Photocatalysts For H2 Production Research Articles

Enhanced photocatalytic hydrogen production by loading histidine on TiO2

Although TiO2 exhibits excellent photocatalytic properties, its application has been limited owing to rapid e−/h+ recombination. Therefore, TiO2 has failed to reach the desired effect in terms of efficient photocatalytic hydrogen production. In this study, a range of titanium dioxide catalysts loaded with histidine (His) were prepared by an easy dipping method, viz. by treating TiO2 in His aqueous solutions with different concentrations at ambient temperature. Photocatalytic hydrogen evolution by splitting water was performed on His-modified TiO2 under 300-W Xe irradiation. His-loaded TiO2 catalysts exhibited improved photocatalytic performance compared with pristine TiO2; TiO2 treated in 1 × 10−3 mol · L−1 His showed the highest photocatalytic H2 evolution activity (4.77 μmol g−1 h−1 gm−2), which was 3.77 times higher than that of pure TiO2. Infrared and XPS analysis showed that His was successfully combined to the TiO2 surface by hydrogen bonding, constructing a fast channel for interfacial charge transfer. In the photocatalytic process, the photogenerated holes could migrate from the valence band of TiO2 to the highest occupied molecular orbital of His, which reduced the recombination rate of photogenerated e−/h+ pairs, consequently, improved H2 production efficiency. Importantly, the composite catalyst exhibited no decline in photocatalytic activity over 30 h of operation. This research provides an alternative approach for creating high-efficiency photocatalysts for H2 production.

Photocatalytic H2 Evolution, CO2 Reduction, and NOx Oxidation by Highly Exfoliated g-C3N4

g-C3N4, with specific surface area up to 513 m2/g, was prepared via three successive thermal treatments at 550 °C in air with gradual precursor mass decrease. The obtained bulk and exfoliated (1ex, 2ex and 3ex) g-C3N4 were characterized and tested as photocatalysts for H2 production, CO2 reduction and NOx oxidation. The exfoliated samples demonstrated graphene-like morphology with detached (2ex) and sponge-like framework (3ex) of layers. The surface area increased drastically from 20 m2/g (bulk) to 513 m2/g (3ex). The band gap (Eg) increased gradually from 2.70 to 3.04 eV. Superoxide radicals (·O2−) were mainly formed under UV and visible light. In comparison to the bulk, the exfoliated g-C3N4 demonstrated significant increase in H2 evolution (~6 times), CO2 reduction (~3 times) and NOx oxidation (~4 times) under UV light. Despite the Eg widening, the photocatalytic performance of the exfoliated g-C3N4 under visible light was improved too. The results were related to the large surface area and low e−-h+ recombination. The highly exfoliated g-C3N4 demonstrated selectivity towards H2 evolution reactions.

Construction of sugar-gourd-shaped CdS/Co1-xS hollow hetero-nanostructure as an efficient Z-scheme photocatalyst for hydrogen generation

Hollow hetero-nanostructures have unique advantages for energy storage and conversion and are highly desirable for their photocatalysis applications. We herein report the synthesis of an unusual sugar-gourd-shaped hollow hetero-nanostructure, Co1-xS hollow polyhedrons (HPs) skewered on CdS nanowires (NWs) (CdS/Co1-xS HHNSs), through a facile two-step strategy of growing ZIF-67 on CdS NWs followed by a sulfidation reaction. The loading amounts of Co1-xS HPs on an individual CdS NW can be adjusted by simply controlling the content of the Co1-xS precursor in the reaction. The coupling of the Co1-xS HPs with the CdS NWs not only broadens the solar light harvesting spectrum from ultraviolet to near infrared region, but also results in abundant Co vacancies as well as sides and facets with lots of active sites. More importantly, in-situ irradiated X-ray photoelectron spectroscopy confirms the direct Z-scheme transfer pathway of photogenerated charge carriers in the hetero-nanostructure, which facilitates the separation of photogenerated electron-hole pairs and induces strong photoreduction ability due to lower aligned conductor band of Co1-xS. As a result, the gular-gourd-shaped CdS/Co1-xS HHNSs are demonstrated as an efficient visible light-driven photocatalyst for H2 production. A H2 production rate of 13.48 mmol g−1 h−1 is achieved with optimized CdS/Co1-xS HHNSs without using any additional co-catalyst. This is the first report on the performance of Co vacancies in Co1-xS hollow nanostructures for photocatalysis which inspire the development of unusual hollow hetero-nanostructures as efficient photocatalysts by means of morphology design and interfacial electronic modulation.

General synthesis strategy for hollow porous prismatic graphitic carbon nitride: a high-performance photocatalyst for H2 production and degradation of RhB

This paper reports a general synthesis strategy for the hollow porous prismatic graphitic carbon nitride obtained from a supramolecular precursor with prismatic morphology, which was synthesized via a one-pot hydrothermal process using melamine or other N-containing compounds as the starting materials, without any other assist reagents. The best g-C3N4 photocatalyst in this work possesses a high hydrogen evolution rate (3486 μmol h−1 g−1, λ > 420 nm and 14643 μmol h−1 g−1 under full spectrum light) and superior photocatalytic activity in degradation of rhodamine B dye, giving a rate constant of 0.05032 min−1, almost 14.7 times than the bulk g-C3N4 under the visible light irradiation. The excellent photocatalytic performance is mainly attributed to the large specific surface area, more negative conduction potential and the faster separation efficiency of the g-C3N4 photocatalyst.

Realizing synergistic effect of electronic modulation and nanostructure engineering over graphitic carbon nitride for highly efficient visible-light H2 production coupled with benzyl alcohol oxidation

Photocatalytic H2 production based on g-C3N4 faces enormous challenging issues including limited visible-light absorption, poor separation and transfer abilities of photo-generated electron-hole pairs. Herein, we realize the synergistic effect of nanostructure engineering and electronic modulation with a supramolecular assembly mediated synthesis of heteroatom doped g-C3N4 hierarchical mesoporous spheres. The favorable doping site and possible effect on electronic structure are disclosed by DFT calculation with supporting experimental analysis. Impressively, S-doped g-C3N4 delivers a 13.2 times higher H2 production rate than bulk g-C3N4 under visible-light. More importantly, as the dual functional photocatalyst for H2 production and selective oxidation of benzyl alcohol, it can exhibit outstanding activity with a H2/benzaldehyde production rate of 3.76/3.87 μmol h−1, respectively. This work not only provide a new rationale for photocatalytic performance enhancement, but also shed new light on the highly efficient utilization of solar energy by coupling H2 generation with value added chemical production.

Mesoporous carbon/graphitic carbon nitride spheres for photocatalytic H2 evolution under solar light irradiation

Herein, two different photocatalytic composites based on ordered (OCS) and disordered (DCS) mesoporous hollow carbon spheres and graphitic carbon nitride (gCN) have been successfully fabricated through facile acid treatment. The influence of carbon shell morphology of the spheres on gCN loading and photocatalytic H2 production under simulated solar light irradiation has been revealed. The amount of evolved H2 was ~6.2 (OCS/gCN) and ~5.3 (DCS/gCN) times higher in comparison to pristine gCN. It was found that graphitic carbon nitride was much more homogenously supported onto ordered mesoporous carbon spheres than disordered ones. The deposition of gCN onto ordered carbon spheres was found to be more efficient to increase carrier concentration, enhance photogenerated charge carrier transport and separation. It is assigned to the formation of the graphitic carbon nitride/carbon heterojunction facilitating the contact surface between the two phases of hybrid. Therefore, via tuning of the morphology of carbon shell being a host for gCN it was possible to find more promising candidate as a photocatalyst in H2 production under solar light irradiation.

Controlling the visible-light driven photocatalytic activity of alloyed ZnSe–AgInSe2 quantum dots for hydrogen production

Alloyed ZnSe–AgInSe2 quantum dots were used as a photocatalyst for H2 production, showing an apparent quantum yield of 3.4% at 600 nm.

Zn2GeO4-x/ZnS heterojunctions fabricated via in situ etching sulfurization for Pt-free photocatalytic hydrogen evolution: interface roughness and defect engineering.

Interface engineering has been regarded as a promising strategy for enhancing the catalytic activities of heterojunction photocatalysts. Herein, we have adopted an in situ etching sulfurization method to construct a Zn2GeO4-x/ZnS intimate heterojunction, which exhibited excellent photocatalytic H2 production in the absence of a Pt co-catalyst. Distinctively, TEM and HRTEM measurements showed that the interface of the Zn2GeO4-x/ZnS heterojunction became rough (topologically) due to in situ etching sulfurization, and etching was found to be strongly dependent on the crystal orientation. Moreover, the surface of the Zn2GeO4 nanorods from flat (100) planes evolved into an irregular coastline-like structure topologized with (110) and (113) high-index planes. ICP and elemental distribution measurements indicated that during the precipitation of ZnS via in situ etching sulfurization, the migration and dissolution of Zn and Ge ions on the Zn2GeO4(100) plane led to the roughening of the interface and the evolution of crystal planes. XPS and EPR analyses showed that Zn2GeO4-x/ZnS contained more oxygen vacancies with structural evolution. The theoretical calculations demonstrated that oxygen defects were prone to be generated on the Zn2GeO4(113) plane and formed the Ge3c3+-VO complexes. Compared to the inactive (100) plane, etching caused the Zn2GeO4(110) planes to have a higher number of threefold coordinated germanium (Ge3c4+) and (113) high-index planes that possessed abundant active sites (Ge3c3+-VO complexes), which dramatically decreased the barrier and reaction energy of H2O dissociation. This work not only provides fundamental insights into the topological interface evolution and facet-dependent defect distribution but also offers a strategy for the design of efficient photocatalysts for H2 production without the use of Pt as a co-catalyst based on a multifunctional interface.

Panchromatic dirhodium photocatalysts for dihydrogen generation with red light.

A series of three dirhodium complexes cis-[Rh2(DPhB)2(bncn)2](BF4)2 (1, DPhB = diphenylbenzamidine; bncn = benzocinnoline), cis-[Rh2(DPhTA)2(bncn)2](BF4)2 (2, DPhTA = diphenyltriazenide), and cis-[Rh2(DPhF)2(bncn)2](BF4)2 (3, DPhF = N,N′-diphenylformamidinate) shown to act as single-molecule photocatalysts for H2 production was evaluated. Complexes 1–3 are able to generate H2 in the absence of any other catalyst in homogenous acidic solution upon irradiation with red light in the presence of the sacrificial electron donor BNAH (1-benzyl-1,4-dihydronicotinamide). The excited state of each complex is reductively quenched by BNAH, producing the corresponding one-electron reduced complex. The latter is also able to absorb a photon and oxidize another BNAH molecule, producing the doubly-reduced, activated form of the catalyst that is able to generate H2. The present work shows the effect of substitution on the bridging ligands on the driving force for reductive quenching and hydricity of the proposed active intermediate, both of which affect the efficiency of hydrogen production. Complexes 1–3 operate following a double reductive quenching mechanism and, importantly, are active with red light. This work lays the foundation for the design of single-molecule photocatalysts that operate from the ultraviolet to the near-infrared, such that solar photons throughout this entire range are harnessed and utilized for solar energy conversion.

Covalent grafting of molecular catalysts on C3N x H y as robust, efficient and well-defined photocatalysts for solar fuel synthesis.

The covalent attachment of molecules to 2D materials is an emerging area as strong covalent chemistry offers new hybrid properties and greater mechanical stability compared with nanoparticles. A nickel bis-aminothiophenol catalyst was grafted onto a range of 2D carbon nitrides (C3NxHy) to form noble metal-free photocatalysts for H2 production. The hybrids produce H2 beyond 8 days with turnover numbers reaching 1360 based on nickel, a more than 3 fold higher durability than reported molecular catalyst-carbon nitride mixtures, and under longer wavelengths (>475 nm). Time-resolved spectroscopy reveals sub-microsecond electron transfer to the grafted catalyst, six orders of magnitude faster compared with similar reports of non-grafted catalysts. The photoelectrons on the catalyst have a ca. 1000 times longer half-time (7 ms) compared with bare carbon nitride (10 μs). The grafting strategy operates across a range of molecular catalyst-carbon nitride combinations, thus paving the way for robust efficient photocatalysts based on low-cost tunable components.

Exfoliated Mo2C nanosheets hybridized on CdS with fast electron transfer for efficient photocatalytic H2 production under visible light irradiation

Visible-light-driven H2 production by water splitting using photocatalysts has aroused widespread interest. For wide applications, an exploration of low-cost and high-performance cocatalysts to replace expensive and rare noble metals is highly desirable. Herein, highly exfoliated 2D Mo2C nanosheets (Mo2C) were prepared by liquid-phase ultrasonic exfoliation for the first time. As a photocatalyst for H2 production under visible light irradiation, Mo2C was hybridized on CdS nanoparticles (CdS) to be the composite Mo2C/CdS. Mo2C/CdS exhibits a remarkable H2 production rate of 7.7 mmol g−1 h−1 which is 6-, 16-, and 4-folds as high as those for bulk Mo2C/CdS, pure CdS, and Pt/CdS, respectively. The apparent quantum efficiency (AQE) of 3 wt% Mo2C/CdS is 86 % at 460 nm. The enhanced activity is attributed to the rapid transfer of photogenerated electrons from CdS to Mo2C as well as the presence of more active sites of Mo2C than bulk Mo2C.

Donor-Acceptor Cyanocarbazole-Based Supramolecular Photocatalysts for Visible-Light-Driven H2 Production.

Highly efficient, stable, and metal-free supramolecular photocatalysts for H2 production are uncommon and of significant interest. In this study, a donor-acceptor (D-A) cyanocarbazole-based supramolecule (2CzPN) is assembled by intermolecular C≡N⋅⋅⋅H-Ar H-bonding interactions to form an efficient, stable, and metal-free photocatalyst for H2 evolution. The catalyst affords an H2 production rate of 91.8 μmol h-1 (20 mg of samples, λ>420 nm) and a corresponding apparent quantum efficiency of 7.5 % at 420 nm. The photon-generated carrier separation of the 2CzPN supramolecule, which is higher than that of the analogous polymer, can be attributed to the strong D-A characteristics and high crystallinity. This study offers the first experimental evidence of visible-light H2 evolution among D-A cyanocarbazole-based supramolecules and it enriches the variety of supramolecular photocatalysts. This stable and effective metal-free 2CzPN supramolecular photocatalyst has obvious advantages among very few supramolecular photocatalysts for H2 production.

Amine Functionalized Metal-Organic Framework Coordinated with Transition Metal Ions: d-d Transition Enhanced Optical Absorption and Role of Transition Metal Sites on Solar Light Driven H2 Production.

Design and development of efficient photocatalysts for H2 production from water and sunlight have gained significant attention as the solar assisted approach is considered to be a promising approach for the generation of clean fuel. However, the poor charge carrier separation and light harvesting ability of existing photocatalysts limits the efficiency of photoconversion of water. In this work, the synthesis of transition metal ions (M2+ = Co2+ , Cu2+ , and Ni2+ ) coordinated with Ti-metal organic frameworks (Ti-MOFs) through a simple post-synthetic coordination method for efficient solar light-driven H2 production is reported. Notably, coordination of M2+ ions with Ti-MOF significantly improves the optical absorption by d-d transitions and the multimetal sites facilitate the fast charge carrier separation, thereby enhancing the solar light-driven H2 production activity. Very interestingly, the rate of solar light-driven H2 production is varied with respect to different metal ions coordination due to the position of d-d bands absorption in the solar spectrum, and the complexing tendency of M2+ ions with sacrificial electron donors. A maximum solar H2 production rate of 1583.55 µmol h-1 g-1 is achieved with a Cu2+ coordinated Ti-MOF, which is ≈13 fold higher than that of the pristine Ti-MOF.

Ce-Doped Graphitic Carbon Nitride Derived from Metal Organic Frameworks as a Visible Light-Responsive Photocatalyst for H2 Production.

Novel fibrous graphitic carbon nitride (g-C3N4) derivatives prepared from metal organic frameworks (MOFs) were doped with Ce3+ (Ce-C3N4) as photocatalytic materials. Ce-C3N4 was characterized using various techniques, revealing its high specific surface area, excellent photocatalytic activity, and stability for H2 evolution under visible light irradiation. The fluorine modified samples show superior photocatalytic activity under visible light irradiation, which is due to the presence of more active sites and enhanced absorption of solar energy. This work provides a new synthetic route for MOF-derived g-C3N4 that can be doped with different metal ions. The fluorine modified Ce-C3N4 is an efficient photocatalyst with potential for many applications related to energy and the environment.

Constructing efficient polyimide(PI)/Ag aerogel photocatalyst by ethanol supercritical drying technique for hydrogen evolution

As one kind of promising photocatalyst, polyimide (PI) still suffers from poor light absorption, low charge transfer efficiency and small specific surface area. Herein, PI/Ag aerogel photocatalysts were designed by the sol-gel method and ethanol supercritical drying technique. As expected, Ag element in PI/Ag aerogel is in two forms, one is introduced into the imide ring of PI in the form of AgC bond, and another is the dopant Ag atoms. The former kind of Ag element can weaken the planar hydrogen bonding and improve the charge transfer efficiency, and the latter can form LSPR effect and promote the photoelectron-holes separation. Apart from that, the high specific surface area of the aerogel photocatalyst also favors the photocatalysis. All these effects boost the photocatalytic activity of the PI/Ag aerogel photocatalysts. The average rate of H2 production is up to 166.1 μmol h−1 g−1 for PI/Ag-1 aerogel, which is ca. 8 times as much as that of PI aerogel. The work supplies an efficient approach for constructing effective aerogel photocatalysts for H2 production.

Construction of Visible Light Responsive CdSe/g-C3N4 Nanocomposites for H2 Production

Utilization of heterogeneous photocatalysts for H2 production using water splitting reaction under visible light is a promising approach for production of large scale sustainable, renewable, and clean energy. CdSe/g-C3N4 nanocomposites were synthesized in this study by employing a simple aqueous synthesis method. Various CdSe contents were tested as a photocatalyst in the H2 production. XRD diffraction results confirmed the hexagonal arrangement of CdSe and its incorporation onto g-C3N4. Spherical shaped CdSe nanoparticles were distributed on the g-C3N4 nanosheets in an orderly fashion. The photocatalytic efficiency of CdSe/g-C3N4 composites was assessed utilizing glycerol as a scavenger. The photocatalytic activity for H2 production was enhanced by increasing the CdSe content, reaching H2 yield of up to 26000 μmol·g-1 using 5% CdSe/g-C3N4 nanocomposite. The produced H2 was higher ∼86 and 52 times than g-C3N4 and CdSe. The H2 production yield also increased with increased 5% CdSe/g-C3N4 photocatalyst load.

Ultrastable metal-free near-infrared-driven photocatalysts for H2 production based on protonated 2D g-C3N4 sensitized with Chlorin e6

Efficient utilization of near-infrared (NIR) energy is a high-priority in the field of energy and nanomedicine, yet the design and assembly of metal-free NIR-driven photocatalysts with biologic safety and good stability is still challenging. Herein, we report the design and synthesis of high-quality metal-free NIR-driven photocatalysts by judiciously combining the US Food and Drug Administration (FDA)-approved clinically useable Chlorin e6 (Ce6) as NIR photosensitizer and the protonated two-dimensional (2D) graphitic carbon nitride (g-C3N4) for the first time. Under the irradiation of visible light (>420 nm) and NIR light (>780 nm), the Ce6@pCN prepared here showed remarkable 10-fold and 174-fold high H2 evolution performance compared with bulk g-C3N4 (1275.6 vs. 127.1 μmol h−1 g−1; 312.6 vs. 1.8 μmol h−1 g−1) and excellent stability. Owing to the high efficiency, stability and nontoxicity, this protonated 2D g-C3N4 sensitized with Ce6 is promising for a wide-range of applications in energy, environmental and biological fields.

MnOx-decorated 3D porous C3N4 with internal donor–acceptor motifs for efficient photocatalytic hydrogen production

Carbon nitride is an intriguing visible-light photocatalyst for H2 production but suffers from low surface area and fast charge recombination. Here, we report a simple yet efficient approach to fine tune the porous structure, modify the electronic structure and carrier behaviors of C3N4 through in-situ growth of MnOx by NaCl template-assisted strategy. This NaCl template method simultaneously constructs 3D porous structure for high surface area, induces Na coordination with N atoms and accompanying cyano group to form an internal donor-acceptor system for efficient charge transfer and separation. Along with the MnOx growth, the band gap of C3N4 is narrowed for stronger visible light absorption and the conduction band level is negatively shifted for stronger reduction capability. Thus, the resulted C3N4 (MSCN) shows much improved activity of about 18 times higher than bulk C3N4 in photocatalytic hydrogen evolution reaction under visible light irradiation.

A high-efficiency photoelectrochemistry of Cu2O/TiO2 nanotubes based composite for hydrogen evolution under sunlight

In present study, a series of single modified TiO2 nanotubes with varying amounts of Cu was synthesized via two-step chemical solution method and studied as photocatalyst for H2 production under sunlight irradiation. The results indicate significant improvement of photocatalytic activity for TiO2 nanotubes modified Cu nanoparticles compared with pristine TiO2 attributed by the formation of the p-n heterojunction between Cu2O and TiO2. This enhanced the photo-generated charge transfer properties, resulting in an improved H2 evolution performance. The modified TiO2 nanotubes with 1.5 wt% Cu (Cu1.5 -TNTs) showed highest photocatalytic activity for H2 generation under sunlight due to structural integrity and uniform distribution of cuprous oxide on the surface of photocatalyst. Additionally, recyclability study for the H2 production of Cu1.5-TNTs material slightly decreased after four times, due to the presence of CuO phase via the oxidation of Cu2O and agglomeration phenomena, rendering them inefficient for H2 evolution performance under sunlight. We believe, addressing these problems would eminently assist in steady-state H2 generation performance under sunlight. Also, expanding the regime of light absorption to visible region for TiO2 1-Dimensional structure decorated Cu2O has higher photocatalytic efficiency for H2 generation in comparison to current 2-Dimensional systems.

Porous CuS/ZnS microspheres derived from a bimetallic metal-organic framework as efficient photocatalysts for H2 production

In this work, porous CuS/ZnS microspheres have been successfully fabricated through a facile metal-organic framework (MOF) derived route. The preparation process involved a solvothermal method for synthesis of bimetallic MOF (Cu-Zn-MOF) and subsequently vulcanization treatment to transform into CuS/ZnS microspheres. The MOF-derived CuS/ZnS microspheres inherited the porous skeleton of the Cu-Zn-MOF precursor, which significantly enhanced the utilization of light and synchronously provided plentiful exposed catalytic active sites. The obtained CuS/ZnS microspheres exhibited excellent photocatalytic activity for hydrogen production via water splitting. The optimized CuS/ZnS photocatalyst demonstrated a H2-production rate of 310.43 μmol h−1 under visible light with an apparent quantum efficiency of 8.5% at 420 nm. Such excellent photocatalytic H2-evolution activity was ascribed to the interfacial charge transfer from the valence band of ZnS to CuS, which facilitated the efficient electron-hole separation and transportation. This study provides a strategy for the rational design of MOF-derived photocatalysts to dramatically increase H2 production efficiency.