Metal-Free 2D/2D Phosphorene/g-C3 N4 Van der Waals Heterojunction for Highly Enhanced Visible-Light Photocatalytic H2 Production

Noble Metal-Free Photocatalysts Consisting of Graphitic Carbon Nitride, Nickel Complex, and Nickel Oxide Nanoparticles for Efficient Hydrogen Generation

A facile and simple synthetic route is developed to prepare earth-abundant and noble metal-free hybrid photocatalysts, which are composed of graphitic carbon nitride (CN), nickel complex, and NiO _x nanoparticles. Bimolecular nucleophilic substitution reaction was employed to attach a nickel complex onto a graphitic CN framework through covalent bonds to support its high loading and dispersion. NiO _x nanoparticles were further incorporated into the catalysts to serve as a hole-transporting medium to improve the separation of photogenerated carriers for higher photocatalytic activity. Both yNiL/CN and yNiL/NiO _x/CN exhibit superb H₂ evolution activity. The optimum H₂ evolution rate of the binary photocatalysts yNiL/CN reaches 303.3 μmol·h^-1·g^-1, whereas that of the ternary photocatalysts yNiL/NiO _x/CN reaches 524.1 μmol·h^-1·g^-1, and the apparent quantum efficiency reaches 1.46% at 450 nm. This finding reveals that coordination of a nickel complex is significant in promoting photocatalytic performance, and the incorporation of NiO _x nanoparticles as a hole-transporting medium is beneficial for separation of the photogenerated charge carriers. The novel hybrid system offers a new horizon for designing transition-metal complex-modified graphitic CN as noble metal-free and highly active photocatalysts for efficient visible light-driven hydrogen generation.

Novel CoAl-LDH/g-C3N4/RGO ternary heterojunction with notable 2D/2D/2D configuration for highly efficient visible-light-induced photocatalytic elimination of dye and antibiotic pollutants

In this study, we fabricate a novel ternary heterojunction comprising CoAl-layered double hydroxide, g-C₃N₄, and reduced graphene oxide (LDH/CN/RGO) with a notable 2D/2D/2D configuration using a simple one-step hydrothermal method. The visible-light-induced LDH/CN/RGO ternary heterojunctions displayed significantly enhanced photocatalytic performance towards the degradation of aqueous Congo red (CR, dye) and tetracycline (TC, antibiotic) contaminants, which is far superior to that observed for pristine CN (base material), LDH, P25 (reference), and binary CN/RGO and LDH/CN heterojunctions. In particular, the LDH/CN/RGO ternary heterojunction with RGO and LDH contents of 1 wt.% and 15 wt.%, respectively, exhibited the highest degradation activity among all the fabricated catalysts, and it also displayed exceptional stability during recycling experiments. The significant enhancement in the photocatalytic performance and good stability of existing LDH/CN/RGO ternary heterojunctions were primarily attributed to the large intimate interfacial contact between constituent CN, LDH, and RGO prompted by their exceptional 2D/2D/2D arrangement, which accelerates the interfacial charge-transfer processes to effectively hinder the recombination of photoexcited charge carriers. The present study provides new insights into the rational design and fabrication of novel g-C₃N₄-based 2D/2D/2D layered ternary heterojunctions as high-performance photocatalysts, and promotes their application in addressing diverse energy and environmental issues.

Unraveling the impact of the Pd nanoparticle@BiVO4/S-CN heterostructure on the photo-physical & opto-electronic properties for enhanced catalytic activity in water splitting and one-pot three-step tandem reaction

Herein, a Pd nanoparticle-embedded SBVCN-37 heterostructure photocatalyst was synthesized and employed in the water-splitting reaction and for the synthesis of imines via a one-pot tandem reaction involving the photocatalytic reduction of nitrobenzene and oxidation of benzyl alcohol, followed by their condensation reaction. The embedded Pd nanoparticles (mean diameter ∼ 5-7 nm) act as an electron mediator and enhance the catalytic activity of SBVCN-37 during the oxidation and reduction reactions. The experimental results confirm that the light-induced holes owing to the favourable redox potential of the catalyst oxidize N2H4 to N2 and liberate H+ ions, which subsequently react with photogenerated electrons to facilitate the reduction of nitrobenzene. The obtained quantum yields for benzyl alcohol oxidation and nitrobenzene reduction were calculated to be 2.08% and 6.53% at λ = 420 nm light illumination, respectively. Furthermore, the obtained apparent quantum yields for the OER and HER were calculated to be 10.22% and 12.72% at 420 nm, respectively, indicating the excellent potential of the investigated photocatalyst for solar fuel production. Photoelectrochemical (PEC) and time-resolved and steady-state photoluminescence measurements reveal that the optimum amount of Pd nanoparticles over SBVCN-37 is the crucial factor for achieving the highest photocurrent response, lowest charge transfer resistance, and efficient carrier mobility, leading to prominent catalytic activity. Furthermore, the Mott-Schottky (M-S) analysis confirmed that the deposition of Pd nanoparticles effectively reduced the over-potential and fine-tuned the band edge potential required for the HER and OER reactions, respectively.

Three-Dimensional Carbon Nitride Nanowire Scaffold for Flexible Supercapacitors

Herein, a 3D composite electrode supported by g-C3N4 nanowire framework as scaffold and poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT: PSS) as conducting polymer is reported for flexible solid-state electrochemical capacitors. Compared to pure PEDOT: PSS, the composite electrodes have a greatly increased specific surface and showed good electrochemical performance. A specific capacitance of 202 F g-1 is achieved, and 83.5% of initial capacitance maintained after 5000 cycles. The device based on the 3D g-C3N4/PEDOT: PSS electrode also exhibits good performance in capacitance, flexibility, and cycling stability.

Sub-5 nm Ultra-Fine FeP Nanodots as Efficient Co-Catalysts Modified Porous g-C3N4 for Precious-Metal-Free Photocatalytic Hydrogen Evolution under Visible Light

Sub-5 nm ultra-fine iron phosphide (FeP) nano-dots-modified porous graphitic carbon nitride (g-C₃N₄) heterojunction nanostructures are successfully prepared through the gas-phase phosphorization of Fe₃O₄/g-C₃N₄ nanocomposites. The incorporation of zero-dimensional (0D) ultra-small FeP nanodots co-catalysts not only effectively facilitate charge separation but also serve as reaction active sites for hydrogen (H₂) evolution. Herein, the strongly coupled FeP/g-C₃N₄ hybrid systems are employed as precious-metal-free photocatalysts for H₂ production under visible-light irradiation. The optimized FeP/g-C₃N₄ sample displays a maximum H₂ evolution rate of 177.9 μmol h^-1 g^-1 with the apparent quantum yield of 1.57% at 420 nm. Furthermore, the mechanism of photocatalytic H₂ evolution using 0D/2D FeP/g-C₃N₄ heterojunction interfaces is systematically corroborated by steady-state photoluminescence (PL), time-resolved PL spectroscopy, and photoelectrochemical results. Additionally, an increased donor density in FeP/g-C₃N₄ is evidenced from the Mott-Schottky analysis in comparison with that of parent g-C₃N₄, signifying the enhancement of electrical conductivity and charge transport owing to the emerging role of FeP. The density functional theory calculations reveal that the FeP/g-C₃N₄ hybrids could act as a promising catalyst for the H₂ evolution reaction. Overall, this work not only paves a new path in the engineering of monodispersed FeP-decorated g-C₃N₄ 0D/2D robust nanoarchitectures but also elucidates potential insights for the utilization of noble-metal-free FeP nanodots as remarkable co-catalysts for superior photocatalytic H₂ evolution.

Enhanced Light-Driven Hydrogen-Production Activity Induced by Accelerated Interfacial Charge Transfer in Donor-Acceptor Conjugated Polymers/TiO2 Hybrid

Donor-acceptor (D-A) conjugated polymers have proved to be desired candidates to couple with inorganic semiconductors for enhanced photocatalytic activity. Herein, the matched energy levels between polymer BFB and TiO₂ make them form BFB-TiO₂ composites with moderate photocatalytic H₂ evolution rate (HER). To further enhance the interfacial interaction, BFB was modified with a carboxylic acid end group, which reacted with surface OH of TiO₂ to form an ester bond. As a result, the functionalized BFBA-TiO₂ composites exhibited superior photocatalytic activity. Especially, HER of 4 % BFBA-TiO₂ can reach up to 228.2 μmol h^-1 under visible light irradiation (λ>420 nm), which is about 2.02 times higher than that of BFB-TiO₂ . The enhanced photocatalytic activity originated from the formed ester bond between polymer and TiO₂ , and photogenerated electrons injection from lowest unoccupied molecular orbital (LUMO) of the exited polymer to conduction band of TiO₂ were accelerated. Therefore, based on an intermolecular interaction mechanism, more suitable D-A conjugated polymers with anchoring groups could be designed to couple with other semiconductors for enhancing photocatalytic activity.

Black Phosphorus, a Rising Star 2D Nanomaterial in the Post-Graphene Era: Synthesis, Properties, Modifications, and Photocatalysis Applications

Semiconductor photocatalysis, a sustainable and renewable technology, is deemed to be a new path to resolve environmental pollution and energy shortage. The development of effective photocatalysts, especially the metal-free photocatalysts, is a critical determinant of this technique. The recently emerged 2D material of black phosphorus with distinctive properties of tunable direct bandgap, ultrahigh charge mobility, fortified optical absorption, large specific surface area, and anisotropic structure has captured enormous attention since the first exfoliation of bulk black phosphorus into mono- or few layered phosphorene in 2014. In this article, the state-of-the-art preparation methods are first summarized for bulk black phosphorus, phosphorene, and black phosphorus quantum dot and then the fundamental structure and electronic and optical properties are analyzed to evaluate its feasibility as a metal-free photocatalyst. Various modifications on black phosphorus are also summarized to enhance its photocatalytic performance. Furthermore, the multifarious applications such as solar to energy conversion, organic removal, disinfection, nitrogen fixation, and photodynamic therapy are discussed and some of the future challenges and opportunities for black phosphorus research are proposed. This review reveals that the rising star of black phosphorus will be a multifunctional material in the postgraphene era.

Fabrication of interlayer β-CD/g-C3N4@MoS2 for highly enhanced photodegradation of glyphosate under simulated sunlight irradiation

Graphitic carbon nitride (g-C3N4) has been considered to be a promising metal-free photocatalyst, although the high recombination rate of charge carriers and poor absorption of visible light have limited its applications. In order to overcome these problems, an interlayer composite photocatalyst that comprised β-cyclodextrin (β-CD), oxygen-doped C3N4 (O-C3N4) and molybdenum disulfide (MoS2) was successfully constructed for the highly enhanced photodegradation of glyphosate in this study. The structure and morphology, optical properties, and photoelectrochemical properties of the prepared photocatalyst were characterized via a series of characterization techniques. The average fluorescence lifetime of the composite photocatalyst was extended from 6.67 ns to 7.30 ns in comparison with that of g-C3N4, which indicated that the composite photocatalyst enhanced the absorption of visible light and also inhibited the recombination of electron-hole pairs. The mass ratio of MoS2 that corresponded to O-C3N4/MoS2-5 enabled the highest removal rate under simulated sunlight irradiation, which was almost twice that achieved using pure g-C3N4. Relative species scavenging experiments revealed that ·O2 - was the main species during the process of photodegradation. Besides, a toxicity test indicated that glyphosate became less toxic or non-toxic after photodegradation. This study provided an effective, feasible and stable photocatalyst driven by simulated sunlight irradiation for the highly enhanced photodegradation of glyphosate.

Rational design of C2N-based type-II heterojunctions for overall photocatalytic water splitting

Photocatalytic water splitting is a promising method for the production of clean energy and searching for efficient photocatalysts has received extensive attention. Fabricating type-II heterojunctions is an effective approach to improve the photocatalytic efficiency. Based on the band edge positions and lattice parameters, we found that several kinds of monochalcogenide monolayers can be used to fabricate type-II heterojunctions with C2N monolayers. C2N/GaTe and C2N/InTe van der Waals (vdW) heterojunctions were investigated as potential photocatalysts for water splitting by means of first-principles computations. Both are type-II heterojunctions, and could promote the efficient spatial separation of electron-hole pairs. Their band edges straddle water redox potentials, satisfying the requirements for photocatalytic water splitting. Besides, the high carrier mobility of C2N/GaTe and C2N/InTe heterojunctions implies that the transfer of carriers to reactive sites is easy, and the recombination probability of photo-generated carriers is reduced. The Gibbs free energy calculations indicate that C2N/GaTe and C2N/InTe heterojunctions, especially C2N/InTe, exhibit high catalytic performance towards hydrogen and oxygen evolution reactions. Particularly, C2N/InTe exhibits a direct band gap with strong absorption in both visible and near ultraviolet regions, indicating that it is a very promising candidate for photocatalytic water splitting. This work would provide a new idea for the development of type-II heterojunctions for photocatalytic water splitting.

A Low-Cost Metal-Free Photocatalyst Based on Black Phosphorus

An efficient metal-free photocatalyst composed of black phosphorus (BP) and graphitic carbon nitride (CN) is prepared on a large scale by ball milling. Using economical urea and red phosphorus (RP) as the raw materials, the estimated materials cost of BP/CN is 0.235 Euro per gram. The BP/CN heterostructure shows efficient charge separation and possesses abundant active sites, giving rise to excellent photocatalytic H2 evolution and rhodamine B (RhB) degradation efficiency. Without using a co-catalyst, the metal-free BP/CN emits H2 consistently at a rate as large as 786 µmol h-1 g-1 and RhB is decomposed in merely 25 min during visible-light irradiation. The corresponding electron/hole transfer and catalytic mechanisms are analyzed and described. The efficient metal-free catalyst is promising in visible-light photocatalysis and the simple ball-milling synthetic method can be readily scaled up.

Self-assembly of graphene-encapsulated antimony sulfide nanocomposites for photoredox catalysis: boosting charge transfer via interface configuration modulation

Charge transfer over Sb₂S₃–GR nanocomposites was tuned by exquisite interface configuration engineering, which results in enhanced photoredox performances under visible light irradiation.

Construction of 2D/2D Ni2P/CdS heterojunctions with significantly enhanced photocatalytic H2 evolution performance

2D/2D Ni₂P/CdS heterojunctions are prepared by combining liquid exfoliation of CdS with a post-annealing procedure and employed as highly efficient photocatalysts for H₂ evolution.

Enhanced photocatalytic performance of black phosphorene by isoelectronic co-dopants

Isoelectronic co-dopants enhance the photocatalytic hydrogen production properties without affecting the band gap feature of pure black phosphorene.

Dual role of a g-C3N4/carbon intra-Schottky junction in charge carrier generation and separation for efficient solar H2 production

Intra-Schottky junction facilitated charge carrier generation and separation in g-C₃N₄ for efficient solar H₂ production.

Spatially separated cocatalysts for efficient charge separation: a hollow Pt/CdS/N–ZnO/CoOx graphene microtubule with high stability for photocatalytic reactions and sustainable recycling

The spatially separated Pt/CdS/N–ZnO/CoO_x graphene microtubule (PCNZCo-GM) with double cocatalysts is prepared by a capillary action assisted hydrothermal method for enhancing charge separation efficiency and photocatalytic oxidation ability.

Characterization of semiconductor photocatalysts

The long-standing popularity of semiconductor photocatalysts stimulated their characterization, which is the subject of this review aiming to help materials chemists and physicists, particularly students, to select suitable characterization methods.

Cross-linked bond accelerated interfacial charge transfer in monolayer zinc indium sulfide (ZnIn2S4)/reduced graphene oxide (RGO) heterostructure for photocatalytic hydrogen production with mechanistic insight

By separating the photo-excited charge carriers, the cross-linked bonds enabled the monolayer ZnIn₂S₄/RGO heterostructure to produce more H₂.

A hollow core–shell structure material NiCo2S4@Ni2P with uniform heterojunction for efficient photocatalytic H2 evolution reaction

Based on a bimetallic sulfide, a hollow core–shell structure material, NiCo₂S₄@Ni₂P, with a uniform type-I heterojunction achieved efficient photocatalytic H₂ evolution.

Graphene-Based Nanocomposites for Efficient Photocatalytic Hydrogen Evolution: Insight into the Interface toward Separation of Photogenerated Charges

Although the reduced graphene oxide (rGO) has been intensively applied for photocatalytic H₂ evolution, no enough attention was given to study the interface between the photocatalyst and rGO, which is the key point to affect the transportation of the photogenerated electron. Herein, in order to research the heterojunction interface, a series of SrTiO₃ photocatalysts with different crystal facets were fabricated to be loaded with the rGO for photocatalytic H₂ evolution. The characterizations and theory calculation verified that the rGO was mainly anchored on the Ti-O bond of the SrTiO₃ in the composite. Therefore, compared to the {001} facets sample, the {110} facets of the SrTiO₃, which exposed more Ti and O atoms, could form a stronger bond with the rGO. Additionally, the density functional theory study deduced that the photoinduced electron could immigrate rapidly from the Ti-O bond to the rGO in the composite, which was in good agreement with the results of photoelectrochemical and photoluminescence experiments. Meanwhile, experimentally, the 1% wt rGO@SrTiO₃ with {110} facets nanocomposite showed the superior photocatalytic H₂ yield rate (3.82 mmol/h/g), which was 2.2 times and 3.2 times higher than that of the pure SrTiO₃ with the same facets and 1% wt rGO@SrTiO₃ with {001} facets, respectively. Both experiments and theoretical calculations unveiled that the synergetic effect of SrTiO₃ facets engineering and the rGO loading effectively prompted the immigration of photoinduced electrons at the nanocomposite interface. This work provides a rational thinking of a high efficiency rGO-based heterogeneous photocatalysts for solar energy conversion.

Visible- and NIR-Light Responsive Black-Phosphorus-Based Nanostructures in Solar Fuel Production and Environmental Remediation

Direct utilization of the full spectrum of renewable solar light, in particular the visible- and near-infrared (NIR) portions, is currently receiving a great deal of attention in solar-to-chemical energy conversion-a clean, economically, and environmentally sustainable process. Black phosphorus (BP), a newly emerging class of ultrathin 2D nanomaterials rediscovered in early 2014, fulfills this purpose due to its unique properties like high charge-carrier mobility and tunable direct-bandgap. To this end, the rational combinations of BP in the form of few-layer nanosheets or ultrasmall quantum dots with a range of organic and inorganic nanomaterials offer more versatile and robust hybrids and nanocomposites that are functional in solar fuel production and environmental remediation. Herein, the most recent and key achievements of BP-based nanostructured photocatalysts in water splitting, organic pollutant degradation, and nitrogen fixation under either visible- or NIR-light illumination are summarized. Furthermore, perspectives on the potential future research directions are provided.

A Hollow Porous CdS Photocatalyst

Efficient light harvesting and charge separation are of great importance in solar-energy conversion on photocatalysts. Herein, the synthesis of a novel hollow porous CdS photocatalyst with effectively restrained electron-hole recombination is reported. By using microporous zeolites as a host and a hard template, ultrasmall Pd and PdS nanoparticles can be anchored separately onto the inner and outer surfaces of a hollow CdS structure. The metallic Pd pulls the photoexcited electrons away from CdS while PdS pushes the holes for more thorough oxidation of the sacrificial agent. The final Pd@CdS/PdS product exhibits superior visible-light-driven photocatalytic H₂ evolution rate of up to 144.8 mmol h^-1 g^-1 . This is among the highest values of all the reported CdS-based catalysts. This synthetic approach may be used to fabricate other highly efficient catalysts with spatially separated cocatalysts.

Unique physicochemical properties of two-dimensional light absorbers facilitating photocatalysis

The emergence of two-dimensional (2D) materials with a large lateral size and extremely small thickness has significantly changed the development of many research areas by producing a variety of unusual physicochemical properties.