Exploiting the advantage of a layered architecture, layered graphitic carbon nitride (CN) and NiFe-layered double hydroxide (LDH) have been coupled in the present investigation to design a series of highly efficient novel CNLDH composites for visible light-induced photocatalytic H 2 and O 2 evolution. The syntheses of these composites were carried out using a facile weight impregnation method while varying the wt% of CN on LDH. The structural, optical, and morphological properties of these composites were characterized by various physicochemical techniques. The results indicate a tuned-in band gap energy within the range of pure LDH to pure CN. In addition, the remarkable quenching of the PL signal and prolonged photogenerated charge lifetime confirmed by TRPL spectra demonstrates the excellent photocatalytic activity of these composites. The activity could be ascribed to the dispersion of exfoliated CN over the brucite layer of LDH, in which strong energy transfer takes place in terms of charge carriers.The visible light-induced photocatalytic H 2 and O 2 evolution study resulted in an enhancement in the activity of the CNLDH10 composite with a H 2 evolution rate of 1488 mmol 2 h À1 and O 2 evolution rate of 886 mmol 2 h À1 . The high photocatalytic activities of these composites may be due to good dispersion of exfoliated CN over the brucite layer of edge-shared MO 6 octahedra, higher life time of charge carriers, low PL intensity, appropriate band gap energy and enhancement in photocurrent density. of electrons and holes. 5 The common layered structures of ZnCr-LDH and layered titanate enable an effective physical contact and a strong electronic coupling between them, which is effective in enhancing the photocatalytic activity of ZnCr-LDH. This motivated us to take the challenge of designing composite photocatalysts by coupling LDH with other layered semiconductor materials, like metal-free polymeric graphitic carbon nitride (g-C 3 N 4 ), which has an appropriate band gap of 2.7 eV and optical absorption in the visible region. 18 The g-C 3 N 4 material also possesses very high thermal, mechanical and chemical stability. Many studies have been focused on the applications of g-C 3 N 4 materials in photocatalytic water splitting together with degradation of organic pollutants. [19][20][21][22] However, recent studies have indicated that these materials exhibit photocatalytic activities for both H 2 and O 2 evolution under visible light irradiation in the presence of a sacricial donor and acceptor. 23 Several composite materials coupled with g-C 3 N 4 have also been reported, such as g-

In this work, a series of heterostructure Ag@Ag 3 PO 4 /g-C 3 N 4 /NiFe layered double hydroxide (LDH) nanocomposites were prepared by a combination of an electrostatic self-assembly and in situ photoreduction method. In this method, positively charged p-type Ag 3 PO 4 was electrostatically bonded to the self-assembled negatively charged surface of the n–n-type g-C 3 N 4 /NiFe (CNLDH) LDH hybrid material with partial reduction of Ag + to metallic Ag nanoparticles (NPs) by the photogenerated electrons and available surface −OH groups of LDH under visible light irradiation. The presence of Ag 3 PO 4 as a p-type semiconductor, the surface plasmon resonance (SPR) effect of metallic Ag NPs, and oxygen vacancies as O v -type defects in NiFe LDH could greatly achieve the quasi-type-II p–n/n–n dual heterojunctions, which was revealed by the shifted conduction band and valence band potentials in Mott–Schottky (M–S) analysis. Among all the optimized heterostructures, CNLDHAgP4 could achieve the highest photocatalytic Cr(VI) reduction rate of 97% and phenol oxidation rate of 90% in 2 h. The heterostructure CNLDHAgP4 photocatalyst possesses a unique morphology consisting of cubic phases of both Ag NPs and Ag 3 PO 4 , which adhered to the thin and curvy layers of the CNLDH hybrid for smooth electronic and ionic charge transport. Furthermore, the intimate Schottky barriers formed at the interface of quasi-type-II p–n/n–n dual heterojunctions were verified by the photoluminescence, linear sweep voltammetry, M–S, electrochemical impedance study, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy studies. The SPR effect of Ag NPs and oxygen vacancies as O v -type defect in NiFe LDH can effectively accelerate the threshold of charge separation and be the main reason for the enhanced activity achieved by the as-fabricated heterostructure photocatalyst.

Designing of an efficient heterostructure photocatalyst for photocatalytic organic pollutant removal and H 2 production has been a subject of rigorous research intended to solve the related environmental aggravation and enormous energy crises. Z-scheme-based charge-transfer dynamics in a p−n heterostructure could significantly replicate the inherent power of natural photosynthesis, which is the key point to affect the transportation of photoinduced exciton pairs. In this finding, a series of p-type MoS 2 loaded with n-type NiFe-layered double hydroxide (LDH) forming a heterostructure MoS 2 /NiFe LDH were designed by electrostatic selfassembled chemistry and an in situ hydrothermal strategy for photocatalytic rhodamine B (RhB) dye degradation and H 2 production. The creation of p−n heterojunctions of type-II and Z-scheme mode of charge transfer modified the optical and electronic property of the as-synthesized MSLDH3, thereafter promoting the generation, separation, and migration of photoinduced electron−hole pairs. The as-synthesized MSLDH3 showed superior photocatalytic activities in degradation of RhB with H 2 evolution, which was enhanced by 3-and 4.5-fold and 10.9 and 19.2 times higher than that of NiFe LDH and MoS 2 , respectively. Last but not the least, heterostructure MSLDH3 possesses practical stability for its resultant enhanced photocatalytic activity with recyclability for everyday life.

A series of heterostructure NiFe LDH/N-rGO/g-C 3 N 4 nanocomposite were fabricated by combining calcinations-electrostatic self-assembly and hydrothermal steps. In this method, negatively charged N-rGO was electrostaticaly bonded to the self-assembled interface of n-n type g-C 3 N 4 /NiFe LDH hybrid. XRD and AFM results revealed successful formation of heterostructure nanocomposite due to the coupling effect of exfoliated NiFe LDH nanosheets with N-rGO and g-C 3 N 4 . Among the as synthesized heterostructure, CNNG3LDH performed superior photocatalytic activities towards 95 and 72% mineralization of RhB and phenol. Furthermore, CNNG3LDH could achieve the highest photocatalytic H 2 evolution rate of 2508 μmolg −1 2h −1 and O 2 evolution rate of 1280 μmolg −1 2h −1 under visible light irradiation. The CNNG3LDH possess lowest PL intensity, reduced arc of the Nyquist plot (43.8 Ώ) and highest photocurrent density (−0.97 mA cm −2 ) which revealed effective charge separation for superior photocatalytic activities. TRPL spectral results reveal the synergistic effect of layered component in CNNG3LDH for achievable higher life time of excitons of ~16.52 ns. In addition, N-rGO mediator based Z-scheme charge transfer mechanisms in CNNG3LDH were verified by the ESR and TA-PL studies. Enriched oxygen vacancy type defects in NiFe LDH and N-rGO mediated Z-scheme charge transfer mechanistic path strongly manifest the superior photocatalytic activities of the heterostructure materials.

Photocatalytic generation of H and O by water splitting remains a great challenge for clean and sustainable energy. Taking into the consideration promising heterojunction photocatalysts, analogous energy issues have been mitigated to a meaningful extent. Herein, we have architectured a highly efficient bifunctional heterojunction material, i.e., p-type Co(OH) platelets with an n-type ZnCr layered double hydroxide (LDH) by an ultrasonication method. Primarily, the Mott-Schottky measurements confirmed the n- and p-type semiconductive properties of LDH and CH material, respectively, with the construction of a p-n heterojunction. The high resolution transmission electron microscopy results suggest that surface modification of ZnCr LDH by Co(OH) hexagonal platelets could form a fabulous p-n interfacial region that significantly decreases the energy barrier for O and H production by effectively separating and transporting photoinduced charge carriers, leading to enhanced photoreactivity. A deep investigation into the mechanism shows that a 30 wt % Co(OH)-modified ZnCr LDH sample liberates maximum H and O production in 2 h, i.e., 1115 and 560 μmol, with apparent conversion efficiencies of H and O evolution of 13.12% and 6.25%, respectively. Remarkable photocatalytic activity with energetic charge pair transfer capability was illustrated by electrochemical impedance spectroscopy, linear sweep voltammetry, and photoluminescence spectra. The present study clearly suggests that low-cost Co(OH) platelets are the most crucial semiconductors to provide a new p-n heterojunction photocatalyst for photocatalytic H and O production on the platform of ZnCr LDH.

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The hybrid structure of nanoparticles (NPs) with nanosheets has the advantage of both anisotropic properties of NPs and large specific surface areas of nanosheets, which is desirable for many technological applications. In this study, MgCrO spinel NPs decorated on highly porous MgO nanosheets forming MgO/MgCr O( x) nanocomposites were synthesized by a one pot coprecipitation method followed by a heat treatment process of the solvated wet gel of MgCr-LDH with polar solvent N, N-dimethylformamide (DMF) at 400 °C. This novel synthetic methodology generates materials consisting of porous metal oxides nanosheets adhered with spinel phase NPs due to the slow generation of gases such as HO, CO, and NH under moderate temperature during the heat treatment process. The synergistic effect of much wider band gap MgO nanosheets and narrow band gap MgCrO NPs added increased stability due to the stronger bonding coordination of MgCrO NPs with MgO nanosheets. The obtained MgO/MgCr O( x) nanocomposites possess large specific surface areas, highly porous structure, and excellent interface between MgCrO NPs and MgO nanosheets, which proved from N sorption isotherm, TEM, HR-TEM study. With metallic ratio of MgCr3:1, MgO/MgCrO(MgCr3:1) nanocomposites exhibit highest H evolution rate of 840 μmolg2h, which was 2 times higher than that of pure MgCrO(420 μmolg2h). The LSV measurement study of MgO/MgCrO (MgCr3:1) nanocomposite shows an enhancement of light current density of 0.22 μA/cm at potential bias of -1.1 V. The Mott-Schottky analysis suggested the band edge positions of the n-type constituents and formation of n-n type heterojunctions in MgO/MgCrO (MgCr3:1) nanocomposite, which facilitates the flow of charge carriers. The EIS and Bode phase plot of MgO/MgCrO (MgCr3:1) nanocomposite signifies the lower interfacial charge transfer resistance and higher lifetime of electrons (2.7 ms) for enhanced H production. Lastly, the enhanced photocatalytic H production activity and long-term stability of MgO/MgCrO(MgCr3:1) could be attributed to maximum specific surface area, porous structure, close intimacy contact angle between two cubic phases of MgCrO NPs and MgO nanosheets, abundant oxygen vacancies sites, reduced charge transfer resistance and suitable band edge potential to drive the thermodynamic energy for H production. This work highlighted an effective strategy for the synthesis of cost-effective 2D porous heterojunctions nanocomposite photocatalyst for promising applications in the field of clean H production utilizing abundant solar energy.

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