As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The constructi...

Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?

A review on g-C3N4-based photocatalysts

Engineering photocatalytic materials for renewable energy generation and environmental decontamination has always been a very exciting prospect to counter the global energy demands and pollution challenges. Graphitic carbon nitride (g-C 3 N 4 ), a polymeric, metal-free semiconductor with a mild band gap (2.7 eV) has become hot-spot in various scientific exploits such as environmental pollution mitigation, energy generation and storage, organic synthesis, sensors, etc. These applications exploit the interesting properties of g-C 3 N 4 such as good visible light absorption, graphene-like structure, good thermal and chemical stability and photocatalytic properties. In this review we begin with an overview of the fundamental aspects of photocatalysis as a pollution remediation strategy. This is followed by an introduction to graphitic carbon nitride as a photocatalyst, preparation strategies and its properties. Subsequently, a comprehensive and critical discussion of the various most recent developments towards enhancing the visible light photocatalytic properties of g-C 3 N 4 for pollution alleviation, selected results and important photocatalytic degradation mechanisms, is given. Summary remarks and future perspective conclude the review.

Graphitic carbon nitride (g-C3N4) nanocomposites: A new and exciting generation of visible light driven photocatalysts for environmental pollution remediation

Biologically-formed hard tissues, referred to as biominerals, have intrigued the materials engineering community for years because of the high degree of crystallographic control that is exerted during the precipitation of the bioinorganic crystals. In recent years, there has been a shift in attention, from prior studies that focused on specific organic-inorganic interactions that modulate the crystal morphology via the conventional crystallization pathway, to recent studies that find that many biominerals are formed via an amorphous precursor pathway. It has become clear that the things we thought we had learned about biominerals before, may or may not be relevant to truly understanding the mechanisms involved in biomineralization. Having witnessed this paradigm shift first hand, I am inclined to provide a review from this historical perspective, where I hope to belay some ideas about where we were, where we are, and where we are going, with respect to understanding how these shells and other biominerals are formed. Therefore, one goal of this review is to try and provide a link between the prior literature and the new literature, which might be useful to newcomers in the field, whom I suspect may find it confusing and difficult to integrate the findings in these different types of studies across this time period. A second goal is to try and integrate some of the knowledge obtained from in vitro model systems, which can be more amenable to obtaining mechanistic information, with the in vivo and ex vivo observational studies on biominerals. A third goal is to demonstrate that there may be certain unifying principles found in biomineral systems that seem widely diverse, such as from diatoms, to mollusk shells, to vertebrate bones and teeth. A final goal (the not so hidden agenda), is to demonstrate that not only is there as strong likelihood that many biominerals are formed by an amorphous precursor, but that the amorphous phase may possess fluidic properties that impart new processing capabilities to the system. Of course those who know my work will readily assess that I am referring to the polymer-induced liquid-precursor (PILP) process, which has been a primary focus in my lab. Along these lines, some new hypotheses are presented regarding the morphogenesis of certain biominerals, such as mollusk nacre, kidney stones, and bones and teeth, along with a review of the literature that provides support to these new ideas. The intent is to stimulate thoughtful discussions in this rapidly emerging area, which seemingly provides a unifying principle in biomineralization.

1.1. Biomineral Overview
Biominerals that have intrigued the materials engineer for years. The diversity of biominerals became particularly evident by the variety of books on biominerals that emerged in the late 80’s to 90’s,1–11 which has continued with a few more recent additions to this nice collection.12–16 Many people are probably not even aware of the diversity of biologically formed minerals, so a sampling is provided in Table 1 to illustrate some of the different chemistries that evolved in the formation of various biological hard tissues. The calcium carbonate (CaCO3) biominerals of invertebrates, such as mollusk shells and sea urchin spines, have been particularly well studied due to their accessibility, and because of the high degree of crystallographic control that is achieved in these biologically formed crystals. The calcium phosphate (CaP) biominerals of the vertebrates, such as bones and teeth, have also been extensively investigated because of their remarkable structure and mechanical properties, and for the more obvious reasons of health related issues.14 Calcium oxalate (CaOx) biominerals can be found in plants, but the majority of studies have focused on the pathological form of CaOx that is found in kidney stones, which often occurs in conjunction with CaP deposits.8,14

Table 1

Examples of the diversity of biominerals. This list is by no means comprehensive, where some 70 different mineral phases have been identified to date.1–11

Biomimetic Model Systems for Investigating the Amorphous Precursor Pathway and Its Role in Biomineralization

2D graphitic carbon nitride (GCN) nanosheets have attracted tremendous attention in photocatalysis due to their many intriguing properties. However, the photocatalytic performance of GCN nanosheets is still restricted by the limited active sites and the serious aggregation during the photocatalytic process. Herein, a simple approach to produce holey GCN (HGCN) nanosheets with abundant in-plane holes by thermally treating bulk GCN (BGCN) under an NH3 atmosphere is reported. These formed in-plane holes not only endow GCN nanosheets with more exposed active edges and cross-plane diffusion channels that greatly speed up mass and photogenerated charge transfer, but also provide numerous boundaries and thus decrease the aggregation. Compared to BGCN, the resultant HGCN has a much higher specific surface area of 196 m2 g−1, together with an enlarged bandgap of 2.95 eV. In addition, the HGCN is demonstrated to be self-modified with carbon vacancies that make HGCN show much broader light absorption extending to the near-infrared region, a higher donor density, and remarkably longer lifetime of charge carriers. As such, HGCN has a much higher photocatalytic hydrogen production rate of nearly 20 times the rate of BGCN.

Holey Graphitic Carbon Nitride Nanosheets with Carbon Vacancies for Highly Improved Photocatalytic Hydrogen Production

Heterojunctions of graphitic carbon nitride (g-C3N4) and Bi2MoO6 were solvothermally synthesized and characterized by X-ray diffraction, Fourier transform-infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy (TEM) and high resolution TEM. The photocatalytic degradation activity of the g-C3N4/Bi2MoO6 composites for Rhodamine B was examined under visible light irradiation. The heterojunction composites exhibited higher photocatalytic activity than pure g-C3N4 or Bi2MoO6. The photocatalytic activity of the composites increased then decreased with increasing Bi2MoO6 content. The g-C3N4/Bi2MoO6 heterojunction with a Bi2MoO6 content of 16.1 wt.% exhibited the highest photocatalytic activity, and its photocatalytic efficiency was more than three times those of pure g-C3N4 or Bi2MoO6. The enhanced photocatalytic activity of the g-C3N4/Bi2MoO6 heterostructure photocatalyst was attributed predominantly to the efficient separation of photoinduced electrons and holes. The g-C3N4/Bi2MoO6 heterojunction photocatalyst exhibited excellent stability and reusability. A detailed mechanism for the enhanced photocatalytic activity is discussed. Superoxide radicals were the major active species. This study provides a visible light responsive photocatalyst with potential in environmental remediation applications.

Synthesis and characterization of g-C3N4/Bi2MoO6 heterojunctions with enhanced visible light photocatalytic activity

Magnetic composites consisting of magnetite (Fe3O4), graphene oxide (GO), and Mg3Al–OH layered double hydroxide (LDH), denoted as MGL composites, with varying GO contents (RGO) were synthesized by a mechano-hydrothermal (MHT) route using Fe3O4, Mg(OH)2, and Al(OH)3 as the inorganic starting materials. The application of the synthesized MGLs for removing the heavy-metal Pb(II) and the hydrophobic organic pesticide 2,4-dichlorophenoxyacetic acid (2,4-D) from aqueous solutions was investigated. Chemical bonding among the GO, Fe3O4, and LDH components was observed in the MGLs. The MGL composites showed good water-dispersity, strong magnetic response, and high sorption capacities and removal efficiencies for both Pb(II) and 2,4-D pollutants. The sorption capacities of the MGL for the pollutants significantly increased with an increase in RGO. Increasing pH could increase the removal efficiency for Pb(II) but decrease that for 2,4-D. The MGLs showed more affinity for Pb(II) than for 2,4-D in the competitive sor...

Synthesis of magnetite-graphene oxide-layered double hydroxide composites and applications for the removal of Pb(II) and 2,4-dichlorophenoxyacetic acid from aqueous solutions

The development of highly active and stable non-noble metal electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) bifunctional performances is crucial for efficient water splitting. In this work, such electrocatalysts are obtained, first in two-step hydrothermal reactions to form Ni3S2/MnS nanosheets on Ni foam (NF/Ni3S2/MnS), then by slightly electroxidizing the Ni3S2/MnS to generate tremella-like Ni3S2/MnS-O with ultrathin nanosheets and abundant oxygen vacancies (denoted as NF/T(Ni3S2/MnS-O)) favorable for electrocatalyzing HER and OER. Benefiting from the abundant oxygen vacancies, tremella-like ultrathin nanosheets with hierarchical porous structure and large active surface area, NF/T(Ni3S2/MnS-O) shows high catalytic performance, including lower overpotential and electrochemistry stability for both HER and OER than so far reported MnOx and other metal oxides/sulfides. The NF/T(Ni3S2/MnS-O) directly used as bifunctional electrodes for overall water splitting exhibits a low voltage of 1.54 V with a low overpotential of 0.31 V at a current density of 10 mA cm−2, a fast H2 and O2 evolution speed of 4.13 and 2.04 μmol min−1 and a good electrochemistry stability with unchanged current density of 10 mA cm−2 for 50 h.

Tremella-like Ni3S2/MnS with ultrathin nanosheets and abundant oxygen vacancies directly used for high speed overall water splitting

Up to now, studies on solid-state Z-scheme two-component heterojunctions have hardly referred to roles of interfacial internal electric fields (IEFs). Herein, solid-state Z-scheme Bi2MoO6/MO (M = Cu, Co3/4, or Ni) heterojunction photocatalysts were simply synthesized, for the first time, with enhanced photocatalytic performance toward organic degradation. Electron microscopy images reveal that the MO homogeneously distributes on surfaces of the Bi2MoO6 hierarchical microspheres, and they close contact with distinct heterojunction interfaces. X-ray photoelectron spectra (XPS) suggest that the electron transfer occurs between the Bi2MoO6 and MO after they contact, resulting in formation of IEFs at their interfaces. Based on band gap values and valance-band XPS and ultraviolet photoelectron spectra, energy band levels of the Bi2MoO6 and MO are defined, and directions of the interfacial IEFs are determined. The IEFs play key roles for the formation of the Z-scheme Bi2MoO6/MO heterojunctions and for the photoactivity enhancement. This work provides a better insight into the formation mechanism of solid-state two-component Z-scheme heterojunctions and a significant experimental guidance for studying other similar systems.

Renjie Zhang

Papers

Synthesis and characterization of g-C3N4/Bi2MoO6 heterojunctions with enhanced visible light photocatalytic activity

Synthesis of magnetite-graphene oxide-layered double hydroxide composites and applications for the removal of Pb(II) and 2,4-dichlorophenoxyacetic acid from aqueous solutions

Tremella-like Ni3S2/MnS with ultrathin nanosheets and abundant oxygen vacancies directly used for high speed overall water splitting

Preparation of solid-state Z-scheme Bi2MoO6/MO (MCu, Co3/4, or Ni) heterojunctions with internal electric field-improved performance in photocatalysis

Synthesis, characterization and enhanced visible light photocatalytic activity of Bi2MoO6/Zn–Al layered double hydroxide hierarchical heterostructures