Cocatalysts For Photocatalytic Water Splitting Research Articles

Implanting nitrogen-doped graphene quantum dots on porous ultrathin carbon nitride for efficient metal-free photocatalytic hydrogen evolution

Mimicking natural photosynthesis to make solar power into hydrogen energy is widely perceived as a promising and sustainable way to alleviate the energy crisis and environmental pollution. However, adding metallic cocatalysts and even noble metal cocatalysts into carbon nitride-based photocatalysts still seriously restrict the practical development of photocatalytic water splitting. To address these issues, we carefully design and construct a metal-free photocatalytic system through grafting 0D nitrogen-doped graphene quantum dots (N-GQDs) on 2D porous ultrathin carbon nitride (PUCN). The incorporated N-GQDs can act as electron collectors to promote the charge separation/transfer on PUCN, and also can extend the photoabsorption region of PUCN. Thus, the metal-free N-GQDs/PUCN photocatalyst exhibits a drastically enhanced hydrogen production rate of 1248 μmol g−1 h−1, which fetches up to about 208 times as high as that of PUCN (6 μmol g−1 h−1). Notably, the photocatalytic performance of N-GQDs/PUCN is close to that of Pt/PUCN with the same cocatalyst amount (1706 μmol g−1 h−1), and metal-free N-GQDs/PUCN photocatalyst outperforms most of the reported carbon nitride-based photocatalysts with metallic cocatalysts for photocatalytic water splitting. This work offers new insight to construct metal-free carbon nitride-based photocatalysts, which will be beneficial for the practical development of photocatalytic water splitting.

Carbon intercalated MoS2 cocatalyst on g-C3N4 photo-absorber for enhanced photocatalytic H2 evolution under the simulated solar light

Despite MoS2 being a promising non-precious-metal cocatalyst, poor electronic conductivity and low activity for hydrogen evolution caused by serious agglomeration have been identified as critical roadblocks to further developing MoS2 cocatalyst for photocatalytic water splitting using solar energy. In this work, the density functional theory calculations reveal that carbon intercalated MoS2 (C-MoS2) has excellent electronic transport properties and could effectively improve catalytic activity. The experiment results show that the prepared tremella-like C-MoS2 nanoparticles have large interlayer spacing along the c-axis direction and high dispersion because of intercalation of the carbon between adjacent MoS2 layers. Furthermore, the heterostructure photocatalyst of C-MoS2@g-C3N4 formed by loading the cocatalyst of C-MoS2 onto g-C3N4 nanosheets exhibits the H2 evolution rate of 157.14 μmolg−1h−1 when containing 5 wt% C-MoS2. The high photocatalytic H2 production activity of the 5 wt% C-MoS2@g-C3N4 can be attributed to the intercalated conductive carbon layers in MoS2, which leads to efficient charge separation and transfer as well as increased activities of the edge S atoms for H2 evolution. We believe that the C-MoS2 will offer great potential as a photocatalytic H2 evolution reaction cocatalyst with high efficiency and low cost.

An unexplored role of the CrOx shell in an elaborated Rh/CrOx core–shell cocatalyst for photocatalytic water splitting: a selective electron transport pathway from semiconductors to core metals, boosting charge separation and H2 evolution

In a Rh/CrOx core–shell cocatalyst for water-splitting photocatalysts, CrOx serves as an electron pathway, transfers the photoexcited electrons from photocatalysts to Rh (reduction site), and improves the H2 evolution activity.

Surface Anchoring and Active Sites of [Mo3S13]2- Clusters as Co-Catalysts for Photocatalytic Hydrogen Evolution.

Achieving light-driven splitting of water with high efficiency remains a challenging task on the way to solar fuel exploration. In this work, to combine the advantages of heterogeneous and homogeneous photosystems, we covalently anchor noble-metal- and carbon-free thiomolybdate [Mo3S13]2– clusters onto photoactive metal oxide supports to act as molecular co-catalysts for photocatalytic water splitting. We demonstrate that strong and surface-limited binding of the [Mo3S13]2– to the oxide surfaces takes place. The attachment involves the loss of the majority of the terminal S22– groups, upon which Mo–O–Ti bonds with the hydroxylated TiO2 surface are established. The heterogenized [Mo3S13]2– clusters are active and stable co-catalysts for the light-driven hydrogen evolution reaction (HER) with performance close to the level of the benchmark Pt. Optimal HER rates are achieved for 2 wt % cluster loadings, which we relate to the accessibility of the TiO2 surface required for efficient hole scavenging. We further elucidate the active HER sites by applying thermal post-treatments in air and N2. Our data demonstrate the importance of the trinuclear core of the [Mo3S13]2– cluster and suggest bridging S22– and vacant coordination sites at the Mo centers as likely HER active sites. This work provides a prime example for the successful heterogenization of an inorganic molecular cluster as a co-catalyst for light-driven HER and gives the incentive to explore other thio(oxo)metalates.

Transition‐Metal‐Based Cocatalysts for Photocatalytic Water Splitting

Recently, semiconductor‐based photocatalytic water splitting has been extensively studied as a promising strategy for converting solar energy into carbon‐neutral and clean H2 fuel. However, the lack of sufficient active sites for surface redox reactions generally results in unsatisfactory photocatalytic water splitting performances over semiconductors. For this problem, cocatalyst provides an encouraging solution and is of great significance in improving photocatalytic performance. Noble metals and their derivatives are mostly utilized as efficient cocatalytic components, but their scarcity and expensiveness severely hamper large‐scale applications. Thereby, the utilization of noble‐metal‐free cocatalysts has aroused immense research attention. Owing to the facile availability, low cost, large abundance, high stability, and efficient performance, transition‐metal‐based materials have been developed as desirable candidates in the photocatalytic water splitting water splitting process. This review gives an outline of some recent advances in the active transition‐metal‐based water splitting cocatalysts. First, the fundamentals of transition‐metal‐based cocatalysts are presented, including the classification, function mechanisms, loading methods, modification strategies, and design considerations. Second, the various cases of depositing reduction cocatalysts, oxidation cocatalysts, and reduction–oxidation dual cocatalysts for water splitting are further discussed. Finally, the crucial challenges and possible research directions of transition‐metal‐based cocatalysts for photocatalytic water splitting are proposed.

Deposition of Size-Selected Gold Nanoclusters onto Strontium Titanate Crystals for Water Splitting under Visible Light

Using a modulated pulse power magnetron sputtering (MPP-MSP) coupled with a quadrupole mass spectrometer (Q-MS), intensive size-selected gold nanoclusters (Aun) ranging from n = 5 to 40 in size are synthesized and soft landed onto a strontium titanate (STO) crystal surface as a co-catalyst for photocatalytic water splitting. The photocatalytic reactivity of the Aun/STO is investigated by measuring the photocurrent density of the sample under visible light radiation. It is found that the Aun co-catalysts enable the visible light response of the Aun/STO photocatalyst. The photocurrent density is sensitively dependent on the size of the Aun on the STO, and Au16 exhibits its maximum photocurrent under visible light. The underlying physics of the size-specific photocurrent are explained in terms of the size-dependent electron affinity of Aun.

Time-resolved infrared spectroscopic investigation of Ga2O3 photocatalysts loaded with Cr2O3-Rh cocatalysts for photocatalytic water splitting

Investigation of the charge dynamics and roles of cocatalysts is crucial for understanding the reaction of photocatalytic water splitting on semiconductor photocatalysts. In this work, the dynamics of photogenerated electrons in Ga2O3 loaded with Cr2O3-Rh cocatalysts was studied using time-resolved mid-infrared spectroscopy. The structure of these Cr2O3-Rh cocatalysts was identified with high-resolution transmission electron microscopy and CO adsorption Fourier-transform infrared spectroscopy, as Rh particles partly covered with Cr2O3. The decay dynamics of photogenerated electrons reveals that only the electrons trapped by the Rh particles efficiently participate in the H2 evolution reaction. The loaded Cr2O3 promotes electron transfer from Ga2O3 to Rh, which accelerates the electron-consuming reaction for H2 evolution. Based on these observations, a photocatalytic water-splitting mechanism for Cr2O3-Rh/Ga2O3 photocatalysts has been proposed. The elucidation of the roles of the Cr2O3-Rh cocatalysts aids in further understanding the reaction mechanisms of photocatalytic water splitting and guiding the development of improved photocatalysts.

Structural Evolution of Ni-Based Co-Catalysts on [Ca2Nb3O10]− Nanosheets during Heating and Their Photocatalytic Properties

Nickel compounds are among the most frequently used co-catalysts for photocatalytic water splitting. By loading Ni(II) precursors, submonolayer Ni(OH)2 was uniformly distributed onto photocatalytic [Ca2Nb3O10]− nanosheets. Further heating of the nanocomposite was studied both ex situ in various gas environments and in situ under vacuum in the scanning transmission electron microscope. During heating in non-oxidative environments including H2, argon and vacuum, Ni nanoparticles form at ≥200 °C, and they undergo Ostwald ripening at ≥500 °C. High resolution imaging and electron energy loss spectroscopy revealed a NiO shell around the Ni core. Ni loading of up to 3 wt% was demonstrated to enhance the rates of photocatalytic hydrogen evolution. After heat treatment, a further increase in the reaction rate can be achieved thanks to the Ni core/NiO shell nanoparticles and their large separation.

Noble-metal-free amorphous CoMoSx modified CdS core-shell nanowires for dramatically enhanced photocatalytic hydrogen evolution under visible light irradiation

Design and construction of highly efficient and stable co-catalysts are critical for the photocatalytic H2 generation from water. In this work, we report the novel CdS/CoMoSx core-shell nanowires (CCMS NWs) prepared by a facile in situ synthesis method. The in situ growth of amorphous CoMoSx shell on the surface of CdS NWs core efficiently promoted the photocatalytic H2 evolution under visible light irradiation. The optimized CCMS sample containing 15 mol% CoMoSx (CCMS-15) achieves a H2 evolution rate as high as 477.96 μmol·h−1 under visible light irradiation, up to 26.3 times over bare CdS (18.2 μmol·h−1). Moreover, the constructed core-shell CdS/CoMoSx photocatalyst shows a long-term stability without noticeable activity degradation. Based on the detailed analyses of PL, UV–vis DRS, transient photocurrent responses, EIS, Mott-Schottky plots and ESR, the significantly enhanced activity is ascribed to the synergistic effect of increased visible light absorption, suppressed photo-generated charge carries recombination and the amorphous defective structure of CoMoSx co-catalyst. In consideration of its facile synthesis, low cost, and superior performance, the amorphous CoMoSx appears to be promising co-catalyst for photocatalytic water splitting.

Amorphous Bimetallic Cobalt Nickel Sulfide Cocatalysts for Significantly Boosting Photocatalytic Hydrogen Evolution Performance of Graphitic Carbon Nitride: Efficient Interfacial Charge Transfer.

Noble metals usually work as the cocatalyst for photocatalytic water splitting, but their rare and expensive properties narrowed their wide development. Transition-metal sulfides have appeared to be promising non-noble metal cocatalysts in the hydrogen evolution reaction (HER) to meet future energy demands. Meanwhile, many studies focus on the fabrication of bimetallic catalysts because of their remarkably superior catalytic activity compared with monometallic substances. Herein, amorphous bimetallic cobalt nickel sulfide (CoNiSx) was fabricated to work as a cocatalyst in the photocatalytic H2 evolution reaction, which can couple with pristine graphitic carbon nitride (g-C3N4). CoNiSx-CN exhibits a larger specific surface area compared with g-C3N4, making it possess more reaction active sites. Moreover, the contacted interface in the CoNiSx-CN composite photocatalyst contributes to higher separation efficiency of photogenerated carriers, which was proved by experimental and theoretical calculations. More importantly, the theoretical calculation also verified that CoNiSx-CN has relatively closer Gibbs free energy to zero than pure g-C3N4 and corresponding monometallic cocatalyzed g-C3N4. Therefore, the prepared CoNiSx-CN composite exhibited a dramatic photocatalytic HER performance of 2.366 μmol mg-1 h-1, which is about 76-fold higher in comparison with pristine g-C3N4 and comparable to g-C3N4 with Pt as a cocatalyst under 420 nm light irradiation. This study reveals a promising and efficient bimetallic cocatalyst for the photocatalytic H2 evolution reaction.

Amorphous NiP as cocatalyst for photocatalytic water splitting

As a new photocatalyst, the photocatalytic hydrogen production performance of strontium titanate (SrTiO3, STO) modified by amorphous NiP was studied. The main purpose was to study the effect of amorphous NiP on hydrogen generation of STO under ultraviolet (UV) light irradiation. In this paper, STO catalysts were synthesized by hydrothermal method and the amorphous NiP was modified on the surface of STO by chemical plating method. The photocatalysts were characterized by X-ray diffraction (XRD), ultraviolet visible diffuse reflectance spectroscopy (DRS), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). The results show that hydrogen generation rate of STO modified by amorphous NiP (NiP/STO) is much higher than that of STO. Furthermore, the deposition time of chemical plating affects the hydrogen generation performance a lot. The results demonstrate that when the deposition time is 10s, the hydrogen generation rate reaches almost 3 times higher than the other deposition time. The DRS results indicate that NiP/STO has narrower band gap than STO. Combination of the results of DRS, Mott-schottky (M-S) and XPS, the mechanism of photocatalytic hydrogen generation was obtained. The cause of higher hydrogen generation rate may be ascribed to the different electronic migration path in NiP/STO.

Bismuth sulphide-modified molybdenum disulphide as an efficient photocatalyst for hydrogen production under simulated solar light

To overcome rapid electron-hole recombination and the need for employing noble metals as co-catalysts for photocatalytic water splitting, the present work reports on the fabrication of bismuth sulphide (Bi2S3)-modified molybdenum disulphide (MoS2) as an efficient hybrid photocatalyst. Under simulated solar light irradiation, the Bi2S3/MoS2 photocatalyst with an optimum molar ratio of Mo to Bi of 50% (mol/mol) achieved a H2 production rate of 61.4μmol/h. The photocatalytic enhancement was attributed to an effective charge transfer mechanism between Bi2S3 and MoS2, as evidenced by photoelectrochemical characterization. A plausible reaction mechanism over the as-prepared Bi2S3/MoS2 photocatalyst was also proposed based on the experimental results obtained.

ChemInform Abstract: Surface and Interface Design in Cocatalysts for Photocatalytic Water Splitting and CO2 Reduction

AbstractReview: 108 refs.

Amorphous transitional metal borides as substitutes for Pt cocatalysts for photocatalytic water splitting

Cocatalysts for H2 production are often made from noble metals, which are expensive and rare. Cocatalysts made from cheap and abundant elements are therefore highly desirable for economically viable H2 production. Here, we demonstrate that amorphous transitional metal borides (TMBs)—made from abundant materials and costing less than 0.1% of the price of Pt cocatalysts—are effective substitutes for Pt-based cocatalysts and result in superior H2 production via water-splitting under visible light irradiation. Under visible-light driven photocatalytic water-splitting, using TMBs as cocatalysts for nanostructured NiCoB/CdS composites achieved an extraordinary H2 production of 144.8mmolh−1g−1, up to 36 times greater than that observed when using CdS alone. The apparent quantum efficiency was measured as 97.42% at 500nm, which is the highest value reported for CdS photocatalysts. The hydrogen atom adsorption energy (ΔE(H)) and hydrogen molecule adsorption energy (ΔE(H2)) of NiB, NiCoB and Pt have been calculated for the first time. Compared with Pt cocatalyst, amorphous TMBs cocatalysts more readily adsorb hydrogen protons and desorb molecular hydrogen during the photocatalytic process. Superior performance of TMBs as cocatalyst, much better than Pt, can be attributed to its powerful trapping electrons ability and highly adsorption of protons. These findings provide a straightforward and effective route to produce cheap and efficient cocatalysts for large-scale water splitting.

Hierarchical Layered WS2 /Graphene-Modified CdS Nanorods for Efficient Photocatalytic Hydrogen Evolution.

Graphene-based ternary composite photocatalysts with genuine heterostructure constituents have attracted extensive attention in photocatalytic hydrogen evolution. Here we report a new graphene-based ternary composite consisting of CdS nanorods grown on hierarchical layered WS2 /graphene hybrid (WG) as a high-performance photocatalyst for hydrogen evolution under visible light irradiation. The optimal content of layered WG as a co-catalyst in the ternary CdS/WS2 /graphene composites was found to be 4.2 wt %, giving a visible light photocatalytic H2 -production rate of 1842 μmol h(-1) g(-1) with an apparent quantum efficiency of 21.2 % at 420 nm. This high photocatalytic H2 -production activity is due to the deposition of CdS nanorods on layered WS2 /graphene sheets, which can efficiently suppress charge recombination, improve interfacial charge transfer, and provide reduction active sites. The proposed mechanism for the enhanced photocatalytic activity of CdS nanorods modified with hierarchical layered WG was further confirmed by transient photocurrent response. This work shows that a noble-metal-free hierarchical layered WS2 /graphene nanosheets hybrid can be used as an effective co-catalyst for photocatalytic water splitting.

Surface and interface design in cocatalysts for photocatalytic water splitting and CO2reduction

This review outlines the recent progress on designing the surface and interface of cocatalysts to create highly efficient photocatalysts for water splitting and CO2reduction.

A copper and chromium based nanoparticulate oxide as a noble-metal-free cocatalyst for photocatalytic water splitting

Nanoparticulate oxides consisting of copper (Cu) and chromium (Cr) were studied as noble-metal-free cocatalysts for photocatalytic water splitting. The structure of the Cu–Cr mixed oxide dispersed on a solid solution of GaN and ZnO (referred to as GaN : ZnO hereafter) was characterized by high-resolution transmission electron microscopy (HR-TEM), X-ray absorption fine-structure (XAFS) spectroscopy, and electrochemical measurements. The mixed-oxide nanoparticle was an effective promoter of photocatalytic overall water splitting on GaN : ZnO, and was loaded by impregnation from an aqueous solution containing Cu(NO3)2·3H2O and Cr(NO3)3·9H2O followed by calcination in air. Impregnation of GaN : ZnO with 1.5 wt% Cu and 2.0 wt% Cr followed by calcination at 623 K for 1 h provided the highest photocatalytic activity, while catalysts modified with either Cu-oxide or Cr-oxide showed little activity. The activity of this photocatalyst was shown to be strongly dependent on the generation of Cu(II)–Cr(III) mixed-oxide nanoparticles with optimal composition and coverage. The results of electrochemical measurements and photocatalytic reactions also indicated that Cu(II)–Cr(III) mixed-oxide nanoparticles on GaN : ZnO are resistant to both the photoreduction of O2 and water formation from H2 and O2, which are undesirable reverse reactions in overall water splitting.

Structural features of p-type semiconducting NiO as a co-catalyst for photocatalytic water splitting

Abstract Perovskite-like NaTaO 3 photocatalysts, synthesized by sol–gel and solid-state methods, were loaded with NiO co-catalyst to enhance water splitting activity under UV illumination. Activity increased significantly with NiO loading and reached a maximum at 3 and 0.7 wt%, respectively, for the sol–gel and solid-state synthesized NaTaO 3 . Beyond this point, photocatalytic activity decreased with further loading. Analysis using X-ray diffraction, high-resolution transmission electron microscopy, and diffuse reflectance spectroscopy shows that the interdiffusion of Na + and Ni 2+ cations created a solid–solution transition zone on the outer sphere of NaTaO 3 . For NiO contents less than 3 wt%, no NiO clusters appeared on the NaTaO 3 surface, and the reduction/oxidation pretreatment did not enhance photocatalytic activity. The high activity resulting from a low NiO loading suggests that the interdiffusion of cations heavily doped the p-type NiO and n-type NaTaO 3 , reducing the depletion widths and facilitating charge transfers through the interface barrier.

Nanosized Au Particles as an Efficient Cocatalyst for Photocatalytic Overall Water Splitting

The effect of the photodeposition of gold particles onto several photocatalysts on the photocatalytic activities was studied. The photocatalytic activities of K4Nb6O17, Sr2Nb2O7, KTaO3 NaTaO3, and NaTaO3 doped with La for water splitting were improved when gold particles were deposited. The latter were nanoparticles, consistent with their surface plasmon absorption. The nanosized gold particle functioned as an efficient cocatalyst for photocatalytic water splitting by assisting H2 evolution.