Dye-sensitized Photocatalytic H2 Evolution Research Articles

In-situ converting CoO@Co/RGO into Co/Co(OH)2/RGO for improved dye-sensitized photocatalytic H2 evolution

It is of huge challenge to develop stable and efficient non-metal based cocatalysts for photocatalytic hydrogen evolution reaction (HER) via dye sensitization. Herein, we demonstrate a composite featuring Co(OH)2 nanosheets embedded with metal Co nanocrystallines on reduced graphene oxide (Co/Co(OH)2/RGO) as a durable cocatalyst, which is obtained from the CoO@Co/RGO composite in dye-sensitized photocatalytic HER process via the dissolution-precipitation reaction of Co element. The large CoO@Co nanoparticles (∼43 nm in diameter) are converted to thin Co(OH)2 nanosheets, and tiny Co nanocrystalllines are exposed on the surface. As a fresh cocatalyst, Co/Co(OH)2/RGO is endowed with favorable adsorption sites of both hydrogen and hydroxyl. Such a phase and structure transformation leads to the high photocatalytic activity (136.1 μmol) of Co/Co(OH)2/RGO cocatalyst via dye sensitization, which is 3.7 times as much as that of pristine CoO@Co/RGO (36.7 μmol). The improved HER performance should be highly related to the more effective interfacial electron transfer ability, as reflected by transient absorption decays, and photocurrent test. Moreover, the Co/Co(OH)2/RGO cocatalyst shows a remarkable stability of photocatalytic H2 evolution, as well as the structural and phase stability. This work provides a novel route to develop highly dispersive and efficient non-noble metal based cocatalysts, and a new vision to understand the dynamic structure–activity relationship of the cocatalysts.

Correction: Distinctive organized molecular assemble of MoS2, MOF and Co3O4, for efficient dye-sensitized photocatalytic H2 evolution

Correction for ‘Distinctive organized molecular assemble of MoS2, MOF and Co3O4, for efficient dye-sensitized photocatalytic H2 evolution’ by Kai Fan et al., Catal. Sci. Technol., 2018, 8, 2352–2363, https://doi.org/10.1039/C8CY00380G.

Tuning metal-support interaction of NiCu/graphene cocatalysts for enhanced dye-sensitized photocatalytic H2 evolution

The modulation of metal-support interaction (MSI) is of particular interest in improving the catalytic performance of metal nanocatalysts. Herein, we adopt an alloying strategy to tune the MSI in graphene-supported Ni catalyst by introducing Cu. The NiCu/graphene cocatalysts (denoted as NxCyG) are facilely synthesized by directly calcinating the mixture of metal chlorides and graphene oxide for dye-sensitized photocatalytic hydrogen evolution reaction (HER). The results reveal the introduction of Cu significantly affects the MSI in NxCyG cocatalysts, and a reasonable Ni/Cu ratio (4:1) significantly enhances the MSI in N4C1G cocatalyst. Such an enhanced MSI facilitates the electron injection from excited dye to RGO and further transfer to the NiCu alloy, and leads to smaller size of NiCu alloy nanoparticles (NPs) with sufficient active sites. Moreover, the synergetic effect between Ni and Cu is conducive to optimize the hydrogen adsorption on NiCu surface. As a result, the N4C1G cocatalyst shows much improved electrocatalytic and photocatalytic HER performance compared with monometallic Ni/graphene and Cu/graphene. The apparent quantum yield (AQY) at 420 nm reaches up to 67.7 % in the dye-sensitization system. We believe that this work develops a facile and effective strategy to tune the MSI of metal/graphene catalysts for enhanced HER performance.

Protonated g-C3N4 cooperated with Co-MOF doped with Sm to construct 2D/2D heterojunction for integrated dye-sensitized photocatalytic H2 evolution

A dye-sensitive photocatalytic H2 evolution reaction (HER) system with photogenerated carrier directed conduction was constructed. The protonated g-C3N4 combines with the sheet-like Co MOF to form a 2D/2D heterojunction via electrostatic self-assembly. The protonated g-C3N4 and 2D Co-MOF directionally adsorb Eosin Y (EY) and triethanolamine (TEOA) molecules through hydrogen bond and complexation to achieve a whole photocatalytic system. The integral structure effectively facilitates the utilization of dye sensitizer and hole sacrificial agent to achieve the effective and stable photocatalytic H2 evolution capacity. The photocatalytic hydrogen evolution rate of g-C3N4 after protonation is 1.88 times as high as that of the original g-C3N4. On the basis of 2D/2D heterojunction, Co MOF is doped with rare earth element Sm. The 4f electrons and the difference valences (Sm3+ and Co2+) further suppress the reorganization of photogenerated excitons to achieve highly efficient photocatalytic HER. The directional coupling of sensitizer and electron sacrificial agent combined with rare earth element doping makes the photocatalytic HER rate of the composite material reached 73.42 μmol.h−1 within 5 h under simulated sunlight.

A Simple Ni-based Metal-organic Framework as Catalyst for Dye-sensitized Photocatalytic H2 Evolution from Water Reduction.

Metal-organic frameworks (MOF) are recently developed coordination porous materials, and their unique structures are very conducive to catalytic reactions. In this paper, p-benzenedicarboxylic acid (PBA)-Ni2+ MOF materials (denoted as PBA-Ni-x, where x represents the initial ratio of PBA to Ni2+ ) were synthesized by a hydrothermal method and characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and N2 gas adsorption. H2 gas was produced using the synthesized MOF as a photocatalyst and Eosin Y as a photosensitizer. The dependence of the special surface area and thickness of the nanosheets of Ni-MOF on the initial ratio of PBA to Ni2+ (PBA/Ni2+ ) was investigated. The BET surface areas of PBA-Ni-1 PBA-Ni-2 and PBA-Ni-3 are 11.00, 24.61 and 13.04m2 g-1 , respectively. And the thicknesses of nanosheets are approximately 600-1000, 200-500 and 300-700nm. Among the three materials, PBA-Ni-2 has the thinnest sheet-like structure and largest surface area. Thus, it displays the highest H2 evolution rate of 20.0μmolh-1 . The noble-metal-free hydrogen production system is valuable for the application of MOF materials in photocatalytic water splitting.

Facile synthesis of Co2(OH)3Cl/cobalt carbide/reduced graphene oxide composites for enhanced dye-sensitized photocatalytic H2 evolution

A hollow Co2(OH)3Cl/cobalt carbide/reduced graphene oxide composite (CG-350) synthesized without a template at low temperature (350 C) exhibits better dye-sensitized photocatalytic H2 evolution activity than the ones obtained at high temperatures.

Confining Mo-activated CoSx active sites within MCM-41 for highly efficient dye-sensitized photocatalytic H2 evolution

Although transition-metal-based sulfides have been identified as efficient catalysts to replace expensive noble metal catalysts for photocatalytic H2 evolution reaction (HER), their activities are still unsatisfied and could be further improved by controlling their microstructures and electronic structures. Herein, we present an effective strategy to confine highly active Mo-activated CoSx (Mo-CoSx) active sites within MCM-41 frameworks by sulfurization of Co-doped MCM-41 during the in situ photoreduction of [MoS4]2- in Erythrosin B-triethanolamine (ErB-TEOA) system. It is found that Co-MCM-41 offers not only abundant coordinatively unsaturated Co sites to be activated by Mo and S but also large surface area to effectively disperse the in situ generated amorphous Mo-CoSx active sites. Under 520 nm irradiation, the most efficient Mo-CoSx/MCM-41-100 (Si/Co = 100) catalyst exhibits ~7, 3, and 4 times higher H2 evolution activity than free MoSx, free Mo-CoSx, and CoSx/MCM-41-100, respectively, and an apparent quantum yield (AQY) of 12.3% for H2 evolution. Furthermore, when Mo-CoSx/MCM-41-100 was sensitized with a more stable fluorescein (FL) dye, the photocatalytic system shows a sustainable H2 evolution activity in a 20 h reaction, showing the good stability of Mo-CoSx/MCM-41-100 catalyst. This work provides a new insight into the design and development of highly active hybrid H2 evolution catalysts based on transition metals for highly efficient and large-scale solar energy conversion to clean H2 energy.

One-step synthesis of nickel oxide/nickel carbide/graphene composite for efficient dye-sensitized photocatalytic H2 evolution

To resolve the energy crisis and environment pollution, it is urgent to explore highly efficient and inexpensive catalysts for hydrogen evolution reaction (HER). In this work, we fabricate nickel oxide/nickel carbide/graphene (labeled as NNG) composite via one-step thermal treatment of NiCl2/graphene oxide (GO) as the precursor at relatively low temperature. GO plays a role of the carbon source to form nickel carbide and increases the distribution of the nickel species. The formed nickel carbide acts as the active sites for hydrogen atoms adsorption (Hads). The NiO contacted with the nickel carbide promotes the Volmer reaction: H2O+e→Hads+OH-. The two different adsorption sites (nickel carbide for Hads, NiO for OH−) jointly lead to the HER synergistic effect in basic solution. The obtained composites exhibit much higher dye-sensitized photocatalytic and electrocatalytic HER activities than nickel carbide, NiO and RGO alone. The highest apparent quantum yield (AQY) of dye-sensitized hydrogen evolution for NNG12 at 420 nm reaches 39.4%. And the electrocatalytic HER overpotential is -0.28 V at the current density of 10 mA cm-2 in 1.0 mol L-1 KOH. To the best of our knowledge, it is the first time to report nickel oxide/nickel carbide as an efficient HER catalyst in alkaline media. This work paves a way for facile and green fabrication of the efficient HER cocatalysts.

High-performance Förster resonance energy transfer-based dye-sensitized photocatalytic H2 evolution with graphene quantum dots as the homogeneous energy donor.

A high-performance dye-sensitized photocatalytic H2 evolution system was developed based on Förster resonance energy transfer (FRET) by employing water-soluble and highly photoluminescent N,S codoped graphene quantum dots (NSGQDs) as the homogeneous energy donor, erythrosin B (ErB) as the sensentizing dye, and platinum nanoparticles (Pt NPs) as the catalyst. NSGQDs absorbed high-energy photons that undergo FRET to transfer the excitation energy to the sensitizing ErB for maximizing light absorption and also served as an electron transfer and loading matrix of Pt NPs for accelarating the electron transfer; as a result, the ErB-sensitized NSGQD-Pt system afforded much higher H2 evolution activity than the NSGQD-free dye-sensitized system.

Quasi-homogenous dye-sensitized photocatalytic H2 evolution catalyzed by in-situ grown cobalt-promoted MoSx catalyst coupled with graphene quantum dots

The development of practical photocatalytic systems for large-scale and on-demand solar-to-hydrogen conversion requires that H2 evolution catalysts consist of earth-abundant materials and can be facilely obtained and modified. In this paper, we report a high-performance and quasi-homogenous dye-sensitized H2 evolution system by coupling graphene quantum dots (GQDs) with cobalt-promoted MoSx (Co-MoSx) catalyst. The Co-MoSx serving as H2 evolution catalyst can be facilely grown by a in-situ photochemical reduction method in the presence of an organic dye as the photosensitizer and simultaneously coupled with GQDs during photocatalytic reactions, forming GQDs/Co-MoSx catalyst. The resulted GQDs/Co-MoSx catalyst exhibits high activity for catalyzing H2 evolution under visible light due to the significant synergistic effects among GQDs, Co dopant, and MoSx. The doping of MoSx with Co ions results in an effective promotional effect on the H2 evolution activity of in-situ generated Co-MoSx catalyst, while the presence of GQDs can not only improve the dispersion of Co-MoSx catalyst by serving as a homogenous matrix but also lead to an efficient Förster resonance energy transfer (FRET) to sensitizing dye by acting as a quasi-homogenous energy donor for enhancing light utilization efficiency. In addition, the GQDs can also act as an electron transfer relay to facilitate the electron transfer from excited dye to Co-MoSx catalyst, thus retarding the back electron transfer and reducing energy loss. This work provides a new route to construct an efficient and quasi-homogenous dye-sensitized H2 evolution system by integrating inexpensive MoSx-based catalyst with transition metal modification and highly photoluminescent and soluble GQDs with outstanding optical properties.

Orderly-designed Ni2P nanoparticles on g-C3N4 and UiO-66 for efficient solar water splitting

Stable and efficient photocatalyst is the key important research goals up to now. On account of the dominant performance of Ni2P, g-C3N4 (graphitized carbonitride) and UiO-66 (Universitetet i Oslo) themselves, an orderly-designed assemble of g-C3N4/UiO-66/Ni2P is successfully designed and assembled with capability of high-efficient dye-sensitized photocatalytic H2 evolution. The electron transport routes are successfully adjusted and the hydrogen evolution is greatly improved. It exhibits synergistic effect on highly efficient photocatalytic hydrogen production. The maximum amount of hydrogen evolution reaches about 200 μmol for 5 h over the g-C3N4/UiO-66/Ni2P photocatalyst under the 5 W LED white light at 420 nm. The H2 evolution rate is 12 times high than over g-C3N4. Such synergistically increased effect in photocatalytic properties is certified by related characterization results such as TEM, SEM, XPS, XRD, UV–vis DRS, Transient photocurrent and FT-IR etc. The above studies show that the Ni2P nanoparticles modified on the g-C3N4/UiO-66 provides the more active sites and improves the efficiency of photo-generated charge separation. In addition, the possible mechanism of photocatalytic hydrogen production is proposed.

Strategy of nitrogen defects sponge from g-C3N4 nanosheets and Ni-Bi-Se complex modification for efficient dye-sensitized photocatalytic H2 evolution

An available strategy of nitrogen defects sponge from g-C3N4 nanosheets is successfully designed and the nitrogen defective ultrathin graphitic-phase (g-C3N4) nanosheets are properly prepared. Moreover, a layer of Ni-Bi-Se complex is effectively modified on the basis of appropriate nitrogen defective ultrathin graphitic-phase (g-C3N4) nanosheets and a more efficient dye-sensitized photocatalytic H2 evolution is obtained. In addition, in need of special note is that the density of nitrogen defects on g-C3N4 photocatalyst can be controlled and adjusted by means of barium hydroxide dosage. Besides the high H2 yield, the Ni-Bi-Se complex modified nitrogen defective ultrathin graphitic-phase (g-C3N4) nanosheets are very stable in the process of photocatalytic hydrogen production. A series of characterization studies such as SEM, XRD, TEM, EDX, XPS, UV–vis DRS, FTIR, transient fluorescence and electro-chemistry etc. show that the more active sites appears and the efficiency of photo-generated charge separation is improved due to the introduction of surface N vacancies and Ni-Bi-Se complex doping. The results of which is well mutual corroboration with each other. Based on above results, a possible reaction mechanism was proposed too.

Facile Synthesis of Graphene Sponge from Graphene Oxide for Efficient Dye-Sensitized H2 Evolution

Graphene is an advanced carbon energy material due to its excellent properties. Reduction of graphene oxide (GO) is the most promising mass production route of graphene/reduced graphene oxide (rGO). To maintain graphene's properties and avoid restacking of rGO sheets in bulk, the preparation of 3-dimensional porous graphene sponge via 2-dimensional rGO sheets is considered as a good strategy. This article presents a facile route to synthesize graphene sponge by thermal treating GO powder at low temperature of 250 °C under N2 atmosphere. The sponge possesses macroporous structure (5-200 nm in size) with BET specific surface area of 404 m(2) g(-1) and high conductivity. The photocatalytic H2 production activity of the rGO sponge with a sensitizer Eosin Y (EY) and cocatalyst Pt was investigated. The rGO sponge shows highly efficient dye-sensitized photocatalytic H2 evolution compared to that obtained via a chemical reduction method. The maximum apparent quantum yield (AQY) reaches up to 75.0% at 420 nm. The possible mechanisms are discussed. The synthesis method can be expanded to prepare other graphene-based materials.

Effect of Water/Acetonitrile Ratio on Dye-Sensitized Photocatalytic H2 Evolution under Visible Light Irradiation

H 2 production in a water-acetonitrile solution containing iodide as an electron donor was investigated over dye-sensitized Pt∕TiO2 photocatalysts under visible light irradiation, as a potential water-splitting system based on an iodide redox mediator. The rates of H2 evolution decreased with increasing proportion of water in the solutions, because of a decrease in the energy gap between the redox potential of I3−∕I− and the highest occupied molecular orbital levels of the dyes, which decreases the efficiency of electron transfer from I− to dye.