Photocatalysts For Hydrogen Production Research Articles

Hydrogen Production from Glycerol Photoreforming on TiO2/HKUST-1 Composites: Effect of Preparation Method

Coupling metal-organic frameworks (MOFs) with inorganic semiconductors has been successfully tested in a variety of photocatalytic reactions. In this work we present the synthesis of TiO2/HKUST-1 composites by grinding, solvothermal, and chemical methods, using different TiO2 loadings. These composites were used as photocatalysts for hydrogen production by the photoreforming of a glycerol-water mixture under simulated solar light. Several characterization techniques were employed, including X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), infrared spectroscopy (FTIR), and time-resolved microwave conductivity (TRMC). A synergetic effect was observed with all TiO2/HKUST-1 composites (mass ratio TiO2/MOF 1:1), which presented higher photocatalytic activity than that of individual components. These results were explained in terms of an inhibition of the charge carrier (hole-electron) recombination reaction after photoexcitation, favoring the electron transfer from TiO2 to the MOF and creating reversible Cu1+/Cu0 entities useful for hydrogen production.

ZnO:InN oxynitride: A novel and unconventional photocatalyst for efficient UV–visible light driven hydrogen evolution from water

A novel UV–visible light active oxynitride photocatalyst, ZnO:InN, has been demostrated to exhibit efficient hydrogen evolution from water as compared to some conventional photocatalysts. A reduction in bandgap (2.82 eV), as estimated from diffuse reflectance spectra, is explained using Valence band XPS studies and attributed to the upshift in valence band maximum. Photocatalytic activity of the samples has been demonstrated by organic dye degradation, where 92% decay was observed in 180 min under direct sunlight. Photoelectrochemical studies under visible light showed significant photoresponse with a photocurrent density of ∼8 μA/cm2. This study, as per our knowledge, is first of its kind where ZnO:InN oxynitride has been explored as a UV–visible light active photocatalyst with significant H2 generation of ∼48 μmol/g by UV–visible light water splitting using methanol scavenger. The results show the promising future applications of ZnO:InN oxynitride as a sunlight active photocatalyst for hydrogen production.

A review on frontiers in plasmonic nano-photocatalysts for hydrogen production

Nanoparticles have plenty of active sites in each particle and short travel distance for photoexcitons proved to be beneficial characteristics for photocatalytic hydrogen generation. Noble metal (Au, Ag, Pt, Pd, Ru, and Rh) attached with semiconductor photocatalyst displayed plasmonic oscillations results in localized surface plasmon resonance (LSPR) effect and schottky junction formed to pump the photoelectrons to surface for reactions. Literature reports evidenced that the choice of synthesis method and experimental conditions directly affects the catalytic and optical properties. Hence, this review discusses comprehensive information on chemical methods reported for the preparation of plasmonic photocatalyst that resulted in co-catalyst and visible light sensitizer properties. Recent developments and achievements on plasmonic photocatalyst for hydrogen production in pure water and sacrificial agent containing water are discussed and highlighted.

Ag2S Quantum Dots as an Infrared Excited Photocatalyst for Hydrogen Production

H2 production using nanoscale semiconductors via photocatalytic water splitting is a much sought-after technology to curb carbon dioxide emission. Among the many challenges found to date is the search for a stable semiconductor photocatalyst responding to visible and preferably visible and IR light. Ag2S is a narrow bandgap semiconductor with a bulk electronic gap smaller than that needed to split water. In this work, using a solvent thermal strategy, we have increased its bandgap energy by shifting up the conduction band edge to make it suitable for the electron transfer reaction to hydrogen ions. The Ag2S quantum dots (QDs) were tested as both electrocatalysts and photocatalysts. As electrocatalysts, Ag2S QDs with an absorption peak at 800 nm (QD800) showed the highest H2 evolution activity with a Tafel slope of 89 mV/dec with an overpotential of 0.32 V. As photocatalysts, H2 was produced at a rate of 858 μmol h–1 gcatal–1 under a white light flux of 100 mW cm–2. Moreover, QD800 was also found to be act...

Two Bulky Conjugated 4′‐(4‐Hydroxyphenyl)‐4,2′:6′,4′′‐terpyridine‐based Layered Complexes: Synthesis, Structure, and Photocatalytic Hydrogen Evolution Activity

Two new layered complexes with the formulas of {[Cu(H2O)(HL)2Cl](NO3)}n (1) and {[Cu(H2O)2(HL)2](NO3)2}n (2) were solvothermally synthesized by the reactions of the bulky conjugated 4′‐(4‐hydroxyphenyl)‐4,2′:6′,4′′‐terpyridine ligand (HL) with different CuII salts, which were further used as photocatalysts to achieve hydrogen production from water splitting. Single‐crystal structural analyses reveal that both complexes feature coplanar (4 4) layers with different connection manners between the HL extended Z‐shaped chains. More interestingly, 1 possessing more negative conduction band potential and higher structural stability exhibits a large hydrogen production rate of 2.43 mmol·g–1·h–1, which is four times higher than that of 2. Thus, the CuII‐based coordination polymers modified by the bulky conjugated organic ligand can become potentially promising non‐Pt photocatalysts for hydrogen production from water splitting.

An Amine-Functionalized Zirconium Metal-Organic Polyhedron Photocatalyst with High Visible-Light Activity for Hydrogen Production.

Metal-organic polyhedra (MOPs) are promising candidates for many potential applications; however, their use as photocatalysts for hydrogen production has yet to be developed. Herein, the photocatalytic performance of a water-stable Zr-MOP, ZrT-1-NH2 , was evaluated, for the first time, through photocatalytic hydrogen evolution under visible-light irradiation. ZrT-1-NH2 shows clearly enhanced photocatalytic activity (510.42 μmol g-1 h-1 ) for hydrogen production, in comparison with that of other homogeneous crystalline materials. If platinum nanoparticles were introduced into the photocatalytic system, the hydrogen production efficiency of ZrT-1-NH2 could be further improved. For ZrT-1-NH2 , the conspicuous improvement in photocatalysis can be attributed to efficient electron-hole separation, targeted electron transfer, and excellent recombination suppression. Furthermore, ZrT-1-NH2 shows excellent stability during photocatalytic hydrogen evolution over five continuous runs. This work illustrates that MOP-based photocatalysts hold promise for broad applications in the domain of clean energy.

Exfoliated Molybdenum Disulfide Encapsulated in a Metal Organic Framework for Enhanced Photocatalytic Hydrogen Evolution

An exfoliated MoS2 encapsulated into metal-organic frameworks (MOFs) was fabricated as a promising noble-metal-free photocatalyst for hydrogen production under visible light irradiation. The as-synthesized samples were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller (BET) surface analysis. It is well known that bulk MoS2 is unsuitable for photocatalysis due to its inadequate reduction and oxidation capabilities. However, exfoliated MoS2 exhibits a direct band gap of 2.8 eV due to quantum confinement, which enables it to possess suitable band positions and retain a good visible-light absorption ability. As a result, it is considered to be an encouraging candidate for photocatalytic applications. Encapsulating exfoliated MoS2 into MOF demonstrates an improved visible light absorption ability compared to pure MOF, and the highest hydrogen production rate that the encapsulated exfoliated MoS2 could reach was 68.4 μmol h-1g-1, which was much higher than that of pure MOF. With a suitable band structure and improved light-harvesting ability, exfoliated MoS2@MOF could be a potential photocatalyst for hydrogen production.

Environmentally Sustainable Synthesis of a CoFe2O4-TiO2/rGO Ternary Photocatalyst: A Highly Efficient and Stable Photocatalyst for High Production of Hydrogen (Solar Fuel).

Herein, a magnetically separable reduced graphene oxide (rGO)-supported CoFe2O4–TiO2 photocatalyst was developed by a simple ultrasound-assisted wet impregnation method for efficient photocatalytic H2 production. Integration of CoFe2O4 with TiO2 induced the formation of Ti3+ sites that remarkably reduced the optical band gap of TiO2 to 2.80 eV from 3.20 eV. Moreover, the addition of rGO improved the charge carrier separation by forming Ti–C bonds. Importantly, the CoFe2O4–TiO2/rGO photocatalyst demonstrated significantly enhanced photocatalytic H2 production compared to that from its individual counterparts such as TiO2 and CoFe2O4–TiO2, respectably. A maximum H2 production rate of 76 559 μmol g–1 h–1 was achieved with a 20 wt % CoFe2O4- and 1 wt % rGO-loaded TiO2 photocatalyst, which was approximately 14-fold enhancement when compared with the bare TiO2. An apparent quantum yield of 12.97% at 400 nm was observed for the CoFe2O4–TiO2/rGO photocatalyst under optimized reaction conditions. This remarkable enhancement can be attributed to synergistically improved charge carrier separation through Ti3+ sites and rGO support, viz., Ti–C bonds. The recyclability of the photocatalyst was ascertained over four consecutive cycles, indicating the stability of the photocatalyst. In addition, it is worth mentioning that the photocatalyst could be easily separated after the reaction using a simple magnet. Thus, we believe that this study may open a new way to prepare low-cost, noble-metal-free magnetic materials with TiO2 for sustainable photocatalytic H2 production.

Rationally designed 2D/2D SiC/g-C3N4 photocatalysts for hydrogen production

Visible-light driven photocatalytic hydrogen production from water is a hotspot in renewable energy.

In situ preparation of CdS decorated ZnWO4 nanorods as a photocatalyst for direct conversion of sunlight into fuel and RhB degradation

In situ synthesis of monoclinic ZnWO4 nanorods decorated with hexagonal CdS nanoparticles by a facile solvothermal method used as photocatalysts for hydrogen production under solar light.

Constructing a novel family of halogen-doped covalent triazine-based frameworks as efficient metal-free photocatalysts for hydrogen production.

Halogens, as typical non-metal dopants, have attracted intensive interests for developing highly active photocatalysts. However, the essential factors and underlying mechanism of halogen modification are still unclear. Herein, we systematically report the development of halogen (F, Cl and Br)-doped covalent triazine-based frameworks (CTFs) via a facile thermal treatment of CTFs and an excess of ammonium halide. The introduction of halogen atoms endowed CTFs with multiple superior effects such as improved optical absorption, promoted charge migration, narrowed band gaps and tuned band positions. The newly developed halogen-doped CTFs showed remarkable photocatalytic activities for H2 evolution under visible-light irradiation. Notably, the most enhanced photocatalytic performance was obtained with Cl-doped CTFs, which exhibited 7.1- and 2.4-fold enhancements compared to un-doped CTFs and Cl-doped g-C3N4, respectively. The electronegativity and atomic radius of the halogen atoms affected the modification of the optical and electronic properties, leading to different photocatalytic performances of F-, Cl- and Br-doped CTFs. The conclusions presented in this work will provide some new insights into the understanding of the doping effect for the improvement of the photocatalytic activity of halogen-doped CTF photocatalysts.

One-step synthesis of hierarchical AuNPs/Cd0.5Zn0.5S nanoarchitectures and their application as an efficient photocatalyst for hydrogen production

The hierarchical AuNPs/Cd0.5Zn0.5S hybrids are directly fabricated via a facile one-step in-situ hydrothermal method. The as-prepared AuNPs/Cd0.5Zn0.5S hybrids demonstrate superior photocatalytic performance toward hydrogen production under visible light irradiation. The hydrogen evolution rate of the 5wt% AuNPs/Cd0.5Zn0.5S synthesized with in-situ hydrothermal method can be 7.1 times greater than that of pure Cd0.5Zn0.5S and as much as 2.5 times of 5wt% AuNPs loaded Cd0.5Zn0.5S synthesized with photodeposition method. Systematical investigations reveal that the enhanced photocatalytic performance of the one-step in-situ prepared AuNPs/Zn0.5Cd0.5S can be attributed to the inherent SPR effect and favorable electron transfer properties of AuNPs, as well as the rational hierarchical nanoarchitectures that allow AuNPs to be uniformly incorporated into Zn0.5Cd0.5S matrix. This one-step in-situ fabrication method provides a simple and efficient route to synthesize well-defined heterocatalysts.

Organic/inorganic nitride heterostructure for efficient photocatalytic oxygen evolution

Given the four-electron water oxidation reaction as the rate-limiting step for water splitting, highly efficient photocatalysts for oxygen evolution have been receiving increasing research attentions. In this study, an organic/inorganic g-C3N4/CoN nitride heterostructure was developed by a facile precipitation-nitridation two-step process. With the CoN loading amounts optimized, the obtained g-C3N4/CoN composite achieves more than 4-fold increase in photocatalytic activity for oxygen evolution, as compared to the pristine g-C3N4, with a highest oxygen evolution rate reaching 607.2 μmol h−1 g−1 under visible light (λ > 420 nm). It was demonstrated that the formed g-C3N4/CoN heterostructure could promote the interfacial charge carrier separation and the loaded CoN acting as an effective cocatalyst could accelerate the water oxidation reaction kinetics, which synergistically contributes to the great enhancement in photocatalytic activity for oxygen evolution. Interestingly, by physically mixing g-C3N4/CoN and g-C3N4/Ni, acting as oxygen and hydrogen production photocatalysts, respectively, the obtained composite could stably produce oxygen and hydrogen in the stoichiometric ratio from pure water under visible light (λ > 420 nm). Although the photocatalytic overall water splitting activity is still very low, this study demonstrates a facile and promising approach to develop visible-light active photocatalysts for simultaneous hydrogen and oxygen production from water, from the perspective of surface modification and bifunctional cocatalyst loading.

Photocatalytic Hydrogen Evolution from Water Using Fluorene and Dibenzothiophene Sulfone-Conjugated Microporous and Linear Polymers

Three series of conjugated microporous polymers (CMPs) were studied as photocatalysts for hydrogen production from water using a sacrificial hole scavenger. In all cases, dibenzo[b,d]thiophene sulfone polymers outperformed their fluorene analogues. A porous network, S-CMP3, showed the highest hydrogen evolution rates of 6076 μmol h–1 g–1 (λ > 295 nm) and 3106 μmol h–1 g–1 (λ > 420 nm), with an external quantum efficiency of 13.2% at 420 nm. S-CMP3 outperforms its linear structural analogue, P35, whereas in other cases, nonporous linear polymers are superior to equivalent porous networks. This suggests that microporosity might be beneficial for sacrificial photocatalytic hydrogen evolution, if suitable linkers are used that do not limit charge transport and the material can be wetted by water as studied here by water sorption and quasi-elastic neutron scattering.

Hydrothermal synthesis of MoS2-NiS/CdS with enhanced photocatalytic hydrogen production activity and stability

CdS is one of the most well-known photocatalysts for hydrogen production, while proper cocatalyst is indispensable to realize high hydrogen production performance. In this work, a three-component composite MoS2-NiS/CdS with dual cocatalysts was synthesized by a relatively facile and environmentally friendly three-step hydrothermal method and characterized by series of techniques. The as-prepared MoS2-NiS/CdS possesses good photostability and excellent photocatalytic hydrogen production performance. Under visible light irradiation of a 300 W Xe lamp (λ ≥ 420 nm), the H2 production rate reached a high value of 2525.2 μmol h−1 and the total amount of H2 evolved in 22 h achieved 40.5 mmol, further confirming that the photocatalysts with dual reduction cocatalysts possess higher photocatalytic performance than those with single cocatalyst. Especially, first study about the effect of pH of lactic acid aqueous solution on hydrogen production activity was carried out and the results indicated that increasing the pH from 1.5 to 3.0 leads to an obvious increase in H2-evolving activity. More importantly, an attempt on the photocatalytic hydrogen production under the irradiation of a 30 W LED lamp demonstrated that the H2 evolved was also up to 32.1 mmol in 48 h, showing the feasibility of using cheap and long lifetime LED lamp as light source for photocatalytic hydrogen production of CdS-based photocatalysts.

Understanding structure-activity relationships in linear polymer photocatalysts for hydrogen evolution

Conjugated polymers have sparked much interest as photocatalysts for hydrogen production. However, beyond basic considerations such as spectral absorption, the factors that dictate their photocatalytic activity are poorly understood. Here we investigate a series of linear conjugated polymers with external quantum efficiencies for hydrogen production between 0.4 and 11.6%. We monitor the generation of the photoactive species from femtoseconds to seconds after light absorption using transient spectroscopy and correlate their yield with the measured photocatalytic activity. Experiments coupled with modeling suggest that the localization of water around the polymer chain due to the incorporation of sulfone groups into an otherwise hydrophobic backbone is crucial for charge generation. Calculations of solution redox potentials and charge transfer free energies demonstrate that electron transfer from the sacrificial donor becomes thermodynamically favored as a result of the more polar local environment, leading to the production of long-lived electrons in these amphiphilic polymers.

Remarkable effect of BaO on photocatalytic H2 evolution from water splitting via TiO2 (P25) supported palladium nanoparticles

Here we report a versatile strategy based on the use of BaO as an electron promoter to boost the photocatalytic activity of TiO2 based, Pd supported photocatalysts for hydrogen production from water splitting. Pd-BaO nanoparticles were deposited on the TiO2 surface by simple chemical reduction method using NaBH4 as reductant. The as-synthesized Pd-BaO/TiO2 photocatalysts showed a remarkably high performance in hydrogen production, approximately 29.6 mmol g−1h−1 using an ethanol-water mixture (5% ethanol and 95% water), which is two times higher than Pd/TiO2 photocatalyst, where the latter is already regarded as the most active metal based TiO2 photocatalysts for H2 production. This high hydrogen production efficiency was attributed to the inherent high catalytic activity of palladium nanoparticles and electronic promotional effect of BaO. The Ba (in the form of BaO) greatly enhanced the electron transfer from the semiconductor conduction band to Pd centers by raising the Fermi level of TiO2 during photo-irradiation. Most importantly, Pd is present exclusively in zero oxidation state, which is ideal for proton reduction in the as-prepared photocatalysts due to close proximity of highly electropositive BaO. The structural morphologies and mechanistic aspects of the Pd-BaO/TiO2 photocatalysts are addressed by using UV–Vis/DRS, XRD, TEM, and XPS characterization techniques.

Construction of CDs/CdS photocatalysts for stable and efficient hydrogen production in water and seawater

Photocatalytic hydrogen production on semiconductors has been intensively researched in the past years, yet it still lacks research on efficient and stable photocatalysis in seawater. In this paper, with cadmium sulfide (CdS) as a sample photocatalyst and carbon dots (CDs) as cocatalyst, we fabricated a series of CDs/CdS composites serving as efficient hydrogen production photocatalysts in both water and seawater. In this case, the optimal CDs/CdS-S composites (nanosheet with cocatalyst content of 0.01 gCDs/gcatalyst) exhibit prominent H2 production rate of 4.64 mmol h−1 g−1 (6.70 mmol h−1 g−1, water) and AQE of 11.8% (19.3%, water) under 420 nm light irradiation in seawater, which are about 265 and 169 (78 and 77, water) times of those of the non-modified irregular CdS in seawater, respectively. The excellent activity and stability are attributed to the competency of CDs that not only dramatically improves the charge separation efficiency, but also plays indispensable role in resisting the distraction from various ionic components in seawater. We hope our work can provide a feasible perspective for constructing high-efficient photocatalyst towards practical applications.

Polyoxometalate-Decorated g-C3 N4 -Wrapping Snowflake-Like CdS Nanocrystal for Enhanced Photocatalytic Hydrogen Evolution.

Photocatalytic hydrogen evolution technology is recognized as a promising approach to relieving the growing energy crisis. Therefore, the development of a stable high-performance photocatalyst has long been the focus of research. In this work, quaternary composite materials involving a snowflake-like CdS nanocrystal wrapped by different amounts of polyoxometalate-decorated g-C3 N4 and polypyrrole (GPP@CdS) have been synthesized as photocatalysts for hydrogen production under visible-light irradiation. It has been revealed that the best composite (40 % GPP@CdS composite) exhibits hydrogen production activity of 1321 μmol, which exceeds that of CdS by a factor of more than two, and can be used in at least seven cycles with negligible loss of activity. The enhanced photocatalytic performance has been primarily attributed to the efficient synergy of CdS, g-C3 N4 , polypyrrole (PPy), and the polyoxometalate Ni4 (PW9 )2 . It should be noted that the introduction of PPy and g-C3 N4 into the title composite simultaneously promotes electron/hole pair separation and photocatalytic stability, whereas Ni4 (PW9 )2 serves as an efficient electron modulator and extra catalytic active site.

Facile synthesis of cobalt-doped (Zn,Ni)(O,S) as an efficient photocatalyst for hydrogen production

Cobalt-doped (Zn,Ni)(O,S) or Co-(Zn,Ni)(O,S) was facilely synthesized at low temperature below 100 °C with different cobalt precursor contents for photocatalytic hydrogen production. The X-ray pattern and elemental mapping proved that cobalt was successfully doped into zinc sites in the (Zn,Ni)(O,S) host lattice. We found the incorporation with a small amount of cobalt into (ZnNi)(O,S) enhanced its photo activity for hydrogen production. The best hydrogen production was achieved for 2.5% Co-(ZnNi)(O,S) with a rate of 8,527 μmol/g·h during a span of 5 h in a 20% (v/v) ethanol/water solution. Based on the results of optical characterizations, the enhancement of hydrogen production was caused by the slow electron-hole recombination and the low charge transfer resistance. A different photocatalytic kinetic mechanism for hydrogen generation from the conventional one with the simultaneous formation of hydrogen and oxygen gases is proposed, based upon the activated surface oxygen anion to initiate or trigger the key reaction of oxidation for water splitting to proceed. Our strategy in preparing catalyst at low process temperature and in doping to activate catalyst is for weakening the lattice oxygen bonding on the catalyst surface in order to firstly initiate the oxidation reaction and the formation of oxygen vacancies. These freshly formed oxygen vacancies play a critical role to trap the water and weaken its OH bonding to form hydrogen gas through the reduction reaction.