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Journal of Alloys and Compounds 802 (2019) 196e209

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Journal of Alloys and Compounds

journal homepage: http: / /www.elsevier .com/locate/ ja lcom

Review

Recent advances in 3D g-C3N4 composite photocatalysts forphotocatalytic water splitting, degradation of pollutants and CO2reduction

Xibao Li a, *, Jie Xiong a, Xiaoming Gao b, Juntong Huang a, **, Zhijun Feng a, Zhi Chen a,Yongfa Zhu c, ***

a School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang, 330063, Chinab Department of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan'an University, Yan'an, 716000, Chinac Department of Chemistry, Tsinghua University, Beijing, 100084, China

a r t i c l e i n f o

Article history:Received 22 January 2019Received in revised form27 May 2019Accepted 15 June 2019Available online 15 June 2019

Keywords:PhotocatalystThree-dimensional g-C3N4

Review

* Corresponding author.** Corresponding author.*** Corresponding author.

E-mail addresses: lixibao@nchu.edu.cn (X. Li), huazhuyf@mail.tsinghua.edu.cn (Y. Zhu).

https://doi.org/10.1016/j.jallcom.2019.06.1850925-8388/© 2019 Elsevier B.V. All rights reserved.

a b s t r a c t

Recently, g-C3N4 has exhibited excellent catalytic performances in photocatalytic water splitting for H2

and O2 generation, degradation of pollutants and CO2 reduction. However, the bulk g-C3N4 demonstratessome disadvantages such as low specific surface area, high defect density, fast recombination possibilityof photogenerated electron-hole pairs, and non-recyclable characteristics, leading to low photocatalyticperformance and efficiency. The three-dimensional (3D) network-like g-C3N4 composite materials con-structed by nanotechnology can effectively improve the adsorption capacity, light response, structurestability and recyclability of photocatalysts, which results in a significant increase in the photocatalyticperformance and utilization. It is a novel way to achieve high-efficient separation of photogeneratedelectron-hole pairs and improve photocatalytic activity. In this review, the recent research progressesespecially the synthesis strategy of 3D g-C3N4 composite photocatalysts and their applications forphotocatalytic water splitting, degradation of organic pollutants and CO2 reduction are firstly and sys-tematically introduced and discussed. The review and prospect of 3D g-C3N4 composite materials canprovide some new ideas and directions for the research and development of 3D g-C3N4 compositephotocatalysts with high activity, strong adsorption, facile recyclability, and no secondary pollution.

© 2019 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1972. Synthesis of 3D g-C3N4 composite photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

2.1. Preparation of 3D g-C3N4 composite photocatalysts by self-assembly strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1982.1.1. Hydrothermal/solvothermal synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1992.1.2. Heating-cold polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1992.1.3. Photopolymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

2.2. Preparation of embedded 3D g-C3N4 composite photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2012.2.1. Template method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2012.2.2. Thermal polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2022.2.3. Freeze-drying method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202

3. Application of 3D g-C3N4 composite photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2033.1. Photocatalytic degradation of organic pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

ngjt@nchu.edu.cn (J. Huang),

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3.2. Photocatalytic water splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2033.3. Photocatalytic CO2 reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

4. Conclusion and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

1. Introduction

Recently, environmental pollution and energy shortages are thetwo major challenges for human being. Therefore, the clean envi-ronmental protection and environment-friendly sustainable en-ergy are of two key technologies for human sustainabledevelopment. Photocatalytic technology has attracted more andmore attention because it can not only make full use of solar energyto split water to produce hydrogen [1e6], but also degrade organicpollutants [7e9] and reduce CO2 [10e16]. The effective applicationof solar photocatalytic technology can effectively alleviate the en-ergy crisis, and has important significance and practical researchvalue for comprehensive environmental improvement and reduc-tion of greenhouse gas emissions [17]. Although photocatalytictechnology has made great progress in environmental treatmentand preparation of clean energy, the low utilization of sunlight andthe high combination of photogenerated electron-hole pairs havegreatly restricted the development of photocatalytic technology[18,19]. As can be seen from Fig. 1, the photocatalytic mechanism isalso constantly improving. The photocatalytic heterojunction isconverted from conventional II-scheme to direct Z-scheme and S-scheme (this heterojunction photocatalyst is mainly made up oftwo n-type semiconductor photocatalysts) [20e26], which greatlyimproves the photocatalytic performance. The key to improving thephotocatalytic activity of photocatalyst is to broaden the photo-absorption region and improve the separation efficiency ofphotoelectron-hole pairs.

At present, the developed photocatalytic materials are mainlydivided into four categories: metal oxides [24e26], sulfides[27e31], preciousmetal semiconductors [32e35], and non-metallicsemiconductors [36e41]. Each catalyst possesses some disadvan-tages, such as heavy metal pollution, high cost, high temperaturerequirements and poor stability in the synthesis process. In 2009,

Fig. 1. Improvement of the photocatalytic mechanism.

Wang and his group [42] synthesized an organic conjugatedsemiconductor photocatalyst, namely graphitic carbon nitride (g-C3N4), which was used to split water under light irradiation toproduce H2 and O2 (l> 420 nm). Due to the features of facilepreparation, high stability, low cost and visible light response[43e49], g-C3N4 has attracted much attention of researchers andthe synthesis of pure g-C3N4 [50e53], doping modification[54e58], heterogeneous composite [59e62] and morphology con-trol [63e66] have been studied in depth. In general, bulk g-C3N4can be synthesized by thermal condensation of various precursors(including melamine, cyanamide, dicyandiamide, thiourea, ureaand their mixtures) with R-C-NH2 units (Fig. 2 [67]). However, thebulk g-C3N4 prepared by thermal condensation has many defects,including low specific surface area, poor dispersion and weak filmforming ability. Due to the insufficient carbon nitrogen ratio, densesurface defects, especially the rapid electron-hole recombinationrate, the photocatalytic activity of g-C3N4 is greatly inhibited.Therefore, researchers havemademany attempts to prepare g-C3N4with specific morphology and surface function, and to improve theutilization of solar energy by nanotechnology, so that the photo-catalytic materials based on g-C3N4 can exhibit better photo-catalytic performance [68e71]. Fig. 3 illustrates the designconsiderations of g-C3N4 based photocatalysts based on differentcharacteristics.

The band gap width of photocatalyst directly affects its photo-catalytic ability and application range [72e75]. From Fig. 4 [76], g-C3N4 has an appropriate band gapwidth (2.7 eV). As a result, g-C3N4has the potential of H2 production, O2 production and CO2 reduc-tion, indicating that photogenerated electrons in g-C3N4 have alarge thermodynamic driving force to reduce various small mole-cules such as H2O, CO2 and O2. Therefore, appropriate electron band

Fig. 2. g-C3N4 prepared by thermal condensation of different nitrogen-rich precursors[67]. Copyright (2015) Wiley Oline Library.

Fig. 3. Design considerations of g-C3N4 based photocatalysts based on differentcharacteristics.

Fig. 4. The redox potential of the relevant reactions of g-C3N4 band edges at pH¼ 7 [76]. Copyright (2015) Royal Society of Chemistry.

X. Li et al. / Journal of Alloys and Compounds 802 (2019) 196e209198

structure is conducive to the wide application of g-C3N4 in photo-catalytic water splitting, CO2 reduction, degradation of pollutant,organic synthesis and disinfection, etc. However, pure g-C3N4 hasthe disadvantages of small specific surface area [77e80], highsurface inertia [81e83], low utilization of solar energy (it is difficultto absorb solar energy with wavelength over 460 nm) [84e88], lowcarrier mobility and fast electron-hole recombination rate [89e91].It has been found that the photocatalytic activity of g-C3N4 matrixcomposites can be optimized by changing synthesis technology[92e95], electronic structure manipulation [96e99], and nano-structure design [100e104].

Compared with zero-dimensional, one-dimensional and two-dimensional structures, three-dimensional (3D) porous frame-work photocatalytic materials can provide higher specific surfacearea, mechanical strength and porosity, and increase their multi-dimensional mass transfer channels and reusability. The prepara-tion of novel 3D porous photocatalysts by combining photocatalystswith a 3D porous skeleton has been extensively studied in theworld [105e113]. 3D porous framework photocatalytic materialshave the following advantages [114e117]: (1) The high specific areacan expose more active sites, provide more reaction sites, thus caneffectively couple adsorption, and employ synergy to improvephotocatalytic performance; (2) The 3D network structure canmake the light reflect in the catalyst and improve the light ab-sorption; (3) The 3D network structure can effectively anchor thecatalyst, prevent the agglomeration of the catalyst, avoid the loss ofthe catalyst, and improve the stability; (4) The porosity of the 3D

network structure is much higher, and the gas resistance is lowerwhen the pollutants in the gas phase are degraded or hydrogen isproduced, which is conducive to the exchange of substrates andproducts; (5) The 3D network structure can fix the desorption ofintermediate and final products; (6) The catalyst with 3D networkstructure in aqueous solution is easy to be separated from othersubstances and has an easy-to-recycle feature.

According to incomplete statistics, since 2009, more than 3000papers related to g-C3N4 have been published, and the number ofcitations has exceeded 100000 (Fig. 5(a)). Although there arerelatively few reports on 3D g-C3N4, the related reports areincreasing year by year (Fig. 5(b)), indicating that investigation on3D g-C3N4 composite materials is gradually being valued by re-searchers. These data highlight the high research hotspots andpotential of g-C3N4 as a photocatalyst.

In this paper, the recent advances in the field of photocatalysis of3D g-C3N4 composite photocatalysts are reviewed. The present

situation of photocatalytic reactions of 3D g-C3N4 composite ma-terials through structural engineering in recent years is illustrated,including the synthesis of 3D g-C3N4 composite photocatalysts, andtheir applications in the photocatalytic water splitting, CO2 reduc-tion and environmental restoration.

2. Synthesis of 3D g-C3N4 composite photocatalysts

Based on a large number of literature, it is found that there aretwo major strategies for the synthesis of 3D g-C3N4 compositephotocatalysts. One is through self-assembly strategy, the other isthrough embedded strategy. The former includes hydrothermal/solvothermal method, heating-cold polymerization method, andphotopolymerization method, etc. The latter includes templatemethod, thermal polymerization method, and freeze-dryingmethod, etc.

2.1. Preparation of 3D g-C3N4 composite photocatalysts by self-assembly strategy

The self-assembly strategy refers to the techniques in which abasic structural unit spontaneously forms an ordered structureunder thermodynamic equilibrium conditions. The self-assemblymethods possess the characteristics of controllable particle size,high purity of products, and simple process operation [118e121].Therefore, self-assembly techniques are widely employed in thepreparation of photocatalytic materials. g-C3N4 nanosheets are

Fig. 5. Since 2009, the number of publications with the titles contain the keywords of (a) "g-C3N4"; (b) "three-dimensional" or "3D" and "g-C3N4" and the number of citations.(Search from Web of Science on Nov. 26, 2018).

X. Li et al. / Journal of Alloys and Compounds 802 (2019) 196e209 199

usually used as the basic structural unit. Hydrothermal synthesis,heating-cold polymerization and photopolymerization areemployed to provide the driving force. The internal hydrogen bondand p-p stacking interaction force are higher than the van derWaals interaction of CeN group in g-C3N4 aromatic ring in theprocess of self-assembly, thus forming a 3D interconnectedstructure.

2.1.1. Hydrothermal/solvothermal synthesisHydrothermal and solvothermal methods have become more

andmore important approaches for the synthesis ofmost inorganic-organic nanocomposites and condensed materials such as sol andgel. In 2014, Ma et al. [122] performed ultrasound treatment of bulkg-C3N4 in a solution containing concentrated hydrochloric acid forabout 2 h (Zeta potential changed from �37.3 mV to þ25.6 mV),then modestly oxidized carbon nanotubes (CNTs, Zetapotential �19.3 mV) was added for the hydrothermal reaction at acertain temperature. The spontaneous assembly of hydrothermalprocesses was driven by electrostatically attraction andp-p stackinginteractions between modestly oxidized CNTs with a large numberof oxygen-containing functional groups (such as COO-) and posi-tively charged g-C3N4 nanosheets (g-C3N4 NSs). Finally, a 3D g-C3N4NS-CNT porous composite was formed. In 2018, Hu et al. [123]successfully prepared a 3D aerogel composed of g-C3N4 modified byperylene imide (PI) and graphene oxide (GO) through a hydrother-mal self-assembly method, which presented a high removal rate ofNO as high as 66%. In the same year, Hu et al. [124] fabricatedCNQDs/GO-InVO4 with porous layered structure (CNQDs is g-C3N4

quantum dot) by a hydrothermal self-assembly method, which alsoexhibited high degradation performance of NO (up to 65%). It wasfound that the average particle size of CNQDs prepared by stepwiseexfoliation of g-C3N4 was as small as 3.0 nm, and CNQDs wereuniformly attached to the GO surface by electrostatic, p-p stackingand hydrogen bonding interaction. In the hydrothermal process,CNQDs/GO was tightly wrapped with cubic InVO4 through thermaldrive and bond energy, thereby forming a 3D aerogel hetero-structure. Fig. 6 presents the preparation process of 3D CNQDs/GO-InVO4 composite. In 2017, Tang et al. [125] successfully prepared afunctionalized 3D CN/GOA aerogel by a hydrothermal synthesis

method with ethylenediamine as a reducing agent and cetyl-trimethylammonium bromide (CTAB) as a surfactant, whichexhibited excellent mechanical properties (Fig. 7). Through photo-catalytic experiments, the 3D CN/GOA aerogel achieved a visible-light degradation rate of 91.1% for methyl orange (MO) and 88.8%for methylene blue (MB) in 40min, and the conversion rate ofbromate at a concentration of 250 mg/L was 80% within 160min.After 5 cycles of experimentation, the mass loss of the 3D macro-scopic material was only 4%.

Hydrothermal and solvothermal methods omit calcination stepsand can directly synthesize highly active materials from solution,such as intermediate and metastable states and special phases,which are easy to be generated by hydrothermal and solvothermalmethods. Therefore, a series of new synthetic products with specialmetastable structures and special condensed states can be syn-thesized and developed.

Although hydrothermal/solvothermal reactions can occur atlower temperatures, the samples need to be fully cooled to roomtemperature before they can be opened and taken out. In otherwords, hydrothermal/solvothermal methods require specialequipment such as hydrothermal kettles. In addition, the wasteliquid produced by hydrothermal/solvothermal process needs to betreated. These are the problems that need to be noticed and over-come in the preparation of 3D g-C3N4 composite photocatalysts byhydrothermal/solvothermal method.

2.1.2. Heating-cold polymerizationIn order to overcome the dependence of hydrothermal/sol-

vothermal methods on special equipment such as hydrothermalkettles, many researchers have made great efforts to prepare 3D g-C3N4 composite photocatalysts by heating-cold polymerization. In2016, Zhang et al. [126] employed the heating-cold polymerizationmethod with water as the solvent, initially heated to 95 �C bymixing a certain proportion of agar and g-C3N4, and then cooled toroom temperature to form a 3D agar-C3N4 hybrid hydrogel withexcellent dispersion (Fig. 8). Similarly, Zhang et al. [127] fabricated a3D C3N4/SiO2 hybrid hydrogel by alkaline solution and acid gelmethod. It was found that the total organic carbon content (TOC)removal rate of C3N4/SiO2 for coking wastewater reached 33%,

Fig. 6. Schematic diagram of preparation process of 3D CNQDs/GO-InVO4 [124]. Copyright (2018) Elsevier.

Fig. 7. The compression performance test of 3D CN/GOA [125]. Copyright (2017)Elsevier.

Fig. 8. Schematic illustration of preparation of C3N4-agar hybrid hydrogel [126].Copyright (2016) Elsevier.

Fig. 9. Performance testing of 50 g weight applied on 3D g-C3N4-agar [128]. Copyright

X. Li et al. / Journal of Alloys and Compounds 802 (2019) 196e209200

which was 5 times as much as that of pure g-C3N4. The ability ofphotocatalytic removal of phenol and MB was 3.1 and 6 times asmuch as that of pure g-C3N4, respectively. Tan et al. [128] also

prepared a 3D g-C3N4-agar hybrid aerogel by the heating-coldpolymerization method. The ciprofloxacin (CIP) and MB werephotocatalyzed by employing the prepared 3D g-C3N4-agar hybridaerogel. After 3 h of photocatalysis, the degradation rate of 3D g-C3N4-agar for CIP reached 92% (the degradation rate of pure g-C3N4for CIP was only 62.1%), and the degradation rate of 3D g-C3N4-agarfor MB was as high as 99%. The 3D g-C3N4-agar hybrid aerogel alsoexhibited excellent mechanical properties, which was demon-strated in Fig. 9.

2.1.3. PhotopolymerizationAs a typical organic conjugated semiconductor material, g-C3N4

can be used as an initiator to provide a radical for the preparation of3D g-C3N4 composite materials by a photopolymerization method.In 2017, Sun et al. [129] successfully prepared the g-C3N4 hydrogelwith a 3D structure by the photopolymerization method, whichemployed g-C3N4 as the initiator, N, N-dimethyl acrylamide (DMA)as the cross-linking agent, and LED light source as the driving force.Liu et al. [130] promoted the cross-linking of N-iso-propylacrylamide (NIPAm) by stripping g-C3N4 nanosheets (CNS)and constructed stereoscopic 3D PNIPAm/CNS hydrogel by photo-polymerization under xenon lamp (l> 420 nm, 50mW/cm2)(Fig. 10). Longer irradiation time, a higher concentration of NIPAm(12wt %) and CNS (0.04 wt %) achieved effective gelation. However,excessive CNS and NIPAm in aqueous solution led to unstable ag-gregation and inhibited the formation of hydrogels. Kumru et al.[131] mixed g-C3N4 with DMA, N, N0-methylenebisacrylamide(CBA) and deionized water as the solvent, which was irradiated bytwo 50W LED lights. Then a uniform 3D gel was formed. (Fig.11(a)).It was proved that the nitrogen atom of the basic amine as a co-

(2019) Elsevier.

Fig. 10. (a) TEM image of CNS, the Tyndall effect of CNS colloidal solution in H2O; (b)initial reaction solution, hydrosol prepared with NIPAm/CNS at room temperature; (c)NIPAm/CNS hydrogel after irradiation with visible light; (d) the CNS induced poly-merization mechanism [130]. Copyright (2017) Royal Society of Chemistry.

Fig. 11. Formation mechanism of g-C3N4 hydrogel with g-C3N4 as initiator, DMA andMBA as crosslinkers [131]. Copyright (2017) American Chemical Society.

Fig. 12. The synthesis schematic of 3DOM g-C3N4 [133]. Copyright (2018) Elsevier.

X. Li et al. / Journal of Alloys and Compounds 802 (2019) 196e209 201

initiator reacted with free radicals on the surface of g-C3N4 bydirectly transferring the group from the g-C3N4 side to the co-initiator side [132]. Then Kumru et al. [131] mixed g-C3N4 withnitrogen-free poly (ethylene glycol) methyl ether methacrylate(PEGMEMA) and poly (ethylene glycol) dimethyl acrylate(PEGDMA) under the same light source, uniform 3D hydrogelcomposites could also be obtained. This revealed that g-C3N4 couldinitially be polymerized to form hydrogels without nitrogen as a co-initiator. As shown in Fig. 11 (b) and (c), CM, CMB, CMp and u-CNrepresent g-C3N4 synthesized by using various precursors,including cyanuric acid (C) andmelamine (M), barbital acid (B), 2,4-diamino-6-phenyl-1,3,5-triazine (Mp) and urea. It was found thatnitrogen-free 3D hydrogels could be formed, indicating that thepolymerization of g-C3N4 monomer mainly occurred on the surface

and had nothing to do with nitrogen-containing functional groups.

2.2. Preparation of embedded 3D g-C3N4 composite photocatalysts

Embedded 3D g-C3N4 composite materials can be analogous toembed the wardrobe into the wall, providing a photocatalytic re-action place like a huge space inside the wardrobe. This structurehas the same characteristics as other 3D photocatalytic materials,such as high specific surface area, high porosity and high utilizationof sunlight. However, compared with other methods, the prepara-tion of embedded 3D g-C3N4 composite materials possesses theadvantages of simple preparation process, wide reaction conditionsand diversified use of raw materials. At present, embedded 3D g-C3N4 composite materials with stable performance can be preparedby template method, thermal polymerization, and freeze-drying,etc.

2.2.1. Template methodTemplate method is an important method to synthesize nano-

composites, and it is also the most widely used method in theresearch of nanomaterials, especially for the preparation of nano-materials with special properties. Template method can design thematerials and structures according to the performance re-quirements and morphologies of synthetic materials to meet theactual needs. In 2016, Lin et al. [133] successfully grew g-C3N4 onSiO2 nanospheres by self-made three-dimensional ordered meso-porous core-shell SiO2 nanospheres, which were then employed asthe hard templates. Cyanamide was used as the raw materials.Finally, hydrofluoric acid was added to remove the SiO2 templates.Three-dimensional ordered macroporous g-C3N4 (3DOM g-C3N4)was successfully prepared (as shown in Fig. 12). Zhang et al. [134]used melamine sponge (MS) as a template, g-C3N4 and GO as rawmaterials, successfully prepared a three-dimensional macro-g-C3N4/GO coated sponge with a freely designed shape. Liang et al.[135] took easy to obtain MS as the in-situ template, filled saturatedurea solution intoMS, and then heated and polymerized at a certaintemperature to form macroscopic three-dimensional porous car-bon nitride monolith (PCNM) (as shown in Fig. 13). The specificsurface area and pore volume of PCNMwere 78m2/g and 0.76 cm3/g, respectively. After 40% volume compression, the original state ofPCNM can still be restored. The hydrogen production rate reached29 mmoL/h, which was 2.84 times as much as powdery g-C3N4. In2018, Zhang et al. [136] successfully prepared a three-dimensionalporous S/Graphene@g-C3N4 (S/GCN) sponge by microemulsion-assisted assembly method. In the experiment, internal oil emul-sion droplets were applied as a soft template to form sulfur-containing pores, and hydrophilic GCN was stacked around theoil droplets to assemble a three-dimensional network. In addition,

Fig. 13. The synthesis schematic of PCNM [135]. Copyright (2015) Wiley Online Library.

X. Li et al. / Journal of Alloys and Compounds 802 (2019) 196e209202

the N-site enriched in the macropores of GCN provided manyadhesion sites for polysulfides, achieving a “physical-chemical”double restriction of diffused polysulfides, and a robust and highlyporous 3D graphene framework was available. An efficient electrontransport path to achieve fast electron movement maintainedstructural integrity.

In summary, due to its unique characteristics such as cheap formass production, fast and convenient for operation, the templatemethod is widely used in the field of 3D g-C3N4 composite photo-catalysts. However, it has some disadvantages such as no directcontrol over preparation, difficulty of fabrication for very complexstructures and needing to construct microchannels and so on.

2.2.2. Thermal polymerizationThermal polymerization is a kind of radical polymerization

method, which directly changes the excitation of monomers intomonomer free radicals by heating, and then initiates monomerpolymerization. In 2012, Zhang et al. [137] mixed melamine with acertain amount of S8, and after grinding, it was calcined at 650 �C for2 h inN2 atmosphere to obtainporous three-dimensional S-doped g-C3N4 (Fig. 14(a)). The photocatalytic hydrogen production rate ofCNeS8 reached 33 mmoL/h, which was about 4.2 times as much asuntreated pure g-C3N4. The results showed that the element S can beused as a chemical promoter for the synthesis of g-C3N4, which can

Fig. 14. (a) Finished products without S8 (left) and with S8 (right) [137]. Copyright (2012) Adoped g-C3N4 [138]. Copyright (2017) Elsevier.

significantly improve the traditional polymerization route, its in-ternal structure, morphology, optical and electronic properties of g-C3N4 for the photocatalytic oxidation and water splitting undervisible light irradiation. In 2017, Tian et al. [138] prepared three-dimensional porous N-doped g-C3N4 (Fig. 14(b)) by hydrothermaland thermal polymerization at a certain temperature, which wasproceeded bymixing the appropriate amount ofmelamine, urea anddeionized water at a certain molar ratio. The prepared three-dimensional porous N-doped g-C3N4 photocatalyst UM3 (molar ra-tio of urea: melamine¼ 3:1) had a specific surface area of 39.1m2/g,which was 9 times as much as that of the pure undoped g-C3N4. Thehydrogen evolution rate of UM3 reached 3579 mmol h�1 g�1, whichwas 23 times as much as undoped g-C3N4 (147 mmol h�1 g�1). It isproved that the synergistic effects of changing thephase transition ofprecursor and introducing urea as another nitrogen source andpore-forming agent by a hydrothermal treatment can increase the specificsurface area of g-C3N4, reduce the band gap width and improve theeffective charge separation, thus exhibiting excellent photocatalytichydrogen evolution activity under visible light irradiation. Jiang et al.[139] synthesized three-dimensional PANI/CNNS photocatalyticcomposite (PANI is polyaniline, CNNS is g-C3N4 nanosheet) by in-situpolymerization. The degradation rate of PANI/CNNS-5% for MB(2� 10�5moL/L) in 4 h was 89.1%. He et al. [140] successfully pre-pared macroscopic three-dimensional g-C3N4 by using 3D printingtechnology, and it could degrade 90% MB (20mg/L) in 60min. In2017, Ou et al. [141] successfully prepared three-dimensional g-C3N4for photocatalytic production of H2 and H2O2 by thermal polymeri-zation and freeze drying without any crosslinking agent. The pre-pared three-dimensional g-C3N4 aerogel possessed a specific surfacearea of 133m2/g, and the yield of H2O2 was 1.44 mmoL/h. It showedthat the outstanding interface charge separation efficiency, photo-electrochemical performance and improved photocatalytic effi-ciency of the 3D g-C3N4 composite photocatalysts are due to thesynergistic effect of large specific surface area, functional groups andthree-dimensional network structure. Fig. 15 is a real image of theprepared three-dimensional g-C3N4 aerogel.

2.2.3. Freeze-drying methodThe freeze-drying method is mostly employed to prepare three-

dimensional porous aerogels and hydrogels with skeleton shape.The synthetic graphene has a typical three-dimensional skeletonstructure [142e145]. Therefore, many researchers have drawnparticular attention to the internal hydrogen bonds and p-p stack-ing of graphene. g-C3N4 and other photocatalysts are constructed asthe fillers to form a macroscopic three-dimensional photocatalyticmaterial. Yan et al. [146] applied ascorbic acid as a reducing agent, g-C3N4, Cu2O andGO as rawmaterials, successfully prepared 3DCu2O/g-C3N4/RGO ternary photocatalytic composite material by freeze-

merican Chemical Society; (b) Schematic illustration for the formation of 3D porous N-

Fig. 15. The real image of 3D g-C3N4 hydrogel and aerogel [141]. Copyright (2017)Wiley Online Library.

X. Li et al. / Journal of Alloys and Compounds 802 (2019) 196e209 203

drying method. Wang et al. [147] also successfully prepared 3D g-C3N4/rGH composite materials by the same method. The resultsshowed that 90% g-C3N4/rGH could adsorb 80% Cr(VI) (30mg/L) in30min. The adsorption rate of Cr(VI) reached 100% within 120minby adsorption and visible light irradiation. After continuous pho-tocatalysis for 25 h, 90% g-C3N4/rGH could still achieve 100%degradation rate for low concentration (2mg/L) Cr(VI). Wan et al.[148] fabricated 3D g-C3N4/GOA composite materials with variousshapes by a simple one-step freeze-drying method. The photo-catalytic experiments showed that the prepared 3D aerogelsexhibited excellent adsorption capacity for oil, organic solvents anddyes, and enhanced visible light photocatalytic activity for dyedegradation and NO oxidation. Fig. 16 shows the 3D g-C3N4/GOaerogels composites with various shapes.

In summary, the self-assembly or co-assembly strategy toconstruct 3D g-C3N4 composite photocatalysts mainly depends onweak interaction of hydrogen bonds, p-p conjugates and electro-static interaction between components. In the process of self-assembly or co-assembly, the basic structural units spontaneouslyorganize or aggregate into a stable structure with regular geometricappearance under the interaction of non-covalent bonds. The pro-cess of self-assembly is not a simple superposition of weak forcesamong a large number of atoms, ions andmolecules, but a close andorderly whole formed by the spontaneous association and aggre-gation of several individuals at the same time. It is a complex syn-ergistic effect of the whole. Therefore, the self-assembly strategyrequires higher structural or surface properties of rawmaterials, andonly for specific materials can they play a role in each other. Becausetheembedded3Dmaterials arefilledwithphotocatalysts attached tothe 3D framework, it can not rely on electrostatic or hydrogen bondinteractions as strictly as self-assembly or co-assembly, so the choiceof rawmaterials can bemore diversified and the reaction conditionsare broader. However, due to the structural defects of the templateitself and the removal of the template, the embedded photocatalystis prone to the problems of uniform distribution of active sites andlow structural strength. There are various methods to prepare 3Dnetwork materials, and each composite material should choose theappropriate construction method according to its composition and

Fig. 16. Different shapes of 3D g-C3N4/GOA composites [148] (a) ultra-light g-C3N4/GOaerogel rested on the leaf, (b) macroscopic g-C3N4-GO aerogels. Copyright (2016) RoyalSociety of Chemistry.

structure characteristics. It is necessary to develop some newmethods for preparing 3D photocatalytic materials with simple andcontrollable process and strong adaptability of raw materials.

3. Application of 3D g-C3N4 composite photocatalysts

3.1. Photocatalytic degradation of organic pollutants

As far as wastewater treatment, photocatalytic compositescontaining g-C3N4 have been widely applied in photocatalyticdegradation of organic pollutants in water, such as rhodamine B(RhB), methyl orange (MO), methylene blue (MB), phenol, antibi-otics and so on. The main degradation mechanism is that thecatalyst is stimulated by solar energy to produce strong oxidativeholes and reducible electrons. Hydroxyl radicals ($OH) and super-oxide radicals ($O2

�) are produced by the holes and electrons. Theseradical groups have strong oxidative or reducible properties andcan effectively mineralize organic pollutants. The composite ma-terials of the three-dimensional g-C3N4 structure simultaneouslyprovide high porosity, specific surface area and pore volume,thereby can effectively adsorb pollutant molecules and achievebetter degradation rate. Table 1 is a summary of the latest progressin the degradation of organic pollutants by some 3D g-C3N4 com-posite photocatalysts.

3.2. Photocatalytic water splitting

The photocatalytic water splitting process follows the mecha-nism of semiconductor photocatalysis. The potential and valenceband level of photocatalyst should be more negative than Hþ/H2,and be more positive than OH�/O2 potential [154]. In order to meetthis standard, photocatalysts should have a minimum bandgap of1.23 eV. Moreover, it should possess the ability of excellent lightcapture, charge separation and large specific surface area to provideabundant surface reaction active sites, thus contributing to thewater splitting to produce H2 and O2. In addition, the potentialrequirement for the half-reaction of water oxidation and reductionmust be met with [72]:

Full reaction: 2H2O(l)/O2(g)þH2(g) DE0¼1.23 V (1)

Half-reaction: Oxidation reaction: 2H2O(l)/O2(g)þ4Hþ(aq)þ4e�

DE0¼1.23 V vs. SHE (2)

Reduction reaction: 4Hþ(aq)þ4e�/2H2(g) DE0¼ 0.00 V vs. SHE(3)

SHE is the standard hydrogen electrode and E0 is the equilibriumpotential under the standard conditions.

Compared with the traditional semiconductor photocatalysts,such as TiO2 [155e158], ZnO [159e162], WO3 [163e167] and CdS[168e171], the research on photocatalytic water splitting by 3D g-C3N4 compositematerials is still in its infancy. In 2009, the hydrogenproduction rate was 10 mmoL/h with g-C3N4 as the water splittingphotocatalyst, and the half-reaction quantum yield under visiblelight was less than 1% [42]. Martin et al. [172] used urea, dicyan-diamide and thiourea as the precursors to prepare g-C3N4 by thermalpolymerization, and hydrogenwas produced under light irradiationat a wavelength longer than 400 nm. The hydrogen production ratecould reach 20000 mmol h�1 g�1, and the quantum yield reached26.1%. Liu et al. [173] achieved a high-efficiency 2e� catalytic processof water splitting by a photocatalyst prepared by combining carbonquantum dots with g-C3N4, and the quantum yield reached 20%. Itcan be seen that g-C3N4 based photocatalytic materials have greatpotential in the field of photocatalytic water splitting. Three-dimensional g-C3N4 based photocatalytic composites possess both

Table 1Effect of 3D g-C3N4 composite materials on photocatalytic degradation of organic pollutants.

Catalyst Light source Catalyst concentration Pollutant concentration Degradation efficiency Ref.

g-C3N4/rGH Adsorption in dark 1 g/L Cr(VI): 30mg/L 80% absorption in 30min (90%g-C3N4/rGH) [147]g-C3N4/GO 500W (l＞420 nm) 1 g/L MO: 20mg/L 92%, 4 h [149]HP-CN 35W metal halide (l＞420 nm) 0.5 g/L RhB: 10mg/L

MO: 10mg/LRhB: 98%, 1.5 hMO: 35%, 3 h

[150]

3DOM g-C3N4 500W (l＞420 nm) 1 g/L RhB: 10mg/L RhB: 100%, 40min [133]g-C3N4-agar hybrid hydrogels 500W (l＞420 nm) 0.5 g/L MB: 3� 10�5 Mphenol: 5 ppm MB: 70%, 1 h (20% g-C3N4/agar)

Phenol: 14%, 10 h (90% g-C3N4/agar)[126]

Cu2O/g-C3N4/RGO 300W (l＞400 nm) 0.05/100(g/mL) MB: 30mg/LMO: 30mg/L

MB: 96%, 120minMO: 83%, 120min

[151]

3D g-C3N4/TiO2 HHs 500W (l＞420 nm) 1 g/L RhB: 10mg/L 93.6%, 120min [152]GA-CQDs/CNN 300W (l＞420 nm) 30/100(mg/mL) MB: 30mg/L

MO: 30mg/LMB: 93%, 180minMO: 91.1%, 240min (GA-CQDs/CNN-24%)

[153]

X. Li et al. / Journal of Alloys and Compounds 802 (2019) 196e209204

physical and structural properties of g-C3N4. Their applications inphotocatalytic water splitting have attracted more and more atten-tion, and related research reports are increasing year by year. Table 2summarizes the latest progress in photocatalytic water splitting forhydrogen production by some 3D g-C3N4 composite materials.

3.3. Photocatalytic CO2 reduction

Converting CO2 into renewable fuels through artificial photo-synthesis is an effective strategy to effectively solve the greenhouseeffect and realize carbon resource recycling [175e178]. In 1978,Halmann et al. [179] first used GaP to reduce CO2 to methanol. Onthis basis, Inoue et al. [180] employed semiconductor materials as aphotocatalyst under the irradiation of ultraviolet light to irradiateCO2 saturated aqueous solution. CO2 was successfully convertedinto methanol (CH3OH) and methane (CH4). Although great prog-ress has been made in photocatalytic CO2 reduction, the chemicalproperties of CO2 are extremely stable as the highest valence stateof C. According to thermodynamic calculation, it takes 1135 kJ/molof energy to reduce CO2 to CH4 [181], and at least 2.14 eV ofreduction potential to reduce CO2 to anionic radicals [182]. Inaddition, the reduction of CO2 is a very complex process. Theproducts include carbon monoxide (CO), formic acid (HCOOH),formaldehyde (HCHO), CH3OH, CH4 and other hydrocarbons [183].Therefore, how to construct high-efficiency photocatalyst andrealize the directional conversion of CO2 is the key and difficultpoint of photocatalytic CO2 reduction.

As a new type of organic conjugated semiconductor photo-catalyst, g-C3N4 remedies the shortcomings of metal semiconductorphotocatalyst, such as high cost and environmental pollution causedby metal ion leaching. Moreover, g-C3N4 has a suitable band gapwidth (2.77 eV in bulk, 2.97 eV in nanosheets [184]), which caneffectively reduce CO2. Therefore, a series of photocatalysts based ong-C3N4 are widely applied in CO2 conversion. From the currentresearch, researchers have mainly focused on g-C3N4 doping[185e187], functionalization [188,189], and forming heterojunctionswith other semiconductor materials [190e192] to improve the

Table 2Effect of 3D g-C3N4 composite materials on photocatalytic H2 production.

Catalyst Light source Reaction solution

g-C3N4

hydrogelLED irradiation TEOA (10 vol%)

3D PCNM 300W (l＞420 nm) TEOA(10 vol%)3D CNeS2.0 300W (l＞420 nm) TEOA(10 vol%)3D porous N doped g-C3N4 300W (l＞420 nm) Aqueous lactic acid

3D g-C3N4/TiO2 HHs 300W (400 nm< l< 780 nm) CH3OH(10 vol%)Ag/g-C3N4 nanofibers 300W (>420 nm) TEOA(10 vol%)

photocatalytic CO2 conversion performance of g-C3N4 based com-posites. In 2015, He et al. [193] prepared a Z-scheme Ag3PO4/g-C3N4photocatalytic material, which reduced CO2 to CO, CH3OH, CH4, andethanol (CH3CH2OH). The reduction rate of CO2 reached57.5 mmol h�1$gcat�1. In the same year, He et al. [194] prepared Z-scheme ZnO/g-C3N4 photocatalytic materials. The photocatalyticreduction rate of CO2 reached 45.6 mmol h�1$gcat�1. Similarly, Z-scheme heterojunction composites composed of g-C3N4 and Pd[195e198], SnO2 [199e202], BiOX(X¼Cl, Br, I) [203e206] exhibitedexcellent photocatalytic performance for CO2 reduction. However,the research on the CO2 reduction by 3D g-C3N4 compositematerialsis in a very initial stage. In 2015, Tong et al. [149] employed a 3Dporous aerogel composed of g-C3N4 and graphene oxide (CNGA) toconvert CO2 into CO by photocatalysis. The yield of 6 hwas as high as23mmoL/g, whichwas 2.3 times asmuch as that of powdery g-C3N4.This indicates that the 3D g-C3N4 composite materials have greatpotential for photocatalytic CO2 reduction. It was found that thehydrogen evolution reaction (HER) often competes with CO2reduction for photogenerated electrons, resulting in a decrease in theselectivity of CO2 reduction [207e209]. Recently, the researchers[210] used a nanosheet structure of carbon nitride (NSeC3N4)instead of the previous mesoporous g-C3N4 as a carrier to obtain anovel photocatalyst RuRu'/Ag/NS-C3N4. Through the tacit synergy ofeach part of the catalyst, the high selectivity (99%) of the photo-catalytic CO2 reduction to formate in the aqueousphaseunder visiblelightwas successfully achieved,whichwas thehighest CO2 reductionselectivity of the similar catalyst reported so far. In a comparativeexperiment with the addition of different salts, the researchersfound that the selectivity of the photocatalyst RuRu'/Ag/NS-C3N4 forthe CO2 reduction to formate and hydrogen (without CO) wasgradually increased in the following order: no salt added (selectivity76%)<NaH2PO4<NaH2PO4þNa2HPO4 <Na2CO3, NaHCO3 orNa2HPO4 (selectivity >93%). This result was related to an increase inpH. The experimental results showed that the pH value of the solu-tion had an important influence on the photocatalytic performanceof RuRu'/Ag/NS-C3N4. A series of comparative experiments showedthat the selectivity of CO2 reduction was linear with the pH of the

H2 evolution Quantum efficiency (%) Ref.

22 mmol/8 h 0.4 [129]

29 mmoL/h 0.284 [135]35 mmoL/h 0.5 [137]

solution (20 vol%) 3579mmol$h�1$g�1

AQE: 27.8 [138]

251.7 mmol h�1 g�1 0.16 [152]130 mmol/4 h 0.66 [174]

X. Li et al. / Journal of Alloys and Compounds 802 (2019) 196e209 205

solution. The conduction band and valence band positions of NS-C3N4 are negatively shifted with increasing pH, which promotes thetransfer of electrons from the conduction band of NS-C3N4 to theRuRu' excited state.

4. Conclusion and perspectives

3D g-C3N4 composite materials are promising photocatalyticmaterials in energy utilization and environmental purification,which are also very conducive to recycling without secondarypollution. In this paper, the latest research progresses in thepreparation of 3D g-C3N4 composite materials, and their applica-tions in photocatalytic water splitting, degradation of organic pol-lutants, and CO2 reduction are mainly introduced. There are manyways to synthesize 3D g-C3N4 composite materials, most of whichcan be summarized into two categories: embedded and self-assembly strategies. There are some other preparation methods,which should be decided according to the characteristics andstructure of the material itself. It can be found that the 3D g-C3N4composite materials possess a stable 3D framework, high porosity,high specific surface area, reasonable particle size distribution andhigh active sites, thus effectively improving the photocatalytic ac-tivity. However, there are still some problems and challenges thatrestrict the large-scale application of 3D g-C3N4 compositephotocatalysts:

(1) Although a great progress has been made in the modificationof g-C3N4, the utilization of visible light by g-C3N4 is stillconcentrated near blue-violet light, and consequently, theefficiency of sunlight utilization or light-harvesting is nothigh. The practical application of 3D g-C3N4 compositephotocatalysts is greatly restricted by photocatalytic corro-sion and catalyst dissolution during photocatalysis. There-fore, we also need to develop 3D g-C3N4 compositephotocatalysts with tighter bonding between componentsand more stable structure.

(2) 3D g-C3N4 composite photocatalysts possess a certain phys-ical strength, but there is still a big gap between the actualapplication of the strength requirements. Moreover, in the 3Dframework, a single component is prone to aggregate, leadingto poor overall dispersion. At present, the hydrothermalmethod and solvothermal methods are mostly applied toprepare 3D g-C3N4 composite photocatalysts, which alsohinder the large-scale preparation of catalysts. Therefore,how to improve the preparation process and shorten thepreparation cycle is still an urgent problem to be solved.

(3) The photocatalytic reactions occur on the surface of thecatalyst, and the researches on the surface reaction mecha-nism and surface modification of 3D g-C3N4 compositephotocatalysts are still in the initial stage. How to improvethe separation and transfer of photogenerated electron-holepairs? How to effectively enhance the utilization of sunlight?All these need to be supported by a sound theoretical basis.

(4) 3D g-C3N4 composite photocatalysts possess a 3D porousstructure, which can adsorb reactants well and providefavorable reaction sites. However, how reactants and productmolecules infiltrate and diffuse into and out of the porouscatalysts? It is an important and urgent task to extrapolateand improve the mechanism of photocatalytic reaction by insitu observation, characterization and theoretical calculationfrom the point of view of thermodynamics and kinetics.

Acknowledgements

The authors acknowledge the financial support from the

National Natural Science Foundation of China (Grant No. 51772140),the Natural Science Foundation of Jiangxi Province, China (GrantNo. 20161BAB206111, 20171ACB21033), the Scientific ResearchFoundation of Jiangxi Provincial Education Department, China(Grant no. GJJ170578).

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