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Visible Light-Driven Photocatalysts for Environmental Applications Based on Graphitic Carbon Nitride

  • Waseem RazaEmail author
  • Khursheed Ahmad
Living reference work entry
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Abstract

With the fast growth of the world population, advancement in living standards and rapid and uncontrolled development in industries have led to unstoppable release of organic, inorganic, and toxic industrial waste containing nonbiodegradable pollutants into the water system. Therefore, above pollutants in freshwater resulted in an environmental issue due to its detrimental effect on human health. Great efforts have been done in order to solve the industrial and environmental problems faced at global scale. Photocatalysis, a green and promising alternative approach, has attracted worldwide scientific interest due to complete degradation of pollutant. Graphitic carbon nitride (g-C3N4) has attracted growing attention due to its fascinating properties, such as nontoxicity, low-cost fabrication, promising electronic band structure, and high thermal and chemical stability as well as visible light harvesting property. However, the photocatalytic performance of bulk g-C3N4 is limited at practical level due to its rapid recombination and delay in the transfer of photogenerated charge carriers. To overcome the innate problems and enhanced the photocatalytic performance of bulk g-C3N4, different methods have been applied. Among all designing a heterojunction semiconductor is a powerful approach to extend the photoresponsive range into visible region as well as promote the charge separation and transfer for enhancing the photocatalytic activity. Therefore, this chapter explores and summarizes the effective approach to construct the heterojunction for photocatalytic water treatment. Overall, it also assumes that this chapter will encourage further research and will open up new possibilities to construct new heterojunctions with g-C3N4.

Keywords

Graphitic carbon nitride Photocatalysis Heterojunction Visible light Pollutants 

Introduction

Semiconductor heterogeneous photocatalysis has been drawing a tremendous consideration as an environmentally friendly technology, and offering a great ability for the worldwide pollution remediation [1, 2]. Photocatalysis is a catalytic process which takes place at the surface of a semiconductor in the presence of the illumination of light. The use of the biggest renewable energy source, i.e., solar light, can solve the problems of energy and the environment at global level [3, 4]. Therefore, the usage of solar energy in semiconductor heterogeneous photocatalysis has been treated as a clean, safe, economical, and green approach for environmental applications [5]. Over the past few decades, traditional TiO2 is the universally employed semiconductor photocatalytic material. However, the industrial application of TiO2 for photocatalytic degradation is limited due to its large bandgap, i.e., 3.2 that can utilize only UV light, which is 2–3% of solar energy and fast recombination rate of photogenerated charge carrier [6, 7, 8, 9]. Therefore, tremendous efforts have been dedicated to develop an efficient and stable photocatalyst.

Very recently polymeric graphitic carbon nitride has been drawing greater attention and established to be a promising metal-free photocatalyst due to its huge chemical and thermal stability, large surface area, easy preparation, suitable bandgap of 2.7, and utilization of blue light up to 450 nm [10, 11]. Moreover, g-C3N4 is made up of earth-abundant elements of C, N, and H with a graphitic π conjugated stacked structure constructed from tri-s-triazine or s-triazine repeating units which are connected to each other via tertiary amines as shown in Fig. 1 [12].
Fig. 1

(a, b) Schematic diagram of g-C3N4 based on S-triazine and S-heptazine [13]

However, the atoms in the layers are organized in honeycomb-like structure having durable covalent bonds, and sheets are formed using weak van der Waals force. The valance band (VB) of g-C3N4 is consist of N 2p state, whereas conduction band (CB) of g-C3N4 is made up of hybridization of N 2p and C 2p states as shown in Fig. 2 [14]. The polymeric g-C3N4 was first prepared by scientist Berzelius; however its name melon was given by Liebig in 1834 [15]. Melon is considered as one of the oldest structures of synthetic polymer. The first structure of this compound was characterized by Franklin in 1922 [16].
Fig. 2

Schematic presentation of energy band structure diagram and density state of g-C3N4 [14]

Moreover, g-C3N4 occurs in five different allotropic forms such as α-C3N4, β-C3N4, cubic-C3N4, pseudocubic-C3N4, and graphitic-C3N4 [17]. Among these graphitic-C3N4 (g- C3N4) is found to be the most stable form at ambient condition [10]. g-C3N4 is made up layers of two-dimensional (2D) graphitic-like planner structure maintaining a distance of 0.326 nm; the carbon of π conjugated systems is substituted with N heteroatoms in graphitic framework. Due to the presence of strong covalent bond between carbon and nitrogen, g-C3N4 is stable in acid and basic medium being made up of only carbon and nitrogen. In 1,3,5-triazine ring, the nitrogen atom Nα is bonded to three-coordinated C-atom which is bonded to nitrogen atom Nβ; all are sp2 hybridized as shown in Fig. 3 [18].
Fig. 3

Schematic representation of single layer of g-C3N4, where nitrogen atoms fill the two different positions in the layer, labelled as α and β [18]

Wang et al. implement a density function theory (DFT) for getting the exact electronic structure of g-C3N4 [10]. It has been reported that valance band and conduction band are consist of nitrogen and carbon Pz orbitals, respectively. In the structure of g-C3N4, nitrogen atom acts as oxidation sites, whereas carbon atoms work as reduction sites. On the absorptiion of sun light equal to or greater than the bandgap of g-C3N4, than excitation of electrons and holes take place for oxidation and reduction reaction at nitrogen and carbon, respectively. Furthermore, the structure of g-C3N4 exhibits relationship between band structure and electronic transition, where nitrogen is supplying the lone pair to band structure to form a lone pair valance band. The lone pair state/lone pair valance band was stabilized by the mixing of π bonding electronic states and lone pair of nitrogen. Hence, for the formation of electronic structure g-C3N4, a lone pair of nitrogen plays a key role. Due to hybridization of nitrogen 2p and carbon 2p orbitals, the photogenerated electrons experience high recombination rate of electron hole pairs due to the presence of holes at nitrogen sites [17]. After the excitation of electron (e¯) from VB to CB leaving behind hole (h+) as shown in Eq. 1.
$$ \mathrm{g}\hbox{-} {\mathrm{C}}_3{\mathrm{N}}_4+\mathrm{h}\upnu =\mathrm{g}\hbox{-} {\mathrm{C}}_3{\mathrm{N}}_4-{\mathrm{h}}^{+}+\mathrm{g}\hbox{-} {\mathrm{C}}_3{\mathrm{N}}_4-{\mathrm{e}}^{-} $$
(1)
Due to fast recombination rate of photogenerated charge carrier, some of the electrons and holes still remain available as arrested by defect sites in g-C3N4 (N-sites), which can transfer to the surface of semiconductor with slightly lower energy than actual electron hole pairs [19]. Due to recombination of electron and hole pairs, energy was lost in the form of releasing heat or light emission. Generally separated electron and hole pairs at the surface of semiconductors act as electron donors and acceptors, which participate in redox reaction for producing radical species [20, 21]. However, the redox ability of the photogenerated charge carrier during surface reaction depend on the potential of VB and CB of semiconductor (i.e., =g-C3N4). According to the DFT calculation, the potentials of VB and CB were found to be +1.57 and −1.12 eV, respectively [22, 23]. The reported value for CB potential of commercial semiconductors were found to be −0.29 for TiO2 and −0.31 for ZnO, which is less negative than g-C3N4 [24, 25, 26, 27]. Therefore, photoexcited electrons can easily produce superoxide radical species after reacting with oxygen as the reduction potential of O2¯/O2 (−0.13 eV vs. NHE) is less than the CB of g-C3N4 [28]. However, hydroxyl radical production cannot be achieved by holes generated by g-C3N4 because the reduction potential for OH/¯OH (+1.99 eV vs. NHE) is much higher than the reduction potential of EVB of g-C3N4 [29]. But few hydroxyl radicals can be generated via multi-electron reaction with absorbed oxygen as shown in the equations [19].
$$ \mathrm{g}\hbox{-} {\mathrm{C}}_3{\mathrm{N}}_4-{\mathrm{e}}^{-}+{\mathrm{O}}_2=\, \mathrm{g}\hbox{-} {\mathrm{C}}_3{\mathrm{N}}_4+{{\mathrm{O}}_2}^{\bullet -} $$
(2)
$$ {\mathrm{O}}_2+2{\mathrm{H}}^{+}+3\mathrm{g}\hbox{-} {\mathrm{C}}_3{\mathrm{N}}_4-{\mathrm{e}}^{-}\, =^{-}\mathrm{OH} +^{\bullet}\mathrm{OH}+\mathrm{g}\hbox{-} {\mathrm{C}}_3{\mathrm{N}}_4 $$
(3)
The photogenerated reactive species such as superoxide radicals and h+ play a key role for the degradation of pollutants into CO2, H2O, and other smaller products during photocatalysis on the surface of g-C3N4 as shown in Fig. 4 [13].
$$ \mathrm{Organic}\ \mathrm{products}+\mathrm{g}\hbox{-} {\mathrm{C}}_3{\mathrm{N}}_4-{\mathrm{h}}^{+}=\mathrm{g}\hbox{-} {\mathrm{C}}_3{\mathrm{N}}_4+{\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O} $$
(4)
$$ \mathrm{Organic}\ \mathrm{products}+{{\mathrm{O}}_2}^{\bullet -}\bar{\mkern6mu}={\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{intermediate} $$
(5)
Fig. 4

Pollutant degradation in the presence of bare graphitic carbon nitride under the visible light illumination [13]

However, the industrial application of pristine g-C3N4 is still hindered by some drawbacks such as high recombination rate of photogenerated charge carriers, low electrical conductivity, quantum efficiency, and specific surface area limiting the sorption capacity of g-C3N4. Moreover, disturbance in the electron delocalization at the surface of a photocatalyst is due to grain boundary effect. To solve these inherent problems of pristine g-C3N4, various efficient ways have been carried out to enhance the photocatalytic performance, such as synthesis of mesoporous and order mesoporous g-C3N4 [30, 31, 32, 33, 34]; synthesis of morphologically and texturally controlled g-C3N4 [35, 36, 37, 38, 39]; and doping of different non-metals such as B, S, P, F, I, etc. within g-C3N4 as well as loading of co-catalysts such as Ag, Pt, Au, etc. [35, 40, 41, 42, 43, 44, 45, 46]. Moreover, the construction of heterojunction/composites with transition metals/other metal oxide semiconductors [47, 48, 49, 50, 51, 52, 53, 54, 55, 56]. Among all approaches, designing a heterojunction-type system by combining different semiconductor with 2D g-C3N4 sheets has been inviting greater attraction because it’s not only beneficial to enhancing the rate of charge transfer and the utilization of sunlight but also generates plentiful potential which is favorable for the separation of photogenerated charge carrier by reducing electron hole recombination rate [57, 58].

To date, various fascinating and important discoveries on superior engineering of g-C3N4 have been reported previously. However, efficient and systematic alteration of pristine g-C3N4 for water treatment on construction of heterojunction has not yet been reported, which is needed to promote further progress for its targeted photocatalytic performance. Hence, this chapter will provide a revised sketch on g-C3N4-based photocatalysts for environmental monitoring via the designing of heterojunction approach. Finally, this chapter will give a perspective for emerging circulations and the advancement of visible light-induced g-C3N4 heterostructures, which will inspire the further investigation on advance opportunity for the designing of new heterojuctions.

g-C3N4-Based Heterojunction as Photocatalysts for Pollutant Remediation

The degradation of pollutants in the presence of g-C3N4 is a multiple reaction pathway and requires five principle steps. Typically, pollutant passes onto the interphase of g-C3N4 and forms the external aqueous solution. After that these pollutants are adsorbed by g-C3N4; where the generation of photogenerated charge carrier takes place after shine light is equal to or greater than the bandgap of g-C3N4. The photogenerated charge carrier produced reactive species (O2¯, OH and h+) after several reaction which takes part in the mineralization of pollutants. After that degraded products were discharged from g-C3N4 surface to the interface. Finally, the immediate degraded pollutants were released to bulk aqueous solution [19, 59]. Therefore, all five steps are critical for the photodegradation point of view and mainly limited by slowest reaction step [59]. However, photocatalytic activity of g-C3N4 also depends on innate personality due to incomplete formation of hydroxyl radicals and high recombination rate. Therefore, speeding up the separation and transfer of photogenerated charge carriers is key a factor in g-C3N4 for enhanced photocatalytic application. Hence, to improve the charge separation and transfer, close contact is needed in between electron donor, acceptor, and g-C3N4, which also reduced the recombination rate efficiently [20]. Herein we will discuss the designing of heterojunction for efficient charge transfer and separation for improved photocatalytic application.

Designing of heterojunctions/composites of g-C3N4 with a suitable semiconductor is an impressive approach for enhanced photocatalytic application because 2D g-C3N4 can provide a suitable scaffold for close contacting with other semiconductors. However, construction of heterojunction is based on band alignment of g-C3N4 and other semiconductors to attain the better performance for photogenerated charge transfer and separation. There are three different possible ways for designing the heterojunctions as explained below.
  1. 1.

    Heterojunctions formation between larger bandgap like TiO2, ZnO, etc. and g-C3N4, where photogenerated electrons can transferred to the CB of longer band gap semiconductors from g-C3N4 which takes part in reduction and holes are transfer to VB of g-C3N4 resulting efficient charge separation as shown in Fig. 5.

     
  2. 2.

    Construction of heterojunction/composite between semiconductors where both are visible light active and generation of electron hole pairs take place as in the case of g-C3N4 and Cu2O. The band gap of g-C3N4 and Cu2O are found to be 2.7 and 2.0 eV; hence photogenerated electrons from higher CB of Cu2O can transfer to lower CB of g-C3N4, whereas photogenerated holes can migrate toward the opposite direction from lower VB of g-C3N4 to higher VB of Cu2O as shown in Fig. 6a [60]. Therefore, double-charge transfer effectively separates the photogenerated electron hole pairs resulting in improvement in photocatalytic performance.

     
  3. 3.

    Architecture of Z-scheme type heterojunction between g-C3N4 and semiconductors with appropriate band-edge potential such as MoO3, which can effectively reduce the recombination rate of photogenerated charge carriers as well as widen the visible light absorption capacity. In this mechanism, the photogenerated electrons of the other semiconductor with lower CB than g-C3N4 is transferred toward the higher VB of g-C3N4 which can recombine each other. Hence, the remaining electron in higher CB of g-C3N4 has vigorous reducibility, and the remaining hole in VB of lower potential of other semiconductor has strong oxidizing power as shown in Fig. 6b [60]. Therefore, these strong reducing electrons and durable oxidizing holes participate in the photocatalytic mechanism.

     
Fig. 5

Schematic presentation of charge transfer and separation in heterojunction between g-C3N4 and longer bandgap semiconductor [60]

Fig. 6

Schematic illustration for charge separation and transfer (a) via heterojunction of g-C3N4 with lower bandgap and (b) via Z-scheme mechanism [60]

Construction of Heterojunctions of g-C3N4 with Longer Bandgap Semiconductors

Different varieties of heterojunctions of g-C3N4 with wide bandgap semiconductors like transition metal oxides have been reported with enhanced photocatalytic performance for environmental applications. However, most of the transition metal oxides having larger and wide band gap are therefore activated only by UV light illumination. Hence, the construction of heterojunctions of such UV-light-activated semiconductors with narrow band semiconductor like g-C3N4 is an appealing approach to increase the photocatalytic activity in visible light region.

It has been reported that g-C3N4-ZnO heterojunction/composite was synthesized via direct calcination of melamine at 520 °C showing strong bond formation of Zn-N between amino groups of triazine and the hydroxyl group of ZnO due to condensation [61]. A simple impregnation method was applied for the synthesis of highly active TiO2/g-C3N4 composite by mixing of g-C3N4 and TiO2 as reported by Yan et al. [62]. Mesoporous TiO2/g-C3N4 microspheres with enhanced visible light photocatalytic activity was synthesized via a facile nanocoating approach by mixing of porous TiO2 as an active supporting scaffold and g-C3N4 as the visible light sensitizer for the degradation of phenol [63]. M. Reli reported the synthesis of novel TiO2/C3N4 photocatalyst for decomposition of N2O using simple mechanical mixing of pure g-C3N4 and commercial TiO2 Evonik P25 [64]. ZnWO4/g-C3N4 heterojunction was synthesized by calcination of methanol suspension, which was made by adding ZnWO4 and g-C3N4 at 250 °C for 1 h for degradation of methylene blue (MB) under visible light [65]. J. Li et al. synthesized the g-C3N4/ZnO/halloysite nanotube (HNT) nanocomposite photocatalyst with improved photocatalytic activity for the degradation of tetracycline using facile calcination method [66]. Firstly, they synthesized the ZnO/HNT by calcination in air, which was achieved via mixture of melamine and ZnO/HNT and calcined at 450 ° to get g-C3N4-ZnO/HNT composite as shown in Fig. 7a. The morphology and microstructure g-C3N4-ZnO/HNT were employed using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution (HR-TEM). It could be seen from Fig. 7b that the SEM image of synthesized composite exhibits the aggregated HNTs’ cylindrical structure with sphere-like ZnO on g-C3N4 layered with smooth surface. However, TEM image shows the formation of heterojunction between g-C3N4 and ZnO, which was further confirmed by HR-TEM showing lattice fringe 0.281 nm corresponding to 100 plane of ZnO, whereas another lattice fringe is assigned to g-C3N4 as shown in Fig. 7c and d. The enhanced photocatalytic activity of the as-synthesized composite g-C3N4-ZnO/HNT was examined by degrading the tetracycline (TC) under visible light illumination. Figure 7e exhibits the degradation rate of TC in the presence of different composites. However, 20% of g-C3N4 exhibits the highest activity toward the degradation of TC up to 87% within 1 h among all composites as well as higher than ZnO/HNTs. It may be due to the introduction of g-C3N4 to ZnO/HNTs which can actually improve the visible light response with enhanced separation of charge carriers and transfer resulting in excellent photocatalytic activity. Moreover stability, which is a very precious data of the quality of photocatalyst and important evidence for further use in the environmental applications, was also carried out for TC degradation under visible light illumination. It could be seen from Fig. 7f that comparative multi-cycle degradation of TC in the presence of g-C3N4-ZnO/HNT and ZnO/HNT indicates good stability of g-C3N4-ZnO/HNT as compared to ZnO/HNT. It may be due to the formation of heterojunction between g-C3N4 and ZnO which increases the transfer and separation of charge carriers.
Fig. 7

(a) Schematic presentation of synthesis of g-C3N4-ZnO/HNT composite, (b) SEM, (c) TEM and (d) HR-TEM images of g-C3N4-ZnO/HNT, (e) photocatalytic degradation of TC in the presence of g-C3N4-ZnO/HNTs with different content of g-C3N4, (f) multi-cyclic experiments for the degradation of TC in the presence of g-C3N4-ZnO/HNT composite with 20% g-C3N4 content under visible light illumination [66]

The enhanced photocatalytic mechanism for g-C3N4-ZnO/HNT composite was also studied for the degradation of TC under visible light irradiation. The efficient charge transfer and separation is a pivotal factor for improved photocatalytic performance. Therefore, potential band edges of CBs and VBs of ZnO and g-C3N4 were calculated and found to be −1.13, −0.5 eV (CB) and +1.57, +2.8 eV (VB) for g-C3N4 and ZnO, respectively. Hence, when composite was illuminated by visible light, then excited electron simultaneously can transfer from CB of g-C3N4 to CB ZnO as the CB potential of g-C3N4 is more negative than ZnO leaving holes in the VB of g-C3N4. Smooth close contact is required between semiconductors for fast transfer of photogenerated charge carriers, which was formed by the calcination method used in this paper which produces some new chemical bonds. The electrons in CB of ZnO and holes in VB of g-C3N4 have sufficient reducing and oxidizing powers to produces reactive species, which are responsible for the degradation of TC.

Ultrasonic-assisted deposition method was applied for the synthesis of SnO2/g-C3N4 heterojunction/composite using melamine and SnCl4·5H2O as the starting materials in methanol at room temperature [67]. They reported that sphere-like SnO2 having a size of 2–3 nm were dispersed on the surface of g-C3N4 sheets exhibiting excellent contact in SnO2/g-C3N4 heterojunction, which is beneficial for outstanding visible light photocatalytic activity. L. Huang et al. reported the architecture of CeO2/g-C3N4 visible light-induced heterojunction photocatalyst using simple mixing of CeO2 and appropriate amount of g-C3N4 via solid-state decomposition and calcination method at 500 °C [68]. Moreover, photocatalytic activity of the as-synthesized heterojunction was investigated by studying the degradation of MB and 4-chlorophenol under visible light illumination. However, 13% CeO2/g-C3N4 displayed the highest photocatalytic activity for pollutant treatment under visible light, which may be due to synergetic effect between CeO2 and g-C3N4 (Fig. 8).
Fig. 8

Schematic illustration for the transfer and separation of charge carrier in g-C3N4-ZnO/HNT composite under visible light illumination [66]

Designing the Heterojunctions Between g-C3N4 and Similar or Smaller Bandgap Semiconductors

Designing and exploitation of economical photocatalytic system based on visible light irritation have been attracting extensive attention in the field of environmental protection. Therefore, Wang et al. reported the facile electrospinning method for the preparation of g-C3N4/BiVO4 composite having 3.9 wt % of g-C3N4 and 450 °C temperature for 40 min electrospinning time [69]. The photocatalytic activity of heterojunction was found to be higher than individual g-C3N4 and BiVO4 due to improved charge separation. Moreover, g-C3N4 sheets not only form heterojunction but also inhibit the agglomeration rate of BiVO4, resulting in good coverage and stability. Decoration of methylamine lead bromide perovskite nanoparticles on protonated (P) g-C3N4 sheets was carried out to form an intrinsic type II band structure of MAPBBr3/P-g-C3N4 first time reported by Pu et al. [70]. The catalytic efficiency of the as synthesized MAPBBr3/P-g-C3N4 nanocomposite was examined by studying the degradation of p-nitrophenol under visible light illumination. The nanocomposite presenting a remarkable photodegradation of p-nitrophenol, which may be due to excellent charge transfer and separation as well as great potential to convert solar energy to chemical energy. Plasmonic Bi metal was deposited in situ on g-C3N4@Bi2WO6 microsphere using hydrothermal method as a low-cost semiconductor-based semiconductor for environmental remediation [71]. Due to surface plasmon resonance effect of Bi metal act as electron-conduction bridged between g-C3N4 and the surface of Bi2WO6 microsphere to improved visible light absorption ability of composite as well as promote the dynamic separation of photogenerated charge carriers. Two-dimensional Caln2S4/g-C3N4 heterojunction composite was synthesized using a facile two-step ultrasonication and hydrothermal method at 120 °C for 24 h with intimate contact for degradation of methyl orange (MO) under visible light illumination [72]. The optimal (50%) Caln2S4/g-C3N4 heterojunction exhibits the highest degradation of MO than pristine as well as other composites, that may be due to the cozy contact between both semiconductors which facilitate the excellent separation and transfer of charge carriers. A plausible mechanism was also proposed for the degradation of MO over Caln2S4/g-C3N4 heterojunction. Upon shining the visible light, photogenerated electrons and holes were transferred to CB and VB of Caln2S4 from CB and VB of g-C3N4, respectively. This heterojunction therefore promotes the separation and transfer of charge carrier resulting in enhanced photodegradation of MO because absorbed MO on the surface of composite immediately is attacked by reactive species (h+, O2•¯ and OH) as shown in Fig. 9.
Fig. 9

Schematic illustration of the possible charge transfer and separation for MO degradation under visible light illumination [72]

Raza et al. reported the synthesis of visible light-induced Na-g-C3N4/DyVO4 nanocomposite with superior photocatalytic activity for the degradation of organic pollutants using a facile two-step method such as ultrasonication followed by hydrothermal method at 180 °C for 10 h. [23] Figure 10a exhibits the preparation of Na-g-C3N4/DyVO4 heterojunction nanocomposite by mixing of previously prepared Na-g-C3N4 and DyVO4 in double distilled water using ultrasonication method at room temperature. The surface morphology and microstructure of the as-prepared heterojunction was studied by SEM, TEM, and energy-dispersive X-ray (EDX) analysis. It could be seen from Fig. 10b that SEM image exhibits the distribution of rod-like DyVO4 on the sheet of g-C3N4 resulting in the formation of heterojunction between two phases as shown in Fig. 10c. Further confirming the interfacial contact and surface, TEM analysis was carried out which exhibits the presence of two different phases as well as displays an aromatic ring-like layered structure which may be made up of tri-s-triazine units. Moreover, the presence of different types of metal and non-metal in the composite was analyzed using EDX analysis, indicating the presence of C, N, O, V, Dy, and Na elements as shown in Fig. 10d.
Fig. 10

(a) Synthesis of Na-g-C3N4 and DyVO4 heterojunction nanocomposite, (b) SEM, (c) TEM, and (d) EDX analysis of Na-g-C3N4 and DyVO4 [23]

The photocatalytic performance of the as synthesized Na-g-C3N4 and DyVO4 nanocomposite was examined by degrading the Rhodamine B (RhB) and 4-nitrophenol (4-NP) in aqueous suspension under visible light illumination [23]. Figure 11a exhibits the change in absorbance of RhB on the illumination of aqueous dye solution at different time intervals in the presence of optimum (15%) Na-g-C3N4 and DyVO4 nanocomposite. The main absorption peak at 553 nm was gradually decreases as the irradiation time increases, indicating the degradation of conjugated xanthene ring in the RhB dye as well as blue shifting in absorbance indicates N-deethylation process. The decrease in the maximum absorption peak indicates that 98% degradation of RhB was achieved within 80 min [23]. Moreover, the photocatalytic efficiency was also measured by studying the degradation of 4-NP under visible light. The degradation rate of 4-NP was examined using UV-visible spectrophotometer, which gives the change in maximum absorbance indicating reduction with irradiation time as shown in Fig. 11b. The results of experiments indicate that around 74% was achieved within 6 h in the presence of 15% Na-g-C3N4/DyVO4 nanocomposite under visible light illumination.
Fig. 11

Time-dependent UV-visible absorption spectra for the degradation of (a) RhB and (b) 4-NP [23]

The superior photodegradation of RhB and 4-NP in the presence of 15% Na-g-C3N4/DyVO4 nanocomposite under visible light illumination was explained in the form of mechanism as shown in Fig. 12. The outstanding photocatalytic performance of the 15% Na-g-C3N4/DyVO4 nanocomposite may be due to the formation of heterojunction between both phases resulting in an excellent transfer and separation of charge carriers which is possible via close and intimate contact. The constructed heterojunction also maintains synergistic effect resulting in prolonged lifetime for photogenerated charge carriers for the participation of degradation of pollutants leading to the highest photocatalytic activity.
Fig. 12

Schematic illustration for transfer and separation of photogenerated charge carrier at heterojunction interface for degradation of pollutants under visible light irradiation [23]

Architecture of Z-Scheme-Type Heterojunction Between g-C3N4 and Other Semiconductors with Pertinent Band-Edge Potential

Designing of 2D composites with layered structure of Z-scheme-type heterojunction is treated as a dynamic approach to attain the high efficiency of photocatalysts. Therefore, new Z-scheme type of LaFeO3/g-C3N4 composite was synthesized using calcination method by simple mixing of g-C3N4 and iron and lanthanum precursor at 450 °C for degradation MB under visible light irradiation [73]. Therefore, photoexcited electrons from CB of LaFeO3 were transferred to the VB of g-C3N4 via the close solid-solid intimate contact where they will be recombined resulting in the separation of the remaining photoexcited charge carriers. However, 5% LaFeO3/g-C3N4 composite presented the highest photodegradation of MB around 95% in 15 min which may be due to excellent separation of photogenerated charge carrier available for degradation. He et al. reported the synthesis of Z-scheme-type Ag3PO4/g-C3N4 composite using a simple in situ deposition method for the conversion of CO2 into valuable fuels under simulated visible light illumination for the first time [74]. The introduction of Ag3PO4 on g-C3N4 boosts the light absorption capacity of the material as well as precisely supports the excellent separation of electron hole pairs via Z-scheme mechanism. Wang el at. prepared the 3D g-C3N4/K2Ti6O13 heterostructure photocatalyst by mixing of 2D g-C3N4 and K2Ti6O13 nanobelts using facile mixing calcination approach for the degradation of MO under simulated sunlight and visible light for the first time [75]. The 3D hybrid architecture displayed excellent enhanced photocatalytic activity which may be due to the formation of Z-scheme resulting an outstanding interfacial charge separation, longer lifetimes, and powerful reduction and oxidation ability of photogenerated charge carriers. A novel RGO/g-C3N4/BiVO4 Z-scheme nanocomposite heterostructure was synthesized using a facile hydrothermal approach with improved photocatalytic activity for the degradation of tetracycline hydrochloride under visible light irradiation [76]. The excellent photocatalytic activity may be due to the formation of Z-scheme heterojunction resulting in efficient charge separation and synergistic effect between heterojunction. Therefore, it is assumed that the development of RGO/g-C3N4/BiVO4 Z-scheme heterojunction is an excellent approach to achieve the high-performance photocatalyst for the decomposition of tetracycline hydrochloride under visible light irradiation. Zhao et al. reported the synthesis of direct Z-scheme Bi2Sn2O7/g-C3N4 composite using high-temperature solid-state reaction approach for the degradation of MB and acid red 18 (AR 18) under visible light irradiation [77]. He at el. reported the synthesis of novel Z-scheme-type MoO3-g-C3N4 composite photocatalyst using simple mixing and calcination method at 400 °C for 2 h for the degradation of MO under visible light illumination [78]. The surface morphology was analyzed using TEM analysis, exhibiting the presence of two different phases where big black particles indicate the presence of MoO3 while attached adjacent particles are corresponding to g-C3N4 as shown in Fig. 13a. High-resolution TEM clearly exhibits the formation of heterojunction between two different phases indicating close contact, the big black particle having fringes about 0.3501 nm corresponding to (040) plane of MoO3 where the other phase is assigned to g-C3N4 as shown in Fig. 13b. In order to figure out the photocatalytic efficiency of MoO3–g-C3N4 composite, the study for the degradation of MO was examined as shown in Fig. 13c. The figure shows the change in concentration as a function of time in the presence of pristine and different wt% of MoO3–g-C3N4 composites, where 1.5 wt% MoO3–g-C3N4 shows the highest photocatalytic activity.
Fig. 13

(a) TEM, (b) HR-TEM of 1.5% MoO3–g-C3N4, (c) photocatalytic activities of pristine and different % of MoO3–g-C3N4 composites for the degradation of MO under visible light irradiation [78]

It has been suggested that the surface area of the photocatalyst plays a crucial role for the determination of photocatalytic performance. However, efficient charge separation of the MoO3–g-C3N4 composite photocatalyst has been used as important factor in the research. Therefore, superb photocatalytic efficiency of the prepared composite is dependent upon the excellent charge separation as well as synergistic effect between MoO3 and g-C3N4 via charge transfer between both semiconductors. However, explanation of final mechanism is a little difficult to disclose because band potential of both MoO3 and g-C3N4 semiconductors shows the condition of the two mechanisms. The photocatalytic activity is greatly affected by band potential of semiconductor; hence the change in the redox ability of MoO3/g-C3N4 composite can greatly influence the reactive species. However, reduction in hydroxyl radicals would be seen if MoO3–g-C3N4 composite would follow the double-charge mechanism. The electrons form CB of g-C3N4 semiconductor will transfer to CB of MoO3, and holes from VB of MoO3 will migrate to VB of g-C3N4 semiconductor as shown in Fig. 14a. However, the CB potential of MoO3 is found to be lower than the standard reduction potential of O2¯/O2, and VB of g-C3N4 is also found to be lower than standard potential of OH/¯OH. Hence, photogenerated electron hole pairs are not able to generate reactive species resulting in reduction in photocatalytic activity of the photocatalyst. Therefore, it is suggested that if MoO3–g-C3N4 composite follows the Z-scheme heterojunction-type mechanism, then surprisingly opposite result was obtained. As shown in Fig. 14b, Z-scheme mechanism gives the higher electron in CB of g-C3N4 and plenty of holes in VB of MoO3, which has good reducing and oxidizing power and able to generate reactive species which can easily degrade the pollutants.
Fig. 14

Schematic illustration for the separation and transport of photogenerated charge carriers under the visible light illumination (a) double-charge transfer mechanism, and (b) Z-scheme mechanism [78]

Conclusions and Further Outlook

In summary, this chapter focused on the principles of physicochemical properties of g-C3N4 semiconductor such as charge carrier dynamics, adsorption of pollutants, stability, and fundamentals on the photocatalytic degradation reactions. Moreover, degradation mechanism and identification and generation of reactive species are thoroughly discussed. Therefore, these factors are playing a crucial role for architecture and designing advance functional photocatalytic system for environmental applications. However, the paramount study of g-C3N4 as metal-free heterogeneous catalyst and their potential application on the catalytic, electrocatalytic, and photocatalytic performance is regularly increasing and wide in scope since its discovery as catalyst. Herein, we have introduced the memorable role of g-C3N4 in the photocatalysis for environmental application. The growing hunger for g-C3N4 may be due to their unique structure, intrinsic work modulations, excellent photoreaction mechanism, and potential applications. However, there are some shortcomings in its function as a photocatalyst for pollutant degradation such as fast recombination rate of photogenerated charge carriers, low surface area, and limited sunlight absorption, besides having unique properties. Recently, tremendous approaches have been applied in order to improve the photocatalytic performance by designing dynamic g-C3N4-based heterojunction photocatalysts. Therefore, architecture and designing of advance functional material for environmental applications via construction of heterojunctions of g-C3N4 with other semiconductors is an appealing approach to the synthesis of heterojunction composites with prolonged charge carrier lifetime.

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© Springer Nature Switzerland AG 2020

Authors and Affiliations

  1. 1.Department of ChemistryIndian Institute of Technology DelhiNew DelhiIndia
  2. 2.Discipline of ChemistryIndian Institute of Technology IndoreSimrolIndia

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