Enhanced visible-light-assisted photocatalytic hydrogen generation by MoS2/g-C3N4 nanocomposites

A facile, one-pot, solvothermal synthesis of MoS₂ microflowers (S1) and the heterostructures MoS₂/g-C₃N₄ with varying ratios of 1:1 (S2), 1:2 (S3) and 1:3 (S4) exhibiting enhanced visible-light-assisted H₂ generation by water splitting has been reported. The compounds were thoroughly characterized by PXRD, FESEM, HRTEM, EDS, UV–vis and XPS techniques. FESEM and HRTEM analyses showed the presence of microflowers composed of nano-sized petals in case of pure MoS₂ (S1), while the MoS₂ microflowers covered with g-C₃N₄ nanosheets in case of MoS₂/g-C₃N₄ heterostructure, S4. XPS analysis of S2 showed the presence of 2H phase of MoS₂ with g-C₃N₄. The Eosin-Y/dye-sensitized visible-light-assisted photocatalytic investigation of the samples in the absence of any noble metal co-catalyst revealed very good water splitting activity of MoS₂/g-C₃N₄ heterostructure, S2 with hydrogen generation rate of 1787 μmol h⁻¹g⁻¹ which is about 6 and 40 times higher than pure MoS₂ and g-C₃N₄ respectively. The relatively higher catalytic activity of the heterostructure, S2 has been ascribed to the efficient spatial separation of photo-induced charge carriers owing to the synergistic interaction between MoS₂ and g-C₃N₄. A possible mechanism for the Eosin-Y-sensitized photocatalytic H₂ generation activity of MoS₂/g-C₃N₄ heterostructures has also been presented. The enhanced activity of S2 was further supported by fluorescence measurements. Thus, the present study highlights the importance of non-noble metal based MoS₂/g-C₃N₄ heterojunction photocatalysts for efficient visible-light-driven H₂ production from water splitting.     

Facile strategy to fabricate Ni2P/g-C3N4 heterojunction with excellent photocatalytic hydrogen evolution activity

Transition metal phosphides are considered as the most prospective replacements for noble metal cocatalysts used for H₂ evolution during photocatalytic water splitting. In this work, Ni₂P/g-C₃N₄ composite photocatalyst was synthesized using a simple in-situ hydrothermal method by one step. Benefiting from the excellent light trapping, efficient transfer of charge carriers and strong stability of Ni₂P nanoparticles, as well as the stable interface contact between Ni₂P and g-C₃N₄, the Ni₂P/g-C₃N₄ exhibit greatly enhanced H₂ evolution performance during photocatalytic water splitting. The optimized H₂ evolution rate can reach 3344 μmol h^?1 g^?1 over 17.5 wt% Ni₂P/g-C₃N₄, which is 68.2 times greater than that of pure g-C₃N₄ and even much greater than that of 15 wt% Pt/g-C₃N₄. The apparent quantum efficiency (QE) is about 9.1% under 420 nm monochromatic. The enhancement mechanism was demonstrated in detail by transient photocurrent responses, photoluminescence spectra and electrochemical impedance spectroscopy. This work develops a facile strategy to fabricate transition metal phosphide/semiconductor heterojunction systems with potential application for photocatalytic H₂ evolution.     

In-situ photo-reducing graphene oxide to create Zn0.5Cd0.5S porous nanosheets/RGO composites as highly stable and efficient photoelectrocatalysts for visible-light-driven water splitting

Nanoporous Zn_0.5Cd_0.5S nanosheets/reduced graphene oxide (Zn_0.5Cd_0.5S/RGO) composites were prepared by a facile in-situ photoreduction method of graphene oxide (GO) in the presence of nanoporous Zn_0.5Cd_0.5S single-crystal-like nanosheets under visible light irradiation. The Zn_0.5Cd_0.5S/RGO photoelectrodes was characterized by TEM, IR and Raman spectra. Electrochemical measurements demonstrated that Zn_0.5Cd_0.5S/RGO photoelectrodes own a higher anodic photocurrent density, a lower zero current potential, and a higher photoelectrochemical response than that of pure Zn_0.5Cd_0.5S photoelectrodes under visible light irradiation under the same conditions. This high photochemical activity is predominately ascribed to the presence of RGO, which serves as the electron collector to efficiently prolong the lifetime of photoinduced electrons from the excited Zn_0.5Cd_0.5S nanosheets. In addition, the content of RGO in the composites had a remarkable influence on the photoelectrochemical behaviors of the photoelectrodes and the optimal RGO content was found to be 5 wt%. Zn_0.5Cd_0.5S/RGO composites at RGO content of 5 wt% reached a stable hydrogen production rate of 12.05 μmol h⁻¹ cm⁻² at an externally applied bias of 0.6 V. Furthermore, the Zn_0.5Cd_0.5S/RGO composites as photoelectrodes were found to be highly stable for hydrogen evolution reaction. The electrons stored in RGO are readily discharged or scavenged on demand by the applied positive bias to the counter electrode, and thus rectify the flow of electrons. Importantly, this work may open up a facile in-situ method for using RGO scaffold to create a stable photoelectrode with enhanced photoelectrochemical activities.     

2D CoP supported 0D WO3 constructed S-scheme for efficient photocatalytic hydrogen evolution

For heterojunction composite photocatalyst, intimate contact interface is the key to the carrier transfer separation conditions. Due to the interface contact, the electron transfer rate between catalysts can be increased during photocatalytic hydrogen production, therefore, we design the close contact of 0D/2D heterojunction, which greatly enhanced the photocatalytic hydrogen production activity of the composite catalyst. The composite catalyst WO₃/CoP was obtained by simple high temperature in situ synthesis. Moreover, it was proved by photoelectric chemistry and fluorescence tests that appropriate conduction band and valence band locations of WO₃ and CoP provided a favorable way for thermodynamic electron transfer. In addition, fluorescence results showed that WO₃ load effectively promoted photoelectron-hole transfer and increased electron lifetime. The formation of S-scheme heterojunctions can make more efficient use of useful photogenerated electrons and prevent the photogenerated electron-hole recombination of CoP itself, further promote the liveness of photocatalytic H₂ evolution. Meanwhile, the study of Metal-organic frameworks (MOFs) materials further promoted the application of MOFs derivatives in the field of photocatalytic hydrogen evolution, and provided a reference for the rational design of composite catalysts for transition metal phosphide photocatalysts.     

MoS2/Ti3C2 heterostructure for efficient visible-light photocatalytic hydrogen generation

The MoS₂/Ti₃C₂ catalyst with a unique sphere/sheet structure were prepared by hydrothermal method. The MoS₂/Ti₃C₂ heterostructure loading 30% Ti₃C₂ has a maximum hydrogen production rate of 6144.7  μmol g⁻¹ h⁻¹, which are 2.3 times higher than those of the pure MoS₂. The heterostructure maintains a high catalytic activity within 4 cycles. The heterostructure not only effectively reduce the recombination of photogenerated electrons and holes, but also provide more activation sites, which promotes the photocatalytic hydrogen evolution reaction (HER). These works can provide reference for the development of efficient catalysts in photocatalytic hydrogen evolution.     

Hydrogen generation by photocatalytic reforming of glucose with heterostructured CdS/MoS2 composites under visible light irradiation

A binary heterostructured CdS/MoS₂ flowerlike composite photocatalysts was synthesized via a simple one-pot hydrothermal method. This photocatalyst demonstrated higher photocatalytic hydrogen production activity than pure MoS₂. The heterojunction formed between MoS₂ and CdS seems to promote interfacial charge transfer (IFCT), suppress the recombination of photogenerated electron–hole pairs, and enhance the hydrogen generation. Based on the good match between the conduction band (CB) edge of CdS and that of MoS₂, electrons in the CB of CdS can be transferred to MoS₂ easily through the heterojunction between them, which prevents the accumulation of electrons in the CB of CdS, inhibiting photocorrosion itself and greatly enhancing stability of catalyst. Hydrogen evolution reaction (HER) using Na₂S/Na₂SO₃ or glucose as sacrificial agents in aqueous solution was investigated. The ratio between CdS and MoS₂ plays an important role in the photocatalytic hydrogen generation. When the ratio between CdS and MoS₂ reaches 40 wt%, the photocatalyst showed a superior H₂ evolution rate of 55.0 mmol g⁻¹ h⁻¹ with glucose as sacrificial agent under visible light, which is 1.2 times higher than using Na₂S/Na₂SO₃ as sacrificial agent. Our experimental results demonstrate that MoS₂-based binary heterostructured composites are promising for photocorrosion inhibition and highly efficient H₂ generation.     

Enhanced optical absorption and photocatalytic water splitting of g-C3N4/TiO2 heterostructure through C&B codoping: A hybrid DFT study

Our theoretical research indicate that the electric field are generated in the direction of (C doped) TiO₂ (101) surface to (B-doped) g-C₃N₄ monolayer for the pristine, C and B doped g-C₃N₄/TiO₂, and higher band-edge potential on the (C doped) TiO₂ (101) surface are observed compared to (B-doped) g-C₃N₄ monolayer. Thus, the pristine (2.591 eV), C-doped (2.663 eV) and B-doped (2.339 eV) g-C₃N₄/TiO₂ are Z-scheme heterostructures, which promotes charge separation and retains a prominent redox ability. After C doping, the C 2p energy level is introduced which facilitate the separation of photoexcited carriers. The B-doped g-C₃N₄/TiO₂ has a reduced bandgap and the mixing of B 2p and N 2p energy levels, promoting the red-shift of the optical absorption edge. The C&B codoped g-C₃N₄/TiO₂ follows type-II charge transfer mode because of their synergistic effect in C and B atoms, which changes the direction of the built-in electric field. It also has a narrow bandgap (1.309 eV) and effectively separate electron-hole pairs leading to strong optical absorption ability in the range of 360 nm–460 nm. The band-edges matching of the semiconductor photocatalyst and the direction of the built-in electric field jointly determine whether the charges are selected to be Z-scheme or II-type transfer mode. Based on g-C₃N₄/TiO₂ for C or/and B (co)doping, their different charge transfer modes have been established and they are expected to show promising photocatalytic water splitting performance.     

Remarkable enhancement of photocatalytic hydrogen evolution over Cd0.5Zn0.5S by bismuth-doping

Bi³⁺ doped Cd_0.5Zn_0.5S photocatalysts were prepared by a simple hydrothermal method, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscope (XPS), energy dispersive X-ray spectroscopy (EDX), BET and UV-Vis absorption spectroscope techniques. When Bi³⁺ doping content is lower, the doping ions lie at the surface lattice sites, whereas when the doping content is higher, the ions also enter the bulk lattice sites. Their photoactivities were evaluated by hydrogen evolution from aqueous solution containing Na₂S and Na₂SO₃ as a hole scavenger under visible light (λ ≥ 420 nm) irradiation. Bi³⁺ doping enhances markedly photocatalytic activity. When Bi³⁺ doping content is 0.10 mole %, the photocatalyst exhibits the highest activity, and the average apparent quantum yield amounts to 9.71% during 30 h irradiation. The possible mechanism was discussed.     

WO3/BiVO4 composite photoelectrode prepared by improved auto-combustion method for highly efficient water splitting

We report on the improvement in the water splitting efficiency of a WO₃/BiVO₄ composite photoelectrode by the application of an improved auto-combustion method to the preparation of porous BiVO₄ thin films. The unique feature of this preparation method is the addition of both NH₄NO₃, as a strong oxidizing agent, and an organic additive into BiVO₄ precursor solution. The local decomposition heat of the organic additive and oxidizing agent created a porous film with small, highly crystalline BiVO₄ particles. The photoelectrode has many advantages over existing ones, such as the high light-harvesting efficiency (LHE), a single BiVO₄ phase, the facile access of the holes to the photoelectrode/electrolyte interface, and the ease of water and oxygen diffusion. The maximum incident photon-to-current efficiency (IPCE) was estimated to be 64% (at 440 nm, 1.23 V vs. RHE) and the applied bias photon-tocurrent efficiency (ABPE) reached as high as 1.28%. This ABPE value is highest among all oxide semiconductor photoelectrodes reported previously, except for the case of a stacking photoelectrode system.     

TiO2 nanotubes for hydrogen generation by photocatalytic water splitting in a two-compartment photoelectrochemical cell

Highly ordered TiO₂ nanotube arrays for hydrogen production have been synthesized by electrochemical anodization of titanium sheets. Under solar light irradiation, hydrogen generation by photocatalytic water splitting was carried out in the two-compartment photoelectrochemical cell without any external applied voltage. The hydrogen gas and oxygen generated on Pt side and on TiO₂ nanotubes side respectively were efficiently separated. The effect of anodization time on the morphology structures, photoelectrochemical properties and hydrogen production was systematically investigated. Due to more charge carrier generation and faster charge transfer, a maximum photoconversion efficiency of 4.13% and highest hydrogen production rate of 97 μmol h⁻¹cm⁻² (2.32 mL h⁻¹cm⁻²) were obtained from TiO₂ nanotubes anodized for 60 min.     

Fe2P nanoparticles as highly efficient freestanding co-catalyst for photocatalytic hydrogen evolution

Design of non-noble-metal artificial photosynthesis system that can split water with high apparent quantum yield (AQY) and robust stability remains a fundamental challenge. Here we report that a physical mixture of Fe₂P nanopaticles (NPs) and CdS nanosheets (NSs) can gives AQY of photocatalytic hydrogen production as high as 90% at 420 nm monochromatic light with ethanol as electron donor at strong alkaline conditions. The highest rate for hydrogen production reached about 220 mmol g^?1 h^?1. In this hybrid photocatalyst system, free standing Fe₂P NPs act as efficient and robust noble-metal-free co-catalysts and ultrathin CdS NSs are used as the photosensitizer. PL and TRPL results demonstrate that photoexcited electron can transfer from the conduction band of the excited CdS to Fe₂P, which aided charge separation and enhanced the hydrogen evolution activity. Femtosecond transient absorption result reveals that the time-averaged interfacial electron transfer (ET) rate constant (<k_ET>) from CdS NSs to Fe₂P is about 7.4 × 10⁹ s^?1 under the guarantee of the scavenging of photoexcited hole immediately, which is one order faster than the electron relaxation rate in pure CdS NSs.     

Ag2CO3-derived Ag/g-C3N4 composite with enhanced visible-light photocatalytic activity for hydrogen production from water splitting

In this paper, Ag-based g-C₃N₄ composites have been successfully fabricated through two deferent synthetic methods: (i) a facile and efficient precipitation-calcination strategy (denoted as D–CN–xAg, x represents the dosage of Ag₂CO₃, the same below), (ii) a calcination method (denoted as Z–CN–xAg). All Ag-based g-C₃N₄ composites exhibit the enhanced photocatalytic activities under visible-light irradiation. Moreover, the optimal dosage of Ag₂CO₃ in the D–CN–xAg composite is found to be 5%, the corresponding hydrogen production capacity is 153.33 μmol g⁻¹ h⁻¹, which is 4.6 times higher than that of Z–CN–5%Ag composite. This might be attributed to appropriate content of metallic Ag and more active sites exposed on the surface of D–CN–5%Ag composite. Meanwhile, combining with photoelectrochemical results, it could be inferred that LSPR effect and the intimate interfacial between metallic Ag and g-C₃N₄ in the system play also important role for the improvement of photocatalytic activity. These results demonstrate that the appropriate loading of metallic Ag originated from Ag₂CO₃ into g-C₃N₄ could accelerate the separation and transfer of photogenerated electron-hole pairs, leading to the improvement of photocatalytic activity for hydrogen production from water splitting. Finally, a possible photocatalytic mechanism is proposed.     

ZIF-67-derived nickel–cobalt phosphide nanocubes/N-doped carbon/nickel form composite for efficient overall water splitting

Hydrogen production from electrochemical water splitting is a promising strategy to generating green energy, which requires development of efficient and stable bifunctional catalysts for hydrogen and oxygen evolution reaction (HER/OER). Herein, dual transition metal phosphides/N-doped carbon/Nickel foam composite (CoNiP/NC-NF) is prepared via direct phosphidation of ZIF-67, in which ZIF-67 can control the size and N-doping content of CoNiP/NC, boosting the bifunctional activities for the OER and HER. Then, the overall water splitting is performed by using CoNiP/NC-NF as the cathode and anode, showing a low cell voltage of 1.60 V to reach current density of 10 mA cm⁻². Experimental studies indicate that ZIF-67 influences the electrocatalytic performance, and theoretical studies identify the active component of CoNiP/NC-NF for HER and OER, respectively.     

Towards high activity of hydrogen production from ammonia borane over efficient non-noble Ni5P4 catalyst

Catalytic hydrolysis of ammonia borane has tremendous potential as an energy-efficient approach to supply hydrogen for energy vehicles and portable electronic devices. Herein, DFT calculation is first performed on electronic properties of Ni₂P and Ni₅P₄ nanocatalysts. It is found that more electrons are transferred from Ni to P for Ni₅P₄, indicating that Ni₅P₄ may show superior performance based on the electron effect. Therefore, Ni₂P and Ni₅P₄ with high purity are synthesized by the phase-controlled thermal decomposition approach. Gratifyingly, the Ni₅P₄ catalyst exhibits the as-expected better catalytic activity than that of Ni₂P catalyst. It also shows low activation energy and good stability. Furthermore, the structures and morphologies of both catalysts are characterized by multi-techniques such as XRD, HRTEM and XPS. The better performance could be ascribed to the higher positive charge of Ni together with the stronger ensemble effect of P. The insights sheds new light on the design of efficient NiP catalysts for hydrogen generation.     

Metalloid Ni2P and its behavior for boosting the photocatalytic hydrogen evolution of CaIn2S4

The development of high-efficiency and low-cost photocatalysts in photocatalytic H₂ evolution systems from water remains challenging. The substitution of a noble metal as the co-catalyst is still one of the important and meaningful issues in this field. Herein, we report a series of CaIn₂S₄ catalysts combined with Ni₂P, which acts as the co-catalyst, for boosting photocatalytic hydrogen evolution under visible light. The integrated system of the Ni₂P/CaIn₂S₄ composite exhibited high efficiency and durability, which were even higher than those of Pt decorated catalysts. The promoting effect of Ni₂P can be ascribed to its excellent reductive ability and analogous metallic character, which can accelerate the transfer and consumption of the photo-generated electrons. Moreover, based on the surface photo-voltage technique and electrochemical tests, the unique mechanism of Ni₂P for the movement of photo-generated charges during the photocatalysis process is proposed for the first time.     

Crystalline phase-dependent photocatalytic water splitting for hydrogen generation on KNbO3 submicro-crystals

Potassium niobate (KNbO₃) submicro-crystals with cubic and orthorhombic phases were hydrothermally prepared and characterized by powder X-ray diffraction, micro-Raman spectroscopy, Fourier transform infrared spectroscopy, nitrogen adsorption–desorption, diffuse reflectance UV–visible spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. The photocatalytic performance of the as-prepared KNbO₃ samples was evaluated toward H₂ generation from an aqueous methanol solution under UV. The surface area-normalized rate of H₂ production over submicro-cubes with cubic phase is two times larger than that of submicro-rods with orthorhombic phase. In addition, both cubic and orthorhombic KNbO₃ submicro-crystals showed much higher reactivity than commercial KNbO₃ bulk-like powders. The underlying mechanism was discussed in terms of crystal structure and electronic structure. The results from this study are potentially applicable to a range of perovskites useful in water splitting as well as other areas of heterogeneous photocatalysis.     

Noble-metal-free Co@Co2P/N-doped carbon nanotube polyhedron as an efficient catalyst for hydrogen generation from ammonia borane

Developing an efficient catalyst for hydrogen (H₂) generation from hydrolysis of ammonia borane (AB) to significantly improve the activity for the hydrogen generation from AB is important for its practical application. Herein, we report a novel hybrid nanostructure composed of uniformly dispersed Co@Co₂P core-shell nanoparticles (NPs) embedded in N-doped carbon nanotube polyhedron (Co@Co₂P/N–CNP) through a carbonization-phosphidation strategy derived from ZIF-67. Benefiting from the electronic effect of P doping, high dispersibility and strong interfacial interaction between Co@Co₂P and N-CNTs, the Co@Co₂P/N–CNP catalyst exhibits excellent catalytic performance towards the hydrolysis of AB for hydrogen generation, affording a high TOF value of 18.4 mol H₂ mol metal^?1 min^?1 at the first cycle. This work provides a promising lead for the design of efficient heterogeneous catalysts towards convenient H₂ generation from hydrogen-rich substrates in the close future.     

Construction of 1D/0D/2D Zn0.5Cd0.5S/PdAg/g-C3N4 ternary heterojunction composites for efficient photocatalytic hydrogen evolution

A high-efficiency and easy-available approach was developed to obtain a ternary heterojunction composites with advanced hydrogen evolution reaction (HER) performance under visible light by water split. PdAg bimetallic nanoparticles make a close contact interface between g-C₃N₄(CN) and Zn_0.5Cd_0.5S(ZCS). Under visible light irradiation, CN and ZCS are both excited to generate electron-hole pairs, PdAg bimetallic nanoparticles act as a bridge between CN and ZCS. Not only can the photogenerated electrons from CN be captured, but they can also be quickly transferred to the surface of ZCS and participate in the photocatalytic reaction to release H₂, and the recombination of charge carriers between the contact interface of ZCS and CN can be significantly inhibited. In addition, the thin CN layer reduces the photocorrosion of the ZCS and enhances the specific surface area of the composite material. After testing, the composite material with 30 wt% ZCS and 4 wt% PdAg demonstrates hydrogen evolution performance, up to 6250.7 μmol g^?1h^?1, which is 753 times the hydrogen evolution rate of single-component CN and 12.6 times of ZCS/CN. Compared with single-component and two-component photocatalysts, the ternary ZCS/PdAg/CN photocatalyst achieves significantly enhanced photocatalytic activity.     

Carbon-incorporated TiO2 photoelectrodes prepared via rapid-anodic oxidation for efficient visible-light hydrogen generation

Carbon-incorporated titanium dioxide (TiO₂) photoelectrodes with different structural features were prepared via rapid-anodic oxidation under different electrical potentials and exposure times. The interstitial carbon arising from the pyrogenation of ethylene glycol electrolytes induced a new C2p occupied state at the bottom of the conduction band, which lowered the band gap energy to ∼2.3 eV and consequently enabled the visible-light responsiveness. Photoelectrodes with nanotubular structures provided higher photoconversion efficiency (η) and hydrogen (H₂) evolution capability than those with irregular structures. The increased aspect ratio, wall thickness, and pore size of the nanotube arrays contributed to η through greater photon excitation and penetration. However, this contribution is limited by the high recombination of the charge carriers at ultra-high aspect ratios. Photoelectrodes with a nanotube length of ∼19.5 μm, pore size of ∼103 nm, wall thickness of ∼17 nm, and aspect ratio of ∼142.5 exhibited remarkable capability to generate H₂ at an evolution rate of up to ∼508.3 μL min⁻¹ cm⁻² and η of ∼2.3%.     

Hydrogen generation from photocatalytic water splitting over TiO2 thin film prepared by electron beam-induced deposition

Photocatalytic TiO₂ thin films were prepared via an electron beam-induced deposition (EBID) method. The effects of post-calcination treatment on the properties of the prepared TiO₂ thin films were studied. X-ray diffraction (XRD), scanning electron microscope-energy dispersive spectrometry (SEM-EDS), and UV–V is absorption spectrometry were performed to reveal the crystallinity, surface morphology, chemical composition, and light absorbance of the prepared TiO₂ thin films. The photoelectrochemical characteristics of the TiO₂ thin films were investigated with a potentiostat. Under UV irradiation, a photocurrent of ˜2.1 mA was observed for the TiO₂ thin film with post-calcination at 500 °C. A water-splitting reaction was conducted over the TiO₂ thin film with the best photoelectrochemical performance. The yields of hydrogen and oxygen were 59.8 and 30.6 μmole, respectively, after 8 h of reaction under UV irradiation.