Construction of ternary Z-scheme covalent triazine framework@Au@TiO2 for enhanced visible-light-driven hydrogen evolution activity

One key challenge in photocatalytic hydrogen production is how to construct high-performance photocatalyst. Covalent triazine framework (CTF) based polymers as photocatalysts show great application potential because of their good photocatalytic activity, high chemical stability, tunable electronic and optical properties, and easy synthesis process. In this paper, we designed the ternary Z-scheme heterojunction Au@TiO₂-X%TrTh based on CTF polymer TrTh, TiO₂ and Au nanoparticle, which exhibit higher photocatalytic hydrogen production rate compared with the corresponding binary heterojunction Au@TiO₂ and TiO₂-12%TrTh. The results of photocatalytic hydrogen production show that the optimized Au@TiO₂-12%TrTh has a remarkable hydrogen production rate of 4288.54 μmol g^?1 h^?1, which is about 312.3 times of Au@TiO₂ and 9.1 times of the TiO₂-12%TrTh. The enhanced hydrogen production activity of the ternary heterojunction comes from the local surface plasmonic resonance effect of Au nanoparticle, lower recombination efficiency of photogenerated electron-holes pairs and Z-scheme electron transfer pathway of Au@TiO₂-12%TrTh. The work provides a new strategy for designing efficient and practical photocatalyst.     

Highly efficient photocatalytic hydrogen evolution by using Rh as co-catalyst in the Cu/TiO2 system

Cu/TiO₂ was modified by adding Rh as co-catalyst and used as a highly efficient photocatalyst for the hydrogen evolution reaction. A low amount of Rh was loaded onto Cu/TiO₂ by the deposition-precipitation with urea (DPU) method to observe the effect on the hydrogen production displayed by different samples. The Rh–Cu/TiO₂ oxide structure exhibited a remarkably high photocatalytic hydrogen evolution performance, which was about twofold higher than that of the Cu/TiO₂ monometallic photocatalyst. This outstanding performance was due to the efficient charge carrier transfer as well as to the delayed electron-hole recombination rate caused by the addition of Rh. The influence of the different parameters of the photocatalyst synthesis and reaction conditions on the photocatalytic activity was investigated in detail. Hydrogen evolution was studied using methanol, ethanol, 2-propanol and butanol as scavengers with an alcohol:water ratio of 20:80. The methanol-water system, which showed the highest hydrogen production, was studied under 254, 365 and 450 nm irradiation; Rh–Cu/TiO₂ showed high photocatalytic activity with H₂ production rates of 9260, 5500, and 1940 μmol h^?1 g^?1, respectively. The Cu–Rh/TiO₂ photocatalyst was active under visible light irritation due to its strong light absorption in the visible region, low band gap value and ability to reduce the electron (e^?) and hole (h⁺) recombination.     

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.     

Enhanced photocatalytic activity in water splitting for hydrogen generation by using TiO2 coated carbon fiber with high reusability

In this study, TiO₂ coated carbon fiber (TiO₂@CF) was synthesized and used for the improvement of hydrogen (H₂) evolution. Obtained results from scanning electron microscopy (SEM), X-ray diffraction (XRD), gas adsorption analysis (BET), UV–vis diffuse (UV–vis), and X-ray photoelectron spectroscopy (XPS) confirmed that the surface area and light absorption of the material was significantly improved. The synthesized TiO₂@CF photocatalyst exhibited improved photocatalytic performance toward hydrogen generation. The enhancement of photocatalytic H₂ evolution capacity by TiO₂@CF was ascribed to its narrowed bandgap energy (2.76eV) and minimized recombination of photogenerated electron-hole pairs The hydrogen production rate by the TiO₂@CF reached 3.238 mmolg^?1h^?1, which was 4.8 times higher than unmodified TiO₂ (0.674 mmolg^?1h^?1). The synthesized TiO₂@CF was relatively stable with no distinct reduction in photocatalytic activity after five recycling runs. The photoluminescence and photocurrent were employed to support the photocatalytic H₂ production mechanism proposed mechanism.Based on these results, TiO₂@CF with unique properties, easy handle, and high reusability could be suggested as an efficient strategy to develop a high-performance photocatalyst for H₂ production.     

Efficient photocatalytic hydrogen evolution on amorphous MoSx-modified CdS/KTaO3 heterojunctions under visible light irradiation

Constructing heterostructures with efficient charge separation is a promising route to improve photocatalytic hydrogen production. In this paper, MoS_x/CdS/KTaO₃ ternary heterojunction photocatalysts were successfully prepared by a two-step method (hydrothermal method and photo deposition method), which improved the photocatalytic hydrogen evolution activity. The results show that the rate of hydrogen evolution for the optimized photocatalyst is 2.697 mmol g^?1·h^?1under visible light, which is 17 times and 2.6 times of the original CdS (0.159 mmol g^?1 h^?1) and the optimal CdS/KTaO₃(1.033 mmol g^?1 h^?1), respectively, and the ternary photocatalyst also shows good stability. The improvement on photocatalytic hydrogen evolution performance can be attributed to the formation of heterojunction between the prepared composite materials, which effectively promotes the separation and migration of photo-generated carriers. Amorphous MoS_x acts as an electron trap to capture photogenerated electrons, providing active sites for proton reduction. This provides beneficial enlightenment for hydrogen production by efficiently utilizing sunlight to decompose water.     

Enhanced photocatalytic hydrogen evolution from reduced graphene oxide-defect rich TiO2-x nanocomposites

Solar-driven photocatalytic hydrogen generation by splitting water molecules requires an efficient visible light active photocatalyst. This work reports an improved hydrogen evolution activity of visible light active TiO_2-x photocatalyst by introducing reduced graphene oxide via an eco-friendly and cost-effective hydrothermal method. This process facilitates graphene oxide reduction and incorporates intrinsic defects in TiO₂ lattice at a one-pot reaction process. The characteristic studies reveal that RGO/TiO_2-x nanocomposites were sufficiently durable and efficient for photocatalytic hydrogen generation under the visible light spectrum. The altered band gap of TiO_2-x rationally promotes the visible light absorption, and the RGO sheets present in the composites suppresses the electron-hole recombination, which accelerates the charge transfer. Hence, the noble metal-free RGO/TiO_2-x photocatalyst exhibited hydrogen production with a rate of 13.6 mmol h^?1g^?1_cat. under solar illumination. The appreciable photocatalytic hydrogen generation activity of 947.2 μmol h^?1g^?1_cat with 117 μAcm^?2 photocurrent density was observed under visible light (>450 nm).     

Direct Z-scheme CoS/g-C3N4 heterojunction with NiS co-catalyst for efficient photocatalytic hydrogen generation

Facilitating the separation of photoexcited electron-hole pairs and enhancing the migration of photogenerated carriers are essential in photocatalytic reaction. CoS/g-C₃N₄/NiS ternary photocatalyst was prepared by hydrothermal and physical stirring methods. The optimized ternary composite achieved a hydrogen yield of 1.93 mmol g^?1 h^?1, 12.8 times that of bare g-C₃N₄, with an AQE of 16.4% at 420 nm. The enhanced photocatalytic activity of CoS/g-C₃N₄/NiS was mainly ascribed to the synergistic interaction between the Z-scheme heterojunction constructed by CoS and g-C₃N₄ and the NiS co-catalyst featuring a large amount of hydrogen precipitation sites, which realized the efficient separation and migration of photogenerated carriers. In addition, the CoS/g-C₃N₄/NiS heterojunction-co-catalyst system exhibited excellent photocatalytic stability and recyclability.     

Polyaniline encapsulated Ti-MOF/CoS for efficient photocatalytic hydrogen evolution

We report the study of conductive polyaniline (PANI) chain embedded Ti-MOF functionalized with CoS as a cocatalyst for hydrogen evolution reaction (HER) application. The post synthetically modified hybrid photocatalyst PANI/Ti-MOF/CoS greatly influences the redox and e^? ? h⁺ separation process and exhibits an impressive rate of HER (～1322 μmol h^?1g^?1), suppressing the pristine Ti-MOF (～62 μmol h^?1g^?1) with apparent quantum yield (AQY) of ～3.2 and transient current response of ～46.4 μA cm^?2. In this system, Ti-MOF provides the circulation of Ti³⁺ and Ti⁴⁺ to the reaction of photocatalytic H₂ generation, where the additional PANI and CoS amended the performance of H₂ production through electron enrichment and thereby improving the stability and integrity of Ti-MOF. The Electrochemical studies demonstrated increased photocurrent by interweaving Ti-MOF crystal with PANI through cation-π interaction thereby enhancing interface connection and then promoting electron transfers. The charge dynamics revealed the initial charge transfer from photoexcited PANI to encapsulated MOF framework to boost the photocatalytic performance of the system. Further, the electron movement at the Ti-MOF/CoS interface is investigated through work function and electrochemical potential of electrons (Fermi level). DFT results demonstrate the importance of CoS in improving the photocatalytic performance of hybrid Ti-MOF catalyst, which leads to superior catalytic behaviour. These results establish that the encapsulation of catalytic active sites inside MOFs with desirable energy band gaps would be an ideal choice for the production of solar fuels.     

Promoted interfacial charge transfer by coral-like nickel diselenide for enhanced photocatalytic hydrogen evolution over carbon nitride nanosheet

Seeking an efficient and non-precious co-catalyst for g-C₃N₄ (CN) remains a great demanding to achieve high photocatalytic hydrogen generation performance. Herein, a composite photocatalyst with high efficiency was prepared by modifying CN with coral-like NiSe₂. The optimal hydrogen evolution rate of 643.16 μmol g^?1 h^?1 is from NiSe₂/CN-5 under visible light. Superior light absorption and interfacial charge transfer properties including suppressed photogenerated carrier recombination and efficient separation of photogenerated electron-hole pairs have been observed, which account for the enhanced photocatalytic performance of CN.     

Rationally designed ternary CdSe/WS2/g-C3N4 hybrid photocatalysts with significantly enhanced hydrogen evolution activity and mechanism insight

Excellent light harvest, efficient charge separation and sufficiently exposed surface active sites are crucial for a given photocatalyst to obtain excellent photocatalytic performances. The construction of two-dimensional/two-dimensional (2D/2D) or zero-dimensional/2D (0D/2D) binary heterojunctions is one of the effective ways to address these crucial issues. Herein, a ternary CdSe/WS₂/g-C₃N₄ composite photocatalyst through decorating WS₂/g-C₃N₄ 2D/2D nanosheets (NSs) with CdSe quantum dots (QDs) was developed to further increase the light harvest and accelerate the separation and migration of photogenerated electron-hole pairs and thus enhance the solar to hydrogen conversion efficiency. As expected, a remarkably enhanced photocatalytic hydrogen evolution rate of 1.29 mmol g⁻¹ h⁻¹ was obtained for such a specially designed CdSe/WS₂/g-C₃N₄ composite photocatalyst, which was about 3.0, 1.7 and 1.3 times greater than those of the pristine g-C₃N₄ NSs (0.43 mmol g⁻¹ h⁻¹), WS₂/g-C₃N₄ 2D/2D NSs (0.74 mmol g⁻¹ h⁻¹) and CdSe/g-C₃N₄ 0D/2D composites (0.96 mmol g⁻¹ h⁻¹), respectively. The superior photocatalytic performance of the prepared ternary CdSe/WS₂/g-C₃N₄ composite could be mainly attributed to the effective charge separation and migration as well as the suppressed photogenerated charge recombination induced by the constructed type-II/type-II heterojunction at the interfaces between g-C₃N₄ NSs, CdSe QDs and WS₂ NSs. Thus, the developed 0D/2D/2D ternary type-II/type-II heterojunction in this work opens up a new insight in designing novel heterogeneous photocatalysts for highly efficient photocatalytic hydrogen evolution.     

LED light-induced enhanced photocatalytic hydrogen evolution on Au@TiO2 core–shell modified by nitrogen doping and reduced graphene oxide

This study focused on the large band gap of TiO₂ for its use as a photocatalyst under light emitting diode (LED) light irradiation. The photocatalytic activities of core–shell structured Au@TiO₂ nanoparticles (NPs), nitrogen doped Au@TiO₂ NPs, and Au@TiO₂/rGO nanocomposites (NCs) were investigated under various light intensities and sacrificial reagents. All the materials showed better photocatalytic activity under white LED light irradiation than under blue LED light. The N-doped core–shell structured Au@TiO₂ NPs (Au@N–TiO₂) and Au@TiO₂/rGO NCs showed enhanced photocatalytic activity with an average H₂ evolution rate of 9205 μmol h^?1g^?1 and 9815 μmol h^?1g^?1, respectively. All these materials showed an increasing rate of hydrogen evolution with increasing light intensity and catalyst loading. In addition, methanol was more suitable as a sacrificial reagent than lactic acid. The rate of hydrogen evolution increased with increasing methanol concentration up to 25% in DI water and decreased at higher concentrations. Overall, Au@TiO₂ core–shell-based nanocomposites can be used as an improved photocatalyst in photocatalytic hydrogen production.     

Efficient photothermal catalytic hydrogen production via plasma-induced photothermal effect of Cu/TiO2 nanoparticles

Developing appropriate photocatalyst with high efficiency is still the basic strategy for practical application of emerging technology. Herein, non-noble metal copper (Cu) nanoparticles were in situ hybrided with TiO₂ by a chemical reduction method. The crystal phase and structure were characterized by XRD, SEM, and TEM measurements. Hydrogen production results showed that Cu nanoparticles significantly improved the photocatalytic hydrogen production rate. The hydrogen production rate was as high as 24160.69 μmol g^?1 h^?1 at 100 °C, which was 36.25 and 8.46 times higher than the hydrogen production rates of pure TiO₂ and 0.13 wt% Cu/TiO₂ at room temperature, respectively. PL spectra, UV–vis spectra, IR images and photoelectrochemical measurements showed that the plasma-induced photothermal effect of Cu/TiO₂ nanoparticles, which raised the temperature of the reaction system and promoted photothermal catalytic performance. Briefly, this work provides a facile fabrication method of noble-metal-free photocatalysts featuring in low-cost and high efficiency. In the future, coupling the photothermal effect of plasmonic Cu to further speed up the kinetics should be another promising research direction for further improving hydrogen production.     

Vertically-heterostructured TiO2-Ag-rGO ternary nanocomposite constructed with {001} facetted TiO2 nanosheets for enhanced Pt-free hydrogen production

TiO₂ nanosheets with high ratio of {001} facets were coupled with reduced graphene oxide (rGO) nanosheets through the link of silver (Ag) nanoparticles, forming a novel ternary nanocomposite photocatalyst with a vertical heterostructure, TiO₂-Ag-rGO. The vertical anchoring of TiO₂-Ag nanosheets between rGO sheets was confirmed by transmission electron microscopy (TEM), Raman spectroscopy, energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Due to excellent separation of electron-hole pairs in the TiO₂ nanosheets, enhanced electron transfer to rGO via Ag nanoparticles, the TiO₂-Ag-rGO nanocomposite exhibited an outstanding performance in photocatalytic hydrogen production, with a hydrogen production rate of 593.56 μmol g^?1 h^?1. This study provides new insights to the development of Pt-free photocatalysts for hydrogen production.     

Rational regulation of vacancy species to manage migration paths of carriers in MoS2/TiO2 heterojunctions for efficient photocatalytic H2 generation

Photocatalytic water splitting to produce hydrogen (H₂), as one means to solve environmental pollution and energy shortage, is limited by the serious recombination of photogenerated electrons and holes, resulting in low solar energy conversion efficiency. Thus, steering the behaviors of charge carriers by rationally designing their transport pathway is essential, which can effectively suppress the recombination of electrons and holes. Herein, we designed a MoS₂/TiO₂ heterojunction with different vacancy species to manage the migration paths of photogenerated charge carriers. As demonstrated by experimental characterizations and density functional theory (DFT) calculations, oxygen and sulfur vacancies can induce defect energy levels in heterostructures, which can capture photogenerated holes and electrons, respectively, resulting in substantially promoted charge separation efficiency and longer lifetime of electrons. As expected, the optimized photocatalyst shows a stable H₂ production rate of 1.41 mmol g^?1 h^?1, which is significantly better than that of the bare MoS₂/TiO₂ heterojunction. This finding informs the significance in rational design of the nanostructures for promoting the photocatalytic performance.     

In2Se3/CdS nanocomposites as high efficiency photocatalysts for hydrogen production under visible light irradiation

Hydrogen energy is an important clean energy. Using visible light to produce hydrogen by semiconductor photocatalysts is one of the current research hotspots. In this work, In₂Se₃/CdS nanocomposite photocatalysts with different mass content of CdS are prepared. The In₂Se₃/CdS photocatalyst with 85.25% CdS mass content exhibits the optimal photocatalytic hydrogen evolution activity (1.632 mmol g^?1 h^?1), which is much higher than that of CdS (0.715 mmol g^?1 h^?1) and In₂Se₃ (trace). Moreover, the In₂Se₃/CdS photocatalyst still maintains a high hydrogen evolution rate after five cycles. The high photocatalytic activity and stability of the In₂Se₃/CdS nanocomposite is due to the formation of heterojunction between In₂Se₃ and CdS. The existence of heterojunction is confirmed by high resolution transmission electron microscopy image and X-ray photoelectron spectra. Theoretical calculations and experimental results indicate that the electron transfer route at the heterojunction is step-scheme. The step-scheme helps the separation of photogenerated electrons and holes, and maximize the hydrogen evolution activity. This work provides a high efficiency step-scheme photocatalyst for hydrogen production.     

Efficient photocatalytic H2 evolution over Cu and Cu3P co-modified TiO2 nanosheet

Schottky junction and p-n heterojunction are widely employed to enhance the charge transfer during the photocatalysis process. Herein, Cu and Cu₃P co-modified TiO₂ nanosheet hybrid (Cu–Cu₃P/TiO₂) was fabricated using an in situ hydrothermal method. The ternary composite achieved the superior H₂ evolution rate of 6915.7 μmol g^?1 h^?1 under simulated sunlight, which was higher than that of Cu/TiO₂ (4643.4 μmol g^?1 h^?1) and Cu₃P/TiO₂ (6315.8 μmol g^?1 h^?1) and pure TiO₂ (415.7 μmol g^?1 h^?1). The enhanced activity can be attributed to the collaboration effect of Schottky junction and p-n heterojunction among Cu/TiO₂ and Cu₃P/TiO₂, which can harvest the visible light, reduce the recombination of charge carriers and lower the overpotential of H₂ evolution, leading to a fast H₂ evolution kinetics. This work develops a feasible method for the exploration of H₂ evolution photocatalyst with outstanding charge separation properties.     

CoNi bimetallic alloy cocatalyst-modified TiO2 nanoflowers with enhanced photocatalytic hydrogen evolution

In terms of improving photocatalytic hydrogen production performance, inexpensive and earth-rich cocatalysts have become promising alternatives to precious metals. Herein, a novel CoNi–TiO₂ photocatalyst composed of TiO₂ nanoflowers and CoNi alloy was prepared by hydrothermal and chemical reduction methods. Various characterizations and test results have confirmed that the further improvement of the photocatalytic performance of the CoNi–TiO₂ photocatalyst is mainly due to the fact that the bimetallic CoNi alloy can accelerate charge transfer and inhibit the recombination of photo-induced carriers. The hydrogen production rate of the prepared CoNi–TiO₂ is about 24 times higher than that of the pristine TiO₂, and its hydrogen production rate value can reach 6580.9 μmol g^?1 h^?1, and showing comparable photocatalytic performance to 0.5 wt% Pt–TiO₂. In addition, combined with the characterization results, a probable mechanism for enhanced photocatalytic performance was proposed. This study provides favorable enlightenment for the design of a series of highly efficient non-precious metal TiO₂-based photocatalysts.     

Fabrication and comparison of highly efficient Cu incorporated TiO2 photocatalyst for hydrogen generation from water

Efficient Cu incorporated TiO₂ (Cu–TiO₂) photocatalysts for hydrogen generation were fabricated by four methods: in situ sol–gel, wet impregnation, chemical reduction of Cu salt, and in situ photo-deposition. All prepared samples are characterized by good dispersion of Cu components, and excellent light absorption ability. Depending on the preparation process, hydrogen generation rates of the as-prepared Cu–TiO₂ were recorded in the range of 9–20 mmol h⁻¹ g_catalyst⁻¹, which were even more superior to some noble metal (Pt/Au) loaded TiO₂. The various fabrication methods led to different chemical states of Cu, as well as different distribution ratio of Cu between surface and bulk phases of the photocatalyst. Both factors have been proven to influence photocatalytic hydrogen generation. In addition, the Cu content in the photocatalyst played a significant role in hydrogen generation. Among the four photocatalysts, the sample that was synthesized by in situ sol–gel method exhibited the highest stability. High efficiency, low cost, good stability are some of the merits that underline the promising potential of Cu–TiO₂ in photocatalytic hydrogen generation.     

In situ tuning of band gap of Sn doped composite for sustained photocatalytic hydrogen generation under visible light irradiation

The significance of Sn dopant on the photocatalytic performance of Iron/Titanium nanocomposite towards photocatalytic hydrogen generation by water splitting reaction is investigated. Iron/Titanium nanocomposite modified by Sn⁴⁺ dopant acts as a suitable photocatalyst for induced visible light absorption facilitating pronounced charge separation efficiency. Various characterization techniques reveal the heterojunction formation of hematite Fe₂O₃ with anatase - rutile mixed phase of TiO₂ employing Sn doping, where Sn⁴⁺ dopant accomplishes the phase transformation of anatase to rutile, entering into the TiO₂ lattice. This extended the lifetime of photogenerated charge carriers and enhanced the quantum efficiency of the photocatalyst. The band gap of the nanocomposite is tuned to ~2.4 eV, favoring visible light absorption. A hydrogen generation activity of 1102.8 μmol, approximately five times higher than the lone system (216.5 μmol) is achieved for the 5% Sn doped system for an average of 5 h. Heterojunctions of hematite with anatase-rutile mixed phase, generated as a consequence of tin doping facilitated the enhanced hydrogen generation activity of photocatalyst.     

Photocatalytic hydrogen production by flower-like graphene supported ZnS composite photocatalysts

Flower-like graphene (FG) prepared by a transformer coupled plasma enhanced chemical vapor deposition method was used as support for the preparation of composite photocatalysts. Small ZnS particles were formed on the surface of FG by a hydrothermal process with ZnCl₂ and Na₂S precursors. The surface morphology, surface area, surface chemistry, crystalline property, optical properties, photogenerated current and photocatalytic hydrogen production activity of the FG-ZnS photocatalysts were investigated by using the X-ray diffraction, scanning electron microscope, transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet-visible diffuse reflectance spectra, photocurrent response, photoluminescence spectra, electrochemical impedance spectra and photocatalytic hydrogen production tests. The maximum hydrogen production rate of FG-ZnS composite photocatalyst ZS-G0.02 was 11600 μmol g^?1h^?1 under UV light irradiation at a graphene/ZnCl₂ precursor weight ratio of 0.02. The flower-like structure of FG may help the light absorption, adsorption of sacrificing agents in the solution, and separation of photogenerated carriers. In comparison with pristine ZnS photocatalyst, the FG-ZnS nanocomposites exhibits enhanced photocatalytic hydrogen production activity.