Photocatalytic Hydrogen Generation Efficiency Research Articles

Enhanced photocatalytic hydrogen generation efficiency through molybdenum-sulfur dual sites synergistic catalysis

Photocatalysis has gained significant attention in recent years for water splitting, with graphitic carbon nitride (g-C3N4) emerging as a promising candidate. However, the low carrier mobility and the lack of active sites for hydrogen evolution limit photocatalytic hydrogen evolution performance. Herein, this work presents a facile approach to achieve single Mo sites with a unique local coordination structure featuring one Mo atom coordinated with two nitrogen atoms and one sulfur atom. Among the prepared photocatalysts, 6 wt% Mo/SCN exhibits the highest photocatalytic hydrogen evolution activity with a yield rate of 4.05 mmol g-1 h-1, which is 13 times higher than that of pure g-C3N4 and superior to most of the g-C3N4 series photocatalysts. Experimental results and density functional theory calculations confirm that the Mo single-atom and S atom have synergistic catalysis under the co-modification, and the formed Mo-S bonds will enhance the ability of the substrate to anchor the Mo single-atom to enhance the stability of the photocatalyst. This work offers insights into transition metal single-atom and synergistic catalysis design for photocatalytic hydrogen evolution.

Charge carrier recombination processes, intragap defect states, and photoluminescence mechanisms in stoichiometric and reduced TiO2 brookite nanorods: an interpretation scheme through in situ photoluminescence excitation spectroscopy in controlled environment.

The study of titanium dioxide (TiO2) in the brookite phase is gaining popularity as evidence has shown the efficient photocatalytic performance of this less investigated polymorph. It has been recently reported that defective anisotropic brookite TiO2 nanorods display remarkable substrate-specific reactivity towards alcohol photoreforming, with rates of hydrogen production significantly (18-fold) higher than those exhibited by anatase TiO2 nanoparticles. To elucidate the basic photo-physical mechanisms and peculiarities leading to such an improvement in the photoactive efficiency, we investigated the recombination processes of photoexcited charge carriers in both stoichiometric and reduced brookite nanorods via photoluminescence excitation spectroscopy in controlled environment. Through an investigation procedure employing both supragap and subgap excitation during successive exposure to oxidizing and reducing gaseous agents, we firstly obtained an interpretation scheme describing the main photoluminescence and charge recombination pathways in stoichiometric and reduced brookite, which includes information about the spatial and energetic position of the intragap states involved in photoluminescence mechanisms, and secondly identified a specific photoluminescence enhancement process occurring in only reduced brookite nanorods, which indicates the injection of a conduction band electron during ethanol photo-oxidation. The latter finding may shed light on the empirical evidence about the exceptional reactivity of reduced brookite nanorods toward the photo-oxidation of alcohols and the concomitant efficiency of photocatalytic hydrogen generation.

Enhancing the water splitting performance of a reduced graphene oxide–platinum nanoparticle hybrid using an intercalating ethylenediamine polar space group

Ethylenediamine (EDA) introduced into rGO-platinum hybrid could lower the band gap and enhance the proton–electron conductivity. rGO-Pt-EDA thus displayed optimum surface area and significantly enhanced photocatalytic hydrogen generation efficiency.

Fabrication of 2D+1D nanoarchitecture for transition metal oxide modified CdS nanorods: A comparative study on their photocatalytic hydrogen-generation efficiency

We report the formation of Mo1−xWxO3-CdS (0 ≤ x ≤1) nanophotocatalysts by a combination of solid-state and solution-impregnation processes. The formation of 2D+1D heterostructured composite was revealed by electron microscopy and the structure of ternary co-catalyst and photocatalysts were confirmed by spectroscopic analyses. The H2 evolution activity of the nanocomposites was assessed via photocatalytic splitting of water under the irradiation of visible light. All the nanocomposites studied here exhibit notable catalytic activity and good photostability using lactic acid as the sacrificial electron donor compared to a pristine compound. Among these nanocomposites, WO3-CdS shows superior activity with H2 evolution rates of 15.19 mmolg−1h−1, 28 times higher than the pure CdS. The WO3-CdS photoactivity is not only superior among all the composites studied here but also highest among the reported WO3 composite catalysts to date. The novel construction of the oxide-based nanocomposite photocatalyst shown here efficiently enhances the catalytic activity by effective separation of charge carriers and inhibits photocorrosion of CdS nanorods. The apparent quantum yield of the hydrogen evolution for WO3-CdS was found to be 8% in the visible spectral range. The disparity of the catalytic ability between MoO3 and WO3 and the variance among the compositions was unraveled through optical band-offset alignment with respect to CdS. Though the 2D+1D novel fabrication is common to all the composites, the difference in the type of band alignment MoO3 (type-I) and WO3 (type-II) with CdS plays a highly significant role in the co-catalytic activity.

Solid-state synthesis of ZnS/ZnO nanocomposites and their decoration with NiS cocatalyst for photocatalytic hydrogen production

Herein was demonstrated the facile solid-state synthesis of ZnS/ZnO nanocomposites using planetary ball milling followed by subsequent thermal treatment. X-ray diffraction analysis has shown that the ZnS/ZnO products are nanocrystalline with an estimated crystallite size of less than 60 nm. A transmission electron microscope has demonstrated the presence of spherical heterostructures and core-shell structured ZnS/ZnO nanocomposites with the size being 40–80 nm in diameter. Fourier transform infrared spectroscopy analysis confirmed the presence of Zn–O and Zn–S bonds. The formation of a thin boundary between the ZnS and ZnO phases, where the sulfur oxidation state varies from −2 to +6, was determined by X-ray photoelectron spectroscopy. Degradation of methylene blue and Orange II organic dyes was used to assess the photocatalytic activity of ZnS/ZnO nanocomposites, and the nanocomposites of ZnS/ZnO, with a mass ratio of ∼1:1, demonstrated the highest visible light photocatalytic activity. For photocatalytic hydrogen production experiments, the surface of ZnS/ZnO nanocomposites was in situ decorated with NiS nanoparticles, where the mass content of NiS was 1.5%. The presence of NiS cocatalyst promotes the separation and transfer of photogenerated charges, thereby increasing the photocatalytic activity of ZnS/ZnO nanocomposites during hydrogen generation. The apparent quantum efficiency of photocatalytic hydrogen generation using ZnS/ZnO-3h-NiS photocatalyst was equal to 1.13%. This contribution shows the possibility to prepare nano-structured photocatalysts via mechanochemistry.

Internal magnetic-field-enhanced photogenerated charge separation in ferromagnetic TiO2 surface heterojunctions

The use of the internal magnetic field of ferromagnets can effectively promote charge separation and transfer (CST) in photoelectrochemical energy conversion. However, photoelectrochemical materials with a ferromagnetic field are scarce, and the internal magnetic field is negligible in nonferromagnetic materials. To address this issue, we propose a rational method for preparing ferromagnetic TiO2 powder using controllable oxygen vacancies in anatase TiO2 with co-exposed {001} and {101} facets. Accordingly, an excellent saturation magnetisation of 0.0014 emu/g in TiO2 is achieved owing to an asymmetric and uneven charge distribution. Compared with that of nonferromagnetic TiO2, the efficiency of photocatalytic hydrogen generation of ferromagnetic TiO2 is improved by 0.64 times. The enhancement of photocatalytic hydrogen generation is due to the different forces exerted on the electrons and holes in the magnetic field, which significantly improve the photogenerated CST efficiency of ferromagnetic TiO2. This result highlights the significant role of the synergistic regulation of the crystal structure and defects in regulating the ferromagnetic characteristics of materials. The findings of this study provide guidance for leveraging point defects to promote CST for high-efficiency solar-energy conversion systems.

Synthesis and photocatalytic performance of g-C3N4/MeTMC-COP composite photocatalyst.

Covalent organic polymers have excellent application prospects in photocatalysis due to their excellent visible light absorption and structural designability. However, their fast recombination efficiency and complex preparation process limit their applications. Because of the above problems, this paper used urea to prepare g-C3N4 by high-temperature thermal polymerization and prepared g-C3N4 composite photocatalyst loaded with MeTMC-COP (g-C3N4/MeTMC-COP) by hydrothermal method. The photocatalytic hydrogen generation and photocatalytic degradation capabilities of composite photocatalysts with various mass ratios were investigated by characterizing the catalyst and using the organic dye Rhodamine B (RhB) as the pollutant. According to the research, the specific surface area of the g-C3N4/MeTMC-COP composite may reach 40.95m2g-1 when the mass ratio of g-C3N4 and MeTMC-COP is 3:1 (25.22m2g-1). It can offer more active sites for the photocatalytic process, and because the fluorescence peak intensity is the lowest, it has the lowest photogenerated electron-hole recombination efficiency. In comparison to g-C3N4, 3:1g-C3N4/MeTMC-COP can breakdown rhodamine B up to 100% after 75min of light irradiation; its photocatalytic hydrogen generation efficiency is 1.62 times that of g-C3N4, and the hydrogen evolution rate is 11.8μmolg-1h-1.

Slow Photon-Enhanced Heterojunction Accelerates Photocatalytic Hydrogen Evolution Reaction to Unprecedented Rates

Slow Photon-Enhanced Heterojunction Accelerates Photocatalytic Hydrogen Evolution Reaction to Unprecedented Rates

Metal-Organic-Frameworks-driven hollow core-shell C03O4/TiO2 Heterojunction for Enhancement of Photocatalytic Hydrogen generation

In recent years, how to further enhance the efficiency of photocatalytic hydrogen generation has become a research hotspot. At present, the metal-organic-frameworks (MOF) has attracted much attention in the field of catalysis due to its special physical and chemical properties. Herein, the hollow core-shell Co3O4/TiO2 heterojunction is prepared by deposition and annealing method with ZIF-67 as self-sacrificial templates and metal precurous. The characterzation of XRD, SEM, and TEM indicate that the hollow core-shell Co3O4/TiO2 heterojunction is synthesized successfully. The characterzation of UV, PL indicate that the Co3O4/TiO2 show superior photoelectric performance. By evaluation, the Co3O4/TiO2 heterogeneous catalyst exhibits a remarkable photocatalytic hydrogen production performance, which is ∼11 and ∼14 times higher than that of pure Co3O4 and TiO2. Studies prove that the formation of heterojunction can promote charge carriers separation, the hollow structure can raise the solar efficiency, offer more surface areas and active sites, all these are beneficial for photocatalytic performance.

Nanostructured Lateral Boryl Substitution Conjugated Donor-Acceptor Oligomers for Visible-Light-Driven Hydrogen Production.

Poor charge separation is the main factor that limits the photocatalytic hydrogen generation efficiency of organic conjugated polymers. In this work, a series of linear donor-acceptor (D-A) type oligomers are synthesized by a palladium-catalyzed Sonogashira-Hagihara coupling of electron-deficient diborane unit and different dihalide substitution sulfur functionalized monomers. Such diborane-based A unit exerts great impact on the resulting oligomers, including distinct semiconductor characters with isolated lowest unoccupied molecular orbital (LUMO) orbits locating in diborane-containing fragment, and elevated LUMO level higher than water reduction potential. Relative to A-A type counterpart, the enhanced dipole polarization effect in D-A oligomers facilitates separation of photogenerated charge carriers, as evidenced by notably prolonged electron lifetime. Owing to π-π stacking of rigid backbone, the oligomers can aggregate into an interesting 2D semicrystalline nanosheet (≈2.74nm), which is rarely reported in linear polymeric photocatalysts prepared by similar carbon-carbon coupling reaction. Despite low surface area (30.3 m2 g-1 ), such ultrathin nanosheet D-A oligomer offers outstanding visible light (λ> 420nm) hydrogen evolution rate of 833 µmol g-1 h-1 , 14 times greater than its A-A analogue (61 µmol g-1 h-1 ). The study highlights the great potential of using boron element to construct D-A type oligomers for efficient photocatalytic hydrogen generation.

Urchin-like TiO2 structures decorated with lanthanide-doped Bi2S3 quantum dots to boost hydrogen photogeneration performance

The formation of heterojunctions between wide- and narrow-bandgap photocatalysts is commonly employed to boost the efficiency of photocatalytic hydrogen generation. Herein, the photoactivity of urchin-like rutile particles is increased by decorating with pristine as well as Er- or Yb-doped Bi2S3 quantum dots (QDs) at varied QD loadings (1–20 wt%) and doping degrees (1–15 mol%), and the best hydrogen evolution performance is achieved at Er and Yb contents of 10 mol%. Specifically, a hydrogen productivity of 1576.7 μmol gcat−1 is achieved after 20-h irradiation for TiO2 decorated by 10 mol% Yb-doped Bi2S3 QDs. Theoretical calculations show that the introduction of defects into the Bi2S3 lattice through Er/Yb doping promotes the creation of new energy levels and facilitates the transport of photogenerated charges during photocatalysis.

Porous g-C3N4/WO3 photocatalyst prepared by simple calcination for efficient hydrogen generation under visible light

A novel porous g-C3N4/WO3 photocatalyst was successfully obtained by adopting urea and WO3⋅0.33H2O as precursor by a simple hydrothermal and calcination method. The structural characteristics, photoelectric properties, and hydrogen generation efficiency of g-C3N4/WO3 were studied. The results shows that best photocatalytic hydrogen generation efficiency of the g-C3N4/WO3 was 963 μmol−1 h−1, it was about 3 times than pure g-C3N4. Hydrated tungsten oxide promotes the formation of porous structure after removing crystal water, and the increased pore volume and pore size provide more reactive activity sites, which contribute to increase the efficiency of hydrogen generation. The mechanism of photocatalytic hydrogen generation was proposed.

Bi4TaO8Cl/Graphene nanocomposite for photocatalytic water splitting

Sillen-Aurivillius structures like Bi4NbO8Cl, Bi4TaO8Cl, and Bi4TaO8Br have been expected as efficient visible light active photocatalysts thanks to their narrow band gaps less than 2.5 eV and suitable negative conduction band potential for hydrogen production reaction, 0.0 V vs NHE. However, despite their excellent potential the photocatalytic hydrogen generation efficiency of them under visible light has remained low. The low activity is usually attributed to the shallow defect levels near the conduction band, causing fast recombinations of photoexcited electrons and holes. In this study, a nanocomposite of Bi4TaO8Cl and graphene is proposed for overcoming this issue. The excellent electron conductivity and abundant delocalized electrons from the conjugated sp2-bonded carbon networks in graphene can facilitate the transfer of electrons from Bi4TaO8Cl conduction band and increase the photocatalytic efficiency. Bi4TaO8Cl/graphene nanocomposite was successfully prepared by a hydrothermal method, and photocatalytic activity enhanced both under UV and visible light.

In situ plasmonic Au nanoparticle anchored nickel ferrite: An efficient plasmonic photocatalyst for fluorescein-sensitized hydrogen evolution under visible light irradiation

Photocatalytic hydrogen generation is a considerable promising technology to decrease climate change effect of CO2 and to solve the increasing global demand for clean energy. Hydrogen generation driven by visible light still faces many challenges although great efforts have been made. Efficient charge separation plays an important role in solar-energy conversion by heterojunction photocatalysts. Herein we report that nickel ferrites covered by gold plasmonics form well-defined plasmonic photocatalysts for fluorescein-sensitized hydrogen production under visible-light irradiation with largely enhanced photoactivity due to fast separation of photogenerated electron-hole pairs. The optimal Au/NiFe2O4 plasmonic photocatalysts with AuNP loading of 1.5wt% shows the hydrogen production rate of 0.256mmolg−1h−1via localized surface plasmon resonance effect of AuNPs. Fluorescein was acted as photosensitizer to extend the visible-light absorption of Au/NiFe2O4 plasmonic photocatalysts and enhance photocatalytic hydrogen generation efficiency under visible-light irradiation. The optimum rate for hydrogen generation reached 3.162mmolg−1h−1 and the rate is about 60-fold and 12-fold higher than that of pure NiFe2O4 and 1.5wt% Au/NiFe2O4 plasmonic photocatalysts, respectively.

Directionality of Ultrafast Electron Transfer in a Hydrogen Evolving Ru–Pd-Based Photocatalyst

Directionality of electron transfer and long-lived charge separation are of key importance for efficient photocatalytic water splitting. Knowledge of the processes that follow photoexcitation is essential for the optimization of supramolecular assembly designs in order to improve the efficiency of photocatalytic hydrogen generation. Photoinduced intramolecular electron transfer processes within the hydrogen-evolving photocatalyst [Ru(bpy)2(tpy)Pd(CH3CN)Cl]2+ (RuPd; bpy = bipyridine, tpy = 2,2′:5′,2″-terpyridine) have been studied by resonance Raman, femtosecond transient absorption, and time-resolved photoluminescence spectroscopies. Comparison of the photophysical properties of RuPd with those of the mononuclear precursor [(bpy)2Ru(tpy)]2+ (Ru) enables establishment of a photophysical model ranging from the femtosecond to the submicrosecond domain. Optical excitation of Ru and RuPd populates both bpy- and tpy-based 1MLCT (metal-to-ligand charge transfer) singlet states, from where intersystem crossing (ISC) into corresponding 3MLCT triplet states occurs. Electron density localized on the peripheral bpy ligands can subsequently flow to the tpy bridging ligand by interligand electron transfer, which process occurs with a time constant of 32.5 (±1.5) ps for RuPd. Not all electron density undergoes this process, most likely due to a competing loss channel on the bpy ligand caused by vibrational relaxation occurring at a time scale of 9.1 (±0.4) ps. The relaxed 3MLCTbpy and 3MLCTtpy states have excited state lifetimes of 400 (±1) ns and 88 (±1) ns, respectively. Electron transfer from the tpy ligand to Pd may take place on a ∼100 ns time scale, but it is also possible that the final relaxed excited state is delocalized over the tpy ligand and the Pd center. The insight that optical excitation populates both the peripheral bpy ligands and the bridging tpy ligand, and that part of the electron density subsequently flows from the former to the latter, is important for the realization of efficient photocatalytic hydrogen generation. The next step is to make the interligand electron transfer process faster, by functionalizing the peripheral ligands with electron-donating moieties, and adapting the nature of the bridging ligand and the catalytic metal center.

Photocatalytic Hydrogen Generation from Pure Water using Silicon Carbide Nanoparticles

AbstractWe report here the photocatalytic hydrogen generation from pure (deionized) water under simulated solar light with silicon carbide (SiC) nanoparticles, without the assistance of sacrificial reagents or precious metal co‐catalysts. We have studied the pH effects of the addition of methanol and the loading of a Pt co‐catalyst on the photocatalytic hydrogen generation efficiency of SiC nanoparticles. We have found that SiC nanoparticles display the best photocatalytic hydrogen generation efficiency in pure water. SiC nanoparticles show activity in both ultraviolet (UV) and visible‐light regions. The addition of methanol in water or loading of Pt nanoparticles on the surface lowers their activity. Thus, SiC nanoparticles may serve as an ideal model photocatalyst for future studies in generating hydrogen from pure water.

Si/poly-CuTAPC coaxial core–shell nanowire array as enhanced visible-light photocatalyst for hydrogen production

A controllable method for coating a dense layer of poly-CuTAPc polymer on the surface of silicon nanowire array was reported and it is shown that the coating of poly-CuTAPc can effectively enhance both the photocatalytic hydrogen generation efficiency and the operation stability.

High quality graphene oxide–CdS–Pt nanocomposites for efficient photocatalytic hydrogen evolution

Graphene oxide–CdS–Pt (GO–CdS–Pt) nanocomposites with different amounts of Pt nanoparticles were successfully synthesized via the formic acid reduction process followed by a two-phase mixing method. The morphology, crystal phase and optical properties of obtained composites were well characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD), UV-vis spectroscopy, Fourier transform IR spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS), respectively. The photocatalytic activity of GO–CdS–Pt composites for hydrogen generation was investigated. The results show that the GO–CdS–Pt composite containing 0.5 at% of Pt exhibits the highest hydrogen evolution rate of 123 mL h−1 g−1 with strong photostability, which is about 2.5 times higher than that of GO–CdS and 10.3 times higher than that of CdS. The increased photocatalytic hydrogen generation efficiency is attributed to the effective charge separation and decreased anti-recombination with the addition of GO and Pt, as well as the low overpotential of Pt for water splitting. Our findings pave a way to design multi-component graphene-based composites for highly efficient H2 generation and other applications.

Photocatalytic hydrogen generation using glucose as electron donor over Pt/Cd x Zn1−x S solid solutions

Cd x Zn1−x S solid solution photocatalysts were prepared by a hydrothermal process. The photocatalysts were characterized by X-ray diffraction (XRD), UV–vis diffusive reflectance spectroscopy (DRS), and transmission electron microscope (TEM) measurements. Using glucose as an electron donor, photocatalytic hydrogen generation over Pt/Cd x Zn1−x S was investigated. The results show that glucose not only improves the efficiency of photocatalytic hydrogen generation but prevents photocorrosion of Cd x Zn1−x S. Glucose was degraded effectively with the hydrogen generation. The factors which affect photocatalytic hydrogen generation, such as composition and structure of Cd x Zn1−x S solid solutions, irradiation time, initial concentration of the glucose, and concentration of NaOH were studied.

Photocatalytic hydrogen generation using glycerol wastewater over Pt/TiO2

Using glycerol as electron donor, photocatalytic hydrogen generation over Pt/TiO2 was investigated. The results show that glycerol can not only improve the efficiency of photocatalytic hydrogen generation but can also be decomposed effectively. The factors which affect photocatalytic hydrogen generation, such as irradiation time, initial concentration of the glycerol solution, pH-value of the suspensions and the coexisting substances were studied. The final oxidation products of glycerol were H2O and CO2. Glyceraldhyde, glycoladehyde, glycolic acid and formaldehyde were identified as the intermediates. A possible reaction mechanism was discussed.