Enhanced solar hydrogen generation using Cu–Cu2O integrated polypyrrole nanofibers as heterostructured catalysts

Nanostructured conducting polymeric materials are beneficial for electron conduction and mass transport, showing high photocatalytic performance under visible light. Herein, we report a colloidal synthesis of copper and copper oxides (Cu₂O) modified polypyrrole nanofibers (PPy) heterostructures, which demonstrates significantly high photocatalytic H₂ generation under visible light. The presence of Cu nanoparticles (NPs) of 50 nm and cubic shaped Cu₂O nanoparticles of size 200 nm endows the heterostructures with a large specific surface area as well as good dispersion of nanoparticles on PPy nanofibers allows the migration of electron during catalysis. Cu₂O/PPy exhibits excellent H₂ production (67 mmol h⁻¹) which is 12 times higher than pure PPy (5.7 mmol h⁻¹). The high catalytic activity of Cu₂O/PPy heterostructure provides a fervent alternative to noble metal-based catalysts for the hydrogen generation and water splitting.     

Photocatalytic H2 evolution of porous silicon derived from magnesiothermic reduction of mesoporous SiO2

In this work, porous silicon (PSi) was synthesized by magnesiothermic reduction of mesoporous SiO₂ (MCM-41) and its photocatalytic hydrogen evolution performance was investigated. The unique mesoporous structure of PSi expands the band gap of silicon and shifts its conduction band to a more negative position. As a result, excellent photocatalytic water splitting efficiency of 604.7 μmol h⁻¹ g⁻¹ under visible-light radiation is recorded for the synthesized PSi photocatalysts without loading noble metal cocatalysts. This study presented a promising visible light response photocatalysts for the generation green renewable hydrogen energy basing on PSi material deriving from simple magnesiothermic reduction of mesoporous SiO₂.     

Rational design of interfacial energy level matching for CuGaS2 based photocatalysts over hydrogen evolution reaction

CuGaS₂(CGS) is effectively synthesized by a one-step solid-phase sintering method. The conduction band maximum, valence band maximum and flat band potential of the obtained CGS are ?1.14, 1.21 and 0.33 V vs RHE, respectively. When the Cu to Ga atom ratio is 1: 1.4 in the raw material, the prepared CuGaS₂ exhibits the highest visible-light (λ?420 nm) H₂ production activity. The hydrogen production rate of CuGaS₂ reaches 1.12 mmol g^?1 h^?1 under visible-light radiation. When Ruthenium is loaded as cocatalyst, the H₂ production rate of CuGaS₂@Ru is promoted to 3.38 mmol g^?1 h^?1. At the same, the interior component of photo-induced carries transfer among photoharvestor and cocatalyst has been revealed. This research provides a facile strategy to fabricate CuGaS₂ with excellent photocatalytic performance for H₂ production.     

Cu2O precipitation-assisted with ultrasound and microwave radiation for photocatalytic hydrogen production

Copper oxides are considered efficient photocatalysts for H₂ generation. In addition, due to their interesting properties such as surface plasmon resonance, they are applied in photo-induced reactions. In heterogeneous photocatalysis, CuO and Cu₂O are the main oxides based in copper that are used as catalysts in water splitting. In this work, Cu₂O is prepared by precipitation method assisted with ultrasound and microwave radiation at 80 °C. For the Cu₂O synthesis, the use of glucose is proposed as a reducing agent due to its abundance in nature, non-toxicity, and low cost. According to the results obtained, the highest glucose concentration and the suspension exposure to microwave irradiation promote the formation of Cu₂O particles with low and homogeneous particle size and a convenient position of their conduction and valence band to produce H₂. The highest H₂ generation using Cu₂O under the aforementioned experimental conditions is 78 μmol g_cat^?1. Additionally, the effect of adding glucose in the photocatalytic reaction is studied in order to provide more electrons to the reaction due to its effect as a hole scavenger, which inhibits the recombination of the electron and hole, promoting a higher H₂ production (400 μmol g_cat^?1).     

A new insight for photocatalytic hydrogen production by a Cu/Ni based cyanide bridged polymer as a co-catalyst on titania support in glycerol water mixture

A two dimensional Cu/Ni based coordination polymer [{Cu^II(4,4ʹ-dipy)₂}{Ni(CN)₄}]_n·0.7(C₂H₆O₂)·1.6(H₂O) (CP-1) (4,4ʹ-dipy = 1,3-di (4-pyridyl)propane) has been demonstrated as a potential co-catalyst on TiO₂ support for hydrogen evolution under UV light. CP-1/TiO₂ composite exhibits considerable hydrogen production in comparison with the pristine CP-1 and TiO₂ (P25), highlighting that the photocatalytic performance is significantly related with the good separation of photo generated e⁻/h⁺ pairs. Different wt. % (2.5, 5 and 7.5%) of CP-1 in CP-1/TiO₂ composites were tested for photocatalytic hydrogen production in 5 vol % glycerol/water mixture. The 5 wt % CP-1/TiO₂ composite displayed the greatest hydrogen production of 9.2 mmolh⁻¹g⁻¹. The concealed mechanism is divulged on the behalf of results obtained by cyclic voltammetry, photoluminescence and diffused reflectance/UV-visible studies which demonstrate that upon irradiation of UV light, electrons transfer from TiO₂ conduction band to CP-1. CP-1 not only grabs the conduction band electrons of titania but also performes as a co-catalyst to reduce the protons into hydrogen. These results are anticipated to direct the forthcoming advancement in creating proficient, cheap semiconductor photocatalysts for solar hydrogen production.     

Construction of MOFs/g-C3N4 composite for accelerating visible-light-driven hydrogen evolution

In this work, we researched the effect of PCN-222 (M = Ni, Fe, Co) (PM) with different metal ligands on their photocatalytic performance. Compared with PFe and PCo, PNi has the highest photocatalytic hydrogen evolution efficiency due to the narrowest bandgap and the highest conduction band (CB) position. Furthermore, PCN-222(M)/g-C₃N₄ (PM/CN) heterojunctions was synthesized by one-pot solvothermal method in which PNi/CN displayed the most outstanding photocatalytic activity for H₂ evolution. PNi/CN-1 displayed the highest photocatalytic activity. Its hydrogen evolution rate is 19.3 and 3.7 times higher than that of PNi and CN, respectively. A mechanism is proposed to expound the roles of PNi and the enhancement of visible-light photocatalytic performance of the PNi/CN. This work presents a new perspective for the development of high performance photocatalysts for hydrogen production under visible-light driven.     

Facile synthesis of CeO2/g-C3N4 nanocomposites with significantly improved visible-light photocatalytic activity for hydrogen evolution

Ceria dioxide supported on graphitic carbon nitride (CeO₂/g-C₃N₄) composites were facilely synthesized to be application for photocatalytic hydrogen (H₂) generation in this present work. The physical and chemical properties of CeO₂/g-C₃N₄ nanocomposites were determined via a series of characterizations. The CeO₂/g-C₃N₄ composites prepared by facile thermal annealing and rotation-evaporation method exhibit excellent photocatalytic H₂ evolution with visible-light illumination. The best hydrogen generation rate of CeO₂/g-C₃N₄ composite with 1.5 wt% Pt is 0.83 mmol h⁻¹ g⁻¹, which is almost same as that of composite with 3 wt% Pt prepared by simple physical mixing method. The significantly developed photocatalytic activity of CeO₂/g-C₃N₄ composite is majorly ascribed to the stronger interfacial effects with the more visible-light absorbance and faster electron transfer. This work reveals that construction of the CeO₂/g-C₃N₄ composite with high disperse and close knit by the facile thermal annealing and rotation-evaporation method could be an effective method to achieve excellent photocatalytic hydrogen evolution performance.     

Improving hydrogen evolution activity of perovskite BaTiO3 with Mo doping: Experiments and first-principles analysis

Hydrogen production through photocatalytic water splitting attracts great attention in fields of energy conversion. To improve the hydrogen evolution efficiency, narrowing the bandgap of photocatalysts by introducing dopant atoms is widely investigated for increasing light absorption. Herein, Mo-doped BaTiO₃ samples are synthesized by a traditional solid-state reaction method and all the samples are modified with Pt by a photo-reduction method. Compared with pure BaTiO₃, Mo doping into BaTiO₃ samples realizes the band-to-band visible-light absorption and shows remarkable improvement in hydrogen production efficiency. Under simulated sunlight irradiation and with 0.4 wt% Pt deposition, BaTiO₃ doped with 2 at% Mo exhibits a hydrogen evolution rate of 63 μmol g⁻¹ h⁻¹, about 2 times improvement in comparison to pure BaTiO₃ (35 μmol g⁻¹ h⁻¹). Further first-principles calculations based on density-function theory demonstrates an apparent downward movement of the conduction band minimum due to the coupling between the Ti 3d and Mo 3d states, leading to the significant bandgap narrowing and enhancement of the visible-light photocatalytic activity.     

Simultaneous hydrogen production and decomposition of dissolved in alkaline water over composite photocatalysts under visible light irradiation

A process of simultaneous hydrogen production and H₂S removal has been investigated over a highly active composite photocatalyst made of bulk CdS decorated with nanoparticles of TiO₂, i.e. CdS(bulk)/TiO₂. The photocatalytic activity was evaluated for hydrogen production from aqueous electrolyte solution containing H₂S dissolved in water or alkaline solution under visible light irradiation. The rate of hydrogen production from the H₂S-containing alkaline solution was similar to the rate obtained from photocatalytic hydrogen production from water containing sacrificial reagents (Na₂S+Na₂SO₃) in the similar concentration. The isotope experiment was carried out with D₂O instead of H₂O to investigate the source of hydrogen from photocatalytic decomposition of H₂S dissolved in H₂O or alkali solution under visible light. Hydrogen originated from both H₂S and H₂O when the reaction solution contained H₂S absorbed in alkaline water.     

Improved photo-electrochemical properties of strained SnO2

In the present study, the potential of strained SnO₂ as a photo-catalyst for hydrogen evolution reaction is predicted using first principles calculation. It was found that tensile strain improves the absorption capability towards visible-light absorption (around 400 nm), increases its mobility which is a positive factor for photocatalysis, and finally improves the redox potential level of H⁺/H₂ with respect to the conduction band minimum. All these improvements make tensile strained-SnO₂ a promising photocatalyst for hydrogen production.     

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.     

Constructing Cu/BN@PANI ternary heterostructure for efficient photocatalytic hydrogen generation: A combined experimental and DFT studies

In this report, (PANI) embedded copper NPs with h-BN has been developed as a new high-performance photocatalysts, minimized the use of precious metals, for hydrogen generation. The facile in-situ synthesis of the ternary Cu/BN@PANI nanocomposite via the hydrothermal process is successfully achieved. The photoexcited charge transfers are controlled by creating suitable junction architectures, as such the ternary composite exhibits remarkably enhanced hydrogen generation. The XRD, XPS, and TEM analysis confirms the coexistence of the Cu/BN@PANI nanocomposites' phase and composition. Cu/BN@PANI-2.5 wt% showed a sustained H₂ evolution rate of 3121 μmol g^?1 h^?1 with quantum efficiency 6.91% and increased photocatalytic current response of 49 mA cm^?2. A higher estimated lifetime in Cu/BN@PANI-2.5 wt% (8.66 ns) suggests enhanced photogenerated charge carrier efficiency across the junction interface within the heterostructures. DFT calculations reveal that the conduction band minimum and valence band maximum of PANI is higher than those of the Cu₇ cluster, indicating desirable band alignment and ensuring high hydrogen evolution reaction activity. The light illuminates on the Cu/BN@PANI composite, the electrons of SP² type h-BN and π conjugated PANI photoexcited to the conduction band of Cu NPs and the excited hole of Cu NPs quickly transfers to the valence band of PANI, which is in agreement with the experimental results.     

Enhancement of visible light-driven hydrogen production over zinc cadmium sulfide nanoparticles anchored on BiVO4 nanorods

Photocatalytic water splitting to produce H₂ is a promising technology for clean energy generation. However, the use of expensive noble metals, toxicity, low charge separation efficiency and wide band gap of semiconductors hampering the widespread commercialization. Herein, we showed the potential of combining BiVO₄ nanorods with ZnCdS forming a hetero-structure which extend the spectral responsive range, separate the charge carriers effectively and enhances photocatalytic activity compared to single-component materials. The two components of hetero-structure forms an interface contact which also mitigate the problems of lower conduction band position of BiVO₄ and fast recombination of charge carriers of ZnCdS. The BiVO₄–ZnCdS hetero-structure was studied through surface morphology, crystallization properties, elemental analysis and optical properties. Under visible light irradiation, the BiVO₄–ZnCdS heterostructure produced 152.5 μmol g^?1 h^?1 hydrogen from water splitting, which was much higher than that of the individual components and stability of the hydrogen production was observed in three consecutive cycles. The as-obtained heterostructure showed improved visible light harvesting ability, prolong life of charges carriers and charge separation efficiency and Z-scheme mechanism features which results in enhanced photocatalytic activity for water splitting.     

CuS-modified ZnO rod/reduced graphene oxide/CdS heterostructure for efficient visible-light photocatalytic hydrogen generation

Solar energy utilization is a promising strategy for the photocatalytic generation of H₂ from water. Herein, a CuS-modified ZnO rod/reduced graphene oxide (rGO)/CdS heterostructure was fabricated via Cu-induced electrochemical growth with Zn powder at room temperature. The resulting powder revealed good interfacial bonding and promoted photoexcited carrier transport. The CuS nanoparticles played a pivotal role in enhancing visible-light responses and demonstrated excellent catalytic performance. A high visible-light photocatalytic H₂ generation rate of 1073 μmol h⁻¹ g⁻¹ was obtained from the CuS–ZnO/rGO/CdS heterostructure containing 0.23% CuS and 1.62% CdS. Increased photoexcited electron lifetimes, improved carrier transport rates, and decreased fluorescence intensities confirmed the synergistic effects of each of the components of the heterostructure. This study provides an innovative strategy for constructing multi-component heterostructures to achieve efficient visible-light H₂ evolution.     

Giant enhancement of photocatalytic H2 production over KNbO3 photocatalyst obtained via carbon doping and MoS2 decoration

This paper was designed for the first time to improve the photocatalytic activity of KNbO₃ via carbon doping and MoS₂ decoration simultaneously. The efficient photocatalytic hydrogen production was realized on the MoS₂/C-KNbO₃ composite under simulated sunlight irradiation in the present of methanol and chloroplatinic acid. The optimal composite presents a H₂ production rate of 1300  μmol·g^?1·h^?1, which reaches 260 times that of pure KNbO₃. Characterization results of the synthesized composite indicates that the introduction of a small amount of carbon into the KNbO₃ lattice greatly hinders the recombination of electron-hole pairs. The decoration of MoS₂ further induces the separation of charge carriers via trapping the electron in the conduction band of C-KNbO₃, which is proven by the EIS and transient photocurrent response analyses. The remarkably enhanced separation efficiency of electron-hole pairs is believed to be the origin of the excellent photocatalytic performance, though other changes in surface area and optical property may also contribute the photocatalytic process. This study provides a feasible way for the design and preparation of novel photocatalysts with high efficiency.     

ZrW2O8 photocatalyst and its visible-light sensitization via sulfur anion doping for water splitting

ZrW₂O₈ prepared by hydrothermal reaction was found to act as a photocatalyst for water splitting under UV light irradiation. It has good activity for water splitting to evolve H₂ and O₂ steadily in the presence of CH₃OH and AgNO₃ as electron donor and electron scavenger respectively. With respect to the band structure for photocatalytic water splitting, ZrW₂O₈ (4.0 eV) was found to be superior to ZrO₂ (5.0 eV) with a wider band gap and WO₃ (2.7 eV) with CB bottom more positive than the reduction potential of H⁺ to H₂. The improvement of the band structure was attributed to the hybridization of W5d and Zr4d in conduction band (CB) as well as the change in crystal structure. Moreover, the absorption edge of ZrW₂O₈ was significantly extended to visible-light region by sulfur anion doping, and H₂ could be evolved over Pt/S-ZrW₂O₈ under irradiation up to 360 nm in the presence of CH₃OH while O₂ could be evolved over S-ZrW₂O₈ under irradiation up to 510 nm in the presence of AgNO₃. The visible-light sensitization was attributed to the S3p states, which increased the width of the VB itself and caused the decrease in the band gap energy.     

Recent advances in covalent organic framework (COF) nanotextures with band engineering for stimulating solar hydrogen production: A comprehensive review

Due to the continuous consumption of fossil fuels, natural reserves are depleting and it has been earnest need for developing new sources of energy. Among the several solar energy conversion techniques, photocatalytic hydrogen (H₂) generation is regarded as one of the most promising routes. Till date, several metal-based semiconductor materials have been investigated, however, H₂ generation is not substantial with the notion of sustainable development. Current research trends show the growing interest in advanced and metal free photocatalyst materials such as covalent organic frameworks (COFs) due to their several benefits such as crystalline porous polymers with pre-designed architectures, large surface area, exceptional stability, and ease of molecular functionalization. By combining COFs with other functional materials, composites may be created that display unique characteristics that exceed those of the separate components. This work provides a comprehensive development on COFs as a photocatalysts and their composites/hybrids for photocatalytic hydrogen generation with a focus on visible-light irradiation. To reduce the dependency on novel metals and overcome the drawbacks of individual material, the creation of composite materials based on covalent-organic frameworks (COFs) are systematically discussed. In addition, advantages in terms of performance, stability, durability of composites/hybrids COFs for photocatalytic hydrogen production in reference to traditional catalysts are investigated. Different composites such as metals loading, morphological development, band engineering, and heterojunctions are systematically discussed. Finally, challenges and opportunities associated with constructing COF-based catalysts as future research prospective for chemistry and materials science are highlighted.     

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.     

Surface plasmon assisted photocatalytic hydrogen generation with Ag decorated g-C3N4 coupled SnO2 nanophotocatalyst under visible-light driven photocatalysis

Photocatalytic hydrogen evolution from water is a feasible technique to solve energy crises and reduce dependance on carbon fuels. As for this, silver nanoparticles were grown on the surface of SnO₂ coupled g-C₃N₄ nanocomposite for the generation of hydrogen gas from water under visible light photocatalysis. The prepared samples were properly characterized to investigate their light absorption characteristics followed by charge generation and separation for water splitting. The optimized nanocomposite produced 270 μmol h⁻¹ g⁻¹ hydrogen which was much superior to pure g-C₃N₄ and SnO₂. These upgraded photocatalytic activities were attached to the extended visible-light absorption due to the presence of Ag nanoparticles characterized by surface plasmon resonance (SPR) and suitable conduction bands position of g-C₃N₄ and SnO₂ for the separation of excited charges. The photoluminescence study, amount of produced hydroxyl free radicals and electrochemical investigation confirmed the long-rooted charge separation capability of the nanocomposites. We believe that this work will have more positive impacts on the synthesis of low cost SPR assisted photocatalysts for energy production and environmental purification.     

Photoelectrochemical properties of doped polyaniline: Application to hydrogen photoproduction

The semi conducting properties of doped polyaniline (emeraldine-salt, PANI) elaborated by chemical route are investigated by the photo-electrochemical technique. The band gap is found to be 1.48 eV and the transition is directly allowed. The electrical conduction obeys to an exponential law with activation energy of 0.13 eV. p-type conductivity is evidenced from the cathodic photocurrent. The energy band diagram clearly shows the spontaneous hydrogen photo evolution. The potential of the conduction band of PANI (−0.93 V_SCE) determined from the capacitance measurements is suitably positioned with respect to H₂O/H₂ level (−0.66 V_SCE). Therefore, the photocatalytic properties of this material has been evaluated according to the hydrogen generation. The best performance is achieved at pH ∼7 with a liberation rate of 0.113 mL h⁻¹ (mg catalyst)⁻¹ and a quantum efficiency of 0.18% under visible light (29 mW cm⁻²). An increase of 56% is obtained on the hetero-system PANI/TiO₂.