The Preparation and Applications of g-C₃N₄/TiO₂ Heterojunction Catalysts

Abstract

In the last few years, because of the better catalytic ability of heterojunction catalysts than that of single-component catalysts, more and more attention has been paid on the research of heterojunction catalysts. Various types of heterojunction catalysts have been reported like Bi₂O₃/Bi₂WO₆, WO₃/BiVO₄, SnO₂/TiO₂, CdS/TiO_2, g-C₃N₄/TiO₂, and Ta₃N₅/Pt/IrO₂. Among them, the heterojunction catalyst g-C₃N₄/TiO₂ has been researched tremendously recently because of its high activity, high thermal and chemical stability, and well-matched energy structure. Many approaches have been developed for its synthesis, such as hydrothermal growth of TiO₂ on g-C₃N₄, ball milling of g-C₃N₄ and TiO₂, and so on. In this chapter, the recent researches on the synthesis of g-C₃N₄/TiO₂ catalyst were summarized. In addition, the applications of g-C₃N₄/TiO₂ catalyst in the field of photocatalysis were introduced in detail.

7.1 Introduction

Nowadays, there have been two global problems all the society is facing, environmental pollution and energy shortage, which have caused great harm to human health and life. To solve these problems, photocatalysis as an effective approach has attracted widespread concern of researchers. In this approach, by utilizing photocatalysts, light as a clean excitation power can be used to induce a series of catalytic reactions, with regard to environment and energy, such as photocatalytic degradation of pollutants in water [1,2,3,4], removal of indoor harmful gases [5], reduction of CO₂ [6,7,8], as well as splitting of water to produce H₂ and O₂ [9].

In many of the photocatalysts studied, TiO₂ has been widely recognized as the most potential one due to its merits of low cost, good stability, nontoxicity, and so on [10,11,12,13]. However, the conventional TiO₂ has shortcomings in the following two aspects: (1) the large bandgap (~3.2 eV) can only absorb UV light with λ ≤ 387 nm, and the absorption of visible light is almost zero, which leads to the low utilization efficiency of sunlight, and (2) the recombination rate of photo-generated electron–hole pairs is high, greatly limiting the photocatalytic performance of TiO₂. At present, various methods for the improvement of the photocatalytic activity of TiO₂ have been reported, such as metal and nonmetal oxide loading [14, 15], noble metal deposition [16, 17], nonmetal element doping [18, 19], modifications of morphology, and so on [20,21,22,23]. These methods can broaden the absorption wavelength of TiO₂ and enhance the absorption efficiency of solar light in some extent. But the recombination of photo-generated electrons and holes results in a lower quantum yield, further seriously affecting the catalytic activity of the catalysts.

Heterojunction catalyst is one of the hot spots in catalytic field in recent years, which usually consists of two different semiconductors contacting with each other to form the structure of heterojunction. The heterojunction structure can promote the transfer of the photo-generated electrons and holes in opposite direction, greatly improving the effective utilization rate of the excitons. Therefore, heterojunction catalyst possesses better catalytic effect than that of single-component catalyst. Nowadays, there have been a great number of materials which can be used to modify TiO₂ by forming the heterojunction structure, such as ZnO [24, 25], SnO₂ [26, 27], g-C₃N₄, etc. [28,29,30,31]. Among them, g-C₃N₄ as a stable nonmetal semiconductor has attracted much attention in the catalytic field. Due to its special triazine structure, g-C₃N₄ exhibits many special properties including low density, high chemical stability, enhanced biological compatibility, good abrasion resistance, and so on. In addition, the relatively narrow bandgap of g-C₃N₄ (about 2.7 eV) extends its light response to visible region (about 450 nm). Zhang et al. reported that the g-C₃N₄ exhibited high photocatalytic performance for water splitting under visible light irradiation [32]. Dong and coworkers facilely synthesized polymeric g-C₃N₄-layered catalyst by directly heating urea or thiourea [33, 34]. In the research on heterojunction catalysts, g-C₃N₄ has gained the majority of researchers’ attention owing to its simple preparation method, abundant kinds of precursors, as well as the advantages of low cost, becoming the first choice to form heterojunction with TiO₂.

In this chapter, the recent developments of the research on the heterojunction photocatalysts formed by g-C₃N₄ and TiO₂ were introduced, including its synthesis methods and applications. Firstly, the synthesis methods were summarized based on the synthesis order of each component during the preparation process and divided into three categories. In each category, the preparation procedures as well as their advantages and drawbacks were introduced in detail. Through these synthesis methods, an efficient heterojunction structure can be obtained between g-C₃N₄ and TiO₂. The photocatalytic activity of the photocatalysts can be greatly enhanced due to the formation of the heterojunction structure, which can effectively promote the separation of photo-generated charge carriers [35, 36]. The excellent photocatalytic activity of the g-C₃N₄/TiO₂ heterojunction photocatalysts enables them to be applied in many aspects. Therefore, the chapter also introduced the applications of g-C₃N₄/TiO₂ heterojunction photocatalysts in the field of photocatalysis, containing depollution of environment, hydrogen generation, photofixation of carbon dioxide, bacteria disinfection, and so on. In the end of the chapter, a short summary and outlook on the development of g-C₃N₄/TiO₂ heterojunction photocatalysts were provided.

7.2 The Preparation Methods of g-C₃N₄/TiO₂ Heterojunction Catalyst

Because the g-C₃N₄/TiO₂ heterojunction photocatalyst consists of two single-component g-C₃N₄ and TiO₂, the synthesis methods of g-C₃N₄/TiO₂ heterojunction catalyst can be classified into three categories according to the order of synthesis of each component: (1) physically mixing TiO₂ and g-C₃N₄, (2) growing g-C₃N₄ on TiO₂ catalyst, and (3) loading TiO₂ on g-C₃N₄ catalyst. No matter what the method is, the final aim is to make g-C₃N₄ and TiO₂ contact with each other, further forming a heterojunction structure between g-C₃N₄ and TiO₂.

7.2.1 Physically Mixing g-C₃N₄ and TiO₂

This preparation method refers to firstly synthesizing g-C₃N₄ and TiO₂, respectively, and then physically mixing the two components together by ball milling or evaporation of dispersion solution. Hongjian Yan et al. prepared TiO₂-C₃N₄ by mixing TiO₂ and g-C₃N₄ powder using a ball milling method with different contents of g-C₃N₄. The TiO₂ was synthesized by the hydrolysis of TiCl₄ in ammonia, and the g-C₃N₄ was prepared by directly heating melamine at an atmosphere of Ar [37]. Yongfa Zhu et al. also fabricated g-C₃N₄/TiO₂ hybrid photocatalysts by a facile ball milling method. In their experiments, g-C₃N₄ was synthesized by directly heating melamine, and g-C₃N₄/TiO₂ photocatalysts were obtained by mixing g-C₃N₄ and TiO₂ powder in a ball mill. Their results showed that a layered structure of g-C₃N₄ was formed on the surface of TiO₂ [38]. Interestingly, they found that the as-prepared catalyst showed highly enhanced photocatalytic performance and the photocatalytic efficiency increased gradually with the increase of milling rate.

Apart from the ball milling, solvent evaporation is also a commonly used physical mixing method for the synthesis of g-C₃N₄/TiO₂ heterojunction photocatalyst. In this preparation process, g-C₃N₄ and TiO₂ are homogeneously dispersed in a solvent such as methanol, and then the solvent is evaporated to make g-C₃N₄ and TiO₂ contact with each other and thus form the heterojunction structure. Jingyu Wang et al. hybridized anatase TiO₂ nanosheets with dominant (001) facets with g-C₃N₄ via this facile solvent evaporation method. The polymeric g-C₃N₄ was synthesized by directly calcinating urea, and the anatase TiO₂ nanosheets with dominant (001) facets were prepared by a solvothermal reaction of tetrabutyl titanate (TBT). After the solvothermal treatment, the well-washed precipitate was dispersed into methanol and mixed with g-C₃N₄, followed by sonication for 30 min to completely disperse the g-C₃N₄. After that, the above sample was stirred in a fume hood for 12 h to evaporate the methanol, and the rest powder was dried at 100 °C for 4 h [39]. Dongjiang Yang et al. synthesized g-C₃N₄/TiO₂(B) nanofibers with exposed (001) plane with the enhanced visible light photoactivity through a facile solvent evaporation operation to the methanol solution of g-C₃N₄ and TiO₂ (B). The g-C₃N₄ was prepared by directly heating melamine in air at the temperature of 550 °C for 4 h, and TiO₂ (B) nanofibers were synthesized using a hydrothermal method combined with a subsequent calcination treatment [40]. Hong Huang et al. prepared heterostructured g-C₃N₄/Ag/TiO₂ microspheres with improved photocatalytic performance under visible light irradiation. As shown in Fig. 7.1, the protonated g-C₃N₄ sheets were synthesized by calcinating melamine and followed by the protonation in HCl solution, and TiO₂ nanomaterial was prepared by a typical hydrothermal method of Ti(OC₄H₉)₄, and then Ag/TiO₂ microspheres were obtained by depositing Ag on the surface of TiO₂ microspheres, which was then mixed with g-C₃N₄ by forming suspension at 70 °C [41].

Fig. 7.1

Scheme for the synthesis of g-C₃N₄/Ag/TiO₂ microspheres [41] (Reprinted with permission from Ref. [41]. Copyright 2014, American Chemical Society)

Guangshe Li et al. reported an effective visible light-driven photocatalyst of brookite TiO₂ (br-TiO₂) hybridized with g-C₃N₄ for the first time via a facile calcination of br-TiO₂ and g-C₃N₄ in air. The optimum photocatalytic activity of the as-prepared samples was higher than that of other phase types of TiO₂ (anatase and rutile) hybridized with g-C₃N₄ [42]. Tianyou Peng et al. synthesized porous g-C₃N₄ by a simple pyrolysis of urea, and Pt-TiO₂ was fabricated by photodepositing Pt on the TiO₂. Then g-C₃N₄–Pt-TiO₂ nanocomposite was synthesized via a facile chemical adsorption followed by a calcination treatment [30].

The physical mixing method for preparing g-C₃N₄/TiO₂ heterojunction catalyst is easy to operate and beneficial for scale-up, which provides a potential for mass production. However, uniform mixing may not be easy to achieve. In the aspect of designing catalyst, morphology control is widely considered as an effective way to improve the catalytic activity, while physical mixing method is difficult to achieve this goal. Moreover, the close contact between g-C₃N₄ and TiO₂ may be not easy to form, causing the poor stability of the heterojunction catalysts.

7.2.2 Growing TiO₂ on g-C₃N₄

In this method, g-C₃N₄ is firstly synthesized by one-step calcinations of precursors, and then the prepared g-C₃N₄ reacts with the precursor of TiO₂ to achieve the in situ growth of TiO₂ on the surface of g-C₃N₄. For example, Deliang Cui et al. fabricated g-C₃N₄/TiO₂ composite through this method. The g-C₃N₄ was synthesized by polymerization of dicyandiamide at the temperature of 600 °C for 5 h under N₂ atmosphere. Then the as-synthesized g-C₃N₄ was taken into the hydrolysis of Ti(OC₄H₉n)₄. After hydrothermal reaction, the hybrid composite of g-C₃N₄/TiO₂ was obtained, which showed better photocatalytic activity than hybrid composite of g-C₃N₄/TiO₂ and the pure TiO₂ for degradation of rhodamine B (RhB) under the UV and visible light irradiation [28]. Similarly, Hongtao Yu et al. prepared g-C₃N₄/TiO₂ hybrid photocatalyst with wide absorption wavelength range from 300 nm to 450 nm by taking g-C₃N₄ into the hydrolysis reaction of TiCl₄. TEM images showed that TiO₂ nanoparticles were dispersed well on the surface of g-C₃N₄ sheet, and the average size of particles was much smaller than that of TiO₂ samples without g-C₃N₄ sheet. The synthesized g-C₃N₄/TiO₂ exhibited much better photocatalytic activity for the degradation of phenol than pristine g-C₃N₄ and TiO₂ [43]. Qianhong Shen et al. developed a novel and facile template-free method to synthesize a network structure of mesoporous g-C₃N₄/TiO₂ nanocomposite with enhanced visible light photocatalytic activity. Firstly, they synthesized g-C₃N₄ by directly heating melamine, and then g-C₃N₄/TiO₂ was obtained by adding g-C₃N₄ into the solution of titanium sulfate Ti(SO₄)₂ and followed by hydrothermal reaction [44].

In recent years, more and more attention has been paid in the research on nitrogen-doped titanium dioxide (N-TiO₂), due to its promising extension for environmental application [2, 45]. Many groups grew the N-TiO₂ on the surface of g-C₃N₄ to form heterojunction. Fatang Li et al. reported an in situ microwave-assisted synthesis method to fabricate N-TiO₂/g-C₃N₄ composites by using H₂TiO₃ as the reactant and NH₃·H₂O as the N-doping source. In their experiments, they firstly took g-C₃N₄ into the H₂TiO₃ solution then followed by a microwave-assisted reaction. The preparation process was as shown in Fig. 7.2. The catalyst had a porous structure and large surface area, which increased the contact area of the catalyst with pollutants. The photocatalytic degradation of rhodamine B (RhB) and methylene blue (MB) with the as-prepared samples was carried out under visible light irradiation to evaluate the photocatalytic activity. Among them, N-TiO₂/g-C₃N₄ composite with 40 wt % of N-TiO₂ showed the highest photocatalytic activity [46].

Fig. 7.2

Diagrammatic sketch for the in situ deposition of N-TiO₂ nanoparticles on g-C₃N₄ sheets [46]. (Reprinted with permission from Ref. [46]. Copyright 2013, American Chemical Society)

Through heating the mixture of the hydrolysis product of TiCl₄ and g-C₃N₄ at different weight ratios, W. F. Zhang et al. successfully prepared N-doped TiO₂/C₃N₄ composite samples. Due to the introduction of g-C₃N₄, the composite samples showed slight visible light absorption. XPS result revealed that some nitrogen was doped into TiO_2, and g-C₃N₄ existed in the composite sample [29]. Similarly, as shown in Fig. 7.3, g-C₃N₄ nanosheets (g-C₃N₄ NSs) hybridized nitrogen-doped TiO₂ (N-TiO₂) nanofibers (GCN/NT NFs) have been synthesized in situ through a simple electrospinning process combined with a modified heat-etching method by Cheng Han et al. [47]. The melamine was thermal polymerized to form g-C₃N₄, which was dispersed into acetic acid solution including poly(vinylpyrrolidone) (PVP) and titanium (IV) n-butoxide (TNBT). Doping nitrogen into TiO₂ narrowed its energy bandgap, and the catalyst could be activated under visible irradiations, leading to higher photocatalytic efficiency.

Fig. 7.3

Schematic illustration of the fabrication of GCN/NT NFs [47]. (Reprinted with permission from Ref. [47]. Copyright 2013, Springer)

In addition, most of TiO₂ nanoparticles grown on the surface of g-C₃N₄ were present as crystals. Solvothermal reaction can control the exposure of high-energy surfaces. For example, Kangle Lv et al. grew TiO₂ hollow nanobox (TiO₂-HNB) assembled from high-energy TiO₂ nanosheets (TiO₂-NS) on g-C₃N₄ to form the g-C₃N₄/TiO₂ hybrid and investigated the effect of contact interfaces of high-energy TiO₂, (101) and (001) facets on the photocatalytic activity. The catalyst was fabricated through a solvothermal strategy using TBA as the solvent [48]. In our previous work, well-dispersed TiO₂ nanocrystals with (001) facets were successfully grown in situ on g-C₃N₄ through a facial solvothermal method, as shown in Fig. 7.4. During the solvothermal process, the ammonium acetate (AMAT) serving as a catalyst for the hydrolysis of tetrabutyl titanate (TBOT) was added into the nonaqueous system. In addition, because carboxylic acid is easy to adsorb on the surface of anatase (001), part of the acetic acid produced by the decomposition of AMAT serves as face-growth inhibitors, slowing the growth of the (001) facet of TiO₂ in the TiO₂ nanoparticles, leading to the exposure of high-energy facets. The characterization results showed an enhanced separation efficiency of photo-generated charge carriers compared with that of pure g-C₃N₄, and well-matched energy levels between TiO₂ and g-C₃N₄ altogether led to the enhancement of photocatalytic activity [49].

Fig. 7.4

(a and b) TEM images of TiO₂/g-C₃N₄(1.5) catalyst, (c) HRTEM image of TiO₂/g-C₃N₄(1.5) catalyst. Inset: the corresponding fast Fourier transform (FFT) pattern [49]. (Reprinted with permission from Ref. [49]. Copyright 2015, Royal Society of Chemistry)

The strategy of growing TiO₂ on g-C₃N₄ is very effective to form the heterojunction structure. The heterojunction which formed in the growing process possesses chemical stability during the multiple cycle experiments. However, due to the rapid hydrolysis process of the titanium precursor, it is difficult to achieve ultradispersed TiO₂ nanocrystals on the surface of g-C₃N₄ by this method [50]. Besides, a great challenge for controlling the microstructures of coupled TiO₂ with desired size distribution and dispersity still retains.

7.2.3 Loading g-C₃N₄ on TiO₂

In this synthesis method, TiO₂ is firstly obtained by hydrolysis, hydrothermal, microwave method, or directly using the commercial P25, and then TiO₂ is impregnated in the precursor solution of g-C₃N₄ to obtain the g-C₃N₄/TiO₂ heterojunction catalyst after drying and calcination. Weide Zhang et al. modified TiO₂ nanorod arrays with g-C₃N₄ via chemical vapor deposition using melamine as a precursor. The rutile TiO₂ nanorod arrays were firstly synthesized by hydrothermal process, and then the TiO₂/FTO was loaded with melamine and followed by heating process in a muffle furnace to obtain g-C₃N₄/TiO₂/FTO. The g-C₃N₄/TiO₂/FTO electrode exhibited high photoelectrocatalytic activity for degradation of RhB. Under visible light irradiation, the photocurrent response of the g-C₃N₄/TiO₂/FTO electrode is about 10 times as that of the TiO₂/FTO electrode, making it a promising nanomaterial for future applications in solar cells, water treatment, as well as photoelectric devices [51]. Min Fu et al. prepared a kind of novel visible light photocatalyst g-C₃N₄/TiO₂ composite by calcinating the mixtures of melamine and commercial TiO₂ at different weight ratios. In their work, the samples at the optimized precursor weight ratio (M_melamine: M_titania = 2.5) exhibited highest adsorption ability and visible light photocatalytic activity, evaluated by photocatalytic degradation of methylene blue (MB) [52]. Furthermore, Min Fu et al. also synthesized novel g-C₃N₄-coated TiO₂ nanocomposites by a facile and cost-effective solid-state method through thermal treatment of the mixture of urea and commercial TiO₂. The as-prepared g-C₃N₄-coated TiO₂ nanocomposites showed efficient visible light photocatalytic activity for degradation of aqueous MB owing to the increased visible light absorption and enhanced MB adsorption [31]. Burapat Inceesungvorn et al. fabricated g-C₃N₄/TiO₂ films by directly heating the mixture of melamine and pre-synthesized TiO₂ nanoparticles at the atmosphere of Ar. The TiO₂ was prepared by hydrolysis of titanium tetraisopropoxide (TTIP) and calcination. The obtained samples showed enhanced photocatalytic degradation of MB. In addition, as Fig. 7.5 shows, the 50 wt% g-C₃N₄/TiO₂ composite with the best loading content exhibited the best performance [53].

Fig. 7.5

MB photolysis and photocatalytic degradation using pure TiO₂, pure g-C₃N₄, and g-C₃N₄/TiO₂ composite films as photocatalysts [53]. (Reprinted with permission from Ref. [53]. Copyright 2014, Elsevier)

Honglei Zhu et al. synthesized a series of g-C₃N₄-P25 composite photocatalysts with different mass ratios using an in situ preparation method. In the method, g-C₃N₄-P25 nanocomposites were obtained by calcinating mixtures of the P25 and dicyandiamide. The optimal g-C₃N₄ content was determined to be 84%. The sample in the optimal weight ratio exhibited almost 3.3 times higher photocatalytic activity than that of individual g-C₃N₄ under visible light irradiation [54]. Our group also prepared g-C₃N₄-modified TiO₂ composites through a simple calcination process of anatase and cyanamide. TEM images of as-synthesized catalyst, presented in Fig. 7.6, show TiO₂ is covered by a thin shell of g-C₃N₄, and the polymer shell on the surface is around 5–10 nm thick. The photocatalytic activities of the composites were evaluated by photocatalytic degradation of Acid Orange 7 (AO7). The photocatalyst showed excellent activity under both visible and UV light. In addition, no nitrogen doping was found in TiO₂ lattice, demonstrating the g-C₃N₄ was surface attached on TiO₂ and ascribing all improvement of photocatalytic activity of g-C₃N₄/TiO₂ composite to the synergy between TiO₂ and g-C₃N₄ [55]. After that, we reported a highly condensed g-C₃N₄-modified TiO₂ photocatalyst prepared by a vacuum calcination method. A close-to-theoretical C/N ratio was detected in the catalyst by element analysis. The results indicated a complete and neat polymerization of the g-C₃N₄ on TiO₂. Excellent photocatalytic activities of as-prepared catalysts have been achieved under both visible and UV light irradiation. The heterojunction can be easily obtained during the calcination process, and the preparation procedures are easy to operate, but the amount of loading g-C₃N₄ is influenced by numerous factors, such as gas condition, flow rate, heating temperature, and heating rate [56].

Fig. 7.6

TEM image of g-C₃N₄-modified TiO₂ composites [55]. (Reprinted with permission from Ref. [55]. Copyright 2015, Elsevier)

Photochemical and electrochemical methods were also developed to load g-C₃N₄ on TiO₂. These methods are hard to control, but this in situ growth strategy has drawn more and more attention in recent years. Xiaoxin Zou et al. synthesized mesoporous TiO₂ spheres with a large surface area and rich surface hydroxyl groups by a light-driven synthetic strategy. It can be used for activating urea under a mild condition to form g-C₃N₄ material [57]. Xiaosong Zhou et al. synthesized a g-C₃N₄/TiO₂ nanotube array (CN/TNT) heterojunction photocatalyst with visible light response via a simple electrochemical method. g-C₃N₄ polymer was deposited into the crystallized TiO₂ nanotubes by electrodeposition [58].

In this method, because TiO₂ is prepared firstly, it is allowed for selection or structure design of TiO₂, but the high-temperature calcination for the formation of g-C₃N₄ is prone to resulting in the aggregation of TiO₂ and may lead to a negative impact on the improvement of photocatalytic activity.

7.3 The Applications of g-C₃N₄/TiO₂ Heterojunction Catalyst

Compared with single-component catalysts, the g-C₃N₄/TiO₂ heterojunction catalysts formed by the combination of g-C₃N₄ and TiO₂ show greatly enhanced photocatalytic activity. Therefore, fabricating the g-C₃N₄/TiO₂ has many promising applications in various fields of photocatalysis. Currently, the researches on the applications of g-C₃N₄/TiO₂ mainly focus on the degradation of organic pollutants, hydrogen generation from water, photocatalytic reduction of CO₂, treatment of heavy metal ion, and inactivation of bacteria.

7.3.1 Degradation of Organic Pollutants

With the rapid development of economy, environmental pollution problems have greatly affected our daily lives, among which the most serious problems are water pollution and air pollution. The majority in the source of pollution is organic pollutants. Therefore, the degradation of organic pollutants is a hot research topic in recent decades. Various kinds of g-C₃N₄/TiO₂ heterojunction catalysts have also been developed and applied to solve these pollution problems.

7.3.1.1 Degradation of Pollutants in Liquid Phase

Many research works have been carried out to examine the photocatalytic degradation of organic dyes such as RhB and AO7 in aqueous solution in the presence of g-C₃N₄/TiO₂ heterojunction catalyst. For instance, methyl blue (MB) was degraded by g-C₃N₄/TiO₂ catalyst which was synthesized by directly heating the mixture of urea and commercial TiO₂ [31]. The catalyst exhibited efficient photocatalytic degradation of MB under visible light irradiation. The degradation efficiency can be adjusted by tuning the treatment temperature in the synthesis process of the composite catalyst, and the g-C₃N₄/TiO₂ nanocomposite prepared at 450 °C exhibited the best photocatalytic performance, which was much higher than the pure TiO₂. In addition to the preparation temperature, the mass ratio between g-C₃N₄ and TiO₂ also had a great influence on the degradation efficiency. Honglei Zhu et al. fabricated g-C₃N₄-P25 composite catalysts with different mass ratios and examined their photocatalytic activity toward the degradation of MB [54]. As shown in Fig. 7.7, the degradation efficiency varied with different g-C₃N₄ and P25 mass ratios. The sample with an optimal g-C₃N₄ content of 88% exhibited the highest photocatalytic activity which was almost 3.3 times higher than that of pure g-C₃N₄ under visible light irradiation.

Fig. 7.7

(a) Photolysis and photocatalytic degradation of MB with TiO₂, g-C₃N₄, and g-C₃N₄-P25 photocatalysts. (b) Degradation rate constants of MB over TiO₂, g-C₃N₄, and g-C₃N₄-P25 photocatalysts [54]. (Reprinted with permission from Ref. [54]. Copyright 2015, Springer)

In addition to MB, other dyes were also degraded by the g-C₃N₄/TiO₂ heterojunction photocatalysts. Photocatalytic degradation of RhB and MB was carried out by Fatang Li et al. to test the visible light photocatalytic activity of N-TiO₂/g-C₃N₄. As Fig. 7.8 shows, N-TiO₂/g-C₃N₄ composite with 40 wt% of N-TiO₂ showed the highest photocatalytic activity. The efficient separation of photo-generated electrons and holes, which resulted from the formation of N-TiO₂/g-C₃N₄ heterostructure, led to the excellent photocatalytic performance [46]. Guohong Wang et al. also degraded RhB using a novel macro-/mesoporous g-C₃N₄/TiO₂ heterojunction photocatalyst. The good photocatalytic activity of this kind of product ascribed to the fact that the sample possessed a large specific surface area and an excellent heterostructure [59]. Xiaosong Zhou et al. synthesized a carbon nitride/TiO₂ nanotube array (CN/TNT), and the catalyst exhibited high photocatalytic activity toward the degradation of methyl orange (MO) [58]. They prepared the photocatalysts denoted as CT_x (x represents the deposition time) by electrodeposition of g-C₃N₄ into the crystallized TiO₂ nanotubes. Their experimental results showed that the photocatalytic activities of CN/TNTs increased as the deposition time increased at the first, then decreased, and the CT_5.0 exhibited the highest photocatalytic activity.

Fig. 7.8

Photocatalytic activities of N-TiO₂, g-C₃N₄, and N-TiO₂/g-C₃N₄ composites on the photodegradation of (a) RhB and (b) MB driven by visible light irradiation [46]. (Reprinted with permission from Ref. [46]. Copyright 2013, American Chemical Society)

Dongjiang Yang et al. synthesized g-C₃N₄/TiO₂(B) nanofibers with selective exposure of high-energy (001) plane and applied it to the degradation of sulforhodamine B (SRB) dye. Figure 7.9 revealed that the diffraction spots of electron diffraction pattern (EDP) of samples could be indexed as (110), (1–10), and (020). The g-C₃N₄/TiO₂(B) system showed better photocatalytic degradation ability than the g-C₃N₄/anatase system, although the photocatalytic activity of the anatase nanofibers was much better than that of the TiO₂(B) nanofibers [40].

Fig. 7.9

(a and b) TEM images of the g-C₃N₄/TiO₂(B)-1 catalyst; (c) EDP of the g-C₃N₄/TiO₂(B)-1 catalyst; (d) HRTEM image of a g-C₃N₄ nanoflake deposited on the TiO₂(B) nanofiber; (e) fast Fourier transformation (FFT) image of the joint area between TiO₂(B) and g-C₃N₄ in image; (D), (f) inverse fast Fourier transformation (IFFT) image of g-C₃N₄/TiO₂(B)-1 sample [40]. (Reprinted with permission from Ref. [40]. Copyright 2014, Royal Society of Chemistry)

In addition to dyes, some other organic compounds such as phenol have also been served as the target pollutants in the g-C₃N₄/TiO₂ photocatalysis system. For example, Hongtao Yu et al. investigated the photocatalytic activity of g-C₃N₄/TiO₂ for the photocatalytic degradation of phenol under visible and UV light. The g-C₃N₄/TiO₂ exhibited higher photocatalytic activity than pure TiO₂ and g-C₃N₄, as shown in Fig. 7.10, and the g-C₃N₄/TiO₂–2 with the mass ratio of g-C₃N₄/TiO₂ = 2 possessed the best photocatalytic activity [43].

Fig. 7.10

(a) Photocatalytic degradation of phenol using g-C₃N₄/TiO₂ catalysts with various weight ratios under full-spectrum irradiation; (b) photocatalytic degradation of phenol by g-C₃N₄, TiO₂, and g-C₃N₄/TiO₂ under full-spectrum irradiation and (c) under visible light irradiation; (d) the kinetic constants of phenol degradation under full-spectrum irradiation [43]. (Reprinted with permission from Ref. [43]. Copyright 2012, Elsevier)

In addition, a kind of TiO₂/g-C₃N₄ catalyst with highly dispersed TiO₂ nanocrystals on g-C₃N₄ has also been used for the photocatalytic degradation of phenol by Jinlong Zhang et al. It was found that high dispersion of TiO₂ with high-energy (001) facet was beneficial for the enhancement of the photocatalytic activity. As Fig. 7.11a shows, the photocatalytic activity of TiO₂/g-C₃N₄ catalysts showed an obvious increase for phenol decomposition compared with the pure TiO₂ and g-C₃N₄. The optimal catalyst TiO₂/g-C₃N₄(1.5) successfully degraded 100% phenol within 50 min, whose rate constant was about 2.2 times that of pure TiO₂ and 2.8 times that of pure g-C₃N₄ (Fig. 7.11b) [49].

Fig. 7.11

Photocatalytic degradation results of 10 mg L⁻¹phenol with different catalysts under simulated sunlight irradiation of 300 W xenon lamp coupled with AM 1.5 [49]. (Reprinted with permission from Ref. [49]. Copyright 2015, Royal Society of Chemistry)

7.3.1.2 Degradation of Pollutants in Gas Phase

The status of air quality has great influence on people’s health. After interior decoration, the concentration of formaldehyde (HCHO) in the air will rise sharply, causing a great impair to people’s health. Jiaguo Yu et al. synthesized g-C₃N₄-TiO₂ photocatalyst and applied it for the photocatalytic oxidation decomposition of HCHO in air. As shown in Fig. 7.12, the weight percentage ratio of urea against P25 in the precursors was tuned to be 0, 20, 100, 200, and 500 (wt%), and the resulting catalysts were labeled as Ux (x represented the urea to P25 weight ratio), and its value was equal to 0, 20, 100, 200, and 500, respectively. Their experimental results showed that the pure g-C₃N₄ exhibited very low photocatalytic activity for HCHO oxidation decomposition, while pure TiO₂ was active for decomposition of HCHO, and the photocatalytic activity of g-C₃N₄-TiO₂ was highly dependent on the amount of incorporated g-C₃N₄. The U100 sample with g-C₃N₄ content of 94% exhibited the highest photocatalytic activity for HCHO decomposition [5].

Fig. 7.12

The photocatalytic decomposition results of HCHO in air using U0, U20, U100, U200, U500, and g-C₃N₄ samples, respectively. (a) The corresponding concentration–time plots of HCHO; (b) the decomposition rate constant for all samples [5]. (Reprinted with permission from Ref. [5]. Copyright 2013, Royal Society of Chemistry)

In addition to formaldehyde, gaseous acetone has also been degraded by the g-C₃N₄/TiO₂ photocatalyst. Xiangli Li fabricated microspherical g-C₃N₄/TiO₂ with high percentage of TiO₂ (001) facets through a solvothermal method and evaluated its photocatalytic activity for the degradation of gaseous acetone [60]. As Fig. 7.13 shows, the g-C₃N₄/TiO₂ catalyst (TCN50) could degrade more than 70% acetone within 120 min under simulated solar light irradiation. The photocatalytic efficiency of g-C₃N₄/TiO₂ for degrading acetone was much higher than that of pure g-C₃N₄ and TiO₂. Moreover, their experimental results also proved that acetone was oxidized by the highly active O₂· into CO₂ and H₂O in their reaction system.

Fig. 7.13

Photocatalytic degradation results of gaseous acetone under the simulated solar light irradiation [60]. (Reprinted with permission from Ref. [60]. Copyright 2016, Elsevier)

In the process of the degradation of the gaseous pollutants, the adsorption capacity of the catalyst was the main factor, which greatly affects the photocatalytic activity. Tailoring the performance of materials via adjusting the morphologies and structures of the catalysts has emerged as a new and important direction of the research on g-C₃N₄/TiO₂ heterojunction catalyst for the photocatalytic degradation of gaseous organic pollutants.

7.3.2 Hydrogen Generation from Water

Due to the fact of the global energy depletion, the development and production of new sources of energy especially the clean energy have attracted more and more experimental interests. Hydrogen is widely considered as a highly effective environmental and green energy, whose production methods have been studied and explored a lot. Among the numerous production methods, water splitting with g-C₃N₄/TiO₂ as photocatalyst has been favored by many researchers due to its merit of environment friendly.

Hongjian Yan et al. fabricated TiO₂-g-C₃N₄ composite catalysts with varying the wt% of g-C₃N₄ and used the samples in photocatalytic H₂ generation. The visible light-induced H₂ evolution rate was remarkably improved by coupling TiO₂ with g-C₃N₄, and the sample TiO₂–50 wt% C₃N₄ showed the highest activity, as shown in Fig. 7.14 [37].

Fig. 7.14

The average hydrogen production rates as a function of wt% of g-C₃N₄ in the TiO₂-g-C₃N₄ composite from the first 3 h of the reaction duration [37]. (Reprinted with permission from Ref. [37]. Copyright 2011, Elsevier)

Tianyou Peng et al. prepared porous g-C₃N₄-Pt-TiO₂, and their experimental results showed that coupling TiO₂ with g-C₃N₄ could remarkably enhance the visible light-induced photocatalytic hydrogen evolution rate. Besides, the g-C₃N₄-Pt-TiO₂ composite with a mass ratio of 70:30 exhibited the maximum photocatalytic activity as well as excellent photostability for hydrogen production under visible light irradiation (Fig. 7.15) [30].

Fig. 7.15

Photocatalytic H₂ evolution rates of Pt-TiO₂, g-C₃N₄, Pt-g-C₃N₄, and g-C₃N₄–Pt-TiO₂ composites in different mass ratios as well as Pt-(g-C₃N₄–TiO₂) and (Pt-g-C₃N₄)/TiO₂ composites with a mass ratio of 70:30 under visible light irradiation [30]. (Reprinted with permission from Ref. [30]. Copyright 2012, Royal Society of Chemistry)

Zhenyi Zhang et al. synthesized ternary heterostructured nanofibers (NFs) consisting of g-C₃N₄ nanosheets (NSs), plasmonic noble metal nanoparticles (Au, Ag, or Pt NPs), and TiO₂ NPs. The ternary composite photocatalyst exhibited improved charge-carrier migration efficiency and achieved highly efficient photocatalytic H₂ evolution [61]. Yanping Hong et al. prepared an anatase boron-doped TiO₂ (B-TiO₂) with exposed (001) facets and composited it with the g-C₃N₄ to form B-TiO₂–001/g-C₃N₄ heterojunctions. The heterojunction photocatalyst had the greatest photocatalytic activity for H₂ production as shown in Fig. 7.16, which was ascribed to the broad range of visible light absorption, the efficiently reduced charge recombination, and relatively higher catalytic activity of (001) facets compared to the (101) facets [62].

Fig. 7.16

Photocatalytic H₂ evolution of the as-synthesized samples: (a) g-C₃N₄, (b) TiO₂–001, (c) B-TiO₂–001, (d) TiO₂–001/g-C₃N₄ (5.0%), (e) P25/g-C₃N₄ (5.0%), (f) B-TiO₂–001/g-C₃N₄ (1.0%), (g) B-TiO₂–001/g-C₃N₄ (2.0%), (h) B-TiO₂–001/g-C₃N₄ (5.0%), (i) B-TiO₂–001/g-C₃N₄ (10%) [62]. (Reprinted with permission from Ref. [62]. Copyright 2016, Elsevier)

In addition, Yan-Yan Song et al. modified the g-C₃N₄/TiO₂ nanotube arrays with Pt nanoparticles. Compared with g-C₃N₄-free aligned TiO₂ nanotube layers, the obtained sample exhibited a strong enhancement for photoelectron–chemical and bias-free H₂ evolution of 15.62 μL h⁻¹ cm⁻², which was almost a 98-fold increase in the H₂ production rate of aligned TiO₂ nanotube layers (0.16 μL h⁻¹ cm⁻²) [63]. Jian-guo Wang et al. designed and fabricated a novel g-C₃N₄/TiO₂ nanobelt heterostructure material. As shown in Fig. 7.17, the g-C₃N₄/TiO₂ nanobelt heterostructure with a mass ratio of 3:1 showed the highest H₂ production rate of 46.6 μmol h⁻¹ [64].

Fig. 7.17

(a) Photocatalytic H₂ evolution rates of the pure TiO₂ NBs, g-C₃N₄, and g-C₃N₄/TiO₂ NB heterostructure with different mass ratios; (b) wavelength dependence of H₂ evolution rate for g-C₃N₄/TiO₂ NBs (3:1) [64]. (Reprinted with permission from Ref. [64]. Copyright 2014, Elsevier)

7.3.3 Other Applications

Apart from the above applications, other applications of the g-C₃N₄/TiO₂ heterojunction catalysts have also been explored, such as the photocatalytic reduction of CO₂, treating heavy metal ion, as well as the inactivation of bacteria.

Guiyuan Jiang et al. fabricated a series of composites of g-C₃N₄ and in situ N-doped TiO₂ and then applied them to photocatalytic reduction of CO₂ under simulated light irradiation with water vapor at room temperature. Their research results showed efficient photocatalytic conversion of CO₂ to CO, and CH₄ was achieved. In addition, the photocatalytic activity and product selectivity were easy to adjust through simply varying the ratios of the precursor for g-C₃N₄ to the precursor for TiO₂ during the synthesis process of the catalyst. Moreover, as shown in Fig. 7.18, compared with g-C₃N₄ and commercial P25, the as-prepared g-C₃N₄-N-TiO₂ heterojunction photocatalysts showed improved photocatalytic performance for the reduction of CO₂, indicating the g-C₃N₄/TiO₂ heterojunction catalysts have good application prospects for mitigating the greenhouse effect and producing hydrocarbon and chemical compounds [65].

Fig. 7.18

(a) CO generation using different samples as a function of UV–vis light irradiation time. (b) CO generation using different samples as a function of UV–vis light irradiation time. (c) CH₄ generation using different samples as a function of UV–vis light irradiation time [65]. (Reprinted with permission from Ref. [65]. Copyright 2014, Elsevier)

Guangshe Li et al. synthesized br-TiO₂/g-C₃N₄ by a facile calcination in air of brookite TiO₂ (br-TiO₂) hybridized with g-C₃N₄. The obtained samples were used for oxidation of toxic As³⁺ [42]. The intimately contacted hybrid br-TiO₂/g-C₃N₄ showed excellent photocatalytic activity in oxidation of As³⁺ to As⁵⁺, which is less harmful than As³⁺. Figure 7.19 indicated that the br-TiO₂/g-C₃N₄ catalyst with 35% weight ratio of the g-C₃N₄ exhibited much higher efficiency than pure br-TiO₂ and g-C₃N₄ for the application of As³⁺ oxidization.

Fig. 7.19

Oxidation efficiency of As³⁺ over the hybrid br-TiO₂/g-C₃N₄–35%, pure g-C₃N₄, and 100% br-TiO₂ under visible light irradiation [42]. (Reprinted with permission from Ref. [42]. Copyright 2014, Royal Society of Chemistry)

Additionally, Taicheng An et al. investigated effective removal of biohazards from water using g-C₃N₄/TiO₂ hybrid photocatalyst [66]. The photocatalyst they synthesized was composed of micron-sized TiO₂ spheres wrapped with lamellar g-C₃N₄. A significantly improved visible light absorption and effectively reduced recombination of photo-generated electron–hole pairs were achieved by the combination of these two components. Using this hybrid photocatalyst, 107 cfu mL⁻¹ of Escherichia coli K-12 could be completely inactivated within 180 min under visible light irradiation. Figure 7.20 showed that bacterial cells were seriously damaged during the photocatalytic inactivation processes, resulting in a severe leakage of intracellular components. Their research revealed that, through this kind of g-C₃N₄/TiO₂ heterojunction catalyst, bacterial cell destruction and water disinfection could be achieved easily. Besides, their results showed that substantial interaction between TiO₂ and g-C₃N₄ in the hybrid photocatalyst was a vital prerequisite for the enhancement of photocatalytic activity, which subsequently increased the trapping of the photoinduced charge carriers, benefiting for the production of reactive species. Furthermore, besides h⁺, other reactive species such as subsequently generated ·O₂ ⁻ and H₂O₂ also attacked biohazards, causing efficient photocatalytic inactivation and completely decomposition of bacteria.

Fig. 7.20

SEM images of E. coli K-12 after the treatment of g-C₃N₄/TiO₂ hybrid photocatalysts at different times under visible light irradiation [66]. (Reprinted with permission from Ref. [66]. Copyright 2015, Elsevier)

7.4 Conclusions

This chapter summarized the recent progress of the research on the preparation methods and catalytic applications of g-C₃N₄/TiO₂ heterojunction catalysts. The synthesis methods of g-C₃N₄/TiO₂ heterojunction catalysts can be simply divided into three classes according to the order of each component prepared in the preparation process, which includes physically mixing g-C₃N₄ and TiO₂, growing TiO₂ on g-C₃N₄, and loading g-C₃N₄ on TiO₂. In spite of the different advantages and disadvantages exciting in the three different methods, every one of them can effectively form the heterojunctions between g-C₃N₄ and TiO₂, resulting in enhanced photocatalytic activity of the catalysts. The g-C₃N₄/TiO₂ heterojunction catalysts with excellent photocatalytic performance have been mainly applied in the photocatalytic degradation of organic pollutants, photolysis of water for producing H₂, photocatalytic reduction of CO₂, as well as the treatment of heavy metal ion and inactivation of bacteria.

Apart from the above research, g-C₃N₄/TiO₂ heterojunction catalysts are still worthy of exploration. Some groups found that there exists the phenomenon that the electron can transfer from dye to TiO₂, which implies that dye self-sensitized degradation also exists in this kind of composite system during the degradation process, providing the possibility for g-C₃N₄/TiO₂ heterojunction catalysts being applied to the dye-sensitized solar cells (DSSC). Moreover, some researchers have tried to design the ternary heterojunction catalysts such as g-C₃N₄/Ag/TiO₂ [41]. The studies on the structure design, the morphology control, and the expansion of applications related to g-C₃N₄/TiO₂ heterojunction catalysts are still significant. The heterojunction catalysts contain multiple components; therefore the stability of the heterojunction is not so satisfactory, and the interaction force between different components is still unknown. The preparation method of heterojunction catalysts looks more complex than the synthesis of other catalyst, and it still remains a great challenge for the development of a simplified synthesis method of the heterojunction catalysts.