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?

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

Surface reaction kinetics and bulk charge separation are both critical to the efficiency of solar water splitting. In addition to the well-documented surface catalytic effect, the promotion of bulk charge separation upon loading of cocatalysts has rarely been reported. This paper describes the synergetic enhancement of surface reaction kinetics and bulk charge separation by introducing discrete nanoisland p-type Co3O4 cocatalysts onto n-type BiVO4, forming a p–n Co3O4/BiVO4 heterojunction with an internal electric field to facilitate charge transport. Being highly dispersed on the surface of photoanodes, the nanoisland cocatalysts could suppress the formation of recombination centers at the photoanode/cocatalyst interface. This cocatalyst-loading method achieved a charge separation efficiency of up to 77% in the bulk and 47% on the surface of catalysts. An AM 1.5G photocurrent of 2.71 mA/cm2 at 1.23 V versus the reversible hydrogen electrode for water oxidation was obtained, which is the highest photocurr...

Enhanced Surface Reaction Kinetics and Charge Separation of p-n Heterojunction Co3O4/BiVO4 Photoanodes.

A review on TiO 2 -based Z-scheme photocatalysts

Bismuth vanadate (BiVO4) is a promising visible-light driven semiconductor photocatalyst with various benefits such as low production cost, low toxicity, high photostability, resistance to photo-corrosion and narrow band gap with a good response to visible-light excite. However, the fast recombination of photoinduced charge carriers restricts their photocatalytic activity. In the past decades, many attempts were adopted to enhance the photocatalytic activity of BiVO4. Significant advances in understanding the fundamental issues and the development of an efficient photocatalyst have been made in current years. In this review, we have provided a comprehensive overview of the latest progress on the morphology control and growth mechanism of BiVO4 micro/nano-structures, doping with metal and non-metal elements and semiconductor coupling along with some highlights in the photodegradation of organic pollutants under visible-light illumination. This review may benefit the researchers and engineers in the arena of material chemistry for designing new BiVO4 based photocatalysts with low production cost and high efficiency.

A review on BiVO4 photocatalyst: Activity enhancement methods for solar photocatalytic applications

The g-C3N4/{010} facets BiVO4 interface Z-scheme photocatalysts is fabricated by ultrasonic dispersion method. The density functional theory (DFT) shows that the differences of the energy levels in the conduction bands and the valence bands between the {010} and {110} facets of BiVO4 is about 0.37 and 0.31 V (vs. NHE, pH = 7), respectively. Therefore, the co-exposed {010} and {110} facets of BiVO4 can form surface heterojunction, which promotes the {010} facets of BiVO4 with negative charge. The zeta potential indicates that layered g-C3N4 with positive charge. The Raman, FT-IR and XPS analysis demonstrates that the layered g-C3N4 is anchored on the {010} facets of BiVO4 through strong interface electrostatic interaction, which leads to form a built-in electric field at the contact interface. Under the built-in electric field driving, photogenerated electrons in the CB of {010} facets of BiVO4 rapidly recombines with the holes in the VB of g-C3N4 to form the interface Z-scheme heterostructure. That is, BiVO4 surface heterojunction ultimately induces the formation of interface Z-scheme heterostructure. The interface Z-scheme heterostructure not only facilitates the space separation of the photogenerated carriers, but also accumulates electrons in the more negative potentiated CB of g-C3N4 and holes in the more positive VB of {110} facets of BiVO4. Consequently, by means of the I-t, LSV and EIS measurements, the g-C3N4/{010} facets of BiVO4 interface Z-scheme photocatalysts presents extraordinary photoelectrochemical performance. More importantly, the degradation rate of g-C3N4/{010} facets of BiVO4 interface Z-scheme photocatalysts can reach the highest 88.3% within 30 min under visible light irradiation, and the mineralization ability (96.03%) is about 2.24 and 3.32 times as high as that of BiVO4 (42.83%) and g-C3N4 (28.89%), respectively.

Photocatalytic properties of the g-C3N4/{010} facets BiVO4 interface Z-Scheme photocatalysts induced by BiVO4 surface heterojunction

BiVO4 powders with hierarchical structures were prepared by the microwave hydrothermal method at different pHs, using Bi(NO3)3·5H2O and NH4VO3 as raw materials. The results show that, when the pH value of the precursor is 0.59, the as-prepared powders are monoclinic BiVO4 crystals with octahedron and decahedron morphologies. Spherical and polyhedral BiVO4 with particle sizes in the range of 2–4 μm can be prepared under the strong acid condition (pH = 0.70–1.21) and possess a mixed crystal consisting of tetragonal and monoclinic phases, whereas rodlike and dendritic BiVO4 with a pure monoclinic phase can be obtained within a very wide pH range (pH = 4.26–9.76). The phase transformation from tetragonal phase to monoclinic phase occurs at pH 3.65. At pH >9.76, the powders are the nonstoichiometric crystals between the mixed-phase BiVO4 and non-BiVO4. The photocatalytic efficiencies were evaluated by the degradation of Rhodamine B (RhB) under UV and simulated sunlight irradiation. The corresponding relationsh...

Effects of pH on the hierarchical structures and photocatalytic performance of BiVO4 powders prepared via the microwave hydrothermal method.

Defect-mediated Z-scheme BiO2-x/Bi2O2.75 heterojunction photocatalysts without electron mediator was prepared via a simple low-temperature hydrothermal method. DFT proved that the existence of oxygen vacancies would affect the geometric and electronic structure of BiO2-x and Bi2O2.75, which played an indispensable role in promoting exciton dissociation. BiO2-x/Bi2O2.75 exhibited a higher redox ability compared with the pure BiO2-x due to the Z-scheme photocatalytic mechanism, which could be ascribed to the formation of the build-in electric field induced by Bi and O defects. The as-synthesized photocatalysts exhibited excellent photocatalytic activity over the full solar spectrum from UV to NIR light due to the LSPR effect of oxygen vacancies, indicating its effective utilization of solar energy. The degradation rates of RhB over the optimal BiO2-x/Bi2O2.75 were 8.49, 10.22 and 3.24 times higher than that of the pure BiO2-x under visible light, simulated sunlight and NIR light irradiation, respectively. The excellent photocatalytic activity was ascribed to the synergistic effects of the LSPR effect of oxygen vacancies and the Z-scheme interfacial heterojunction. It was believed that this work provided a new idea to design high active and full solar spectrum-driven photocatalysts for energy conversion and environmental remediation.

Defect-mediated Z-scheme BiO2-x/Bi2O2.75 photocatalyst for full spectrum solar-driven organic dyes degradation

Highly efficient mpg-C3N4/BiPO4 heterostructured photocatalysts were successfully synthesized via a facile ultrasonic chemical method. The structure, morphology, surface composition and chemical state of mpg-C3N4/BiPO4 hybrid photocatalysts were characterized by XRD, TEM, FTIR and XPS, respectively. Their photocatalytic activity was evaluated under visible light, UV light and solar light irradiation against the photodegradation of rhodamine B (RhB). BiPO4 nanorod was anchored on the surface of mpg-C3N4 nanosheets and the strong electrostatic interaction between mpg-C3N4 and BiPO4 facilitated the formation of mpg-C3N4/BiPO4 heterostructure. Under visible light irradiation, O2− and h+ were the two primary active species. BiPO4 served as an electron trap to efficiently promote the separation of photogenerated electron-hole pairs. The electrons in the CB of both BiPO4 and mpg-C3N4 could react with O2 adsorbed onto the surface of BiPO4/mpg-C3N4 and reduce it to form O2−, the holes left behind in the VB of mpg-C3N4 could not directly oxidize OH− or H2O to form OH. The 9.5CN/0.5BPO could degrade 92.3% of RhB in 30 min, which was about 1.3 times as high as that of mpg-C3N4. The TOC removal rate of 9.5CN/0.5BPO was 1.23 times higher than that of mpg-C3N4. Under UV light irradiation, O2−, h+ and OH were the primary active species, the inhibition degree followed the order BQ > EDTA-2Na > TBA. The electrons in the CB of both BiPO4 and mpg-C3N4 could react with O2 to generate O2−, the holes in the VB of BiPO4 could react with OH− or H2O to produce OH. The 9.5CN/0.5BPO could degrade 94.3% of RhB in 6 min, which was approximately 4.2 times higher than that of BiPO4 and 1.5 times larger than that of mpg-C3N4. The TOC removal rate of 9.5CN/0.5BPO was 6.98 times higher than that of P25. HPLC analysis proved that the destruction of the conjugated structure of RhB would be the major process. Based on the experimental results, a possible charge transfer mechanism for the enhanced visible and UV light photocatalytic activity was proposed.

Ultrasonic chemical synthesis of hybrid mpg-C3N4/BiPO4 heterostructured photocatalysts with improved visible light photocatalytic activity

The Nd/Er co-doped tetragonal BiVO4 photocatalysts are synthesized by a microwave hydrothermal method, and the crystal structures, morphologies and optical properties are characterized. The substitution of Nd3+ and Er3+ for Bi3+ sites leads to distortion of the [VO4] tetrahedron chains and induces the monoclinic structure transforming into the tetragonal structure, with the morphology evolving from nano particle agglomerations to disorganized regular rods agglomerations. The impurity energy levels induced by Nd3+ and Er3+ in the energy band of BiVO4 act as electron traps and have further energy transfer in the up-conversion processes to facilitate the photocarriers’ separation. The more positive VB positions in the co-doped BiVO4 band structures can also effectively enhance oxidation capacity. Based on the above synergetic effect, the degradation rate the degradation rate of co-doped tetragonal BiVO4 photocatalyst can reach the highest 96% within 150 min under simulated sunlight and 20 min under the NIR irradiation.

Guoqiang Tan

Papers

Photocatalytic properties of the g-C3N4/{010} facets BiVO4 interface Z-Scheme photocatalysts induced by BiVO4 surface heterojunction

Effects of pH on the hierarchical structures and photocatalytic performance of BiVO4 powders prepared via the microwave hydrothermal method.

Defect-mediated Z-scheme BiO2-x/Bi2O2.75 photocatalyst for full spectrum solar-driven organic dyes degradation

Ultrasonic chemical synthesis of hybrid mpg-C3N4/BiPO4 heterostructured photocatalysts with improved visible light photocatalytic activity

Enhanced photocatalytic mechanism of the Nd-Er co-doped tetragonal BiVO4 photocatalysts