3 D Co3 (PO4 )2 -Reduced Graphene Oxide Flowers for Photocatalytic Water Splitting: A Type II Staggered Heterojunction System

NiO/MnFe2O4 Nanocomposite Photoluminescence, Structural, Morphological, Magnetic, and Optical Properties: Photocatalytic Removal of Cresol Red under Visible Light Irradiation

In this study, pure nickel oxide (NiO), manganese ferrite (MnFe2O4 or MFO), and binary nickel oxide/manganese ferrite (NiO/MFO1-4) nanocomposites (NCs) were synthesized using the Sol-Gel method. A comprehensive investigation into their photoluminescence, structural, morphological, magnetic, optical, and photocatalytic properties was conducted. Raman analysis, UV-Vis spectroscopy, Fourier-transform infrared spectroscopy, scanning electron microscopy, and X-ray diffraction techniques were used to characterize the materials. The synthesized samples exhibited superparamagnetic behavior, as revealed by our analysis of their magnetic properties. A lower recombination rate was shown by the photoluminescence analysis, which is helpful for raising photocatalytic activity. The photocatalytic activity was evaluated for the degradation of Cresol Red (CR) dye. 91.6% of CR dye was degraded by NiO/MFO-4 nanocomposite, and the NC dosage as well as solution pH affected the photocatalytic performance significantly. In four sequential photocatalytic cycles, the magnetically separable NCs were stable and recyclable. The enhanced photocatalytic activity and magnetic separability revealed the potential application of NiO/MFO-4 as an efficient photocatalyst for the removal of dyes from industrial wastewater under solar light irradiation.

Carbon nitride quantum dots-modified cobalt phosphate for enhanced photocatalytic H2 evolution

In the present work, carbon nitride quantum dots (CNQDs)-modified cobalt phosphate (CoPi) composites CNQDs/CoPi-x (x = 1, 2, 3) were prepared at room temperature and characterized by FTIR, XRD, UV-Vis DRS, EIS, SEM, TEM/HR-TEM, XPS, and N2 gas adsorption. The morphologies and surface areas of CNQDs/CoPi-x have no remarkable change after modification of CNQDs, compared with pure CoPi. The obtained CNQDs/CoPi-x shows enhanced activity and stability of photocatalytic H2 evolution compared to pure CoPi using Eosin Y (EY) as a sensitizer and triethanolamine as an electron donor. The CNQDs/CoPi-2 possesses the highest hydrogen evolution rate, 234.5 μmol h-1  g-1 , upon visible light, which outshines that of CoPi by 2.4 times. It was believed that the enhanced photocatalytic performances of the CNQDs/CoPi-2 could result from the boosted electron transfer from radical EY·- to CNQDs/CoPi-2 by the employment of CNQDs; in addition, the visible-light activity of CNQDs contributes to hydrogen evolution. The mechanism of photocatalytic hydrogen production was discussed. This study may contribute toward the development of production of "green hydrogen" using solar.

Interfacial Chemical Bond Modulation of Co3(PO4)2-MoO3-x Heterostructures for Alkaline Water/Seawater Splitting

The development of a high current density with high energy conversion efficiency electrocatalyst is vital for large-scale industrial application of alkaline water splitting, particularly seawater splitting. Herein, we design a self-supporting Co₃(PO₄)₂-MoO_3-x/CoMoO₄/NF superaerophobic electrode with a three-dimensional structure for high-performance hydrogen evolution reaction (HER) by a reasonable devise of possible "Co-O-Mo hybridization" on the interface. The "Co-O-Mo hybridization" interfaces induce charge transfer and generation of fresh oxygen vacancy active sites. Consequently, the unique heterostructures greatly facilitate the dissociation process of H₂O molecules and enable efficient hydrogen spillover, leading to excellent HER performance with ultralow overpotentials (76 and 130 mV at 100 and 500 mA cm^-2) and long-term durability of 100 h in an alkaline electrolyte. Theoretical calculations reveal that the Co₃(PO₄)₂-MoO_3-x/CoMoO₄/NF promotes the adsorption/dissociation process of H₂O molecules to play a crucial role in improving the stability and activity of HER. Our results exhibit that the HER activity of non-noble metal electrocatalysts can be greatly enhanced by rational interfacial chemical bonding to modulate the heterostructures.

Access to carbon nanofiber composite hydrated cobalt phosphate nanostructure as an efficient catalyst for the hydrogen evolution reaction

Attractive technology for producing sustainable hydrogen with water electrolyzers was foreseen as one of the most promising ways to meet the increasing demands of renewable resources and electricity storage. Mainly used for the efficient generation of H₂, water electrolysis involving hydrogen evolution reactions (HERs) depends on efficient and affordable electrocatalysts. Hydrogen is an effective fuel that can be produced by splitting water. Hence, the search for highly efficient HER catalysts is a major challenge as efficient hydrogen evolution catalysts are sought to replace catalysts such as platinum. Here, we describe a low-cost and highly effective electrocatalyst for the proper incorporation of the HER electrocatalyst with low overpotential, effective charge transfer kinetics, low Tafel slope, and good durability. By using a simple hydrothermal approach to produce Co₃(PO₄)₂.8H₂O/CNF, it is possible to attach Co₃(PO₄)₂.8H₂O to the surface of carbon nanofibers (CNFs), which also exhibit remarkable HER activity at an overpotential of 133 mV and produce a current density of 10 mA/cm² and a 48 mV/decade for the Tafel slope. Large electrochemical surface areas and easy charge transfer from Co₃(PO₄)₂.8H₂O to the electrode through conductive Co₃(PO₄)₂.8H₂O/CNF composites are the reasons for the improved performance of Co₃(PO₄)₂.8H₂O/CNF.

Photocatalytic and photo-electrochemical ammonia synthesis over dimensional oriented cobalt titanate/nitrogen-doped reduced graphene oxide junction interface catalyst

Nitrogen reduction to ammonia is vital for chemical industries and renewable clean energy. Denying the harsh reaction conditions adopted in the Haber-Bosch process and stimulation research for ammonia production through sustainable technologies is a smart approach. Hitherto, photocatalyst acquiring the potential to attain high nitrogen reduction reaction (NRR) efficiency is a challenging task. Here, this study demonstrated cobalt titanate (CoTiO₃) rods (p-type) straddled with two-dimensional (2D) sheets of nitrogen-doped reduced graphene oxide (N-rGO, n-type) via, reflux method; realizing the advantages of dissimilar dimensionalities and strong interfacial junction coupling for efficient NRR under visible light irradiation. The successful interface junction establishment between CoTiO₃ and N-rGO has been witnessed from Raman, x-ray photoelectron spectroscopy (XPS), and Mott-Schottky analysis. Moreover, a well-defined type-II band structure is capable to curl the charge anti-recombination process; reflected in upgraded photo-catalytic/electrocatalytic upshots. The CoTiO₃ modified with an optimized concentration of N-rGO exhibits high stability with an improved photocatalytic (1722.22 μmolL^-1h^-1) and photo-electrocatalytic (16.8 µg cm^-1h^-1) nitrogen reduction to ammonia production; multiple times higher than counterparts. This improved photo-activity of CoTiO₃/N-rGO junction hybrid stems from the built-in electric field existing across the dissimilar junction interface, triggering charge transfer channels for reduction reaction in mild reaction conditions. The result of these materials might strategies the way for future development of new functionalities bearing highly active catalyst materials for sustainable energy-related conversion.

Dumbbell-Shaped Ternary Transition-Metal (Cu, Ni, Co) Phosphate Bundles: A Promising Catalyst for the Oxygen Evolution Reaction

Development of economical and high-performance electrocatalysts for the oxygen evolution reaction (OER) is of tremendous interest for future applications as sustainable energy materials. Here, a unique member of efficient OER electrocatalysts has been developed based upon structurally versatile dumbbell-shaped ternary transition-metal (Cu, Ni, Co) phosphates with a three-dimensional (3D) (Cu₂(OH)(PO₄)/Ni₃(PO₄)₂·8H₂O/Co₃(PO₄)₂·8H₂O) (CNCP) structure. This structure is prepared using a simple aqueous stepwise addition of metal ion source approach. Various structural investigations demonstrate highly crystalline nature of the composite structure. Apart from the unique structural aspect, it is important that the CNCP composite structure has proved to be an excellent electrocatalyst for OER performance in comparison with its binary or constituent phosphate under alkaline and neutral conditions. Notably, the CNCP electrocatalyst displays a much lower overpotential of 224 mV at a current density of 10 mA cm^-2 and a lower Tafel slope of 53 mV dec^-1 with high stability in alkaline medium. In addition, X-ray photoelectron spectroscopy analysis suggested that the activity and long-term durability for the OER of the ternary 3D metal phosphate are due to the presence of electrochemically dynamic constituents such as Ni and Co and their resulting synergistic effects, which was further supported by theoretical studies. Theoretical calculations also reveal that the incredible OER execution was ascribed to the electron redistribution set off in the presence of Ni and Cu and the most favorable interaction between the *OOH intermediate and the active sites of CNCP. This work may attract the attention of researchers to construct efficient 3D ternary metal phosphate catalysts for various applications in the field of electrochemistry.

Hierarchical Co3(PO4)2/CuI/g-CnH2n-2 S-Scheme Heterojunction for Efficient Photocatalytic Hydrogen Evolution

Graphdiyne (GD), a new type of carbon allotrope formed by sp and sp² hybrid carbon atoms, has attracted wide attention due to its high π-conjugation degree, special band structure, and uniformly distributed pores. In traditional synthesis methods, hexaethylbenzene was coupled on the substrate catalytic material (copper foil or foamed copper) to generate graphdiyne. In this work, CuI was used as the substrate catalytic material, and the CuI-GD composite was synthesized by cross-coupling in the pyridine solution of hexaethylbenzene. For the first time, Co₃(PO₄)₂ was modified by the CuI-GD composite material to prepare a Co₃(PO₄)₂/CuI-GD S-scheme heterojunction catalyst, which avoided the complicated process of removing the substrate catalytic material. Under the action of the internal electric field, electrons are induced to move quickly and directionally, and the powerful photogenerated electrons in the conduction band (CB) of GD and the holes in the valence band (VB) of CuI are retained to participate in the photocatalytic reaction. These advantages were combined with the high-energy acetylene bond in GD, which accelerated the catalytic reaction of the Co₃(PO₄)₂/CuI-GD heterostructure. Electrochemical and fluorescence analysis showed that Co₃(PO₄)₂/CuI-GD has faster electron and hole separation efficiency, lower hydrogen evolution overpotential, and higher carrier utilization. Therefore, Co₃(PO₄)₂/CuI-GD exhibited good hydrogen evolution activity. This work shows that GD has broad prospects in designing high-performance photocatalyst systems.

One-step preparation of cobalt phosphate at room temperature for effective photocatalytic H2 evolution

Cobalt phosphate materials were prepared in the present work in one step at room temperature using different raw materials and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and N₂ gas adsorption. Cobalt phosphates exhibit 3D flower-like structures, and the assembly of nanosheets (petals of the "flowers") of cobalt phosphate prepared with sodium phosphate and cobalt acetate as raw materials (denoted as Co-P(A)) is more incompact than that of cobalt phosphate prepared with diammonium hydrogen phosphate and cobalt nitrate as raw materials (denoted as Co-P(B)) due to the former's mildly basic environment. The cobalt phosphates show relatively high photocatalytic activity for H₂ evolution reaction (HER) in the presence of Eosin Y as a sensitizer in an aqueous triethanolamine solution. The activity of Co-P(A) (0.40 mmol h^-1 g^-1) exceeds that of Co-P(B) (0.19 mmol h^-1 g^-1), which can be attributed to a more dispersive nanosheet and larger BET-specific surface area of Co-P(A). The mechanisms of photocatalytic HER and the formation of flower-like Co₃(PO₄)₂ were discussed. The present system comprising of only abundant elements contributes toward the development of cost-efficient solar HER to achieve sustainable development.

Synthesis and enhanced photocatalytic activity of the flower-like CdS/Zn3(PO4)2 Z-scheme heteronanostructures

CdS/Zn3(PO4)2 Z-scheme heteronanostructures were prepared through a simple hydrothermal route and precipitation methods, and the efficiency for the photocatalytic degradation of MB solution can be improved greatly.

Nanostructure-assisted charge transfer in α-Fe2O3/g-C3N4 heterojunctions for efficient and highly stable photoelectrochemical water splitting

The development of semiconductor heterojunctions is a promising and yet challenging strategy to boost the performance in photoelectrochemical (PEC) water splitting. This paper describes the fabrication of a heterojunction photoanode by coupling α-Fe₂O₃ and g-C₃N₄via aerosol-assisted chemical vapour deposition (AACVD) followed by spin coating and air annealing. Enhanced PEC performance and stability are observed for the α-Fe₂O₃/g-C₃N₄ heterojunction photoanode in comparison to pristine α-Fe₂O₃ and the reason is systematically discussed in this paper. Most importantly, the fabricated α-Fe₂O₃/g-C₃N₄ film shows impressive stability, retaining more than 90% of the initial current over 12 h operating time. The excellent stability of the heterojunction photoanode is achieved due to the unique nanoflake structure of α-Fe₂O₃ induced by AACVD. This nanostructure promotes good adhesion with the g-C₃N₄ particles, as the particles tend to be trapped within the α-Fe₂O₃ valleys and eventually create strong and large interfacial contacts. This leads to improved separation of charge carriers at the α-Fe₂O₃/g-C₃N₄ interface and suppression of charge recombination in the photoanode, which are confirmed by the transient decay time, charge transfer efficiency and electrochemical impedance analysis. Our findings demonstrate the importance of nanostructure engineering for developing heterojunction structures with efficient charge transfer dynamics.

Fabrication of ternary Ag3PO4/Co3(PO4)2/g-C3N4 heterostructure with following Type II and Z-Scheme dual pathways for enhanced visible-light photocatalytic activity

A novel ternary Ag₃PO₄/Co₃(PO₄)₂/g-C₃N₄ (APO/CPO/CN) heterostructure photocatalyst was successfully synthesized via a simple precipitation method for photocatalytic degradation of tetracycline (TC) under visible light irradiation. The experimental result reveals that the ternary APO/CPO/CN heterojunction showed enhanced photocatalytic performance compared with single semiconductor CPO and CN, binary composite CPO/CN. And APO/CPO/CN-15 % composite exhibits highest photocatalytic degradation efficiency, which can degrade TC around 88 % under visible light within 120 min. The enhanced photocatalytic performance is due to the synergy effects between CPO, CN and APO with the aid of following Z-scheme and Type II heterojunction dual pathways for effective separation of photogenerated charges. This work provides a new approach in the rational design of ternary heterojunction photocatalyst with multilevel electron transfer for environmental decontamination.

An energy band compactable B-rGO/PbTiO3 p-n junction: a highly dynamic and durable photocatalyst for enhanced photocatalytic H2 evolution

Reduced graphene oxide (rGO) intentionally doped with boron atoms is a promising tactic to extract bandgap energy and p-type semiconducting behavior from graphene-based materials. Moreover, the integration of p-type boron-doped rGO with an n-type material through a heterojunction interface exhibits complementary properties to restrict the fast recombination of charge carriers and enhance the photoreaction towards energy applications. Herein, we have prepared boron-doped rGO/PbTiO3 p-n heterojunctions via a hydrothermal method. The successful formation of an excellent p-n heterojunction was demonstrated by TEM, XPS and Raman analysis. The constructed boron-doped rGO/PbTiO3 p-n heterojunctions exhibit dramatic increases in photoelectrochemical and photocatalytic performance due to the presence of a space charge region at the interface of the two materials. The scenario also revealed the double-edge sword functions of B-rGO: the material itself (i) acts as a visible light active photocatalyst with a band gap energy of 2.7 eV and (ii) participates in enhanced charge transfer via the band edge alignment between B-rGO and PbTiO3, as elucidated from photoluminescence and electrochemical impedance studies. Furthermore, the optimal 2B-rGO/PT p-n heterojunction possesses outstanding repeatability and exhibited the highest rate of hydrogen evolution, i.e. 293.79 μmol h-1 under visible light irradiation. The results for these materials may provide advanced insight into the design of next-generation high-efficiency black graphene-based heterojunctions.

Confined growth of Co–Pi co-catalyst by organic semiconductor polymer for boosting the photoelectrochemical performance of BiVO4

The significant recombination of carriers and low OER kinetics depress the solar to chemical energy conversion efficiency over BiVO₄.

Hierarchically Structured FeNiOx Hy Electrocatalyst Formed by In Situ Transformation of Metal Phosphate for Efficient Oxygen Evolution Reaction

A simple and low-cost fabrication method is needed to obtain effective and robust heterogeneous catalysts for the oxygen evolution reaction (OER). In this study, an electrocatalyst FeNiO_x H_y with a hierarchical structure is synthesized on nickel foam by a simple fabrication method through anion exchange from a metal phosphate to a metal hydroxide. The as-fabricated FeNiO_x H_y electrode requires overpotentials of 206 and 234 mV to deliver current densities of 10 and 50 mA cm^-2 , respectively. The catalytic performance of FeNiO_x H_y is superior to that of most previously reported FeNi-based catalysts, including NiFe layered double hydroxide. The catalyst also shows good long-term durability at a current density of 50 mA cm^-2 over 50 h with no activity decay under 1 m KOH. By comparison to the directly electrodeposited FeNi hydroxide in morphology and electrochemical properties, the improved activity of the catalyst could be mainly attributed to an enhancement of its intrinsic activity, which was caused by the anion exchange of phosphate to (oxy)hydroxide. Further studies by cyclic voltammetry indicated a stronger interaction between Ni and Fe from the negative shift of the oxidation peak of Ni²⁺ /Ni³⁺ in comparison with reported FeNiO_x H_y , which promoted the generation of active Ni³⁺ species more easily. This work may provide a new approach to the simple preparation of effective and robust OER catalysts by anion exchange.

Repercussion of Solid state vs. Liquid state synthesized p-n heterojunction RGO-copper phosphate on proton reduction potential in water

The same copper phosphate catalysts were synthesized by obtaining the methods involving solid state as well as liquid state reactions in this work. And then the optimised p-n hybrid junction photocatalysts have been synthesized following the same solid/liquid reaction pathways. The synthesized copper phosphate photocatalyst has unique rod, flower, caramel-treat-like morphology. The Mott-Schottky behavior is in accordance with the expected behavior of n-type semiconductor and the carrier concentration was calculated using the M-S analysis for the photocatalyst. And for the p-n hybrid junction of 8RGO-Cu₃(PO₄)₂-PA (PA abbreviated for photoassisted synthesis method), 8RGO-Cu₃(PO₄)₂-EG(EG abbreviated for Ethylene Glycol based synthesis method), 8RGO-Cu₃(PO₄)₂-PEG (PEG abbreviated for Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol based synthesis method)the amount of H₂ synthesized was 7500, 6500 and 4500 µmol/h/g, respectively. The excited electrons resulting after the irradiation of visible light on the CB of p-type reduced graphene oxide (RGO) migrate easily to n-type Cu₃(PO₄)₂ via. the p-n junction interfaces and hence great charge carrier separation was achieved.

Advanced bi-functional CoPi co-catalyst-decorated g-C3N4 nanosheets coupled with ZnO nanorod arrays as integrated photoanodes

In this work, a CoPi-decorated type II heterojunction composed of one-dimensional (1D) ZnO nanorod arrays (NRAs) coated with two-dimensional (2D) carbon nitride (g-C₃N₄) was successfully prepared and used as photoanode.

Z-Scheme in a Co3(PO4)2/α-Fe2O3 photocatalysis system for overall water splitting under visible light

A Z-scheme Co₃(PO₄)₂/α-Fe₂O₃ structure was designed as an efficient photocatalyst for overall water splitting under visible light.

Construction of a composite photocatalyst with significantly enhanced photocatalytic performance through combination of homo-junction with hetero-junction

BiOCl modified twinned BiPO₄ exhibited significantly enhanced photocatalytic activity because of the synergistic effect of homo-junction and hetero-junction.

Graphene-Supported Pyrene-Modified Cobalt Corrole with Axial Triphenylphosphine for Enhanced Hydrogen Evolution in pH 0-14 Aqueous Solutions

A cobalt complex of 5,15-bis(pentafluorophenyl)-10-(4)-(1-pyrenyl)phenyl corrole that contains a triphenylphosphine axial ligand (1-PPh₃ ) was synthesized and examined as an electrocatalyst for the hydrogen evolution reaction (HER). If supported on graphene (G), the resulting 1-PPh₃ /G material can catalyze the HER in aqueous solutions over a wide pH range of 0-14 with a high efficiency and durability. The significantly enhanced activity of 1-PPh₃ /G, compared with that of its analogues 1-py/G (the Co-bound axial ligand is pyridine instead of triphenylphosphine) and 2-py/G (Co complex of 5,10,15-tris(pentafluorophenyl)corrole), highlights the effects of the pyrenyl substituent and the triphenylphosphine axial ligand on the HER activity. On one hand, the pyrenyl moiety can increase the π-π interactions between 1 and graphene and thus lead to a fast electron transfer from the electrode to 1. On the other hand, the triphenylphosphine axial ligand can increase the electron density (basicity) of Co and thus make the metal center more reactive to protons at the trans position through a so-called "push effect". This study concerns a significant example that shows the trans effect of the axial ligand on the HER, which has been investigated rarely. The combination of various ligand-design strategies in one molecule has been realized in 1-PPh₃ to achieve a high catalytic HER performance. These factors are valuable to be used in other molecular catalyst systems.

Mesoporous Nanosheet Networked Hybrids of Cobalt Oxide and Cobalt Phosphate for Efficient Electrochemical and Photoelectrochemical Oxygen Evolution

A novel mesoporous nanosheet networked hybrid comprising Co₃ O₄ and Co₃ (PO₄ )₂ is successfully synthesized using a facile and scalable method through calcinating the carbon, cobalt hydroxy carbonate, and cobalt phosphate composite precursor. Electron transfer from Co₃ O₄ to Co₃ (PO₄ )₂ , together with the special networked structure and the porous nature of the nanosheets enable the Co₃ (PO₄ )₂ -Co₃ O₄ hybrid to have a high oxygen evolution reaction (OER) activity and outstanding stability in alkaline electrolyte, e.g., an overpotential of 270 mV at current density of 10 mA cm^-2 , and a Tafel slope of 39 mV dec^-1 , which are superior to most non-noble metal-based OER electrocatalysts reported thus far and as well the commercial RuO₂ electrocatalyst. Furthermore, Co₃ (PO₄ )₂ -Co₃ O₄ hybrid is demonstrated to be used as an efficient cocatalyst to enhance the photoelectrochemical OER performance of BiVO₄ photoanode. A significantly increased photocurrent density of 3.0 mA cm^-2 at 1.23 V (vs reversible hydrogen electrode, RHE), and a potential reduction of 530 mV with respect to that of bare BiVO₄ at the photocurrent density of 0.5 mA cm^-2 are achieved. The electron transfer-induced enhancement of OER by a hybrid structure may pave the new routes for the design and synthesis of low-cost catalysts for electrochemical and photoelectrochemical oxygen evolution.