorption-calcination process was superior to produce active photocatalyst, since it was prone to produce photocatalyst with more highly dispersed CuO.     

Influence of metal oxide on LiBH4/2LiNH2/MgH2 system for hydrogen storage properties

In this study, various nanoscale metal oxide catalysts, such as CeO₂, TiO₂, Fe₂O₃, Co₃O₄, and SiO₂, were added to the LiBH₄/2LiNH₂/MgH₂ system by using high-energy ball milling. Temperature programmed desorption and MS results showed that the Li–Mg–B–N–H/oxide mixtures were able to dehydrogenate at much lower temperatures. The order of the catalytic effect of the studied oxides was Fe₂O₃ > Co₃O₄ > CeO₂ > TiO₂ > SiO₂. The onset dehydrogenation temperature was below 70 °C for the samples doped with Fe₂O₃ and Co₃O₄ with 10 wt.%. More than 5.4 wt.% hydrogen was released at 140 °C. X-ray diffraction indicated that the addition of metal oxides inhibited the formation of Mg(NH₂)₂ during ball milling processes. It is thought that the changing of the ball milling products results from the interaction of oxide ions in metal oxide catalysts with hydrogen atoms in MgH₂. The catalytic effect depends on the activation capability of oxygen species in metal oxides on hydrogen atoms in hydrides.     

Visible light induced hydrogen evolution on new hetero-system ZnFe2O4/SrTiO3

The physical properties and photoelectrochemical characterization of the spinel ZnFe₂O₄, elaborated by chemical route, have been investigated for the hydrogen production under visible light. The forbidden band is found to be 1.92 eV and the transition is indirectly allowed. The electrical conduction occurs by small polaron hopping with activation energy of 0.20 eV. p-type conductivity is evidenced from positive thermopower and cathodic photocurrent. The flat band potential (0.18 V_SCE) determined from the capacitance measurements is suitably positioned with respect to H₂O/H₂ level (−0.85 V_SCE). Hence, ZnFe₂O₄ is found to be an efficient photocatalyst for hydrogen generation under visible light. The photoactivity increases significantly when the spinel is combined with a wide band gap semiconductor. The best performance with a hydrogen rate evolution of 9.2 cm³ h⁻¹ (mg catalyst)⁻¹ occurs over the new hetero-system ZnFe₂O₄/SrTiO₃ in Na₂S₂O₃ (0.025 M) solution.