Abstract
1. Introduction
With the rapid development of the world economy and the increase of the population, the energy crisis and environmental pollution have gradually become the key challenges for the sustainable development of mankind. As a powerful and sustainable strategy, photocatalysis has been regarded as one of the most promising energy conversion processes to utilize green and inexhaustible solar energy for exploiting ideal fuels and simultaneously promoting environmental remediation[
In the development of robust visible-light-responsive photocatalysts for utilizing solar energy, graphitic carbon nitride (g-C3N4), a metal-free polymeric semiconductor with triazine or heptazine as a basic structural unit (Fig. 1)[
Figure 1.(Color online) The schematic structure of triazine (a) and tri-s-triazine (heptazine) (b) in g-C3N4. Reprinted from Ref. [
Up to now, although many interesting reviews on the strategies to improve the photocatalytic property of g-C3N4 have been reported, a crucial review focusing on the electronic structure tuning of g-C3N4 is lacking to provide the researchers with a panorama of the latest advances in this field. Herein, we present a comprehensive and updated review on the most recent progress in the electronic structure tuning of g-C3N4 for highly efficient photocatalysis. The diverse design and regulation strategies for optimizing the electronic structure of g-C3N4 are elaborated on and the effect of the electronic structure tuning on the properties and photocatalytic activities of g-C3N4 are also summarized. Moreover, this review will present some novel insights on the crucial challenges, opportunities, and inspiring perspectives for the design of highly efficient g-C3N4-based photocatalysts, which should be a guide for future research in this hot area.
2. Role of electronic structure modulation in optimizing the photocatalytic activity of g-C3N4
It is a well-known fact that modulating the electronic structure of g-C3N4 serves as a key strategy to optimize not only light absorption ability but also the redox reaction kinetics. Precisely, according to semiconductor theory, a semiconductor material can only absorb the photons whose energy is equal to or greater than its bandgap energy. As shown in Fig. 2, when it was used as a photocatalyst and exposed to sunlight, the electrons can be excited to the higher conduction band (CB) and participate in reduction reactions, resulting in the formation of holes in the valence band (VB) where they can take part in oxidation reactions. Usually, if a semiconductor has a large bandgap energy (>2.8 eV), it can only absorb solar light in the ultraviolet (UV) region (λ < 420 nm), which is only about 5% of the total solar spectrum. While if a semiconductor material has a small bandgap energy (0 < Eg < 2.8 eV), its absorbable spectrum can reach the visible region (420 < λ < 780 nm) or near-infra-red (NIR) region (780 < λ < 2500 nm), which accounts for about 43% and 52% of the total solar spectrum, respectively [
Figure 2.(Color online) The process of overall solar water splitting over a semiconductor photocatalyst.
3. Strategies of electronic structure tuning
The photocatalytic properties of materials are directly related to their electronic structures, which determine the spectral response properties and chemical kinetics of photocatalysts. In recent years, with the development of materials science and engineering, electronic structure tuning has received a lot of attention, especially in the field of photocatalysis. Ever since it has been used as a photocatalyst, tremendous effort has been devoted to designing and optimizing the photocatalytic performances of g-C3N4. The approaches, including vacancy modification, doping, crystallinity modulation and synthesis of new molecular structure, are the main strategies to tune the electronic structure of g-C3N4.
3.1. Vacancy modification
Recent studies found that defect engineering by the introduction of carbon and/or nitrogen vacancy defects into the motif of g-C3N4 has a pronounced effect on its electronic structure and photocatalytic performance, which is mainly due to that the VB and CB of g-C3N4 are composed of nitrogen PZ orbitals and carbon PZ orbitals, respectively[
In order to investigate the effect of band structure changing, induced by vacancy defects on light absorption of g-C3N4, Yang and coworkers synthesized a N-vacancy-rich g-C3N4 through a hard template method with a high pressure and high temperature (HPHT) technique. The measurements showed that the bandgap energy of the resulted sample was significantly narrowed and reduced from 2.97 to 1.88 eV due to the up-shifted VB and down-shifted CB edge potential. As a result, the light-responding range of N-vacancy-mediated g-C3N4 was greatly extended and showed a maximum absorption of 660 nm[
Figure 3.(Color online) (a) The UV–vis absorption spectra, (b) converted Kubelka–Munk vs. light energy plots and (c) XPS valence band spectra of CN and CNQs. (d) The schematic band structures of CN and CNQ 680. Reprinted from Ref. [
Generally, the C or N defects can be successfully introduced into the framework when the bulk g-C3N4 is thermally treated under H2, Ar, NH3 or N2 atmosphere[
To meet this challenge, many new defect engineering strategies that can control the concentration of the defect have been developed. As shown in Fig. 4, our group has designed a novel and facile urea and KOH-assisted thermal polymerization (UKATP) strategy for the preparation of defect-modified thin-layered and porous g-C3N4 (DTLP-CN), wherein the thickness of g-C3N4 was dramatically decreased, and nitrogen vacancies, cyano groups and mesopores were simultaneously introduced. Especially, the roles of N defects and structures of g-C3N4 can be targeted, regulated and optimized by changing the mass ratio of precursors. Consequently, the band structure and charge carrier transportation of g-C3N4 were well-optimized, and the photocatalytic hydrogen evolution performance of the DTLP-CN was significantly improved more than 48.5 times with the average apparent quantum yield (AQY) of as high as 0.82% at 500 nm[
Figure 4.(Color online) Schematic illustration of synthesis methods of DTLP-CN via thermal polymerization of melamine, urea, and KOH. Reprinted from Ref. [
3.2. Doping
Owing to regulating the physical-chemical property of semiconductors effectively, doping is widely used to optimize the photocatalysts for efficient solar energy conversion. Recently, doping has been proven as a valid method to regulate the electronic structure of g-C3N4, which can be facilely realized by incorporating foreign elements or a structure-matching organic group into the framework of g-C3N4[
Many non-metal elements, such as O, B, P, S, F, Br and I, have been incorporated into the motif of g-C3N4[
Figure 5.(Color online) (a) Schematic structure of the O-doped g-C3N4-based photocatalyst. (b) Band structure diagrams of g-C3N4 and O-doped g-C3N4. (c) Schematic of the fabrication of BDCNN originated from CNN and (d) the charge-transfer process in BDCNN-based heterojunction upon light irradiation. Reprinted from Refs. [
Similar to the non-metal doping, the introduction of metal atoms such as Na, K, Fe, Co, Cu, Mn, Zn, Ni, and Ag into the framework of g-C3N4 is also a successful strategy to modulate the band structure, extend the light absorption and improve the photocatalytic performance of g-C3N4[
Figure 6.(Color online) (a) UV–vis diffuse reflectance spectra, (b) the band gap from (
From the above studies, we can conclude that the electronic structure and photocatalytic properties of g-C3N4 can be modulated by doping of non-metal or metal elements, however, there are still many problems to be solved, such as the presence of surface trapping center, doping site, lower oxidizing and reducing ability. Additionally, many defects may originate from the excessive doping of metals and non-metals, which may reduce the separation performance of charge carriers because of doping asymmetry[
3.3. Crystallinity modulation
The performances of g-C3N4, such as band structure, charge carrier migration, absorptivity and photoelectronic characteristics, can be effectively optimized by regulating its crystallinity, thus, the degree of crystallinity is closely related to the photoelectrochemical property of semiconductor[
In order to understand the effect of crystallinity on the electronic structure and photocatalytic performance of g-C3N4, Zhang and coworkers proposed a facile ion thermal strategy to enhance the crystallinity of g-C3N4 where most of the photons can be used to drive photocatalytic reactions. The measurements demonstrated that this thermal condensation in the molten salt method can result in highly crystalline g-C3N4 with a maximum π–π layer stacking distance of 0.292 nm. Moreover, the band gap can be regulated from 2.74 to 2.56 eV by the addition of oxamide. As a result, the visible-light response range of CN-OA-m is extended to 700 nm and its H2 evolution activity is dramatically improved[
Figure 7.(Color online) (a) UV–visible diffuse reflectance spectrum (DRS) and (b) HOMO and LUMO positions of CN, CN-LiNa, CN-NaK, and CN-LiK. (c) UV–vis DRS and (b) bandgap structures for CN, crystalline CN, CCN and crystalline CCN. Reprinted from Refs. [
3.4. Development of new molecular structure
Carbon nitride is usually composed of triazine building units, which forms melon or crystallinity-modulated poly triazine imide (PTI) and poly heptazine imide (PHI)[
Figure 8.(Color online) Schematic illustrations of basic structural units of polymeric carbon nitride with different C and N stoichiometric ratios: (a) triazine-based graphitic carbon nitride, (b) heptazine-based graphitic carbon nitride, (c, d) polymeric C3N5, (e) C3N6, (f) C3N7, and (g) C3N3. Reprinted from Ref. [
Recently, C4N has attracted significant attention as a new class of low-band-gap polymeric carbon nitride owning to its excellent physiochemical properties for efficient solar energy conversions. Li and coworkers firstly fabricated the exfoliated C4N nanosheets for highly efficient oxygen reduction via a top-down method. The obtained C4N demonstrated a small band gap of 1.41 eV and an extended absorption band at around 500 nm due to n→π* transition[
Figure 9.(Color online) (a) Synthesis scheme of C3N5. (b) UV–Vis DRS for C3N5 compared with bulk g-C3N4. (c) Steady-state PL spectra of melem, g-C3N4 and C3N5. Reprinted from Ref. [
4. Conclusions and perspectives
In this review, the strategies and recent progresses in the electronic structure tuning of g-C3N4 for highly efficient photocatalysis are summarized, which is critical for highly efficient solar energy conversion, such as water splitting, organism degradation, and CO2 reduction, etc. Specifically, the regulation strategies based on vacancy modification, doping, crystallinity modulation and molecular structure construction are elaborated in detail. Thus, with the rational designing and modifying of the electronic band structure of g-C3N4, the light harvesting, charge separation and photocatalytic properties of g-C3N4 would be dramatically enhanced. This review provides a multi-angle cognition to cater to actual production demand in the field of solar energy conversion.
Despite plenty of progress on electronic structure having been made, there are some trends or challenges of pivotal issues, which are elaborated on in the following:
(1) As summarized above, various strategies have been demonstrated to tune the electronic band structure of g-C3N4 so as to improve light harvesting and accelerate photo-generated carriers transfer kinetics for enhanced photocatalysis. Nevertheless, the approaches that synergistically use different regulatory strategies to precisely regulate the different features of g-C3N4 need to be researched further.
(2) With the development of g-C3N4 research, the strategies that combine the regulation of electronic structure with the construction of g-C3N4-based heterostructure have attracted great attention. However, the synergetic effect and interaction mechanism of the different components have not been clearly elucidated. Therefore, much attention on new experimental methods and theoretical calculations should be paid to clarify these problems.
(3) Although it is believed that g-C3N4 has strong chemical and thermal stability, the existing photocatalytic decays are often ignored when the electronic structure is modulated. Therefore, the stability of modified g-C3N4 should be of concern for future research.
Acknowledgements
This work was mostly supported by the National Natural Science Foundation of China (Nos. 21975245, 51972300, 61674141, 12004094, and 21976049), the Key Research Program of Frontier Science, CAS (QYZDB-SSW-SLH006), the National Key Research and Development Program of China (Nos. 2017YFA0206600 and 2018YFE0204000), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB43000000), the Natural Science Foundation of Hebei Province (F2019402063), the Youth Foundation of Hebei Province Department of Education (QN2019326), the Science and Technology Research and Development Program of Handan city (21422111246), and the Key Project of Handan University (2018101). K.L. acknowledges the support from the Youth Innovation Promotion Association, Chinese Academy of Sciences (2020114). Y. H. also acknowledges the support from the Doctoral Special Fund Project of Hebei University of Engineering.
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