# N-TiO2-δ/g-C3N4 Dual Photocatalysts for Efficient Oxytetracycline Hydrochloride Photodegradation and CO2 Photoreduction

Authors: Thanh-Binh Nguyen; Thuy Hang Dinh Thi; Kim Hue Dinh Thi; Hien Bui Minh; Ngoc Quynh Nguyen Thi; Duy Vu Ngoc; Bang Nguyen Dinh
Journal: Adsorption Science & Technology
 (2022)

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## Abstract

A series ofx% (wt) N-TiO2-δ/g-C3N4 composites was synthesized by calcination and hydrothermal methods (labeled xTiCN, x: 5, 10, and 15). All composites were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, UV-vis diffuse reflectance spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The photocatalytic activity of these composites was evaluated through oxytetracycline hydrochloride (denoted as OTC) photodegradation and CO2 photoreduction. The xTiCN composites exhibited higher OTC photodegradation than bulk g-C3N4. 10TiCN was slightly more active than 5TiCN and 15TiCN, with a photodegradation yield of 97% after 5 h of light irradiation and constant rate of 0.647 h-1. For CO2 photoreduction, it was observed that 5TiCN exhibited the highest activity among the synthesized composites, with 7.0 ppm CH4 formed. This CH4 concentration was 7.8 times higher than the concentration formed by bulk g-C3N4 (0.9 ppm). A Z-scheme mechanism was proposed to explain the enhanced photocatalysis by x% (wt) N-TiO2-δ/g-C3N4 composites. The Z-scheme structure increased redox ability, caused better separation of photogenerated electron-hole pairs, and broadened the light absorption zone of the photocatalysts.


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## Body

## 1. Introduction

Antibiotics are widely used to control bacterial infections in medical, agricultural, and veterinary sectors [1]. Oxytetracycline hydrochloride (OTC) is a commonly used tetracycline antibiotic. Large amounts of OTC have been released into the environment due to its extensive use [2, 3]. It has a long half-life due to its naphthacene core, persisting in the environment for long periods of time. Its environmental residue, especially in water sources, being carcinogenic, and causing antibiotic resistance in bacteria, is dangerous for human health and the ecosystem [3, 4]. Along with antibiotic water pollution, air pollution has also increased in recent times. With rapid worldwide industrial development, large amounts of CO2 are released into the atmosphere, causing the greenhouse effect and global warming [5, 6]. Climate change, due to pollution, has caused severe meteorological phenomena, such as typhoons and floods. Thus, CO2 emission reduction and its environmental remediation are an urgent necessity.Among different ways to solve these above pollution problems, photocatalysis is an attracted one in recent times [7–12]. The photocatalysis can oxidize OTC to nontoxic compounds [13] and reduce CO2 to useful chemical compounds, such as CH4 and CH3OH [14], reducing water and air pollution. It is a simple and green process, requiring only light irradiation for catalyst activation. The TiO2 photocatalyst is commonly used because of its high photocatalytic efficiency, good stability, and nontoxicity [15, 16]. However, it is only activated under 380 nm light irradiation. UV light intensity in the sunlight spectrum is approximately 5%; therefore, the TiO2 photocatalyst requires a UV light source, increasing the process cost. To overcome this drawback, numerous studies report reducing the TiO2 bandgap, activating TiO2 at longer wavelengths (such as visible light), or combining TiO2 with narrow bandgap semiconductors. In the first strategy, the TiO2 oxide is doped with nonmetallic elements (N, F) or transition metals (Fe, Co) [17–22]. In the second method, TiO2 is combined with other semiconductors, such as CuO, BiOBr, metal Au, Pt, or both [23–29]. When combining with an oxide-owned narrower bandgap energy, the new composite will be activated by a longer light wavelength, such visible light, and prevent electron-hole pair recombination. With metal deposition, the metallic phase conduction band attracts free electrons, reducing the recombination of electron-hole pairs.Recently, graphitic carbon nitride (g-C3N4), a nonmetallic photocatalyst, has attracted immense interest [30–36]. Synthesized by facile and cost-effective methods, it exhibits high chemical and thermal stability. In particular, it exhibits potential for antibiotic-photocatalytic degradation in aqueous solutions and CO2 photoreduction [37, 38]. A low bandgap of 2.7 eV enables their visible-light activation. However, rapid recombination of photogenerated electron-hole pairs is a limitation of the g-C3N4 photocatalyst. The combining of g-C3N4 with other semiconductors to prevent this recombination is an interesting strategy [39].Among the aforementioned photocatalyst improvement strategies, the construction of a heterojunction between two semiconductors is a promising method. In this structure, the photogenerated electron-hole pairs are separated into different zones, thus preventing recombination and enhancing photocatalytic activity. Further, the carrier transfer behavior has been altered by several photocatalytic mechanisms such as type-I, type-II, and, quite recently, theZ-scheme [40]. Among these mechanisms, the Z-scheme has garnered considerable attention as it not only exhibits electron-hole pair separation in two different zones but also enhances the redox properties. For instance, Guan et al. demonstrated that the activity of 20% LaFeO3/BiOBr was ~21.0 and~1.3 times that of bare LaFeO3 and BiOBr, respectively [41]. Additionally, in 2022, Cheng’s group also revealed that the optimal photocatalyst 20% Bi4Ti3O12/CdS exhibited an activity 1.6 and 3.3 times that of bare CdS and Bi4Ti3O12, respectively [42]. An improved photocatalytic activity was also observed on the Bi2O2CO3 photocatalyst [43]. The enhanced composite photoactivity was explained by improved photoexcited carrier separation in the composite. Therefore, with a similar targeted photocatalyst structure, TiO2- and g-C3N4-based composites were also developed, which exhibited positive results. Wang et al. demonstrated that the tetracycline (TCL) photodegradation on TiO2@g-C3N4 was 75% and 12% greater than that on TiO2 and g-C3N4, respectively [44]. Similarly, for CO2 photoreduction, Reli et al. showed a twofold increase in CH4 formation on TiO2/g-C3N4 (0.3/1) as compared to g-C3N4 [45]. These studies indicate that along with the nature of coupled semiconductors, the morphology and interface interaction between two semiconductors play an important role in photocatalytic improvement.In view of the above, this study demonstrates the coupling of TiO2 and g-C3N4 to obtain heterojunction photocatalysts, x% TiO2/g-C3N4. On the one hand, TiO2 has the disadvantage of a large bandgap energy, Eg=3.2eV; on the other hand, g-C3N4, despite having a lower bandgap energy, Eg=2.7eV, demonstrates a fast recombination of electron-hole pairs. Therefore, the formation of a heterojunction composite through semiconductor coupling could allow overcoming the disadvantages of each constituent. Additionally, the CB/VB potential position of TiO2 (-0.17 V/+3.0 V) and g-C3N4 (-1.15 V/+1.5 V) [44] would allow the establishment of a staggered band structure and the formation of a Z-scheme photocatalyst. The hydrothermal method, prevalently employed for tuning the morphology of synthesized compounds, was used to prepare the composite. Subsequently, calcination was conducted to enhance the interaction between TiO2 and g-C3N4 such as an in situ nitrogen doping into TiO2. The photocatalytic activity was estimated through OTC oxidation in the liquid phase. OTC is the most stable compound in the TCL group, and as per our knowledge, no studies on OTC oxidation were reported using this composite type. To further evaluate the synthesized photocatalyst composites, CO2 photoreduction was also investigated in the gas phase.


## 2. Materials and Methods

### 2.1. Materials

Chemical compounds, melamine (C6H6N6) (Sigma-Aldrich), C16H36O4Ti (Sigma-Aldrich), CH3COOH (China), C2H5OH (China), TiO2 (Evonik P25), and oxytetracycline hydrochloride (OTC) (Sigma-Aldrich), of analytical purity were used as obtained.


### 2.2. Synthesis of g-C3N4 and N-TiO2-δ/g-C3N4 Composites

#### 2.2.1. Synthesis of g-C3N4 and N-TiO2

Melamine calcination at 550°C, for 3 h, in nitrogen gas medium, was used to synthesize g-C3N4.The nitrogen-doped TiO2 (N-TiO2) synthesis was inspired by the work of Viswanath et al. [46]. In this typical procedure, titanium (IV) butoxide in ethanol solution and melamine in hot water-ethanol (1 : 3 volume ratio) solution were mixed, then stirred for 24 hours and aged for 5 days. The obtained gel was dried and calcined at 400°C for 3 hours.


#### 2.2.2. Synthesis of N-TiO2-δ/g-C3N4

Calculated amounts of g-C3N4, titanium (IV) butoxide, and acetic acid (1 : 30 in volume) were mixed in a 250 ml beaker, with 15 min magnetic stirring, and autoclaved at 140°C for 12 h. The resulting solution was centrifuged and rinsed several times with ethanol. The solid obtained was oven dried at 60°C for 24 h and calcined at 400°C for 3 h, under nitrogen gas. The x% (wt) N-TiO2-δ/g-C3N4 composites were denoted as xTiCN (x: 5, 10, and 15).


### 2.3. Photocatalytic Procedure

#### 2.3.1. OTC Photodegradation

Into a beaker containing 100 ml 10 ppm OTC solution (C0), g-C3N4 or xTiCN composite (0.1 g) was added with stirring (Figure 1(a)) and left in the dark for 1 h to attain adsorption equilibrium. The mixture was then illuminated with a 200 W LED lamp. Every 1 hour, 5 ml sample was taken, filtered, and analyzed by a UV-vis spectrophotometer to detect OTC content (Ct) of the reaction mixture. The photostability test was performed 5 times. The separation of the catalyst and the reaction solution was carried out by centrifugation (6000 rpm).Figure 1
Scheme of the photocatalytic reactor used for OTC oxidation (a) and CO2 reduction (b).
(a)(b)


#### 2.3.2. CO2 Photoreduction

Into a glass beaker (5 cm diameter) containing 15 ml deionized water, g-C3N4 or xTiCN composite (0.1 g) was added. After 15 min stirring, the mixture was evaporated in an oven, at 70°C, to obtain a well-dried, homogeneously dispersed powder. The catalyst-containing beaker was placed in a handmade closed stainless steel reactor (169 cm3 volume), equipped with a 6 cm diameter quartz window, and purged with high-purity (99.999%) 500 ml/min CO2 flow for 30 min. The reactor was illuminated with a 150 W Xenon lamp (Newport model 67005) for 18 h. Gaseous products were analyzed using a gas valve system connected to a gas chromatograph, equipped with a thermal conductivity detector and flame ionization detector (TCD-FID) (Scion 456) (Figure 1(b)).


### 2.4. Characterizations

X-ray diffraction (XRD) (Bruker D8), Fourier-transform infrared spectroscopy (FTIR) (8101M Shimadzu), N2 adsorption-desorption (TriStar 3000-Micromeritics), UV differential reflectance spectroscopy (UV-DRS) (Jaco V-530), transmission electronic microscopy (TEM, JEM 1400 – Plus Jeon), and X-ray photoelectron spectroscopy (XPS) (Thermo Scientific MultiLab 2000) were used to characterize g-C3N4 and xTiCN composites.


## 2.1. Materials

Chemical compounds, melamine (C6H6N6) (Sigma-Aldrich), C16H36O4Ti (Sigma-Aldrich), CH3COOH (China), C2H5OH (China), TiO2 (Evonik P25), and oxytetracycline hydrochloride (OTC) (Sigma-Aldrich), of analytical purity were used as obtained.


## 2.2. Synthesis of g-C3N4 and N-TiO2-δ/g-C3N4 Composites

### 2.2.1. Synthesis of g-C3N4 and N-TiO2

Melamine calcination at 550°C, for 3 h, in nitrogen gas medium, was used to synthesize g-C3N4.The nitrogen-doped TiO2 (N-TiO2) synthesis was inspired by the work of Viswanath et al. [46]. In this typical procedure, titanium (IV) butoxide in ethanol solution and melamine in hot water-ethanol (1 : 3 volume ratio) solution were mixed, then stirred for 24 hours and aged for 5 days. The obtained gel was dried and calcined at 400°C for 3 hours.


### 2.2.2. Synthesis of N-TiO2-δ/g-C3N4

Calculated amounts of g-C3N4, titanium (IV) butoxide, and acetic acid (1 : 30 in volume) were mixed in a 250 ml beaker, with 15 min magnetic stirring, and autoclaved at 140°C for 12 h. The resulting solution was centrifuged and rinsed several times with ethanol. The solid obtained was oven dried at 60°C for 24 h and calcined at 400°C for 3 h, under nitrogen gas. The x% (wt) N-TiO2-δ/g-C3N4 composites were denoted as xTiCN (x: 5, 10, and 15).


## 2.2.1. Synthesis of g-C3N4 and N-TiO2

Melamine calcination at 550°C, for 3 h, in nitrogen gas medium, was used to synthesize g-C3N4.The nitrogen-doped TiO2 (N-TiO2) synthesis was inspired by the work of Viswanath et al. [46]. In this typical procedure, titanium (IV) butoxide in ethanol solution and melamine in hot water-ethanol (1 : 3 volume ratio) solution were mixed, then stirred for 24 hours and aged for 5 days. The obtained gel was dried and calcined at 400°C for 3 hours.


## 2.2.2. Synthesis of N-TiO2-δ/g-C3N4

Calculated amounts of g-C3N4, titanium (IV) butoxide, and acetic acid (1 : 30 in volume) were mixed in a 250 ml beaker, with 15 min magnetic stirring, and autoclaved at 140°C for 12 h. The resulting solution was centrifuged and rinsed several times with ethanol. The solid obtained was oven dried at 60°C for 24 h and calcined at 400°C for 3 h, under nitrogen gas. The x% (wt) N-TiO2-δ/g-C3N4 composites were denoted as xTiCN (x: 5, 10, and 15).


## 2.3. Photocatalytic Procedure

### 2.3.1. OTC Photodegradation

Into a beaker containing 100 ml 10 ppm OTC solution (C0), g-C3N4 or xTiCN composite (0.1 g) was added with stirring (Figure 1(a)) and left in the dark for 1 h to attain adsorption equilibrium. The mixture was then illuminated with a 200 W LED lamp. Every 1 hour, 5 ml sample was taken, filtered, and analyzed by a UV-vis spectrophotometer to detect OTC content (Ct) of the reaction mixture. The photostability test was performed 5 times. The separation of the catalyst and the reaction solution was carried out by centrifugation (6000 rpm).Figure 1
Scheme of the photocatalytic reactor used for OTC oxidation (a) and CO2 reduction (b).
(a)(b)


### 2.3.2. CO2 Photoreduction

Into a glass beaker (5 cm diameter) containing 15 ml deionized water, g-C3N4 or xTiCN composite (0.1 g) was added. After 15 min stirring, the mixture was evaporated in an oven, at 70°C, to obtain a well-dried, homogeneously dispersed powder. The catalyst-containing beaker was placed in a handmade closed stainless steel reactor (169 cm3 volume), equipped with a 6 cm diameter quartz window, and purged with high-purity (99.999%) 500 ml/min CO2 flow for 30 min. The reactor was illuminated with a 150 W Xenon lamp (Newport model 67005) for 18 h. Gaseous products were analyzed using a gas valve system connected to a gas chromatograph, equipped with a thermal conductivity detector and flame ionization detector (TCD-FID) (Scion 456) (Figure 1(b)).


## 2.3.1. OTC Photodegradation

Into a beaker containing 100 ml 10 ppm OTC solution (C0), g-C3N4 or xTiCN composite (0.1 g) was added with stirring (Figure 1(a)) and left in the dark for 1 h to attain adsorption equilibrium. The mixture was then illuminated with a 200 W LED lamp. Every 1 hour, 5 ml sample was taken, filtered, and analyzed by a UV-vis spectrophotometer to detect OTC content (Ct) of the reaction mixture. The photostability test was performed 5 times. The separation of the catalyst and the reaction solution was carried out by centrifugation (6000 rpm).Figure 1
Scheme of the photocatalytic reactor used for OTC oxidation (a) and CO2 reduction (b).
(a)(b)


## 2.3.2. CO2 Photoreduction

Into a glass beaker (5 cm diameter) containing 15 ml deionized water, g-C3N4 or xTiCN composite (0.1 g) was added. After 15 min stirring, the mixture was evaporated in an oven, at 70°C, to obtain a well-dried, homogeneously dispersed powder. The catalyst-containing beaker was placed in a handmade closed stainless steel reactor (169 cm3 volume), equipped with a 6 cm diameter quartz window, and purged with high-purity (99.999%) 500 ml/min CO2 flow for 30 min. The reactor was illuminated with a 150 W Xenon lamp (Newport model 67005) for 18 h. Gaseous products were analyzed using a gas valve system connected to a gas chromatograph, equipped with a thermal conductivity detector and flame ionization detector (TCD-FID) (Scion 456) (Figure 1(b)).


## 2.4. Characterizations

X-ray diffraction (XRD) (Bruker D8), Fourier-transform infrared spectroscopy (FTIR) (8101M Shimadzu), N2 adsorption-desorption (TriStar 3000-Micromeritics), UV differential reflectance spectroscopy (UV-DRS) (Jaco V-530), transmission electronic microscopy (TEM, JEM 1400 – Plus Jeon), and X-ray photoelectron spectroscopy (XPS) (Thermo Scientific MultiLab 2000) were used to characterize g-C3N4 and xTiCN composites.


## 3. Results and Discussion

### 3.1. Structural Characterization

XRD patterns ofxTiCN composites are shown in Figure 2(a). The formation of the g-C3N4 crystalline phase, with a characteristic peak at 27.8°, was observed in all XRD patterns, confirming the g-C3N4 structure after composite synthesis. Characteristic peaks at 25.5°, 38.0°, 48.1°, 54.2°, and 62.8°, corresponding to the TiO2 anatase phase, were observed in the XRD patterns of all composites. In the N-TiO2 XRD patterns, a small quantity of the rutile phase was recognized at 2 theta of 27.5 and 36.1°. To identify different components, FTIR characterization was carried out, and the spectra are shown in Figure 2(b). In the g-C3N4 spectrum, the peak at 802 cm-1 was ascribed to the s-triazine bending mode [30]. The peaks at 1228 and 1311 cm-1 were attributed to the C–N stretching vibrations [47, 48]. The peaks at 1392, 1535, and 1625 cm-1 originated from the –C=N stretching vibrations in aromatic rings. The broad band at 3000–3500 cm-1 corresponded to the stretching vibrations of the absorbed water hydroxyl group (–O–H) and terminal amino groups (–NH2) [47, 48]. Finally, the broad peak at 534 cm-1 was attributed to the Ti–O bond vibrations [49].Figure 2
XRD patterns (a) and FTIR spectra (b) of g-C3N4 and xTiCN composites.
(a)(b)The light absorption abilities of the composites were analyzed by UV-vis DR spectra. Figure3(a) shows the UV-vis DR spectra of g-C3N4 and xTiCN. The bandgap energies, calculated using the Kubelka–Munk function (results are shown in Figure 3(b)), were 2.58, 2.60, 2.63, 2.66, and 2.90 eV for g-C3N4, 5TiCN, 10TiCN,15TiCN, and N-TiO2, respectively. Increasing the TiO2 content broadened the composite bandgap energy owing to a greater contribution of the large band gap energy by TiO2 (3.2 eV) as compared to g-C3N4 (2.58 eV). However, generally, all photocatalyst composites could be activated by visible light. The corresponding differential curves of UV-vis DR spectra are displayed in Figure 3(c). The absorption edges (λabs) of pristine TiO2 (P25) and g-C3N4 are at wavelengths of 396.3 and 437.5 nm, respectively. In contrast, the xTiCN composites exhibit a slight shift of the g-C3N4 adsorption edge peak to a shorter wavelength, while that of TiO2 slightly shifts to a longer wavelength. The longer-wavelength-shifted adsorption edge of the TiO2 constituent is possibly due to nitrogen doping in TiO2 during the composite synthesis. These observations suggest an interaction between g-C3N4 and TiO2, forming in situ-doped N-TiO2. In the case of 5TiCN, the observed adsorption edge peaks of TiO2 andg-C3N4 correspond to wavelengths of 402.0 and 435.3 nm, respectively. Based on the relationship Eg=1240/λabs, the calculated bandgap energies (Eg) of TiO2 and g-C3N4 are 3.08 and 2.85 eV, respectively. For the N-TiO2 sample prepared by the hydrothermal method, the differential curve of the UV-vis DR spectrum exhibited a single weak and broad peak at 392.5 nm (or Eg=3.16eV). This peak is attributed to the adsorption edge of TiO2.Figure 3
UV-vis DR spectra (a), energy bandgap determination by Kubelka–Munk function (b), and its corresponding differential curves (c) of g-C3N4, N-TiO2, and xTiCN composites; N2 adsorption-desorption isotherms and corresponding pore distribution curves of g-C3N4 (solid line) and 5TiCN composite (dash line) (d).
(a)(b)(c)(d)The g-C3N4 and 5TiCN composites were selected to characterize its specific surface area, one of the important properties of heterogeneous catalysis. The results are presented in Figure 3(d). It is observed that two samples show the type-4 isotherms with H3 hysteresis loop, which indicate the presence of a mesopore. This is suitable with the obtained pore distribution curves. The BET surface areas are 18 and 20 m2/g for g-C3N4 and 5TiCN, respectively. Hence, the composite preparation did not seem to change the g-C3N4 structure.The interaction between the TiO2 and g-C3N4 phases is better understood from the TEM images presented in Figure 4. It is observed that the TiO2 particles (dark areas in Figures 4(a) and 4(b)) were formed in various sizes and shapes. Some TiO2 particles were deposited on the g-C3N4 layer (e.g., position of cycle 1), while the others were covered by g-C3N4 multisheets, forming a core-shell structure (e.g., position of cycle 2), thereby enhancing the interaction surface between the TiO2 and g-C3N4 phases.Figure 4
TEM images of 5TiCN.The elemental composition and oxidation state of the catalyst influence catalytic performance. Therefore, the 5TiCN composite was characterized using XPS, as shown in Figure5.Figure 5
X-ray photoelectron spectra of 5TiCN: (a) survey spectra; (b) high-resolution spectra of Ti2p, (c) high-resolution spectra of O1s; (d) high-resolution spectra of N1s; (e) high-resolution spectra of C1s.
(a)(b)(c)(d)(e)Three peaks at binding energies of 458.5, 459.1, and 464.3 eV, corresponding to orbitals Ti4+2p3/2, Ti3+2p1/2, and Ti4+2p1/2, respectively, were observed in the Ti2p high-resolution spectrum [50, 51]. As reported by Jia et al., binding energies of two prominent peaks (at 458.5 and 464.3 eV) exhibited 0.2 eV shifts compared to spectral peaks of pure TiO2. This could be due to the substitution of O2- by N3-, leading to the formation of N-Ti-O bonds [52]. This indicated N doping of TiO2 during composite synthesis, decreasing the TiO2 bandgap energy, leading to visible-light activation [17, 18]. The small peak of Ti3+2p1/2 confirmed oxygen vacancies in TiO2 [51]. Hence, TiO2-δ is a more accurate molecular formula than TiO2.Deconvolution peaks for the O1s spectrum exhibited two peaks at 531 and 532 eV, assigned to O-H of absorbed water and the Ti–O bond [53], respectively. N1s spectrum exhibited three peaks at 398.6, 399.7, and 400.8 eV, ascribed to the sp2C of C-N=C, tertiary N of N-(C)3 group, and N-C=N bonds, respectively. The C1s spectrum exhibited three peaks at 284.9, 286.3, and 288.2 eV, corresponding to C-C, C-NH2, and N-C=N bonds, respectively [53, 54].


### 3.2. Evaluation of Photocatalytic Activity

#### 3.2.1. Photooxidation of OTC

Before performing the photocatalytic test, the adsorption equilibria were carried out (Figure6(a)). The results demonstrated that all composites reached rapidly the adsorption equilibrium after only about 15 minutes, while 60 minutes was required for N-TiO2. The calculation indicated that the equilibrium adsorption quantities of OTC at 60 min were 11%, 14%, 17%, 12%, and 88% for g-C3N4, 5TiCN, 10TiCN, 15TiCN, and N-TiO2, respectively. It remarked that there was a strong adsorption phenomenon of OTC on N-TiO2.Figure 6
OTC adsorption equilibrium (a) onxTiCN and UV-vis spectra of OTC during test time on 5TiCN composite (b).
(a)(b)The photocatalytic activity of the catalysts was investigated through OTC photodegradation and CO2 photoreduction. Figure 6(b) shows the photodegradation of OTC and UV-vis spectra of OTC solutions during test time, using 5TiCN. OTC concentrations were calculated from the absorbance intensity of UV-vis spectra at the 357 nm wavelength.Figure7 shows OTC photodegradation efficiency and kinetics. All composite photocatalysts exhibited excellent OTC degradation activity, with yields of 93%, 97%, and 92% for 5TiCN, 10TiCN, and 15TiCN, respectively. For N-TiO2, the adsorption phenomenon dominated, reaching 90% adsorbed OTC quantity after 1 hour of equilibrium, and the efficiency of OTC removal increased only ~4% when turning on the light for 5 hours. Hence, the photocatalytic reaction on N-TiO2 was negligible. This behavior was possibly due to the formation of the melon structure formed during the synthesis, besides the process of nitrogen doping on TiO2 [55]. The melon structure, which is not a semiconductor, could cover TiO2/N-TiO2 particles, preventing the photocatalytic process. A blank test (without catalyst) was also carried out for comparison. In the blank test, the OTC concentration decreased about 9% by photolysis. Hence, after 5 h light irradiation, the 10TiCN composite was slightly more active than those of 5TiCN and 15TiCN. It is noted that the adsorption phenomenon contributes a small part in the conversion calculation, only 14% in the case of 5TiCN as mentioned in the above adsorption equilibrium study. For these OTC conversion reactions, reaction kinetics were described by the following equation [56]:
(1)lnC0Ct=kt+lnC0C0′,where C0, C0′, and Ct are the initial, equilibrium, and time t concentrations of OTC during the test, respectively. Rate constants (k) were 0.389, 0.457, 0.647, and 0.451 h-1 for g-C3N4, 5TiCN, 10TiCN, and 15TiCN, respectively (from fitted lines in Figure 7(b)). The reaction kinetic on N-TiO2 was not investigated as the catalytic activity was negligible with respect to the adsorption phenomenon. Based on rate constant values, composite activities followed the following order: 10TiCN > 5TiCN ≈ 15TiCN > g-C3N4. The rate constant of 10TiCN was approximately 1.7 times that of g-C3N4.Figure 7
(a) Photodegradation efficiency of OTC onxTiCN composites versus irradiation time and (b) kinetic of OTC degradation reaction on xTiCN composites.
(a)(b)There are no reports of OTC photodegradation on TiO2-g-C3N4 composites, while there are very few reports on tetracycline (TCL) photodegradation using such composites. Wang’s research group reported 20 ppm TCL solution photodegradation using composites, such as TiO2@g-C3N4 heterojunction, TiO2@g-C3N4 core-shell quantum heterojunction, and TiO2-x/ultrathin g-C3N4/TiO2-x direct Z-scheme heterojunction [44, 57, 58]. TCL photodegradation rate on the TiO2@g-C3N4 core-shell quantum heterojunction composite was 2 and 2.3 times higher than those on TiO2 and bulk g-C3N4, respectively. Photoactivity of TiO2-x/ultrathin g-C3N4/TiO2-x direct Z-scheme heterojunction system was 20.1 and 1.3 times higher than those of TiO2 and g-C3N4, respectively. Wang et al. reported a 3 times higher TCL degradation rate on N-TiO2/O-doped N-vacancy g-C3N4 than on N-vacancy g-C3N4 [59]. Rao et al. reported a 4.4 times higher rate constant of 10 ppm TCL solution photodegradation using a hierarchical structure of anatase-rutile TiO2/g-C3N4 (ARC), compared to g-C3N4 [60].In general, the TiO2-g-C3N4 heterojunction structure improved photoactivity remarkably compared to those of bulk TiO2 and g-C3N4. OTC photodegradation yield reached up to 97%, with a 1.7 times higher rate constant with 10TiCN than with g-C3N4.The influence of pH on the photoactivity was evaluated. The researches focused on 5TiCN (Figure8(a)). After 5 hours of irradiation, the photoactivity at pH 11 and 5 was slightly improved, in comparison with pH 7. In the study of Yu et al., the isoelectric point of g-C3N4 was about pH 5 [61]. It means that the g-C3N4 surface charge is positive at pH<5 and negative at pH>5. The OTC has the pKa1=3.22, pKa2=7.68, and pKa3=8.94 [62]. Therefore, the OTC exhibited a positive charge at pH<3.22 in the form of H3TC+ and negative one at pH>9 in the main form of OTC-2. Thus, the charges on the g-C3N4 surface and OTC molecules in experimental pH conditions are the same, which prevent adsorption of OTC on the g-C3N4 surface by electrostatic repulsion force. That could explain why the pH 3 and pH 7 did not influence strongly on the OTC conversion as observed.Figure 8
Photodegradation efficiency of OTC in function of pH (a) and photostability test on 5TiCN (b).
(a)(b)The stability tests were carried out for the 5TiCN composite (Figure8(b)). The OTC conversion decreased from 93.4% to 89.4% after 5 cycles. This is the promising result in an application view.


#### 3.2.2. Photoreduction of CO2

To investigate the dual photocatalytic behavior of N-TiO2-δ/g-C3N4 composites, they were used in the photoreduction of CO2 by H2O in the gaseous phase (Figure 9). Concentration of CH4 (the only product detected) was monitored to analyze the photocatalytic behavior of the synthesized composites. 5TiCN exhibited the highest activity, with 7.0 ppm CH4 concentration, followed by 10TiCN (5.2 ppm CH4), 15TiCN (4.3 ppm CH4), and g-C3N4 (0.9 ppm CH4). No product was detected in the N-TiO2 test. As mentioned in the OTC photooxidation results above, this low photoactivity of N-TiO2 could be explained by the formed melon structure that covered TiO2/N-TiO2 particles and possibly the lower CB potential position of N-TiO2 than the standard reduction potential of CO2/CH4 [55]. The detected CH4 concentration was quite low over g-C3N4. Thus, the test was performed three times, and average value was taken. Hence, the CH4 concentration was 7.8 times higher for 5TiCN (composite exhibiting maximum photocatalysis) than for pristine g-C3N4.Figure 9
CH4 concentration on g-C3N4 and xTiCN composite.There are very few reports on photoreduction using a photocatalyst formed by TiO2 and g-C3N4 [45, 63–69]. CO2 photoreduction in the gaseous phase [45, 63–65] and liquid phase [66–69] has been reported. Gas phase studies, with CH4 and CO products, indicate better photocatalytic activity for the TiO2-g-C3N4-combined photocatalyst, compared to bulk TiO2 and g-C3N4. Zhou et al. reported a 4 times higher CO formation on g-C3N4-N-TiO2 (14.73 μmol) than on P25 (TiO2) [63]. In liquid phase photoreductions, besides CH4 and CO, other oxygenated hydrocarbons (CH3OH, HCOOH, and CH3COOH) are formed. TiO2-g-C3N4-combined photocatalysts exhibit also higher photoactivity compared to single-phase TiO2 or g-C3N4. Badiei et al. reported an 11.3 μmol·g-1·h-1 CH3OH formation for g-C3N4@TiO2, which was 5 and 10 times higher than those for g-C3N4 and P-25 (TiO2), respectively [66]. Lu et al. reported a 283.9 μmol·h-1·g-1 CO formation for 2D g-C3N4/TiO2, which was 292.2, 6.8, and 5.7 times higher than those for TiO2, bulk g-C3N4, and mechanically mixed TiO2/g-C3N4, respectively [67]. These results cannot be compared due to different experimental conditions. However, all studies indicate higher photoactivity using TiO2-g-C3N4-combined photocatalysts. In this study, CH4 production on 5TiCN was 7.8 times higher than that on bulk g-C3N4.


#### 3.2.3. Photocatalytic Mechanism

An outstanding photocatalytic activity was exhibited by thexTiCN composites during OTC photodegradation and CO2 photoreduction as compared to the pristine g-C3N4 and N-TiO2. Hence, the coupling mechanism of g-C3N4 and TiO2 is worth investigating. From the UV-vis DR spectrum, the semiconductor conduction/valance edge energies (ECB/EVB) can be calculated using the electronegativity theory [70]. According to this theory, we have the following empirical formulas:
(2)ECB=X−Ee−0.5Eg,EVB=ECB+Eg,where X, Ee, and Eg are the semiconductor electronegativity (5.81 eV for TiO2 [71] and 4.72 eV for g-C3N4 [30]), free electron energy corresponding to the hydrogen scale (4.5 eV), and semiconductor bandgap energy, respectively. As determined above from the differential curve of the UV-vis DR spectrum for 5TiCN, the bandgap energies are 3.08 and 2.85 eV, respectively, for the N-TiO2-δ and g-C3N4 constituents. Using equation (2), ECB/EVB is -0.23 V/+2.85 V for N-TiO2-δ and -1.21 V/+1.64 V for g-C3N4 (-0.14/+2.76 for separately synthesized N-TiO2). With these calculated Eg and ECB/EVB values, the energy band diagram before the coupling of the N-TiO2-δ and g-C3N4 phases is presented in Figure 10(a). As thoroughly discussed by Yang [70], when coupling two semiconductors, the built-in electric field formation and photoexcited carrier transfer occur by different routes depending on the semiconductor types, their Fermi level, and CB/VB potential positions. In xTiCN composites, N-TiO2 and g-C3N4 behave as n-type semiconductors [72, 73], and thus, their Fermi level is near the top conduction edge energy. As the N-TiO2-δ Fermi level is higher than that of g-C3N4, spontaneous electron diffusion from N-TiO2-δ to g-C3N4 occurs during their coupling and generates a built-in electric field with positive charge accumulation in the g-C3N4 interface zone and a negative one in the N-TiO2-δ interface zone. This diffusion is gradually suppressed by the built-in electric field itself, and finally, a thermal equilibrium state is established in the N-TiO2-δ/g-C3N4 heterojunction [70, 74]. On irradiating the composite, photogenerated electron (e-) and hole (h+) pairs formed on the CB and VB of each constituent. Subsequently, the built-in electric field promoted the photogenerated electrons on the CB of N-TiO2-δ to the VB of g-C3N4. This transfer is displayed in Figures 10(b) and 10(c) and is well-known as the Z-scheme mechanism. In this system, the charge transfer between two phases caused the separation of photogenerated electrons and holes into different zones—electrons in the CB of g-C3N4 and holes in the VB of TiO2—preventing their recombination or, simply put, prolonging their lifetime. Notably, this type of charge transfer enriched the photogenerated electrons on the negative CB of g-C3N4 and photogenerated holes on the positive VB of TiO2, thereby strengthening the redox property of the composite. Additionally, a low bandgap of g-C3N4 broadened the light absorption region and improved the light utilization efficiency. All these factors contributed to the photocatalytic enhancement of N-TiO2-δ/g-C3N4 composites.Figure 10
Schematic illustrations of the energy band structures of N-TiO2-δ and g-C3N4 before contact (a), N-TiO2-δ/g-C3N4 heterojunction composite for OTC photooxidation mechanism (b), and CO2 photoreduction mechanism (c).
(a)(b)(c)Note that the type-II andZ-scheme mechanisms entail a similar photocatalyst composite structure. However, in the type-II mechanism, reduction takes place on the CB of the semiconductor with a less negative CB potential. Hence, for the xTiCN composite, if the type-II mechanism had occurred, the CO2 would have been reduced on the CB of the N-TiO2-δ constituent. This is unlikely as the reduction potential of the N-TiO2-δ constituent (-0.23 V) is less negative than that of the CO2/CH4 (-0.24 V) [14]. Nevertheless, the obtained experimental results show a remarkably higher CH4 content formation than that of bare g-C3N4. This evidence suggests that the xTiCN photocatalyst composites entail the Z-scheme mechanism. It is also found that CO2 photoreduction could not take place on separately synthesized N-TiO2 as its CD potential position (-0.14 V) is less negative than that of CO2/CH4 (-0.24 V).The photogenerated electron-hole pair transfers are summarized as follows:N‐TiO2‐δ+hν⟶e−N‐TiO2‐δCB+h+N‐TiO2‐δVBg‐C3N4+hν⟶e−g‐C3N4CB+h+g‐C3N4VBe−N‐TiO2‐δCB⟶h+g‐C3N4VBchargetransferThen, for oxidation of OTC,e−g‐C3N4CB+O2⟶O⋅2h+N‐TiO2‐δVB+OTC⟶byproduct+CO2+H2Oor for reduction of CO2,8e−g‐C3N4CB+CO2+8H+⟶CH4+2H2O8h+TiO2‐δVB+8H2O⟶8H++8⋅OH8⋅OH⟶4H2O+2O2


## 3.1. Structural Characterization

XRD patterns ofxTiCN composites are shown in Figure 2(a). The formation of the g-C3N4 crystalline phase, with a characteristic peak at 27.8°, was observed in all XRD patterns, confirming the g-C3N4 structure after composite synthesis. Characteristic peaks at 25.5°, 38.0°, 48.1°, 54.2°, and 62.8°, corresponding to the TiO2 anatase phase, were observed in the XRD patterns of all composites. In the N-TiO2 XRD patterns, a small quantity of the rutile phase was recognized at 2 theta of 27.5 and 36.1°. To identify different components, FTIR characterization was carried out, and the spectra are shown in Figure 2(b). In the g-C3N4 spectrum, the peak at 802 cm-1 was ascribed to the s-triazine bending mode [30]. The peaks at 1228 and 1311 cm-1 were attributed to the C–N stretching vibrations [47, 48]. The peaks at 1392, 1535, and 1625 cm-1 originated from the –C=N stretching vibrations in aromatic rings. The broad band at 3000–3500 cm-1 corresponded to the stretching vibrations of the absorbed water hydroxyl group (–O–H) and terminal amino groups (–NH2) [47, 48]. Finally, the broad peak at 534 cm-1 was attributed to the Ti–O bond vibrations [49].Figure 2
XRD patterns (a) and FTIR spectra (b) of g-C3N4 and xTiCN composites.
(a)(b)The light absorption abilities of the composites were analyzed by UV-vis DR spectra. Figure3(a) shows the UV-vis DR spectra of g-C3N4 and xTiCN. The bandgap energies, calculated using the Kubelka–Munk function (results are shown in Figure 3(b)), were 2.58, 2.60, 2.63, 2.66, and 2.90 eV for g-C3N4, 5TiCN, 10TiCN,15TiCN, and N-TiO2, respectively. Increasing the TiO2 content broadened the composite bandgap energy owing to a greater contribution of the large band gap energy by TiO2 (3.2 eV) as compared to g-C3N4 (2.58 eV). However, generally, all photocatalyst composites could be activated by visible light. The corresponding differential curves of UV-vis DR spectra are displayed in Figure 3(c). The absorption edges (λabs) of pristine TiO2 (P25) and g-C3N4 are at wavelengths of 396.3 and 437.5 nm, respectively. In contrast, the xTiCN composites exhibit a slight shift of the g-C3N4 adsorption edge peak to a shorter wavelength, while that of TiO2 slightly shifts to a longer wavelength. The longer-wavelength-shifted adsorption edge of the TiO2 constituent is possibly due to nitrogen doping in TiO2 during the composite synthesis. These observations suggest an interaction between g-C3N4 and TiO2, forming in situ-doped N-TiO2. In the case of 5TiCN, the observed adsorption edge peaks of TiO2 andg-C3N4 correspond to wavelengths of 402.0 and 435.3 nm, respectively. Based on the relationship Eg=1240/λabs, the calculated bandgap energies (Eg) of TiO2 and g-C3N4 are 3.08 and 2.85 eV, respectively. For the N-TiO2 sample prepared by the hydrothermal method, the differential curve of the UV-vis DR spectrum exhibited a single weak and broad peak at 392.5 nm (or Eg=3.16eV). This peak is attributed to the adsorption edge of TiO2.Figure 3
UV-vis DR spectra (a), energy bandgap determination by Kubelka–Munk function (b), and its corresponding differential curves (c) of g-C3N4, N-TiO2, and xTiCN composites; N2 adsorption-desorption isotherms and corresponding pore distribution curves of g-C3N4 (solid line) and 5TiCN composite (dash line) (d).
(a)(b)(c)(d)The g-C3N4 and 5TiCN composites were selected to characterize its specific surface area, one of the important properties of heterogeneous catalysis. The results are presented in Figure 3(d). It is observed that two samples show the type-4 isotherms with H3 hysteresis loop, which indicate the presence of a mesopore. This is suitable with the obtained pore distribution curves. The BET surface areas are 18 and 20 m2/g for g-C3N4 and 5TiCN, respectively. Hence, the composite preparation did not seem to change the g-C3N4 structure.The interaction between the TiO2 and g-C3N4 phases is better understood from the TEM images presented in Figure 4. It is observed that the TiO2 particles (dark areas in Figures 4(a) and 4(b)) were formed in various sizes and shapes. Some TiO2 particles were deposited on the g-C3N4 layer (e.g., position of cycle 1), while the others were covered by g-C3N4 multisheets, forming a core-shell structure (e.g., position of cycle 2), thereby enhancing the interaction surface between the TiO2 and g-C3N4 phases.Figure 4
TEM images of 5TiCN.The elemental composition and oxidation state of the catalyst influence catalytic performance. Therefore, the 5TiCN composite was characterized using XPS, as shown in Figure5.Figure 5
X-ray photoelectron spectra of 5TiCN: (a) survey spectra; (b) high-resolution spectra of Ti2p, (c) high-resolution spectra of O1s; (d) high-resolution spectra of N1s; (e) high-resolution spectra of C1s.
(a)(b)(c)(d)(e)Three peaks at binding energies of 458.5, 459.1, and 464.3 eV, corresponding to orbitals Ti4+2p3/2, Ti3+2p1/2, and Ti4+2p1/2, respectively, were observed in the Ti2p high-resolution spectrum [50, 51]. As reported by Jia et al., binding energies of two prominent peaks (at 458.5 and 464.3 eV) exhibited 0.2 eV shifts compared to spectral peaks of pure TiO2. This could be due to the substitution of O2- by N3-, leading to the formation of N-Ti-O bonds [52]. This indicated N doping of TiO2 during composite synthesis, decreasing the TiO2 bandgap energy, leading to visible-light activation [17, 18]. The small peak of Ti3+2p1/2 confirmed oxygen vacancies in TiO2 [51]. Hence, TiO2-δ is a more accurate molecular formula than TiO2.Deconvolution peaks for the O1s spectrum exhibited two peaks at 531 and 532 eV, assigned to O-H of absorbed water and the Ti–O bond [53], respectively. N1s spectrum exhibited three peaks at 398.6, 399.7, and 400.8 eV, ascribed to the sp2C of C-N=C, tertiary N of N-(C)3 group, and N-C=N bonds, respectively. The C1s spectrum exhibited three peaks at 284.9, 286.3, and 288.2 eV, corresponding to C-C, C-NH2, and N-C=N bonds, respectively [53, 54].


## 3.2. Evaluation of Photocatalytic Activity

### 3.2.1. Photooxidation of OTC

Before performing the photocatalytic test, the adsorption equilibria were carried out (Figure6(a)). The results demonstrated that all composites reached rapidly the adsorption equilibrium after only about 15 minutes, while 60 minutes was required for N-TiO2. The calculation indicated that the equilibrium adsorption quantities of OTC at 60 min were 11%, 14%, 17%, 12%, and 88% for g-C3N4, 5TiCN, 10TiCN, 15TiCN, and N-TiO2, respectively. It remarked that there was a strong adsorption phenomenon of OTC on N-TiO2.Figure 6
OTC adsorption equilibrium (a) onxTiCN and UV-vis spectra of OTC during test time on 5TiCN composite (b).
(a)(b)The photocatalytic activity of the catalysts was investigated through OTC photodegradation and CO2 photoreduction. Figure 6(b) shows the photodegradation of OTC and UV-vis spectra of OTC solutions during test time, using 5TiCN. OTC concentrations were calculated from the absorbance intensity of UV-vis spectra at the 357 nm wavelength.Figure7 shows OTC photodegradation efficiency and kinetics. All composite photocatalysts exhibited excellent OTC degradation activity, with yields of 93%, 97%, and 92% for 5TiCN, 10TiCN, and 15TiCN, respectively. For N-TiO2, the adsorption phenomenon dominated, reaching 90% adsorbed OTC quantity after 1 hour of equilibrium, and the efficiency of OTC removal increased only ~4% when turning on the light for 5 hours. Hence, the photocatalytic reaction on N-TiO2 was negligible. This behavior was possibly due to the formation of the melon structure formed during the synthesis, besides the process of nitrogen doping on TiO2 [55]. The melon structure, which is not a semiconductor, could cover TiO2/N-TiO2 particles, preventing the photocatalytic process. A blank test (without catalyst) was also carried out for comparison. In the blank test, the OTC concentration decreased about 9% by photolysis. Hence, after 5 h light irradiation, the 10TiCN composite was slightly more active than those of 5TiCN and 15TiCN. It is noted that the adsorption phenomenon contributes a small part in the conversion calculation, only 14% in the case of 5TiCN as mentioned in the above adsorption equilibrium study. For these OTC conversion reactions, reaction kinetics were described by the following equation [56]:
(1)lnC0Ct=kt+lnC0C0′,where C0, C0′, and Ct are the initial, equilibrium, and time t concentrations of OTC during the test, respectively. Rate constants (k) were 0.389, 0.457, 0.647, and 0.451 h-1 for g-C3N4, 5TiCN, 10TiCN, and 15TiCN, respectively (from fitted lines in Figure 7(b)). The reaction kinetic on N-TiO2 was not investigated as the catalytic activity was negligible with respect to the adsorption phenomenon. Based on rate constant values, composite activities followed the following order: 10TiCN > 5TiCN ≈ 15TiCN > g-C3N4. The rate constant of 10TiCN was approximately 1.7 times that of g-C3N4.Figure 7
(a) Photodegradation efficiency of OTC onxTiCN composites versus irradiation time and (b) kinetic of OTC degradation reaction on xTiCN composites.
(a)(b)There are no reports of OTC photodegradation on TiO2-g-C3N4 composites, while there are very few reports on tetracycline (TCL) photodegradation using such composites. Wang’s research group reported 20 ppm TCL solution photodegradation using composites, such as TiO2@g-C3N4 heterojunction, TiO2@g-C3N4 core-shell quantum heterojunction, and TiO2-x/ultrathin g-C3N4/TiO2-x direct Z-scheme heterojunction [44, 57, 58]. TCL photodegradation rate on the TiO2@g-C3N4 core-shell quantum heterojunction composite was 2 and 2.3 times higher than those on TiO2 and bulk g-C3N4, respectively. Photoactivity of TiO2-x/ultrathin g-C3N4/TiO2-x direct Z-scheme heterojunction system was 20.1 and 1.3 times higher than those of TiO2 and g-C3N4, respectively. Wang et al. reported a 3 times higher TCL degradation rate on N-TiO2/O-doped N-vacancy g-C3N4 than on N-vacancy g-C3N4 [59]. Rao et al. reported a 4.4 times higher rate constant of 10 ppm TCL solution photodegradation using a hierarchical structure of anatase-rutile TiO2/g-C3N4 (ARC), compared to g-C3N4 [60].In general, the TiO2-g-C3N4 heterojunction structure improved photoactivity remarkably compared to those of bulk TiO2 and g-C3N4. OTC photodegradation yield reached up to 97%, with a 1.7 times higher rate constant with 10TiCN than with g-C3N4.The influence of pH on the photoactivity was evaluated. The researches focused on 5TiCN (Figure8(a)). After 5 hours of irradiation, the photoactivity at pH 11 and 5 was slightly improved, in comparison with pH 7. In the study of Yu et al., the isoelectric point of g-C3N4 was about pH 5 [61]. It means that the g-C3N4 surface charge is positive at pH<5 and negative at pH>5. The OTC has the pKa1=3.22, pKa2=7.68, and pKa3=8.94 [62]. Therefore, the OTC exhibited a positive charge at pH<3.22 in the form of H3TC+ and negative one at pH>9 in the main form of OTC-2. Thus, the charges on the g-C3N4 surface and OTC molecules in experimental pH conditions are the same, which prevent adsorption of OTC on the g-C3N4 surface by electrostatic repulsion force. That could explain why the pH 3 and pH 7 did not influence strongly on the OTC conversion as observed.Figure 8
Photodegradation efficiency of OTC in function of pH (a) and photostability test on 5TiCN (b).
(a)(b)The stability tests were carried out for the 5TiCN composite (Figure8(b)). The OTC conversion decreased from 93.4% to 89.4% after 5 cycles. This is the promising result in an application view.


### 3.2.2. Photoreduction of CO2

To investigate the dual photocatalytic behavior of N-TiO2-δ/g-C3N4 composites, they were used in the photoreduction of CO2 by H2O in the gaseous phase (Figure 9). Concentration of CH4 (the only product detected) was monitored to analyze the photocatalytic behavior of the synthesized composites. 5TiCN exhibited the highest activity, with 7.0 ppm CH4 concentration, followed by 10TiCN (5.2 ppm CH4), 15TiCN (4.3 ppm CH4), and g-C3N4 (0.9 ppm CH4). No product was detected in the N-TiO2 test. As mentioned in the OTC photooxidation results above, this low photoactivity of N-TiO2 could be explained by the formed melon structure that covered TiO2/N-TiO2 particles and possibly the lower CB potential position of N-TiO2 than the standard reduction potential of CO2/CH4 [55]. The detected CH4 concentration was quite low over g-C3N4. Thus, the test was performed three times, and average value was taken. Hence, the CH4 concentration was 7.8 times higher for 5TiCN (composite exhibiting maximum photocatalysis) than for pristine g-C3N4.Figure 9
CH4 concentration on g-C3N4 and xTiCN composite.There are very few reports on photoreduction using a photocatalyst formed by TiO2 and g-C3N4 [45, 63–69]. CO2 photoreduction in the gaseous phase [45, 63–65] and liquid phase [66–69] has been reported. Gas phase studies, with CH4 and CO products, indicate better photocatalytic activity for the TiO2-g-C3N4-combined photocatalyst, compared to bulk TiO2 and g-C3N4. Zhou et al. reported a 4 times higher CO formation on g-C3N4-N-TiO2 (14.73 μmol) than on P25 (TiO2) [63]. In liquid phase photoreductions, besides CH4 and CO, other oxygenated hydrocarbons (CH3OH, HCOOH, and CH3COOH) are formed. TiO2-g-C3N4-combined photocatalysts exhibit also higher photoactivity compared to single-phase TiO2 or g-C3N4. Badiei et al. reported an 11.3 μmol·g-1·h-1 CH3OH formation for g-C3N4@TiO2, which was 5 and 10 times higher than those for g-C3N4 and P-25 (TiO2), respectively [66]. Lu et al. reported a 283.9 μmol·h-1·g-1 CO formation for 2D g-C3N4/TiO2, which was 292.2, 6.8, and 5.7 times higher than those for TiO2, bulk g-C3N4, and mechanically mixed TiO2/g-C3N4, respectively [67]. These results cannot be compared due to different experimental conditions. However, all studies indicate higher photoactivity using TiO2-g-C3N4-combined photocatalysts. In this study, CH4 production on 5TiCN was 7.8 times higher than that on bulk g-C3N4.


### 3.2.3. Photocatalytic Mechanism

An outstanding photocatalytic activity was exhibited by thexTiCN composites during OTC photodegradation and CO2 photoreduction as compared to the pristine g-C3N4 and N-TiO2. Hence, the coupling mechanism of g-C3N4 and TiO2 is worth investigating. From the UV-vis DR spectrum, the semiconductor conduction/valance edge energies (ECB/EVB) can be calculated using the electronegativity theory [70]. According to this theory, we have the following empirical formulas:
(2)ECB=X−Ee−0.5Eg,EVB=ECB+Eg,where X, Ee, and Eg are the semiconductor electronegativity (5.81 eV for TiO2 [71] and 4.72 eV for g-C3N4 [30]), free electron energy corresponding to the hydrogen scale (4.5 eV), and semiconductor bandgap energy, respectively. As determined above from the differential curve of the UV-vis DR spectrum for 5TiCN, the bandgap energies are 3.08 and 2.85 eV, respectively, for the N-TiO2-δ and g-C3N4 constituents. Using equation (2), ECB/EVB is -0.23 V/+2.85 V for N-TiO2-δ and -1.21 V/+1.64 V for g-C3N4 (-0.14/+2.76 for separately synthesized N-TiO2). With these calculated Eg and ECB/EVB values, the energy band diagram before the coupling of the N-TiO2-δ and g-C3N4 phases is presented in Figure 10(a). As thoroughly discussed by Yang [70], when coupling two semiconductors, the built-in electric field formation and photoexcited carrier transfer occur by different routes depending on the semiconductor types, their Fermi level, and CB/VB potential positions. In xTiCN composites, N-TiO2 and g-C3N4 behave as n-type semiconductors [72, 73], and thus, their Fermi level is near the top conduction edge energy. As the N-TiO2-δ Fermi level is higher than that of g-C3N4, spontaneous electron diffusion from N-TiO2-δ to g-C3N4 occurs during their coupling and generates a built-in electric field with positive charge accumulation in the g-C3N4 interface zone and a negative one in the N-TiO2-δ interface zone. This diffusion is gradually suppressed by the built-in electric field itself, and finally, a thermal equilibrium state is established in the N-TiO2-δ/g-C3N4 heterojunction [70, 74]. On irradiating the composite, photogenerated electron (e-) and hole (h+) pairs formed on the CB and VB of each constituent. Subsequently, the built-in electric field promoted the photogenerated electrons on the CB of N-TiO2-δ to the VB of g-C3N4. This transfer is displayed in Figures 10(b) and 10(c) and is well-known as the Z-scheme mechanism. In this system, the charge transfer between two phases caused the separation of photogenerated electrons and holes into different zones—electrons in the CB of g-C3N4 and holes in the VB of TiO2—preventing their recombination or, simply put, prolonging their lifetime. Notably, this type of charge transfer enriched the photogenerated electrons on the negative CB of g-C3N4 and photogenerated holes on the positive VB of TiO2, thereby strengthening the redox property of the composite. Additionally, a low bandgap of g-C3N4 broadened the light absorption region and improved the light utilization efficiency. All these factors contributed to the photocatalytic enhancement of N-TiO2-δ/g-C3N4 composites.Figure 10
Schematic illustrations of the energy band structures of N-TiO2-δ and g-C3N4 before contact (a), N-TiO2-δ/g-C3N4 heterojunction composite for OTC photooxidation mechanism (b), and CO2 photoreduction mechanism (c).
(a)(b)(c)Note that the type-II andZ-scheme mechanisms entail a similar photocatalyst composite structure. However, in the type-II mechanism, reduction takes place on the CB of the semiconductor with a less negative CB potential. Hence, for the xTiCN composite, if the type-II mechanism had occurred, the CO2 would have been reduced on the CB of the N-TiO2-δ constituent. This is unlikely as the reduction potential of the N-TiO2-δ constituent (-0.23 V) is less negative than that of the CO2/CH4 (-0.24 V) [14]. Nevertheless, the obtained experimental results show a remarkably higher CH4 content formation than that of bare g-C3N4. This evidence suggests that the xTiCN photocatalyst composites entail the Z-scheme mechanism. It is also found that CO2 photoreduction could not take place on separately synthesized N-TiO2 as its CD potential position (-0.14 V) is less negative than that of CO2/CH4 (-0.24 V).The photogenerated electron-hole pair transfers are summarized as follows:N‐TiO2‐δ+hν⟶e−N‐TiO2‐δCB+h+N‐TiO2‐δVBg‐C3N4+hν⟶e−g‐C3N4CB+h+g‐C3N4VBe−N‐TiO2‐δCB⟶h+g‐C3N4VBchargetransferThen, for oxidation of OTC,e−g‐C3N4CB+O2⟶O⋅2h+N‐TiO2‐δVB+OTC⟶byproduct+CO2+H2Oor for reduction of CO2,8e−g‐C3N4CB+CO2+8H+⟶CH4+2H2O8h+TiO2‐δVB+8H2O⟶8H++8⋅OH8⋅OH⟶4H2O+2O2


## 3.2.1. Photooxidation of OTC

Before performing the photocatalytic test, the adsorption equilibria were carried out (Figure6(a)). The results demonstrated that all composites reached rapidly the adsorption equilibrium after only about 15 minutes, while 60 minutes was required for N-TiO2. The calculation indicated that the equilibrium adsorption quantities of OTC at 60 min were 11%, 14%, 17%, 12%, and 88% for g-C3N4, 5TiCN, 10TiCN, 15TiCN, and N-TiO2, respectively. It remarked that there was a strong adsorption phenomenon of OTC on N-TiO2.Figure 6
OTC adsorption equilibrium (a) onxTiCN and UV-vis spectra of OTC during test time on 5TiCN composite (b).
(a)(b)The photocatalytic activity of the catalysts was investigated through OTC photodegradation and CO2 photoreduction. Figure 6(b) shows the photodegradation of OTC and UV-vis spectra of OTC solutions during test time, using 5TiCN. OTC concentrations were calculated from the absorbance intensity of UV-vis spectra at the 357 nm wavelength.Figure7 shows OTC photodegradation efficiency and kinetics. All composite photocatalysts exhibited excellent OTC degradation activity, with yields of 93%, 97%, and 92% for 5TiCN, 10TiCN, and 15TiCN, respectively. For N-TiO2, the adsorption phenomenon dominated, reaching 90% adsorbed OTC quantity after 1 hour of equilibrium, and the efficiency of OTC removal increased only ~4% when turning on the light for 5 hours. Hence, the photocatalytic reaction on N-TiO2 was negligible. This behavior was possibly due to the formation of the melon structure formed during the synthesis, besides the process of nitrogen doping on TiO2 [55]. The melon structure, which is not a semiconductor, could cover TiO2/N-TiO2 particles, preventing the photocatalytic process. A blank test (without catalyst) was also carried out for comparison. In the blank test, the OTC concentration decreased about 9% by photolysis. Hence, after 5 h light irradiation, the 10TiCN composite was slightly more active than those of 5TiCN and 15TiCN. It is noted that the adsorption phenomenon contributes a small part in the conversion calculation, only 14% in the case of 5TiCN as mentioned in the above adsorption equilibrium study. For these OTC conversion reactions, reaction kinetics were described by the following equation [56]:
(1)lnC0Ct=kt+lnC0C0′,where C0, C0′, and Ct are the initial, equilibrium, and time t concentrations of OTC during the test, respectively. Rate constants (k) were 0.389, 0.457, 0.647, and 0.451 h-1 for g-C3N4, 5TiCN, 10TiCN, and 15TiCN, respectively (from fitted lines in Figure 7(b)). The reaction kinetic on N-TiO2 was not investigated as the catalytic activity was negligible with respect to the adsorption phenomenon. Based on rate constant values, composite activities followed the following order: 10TiCN > 5TiCN ≈ 15TiCN > g-C3N4. The rate constant of 10TiCN was approximately 1.7 times that of g-C3N4.Figure 7
(a) Photodegradation efficiency of OTC onxTiCN composites versus irradiation time and (b) kinetic of OTC degradation reaction on xTiCN composites.
(a)(b)There are no reports of OTC photodegradation on TiO2-g-C3N4 composites, while there are very few reports on tetracycline (TCL) photodegradation using such composites. Wang’s research group reported 20 ppm TCL solution photodegradation using composites, such as TiO2@g-C3N4 heterojunction, TiO2@g-C3N4 core-shell quantum heterojunction, and TiO2-x/ultrathin g-C3N4/TiO2-x direct Z-scheme heterojunction [44, 57, 58]. TCL photodegradation rate on the TiO2@g-C3N4 core-shell quantum heterojunction composite was 2 and 2.3 times higher than those on TiO2 and bulk g-C3N4, respectively. Photoactivity of TiO2-x/ultrathin g-C3N4/TiO2-x direct Z-scheme heterojunction system was 20.1 and 1.3 times higher than those of TiO2 and g-C3N4, respectively. Wang et al. reported a 3 times higher TCL degradation rate on N-TiO2/O-doped N-vacancy g-C3N4 than on N-vacancy g-C3N4 [59]. Rao et al. reported a 4.4 times higher rate constant of 10 ppm TCL solution photodegradation using a hierarchical structure of anatase-rutile TiO2/g-C3N4 (ARC), compared to g-C3N4 [60].In general, the TiO2-g-C3N4 heterojunction structure improved photoactivity remarkably compared to those of bulk TiO2 and g-C3N4. OTC photodegradation yield reached up to 97%, with a 1.7 times higher rate constant with 10TiCN than with g-C3N4.The influence of pH on the photoactivity was evaluated. The researches focused on 5TiCN (Figure8(a)). After 5 hours of irradiation, the photoactivity at pH 11 and 5 was slightly improved, in comparison with pH 7. In the study of Yu et al., the isoelectric point of g-C3N4 was about pH 5 [61]. It means that the g-C3N4 surface charge is positive at pH<5 and negative at pH>5. The OTC has the pKa1=3.22, pKa2=7.68, and pKa3=8.94 [62]. Therefore, the OTC exhibited a positive charge at pH<3.22 in the form of H3TC+ and negative one at pH>9 in the main form of OTC-2. Thus, the charges on the g-C3N4 surface and OTC molecules in experimental pH conditions are the same, which prevent adsorption of OTC on the g-C3N4 surface by electrostatic repulsion force. That could explain why the pH 3 and pH 7 did not influence strongly on the OTC conversion as observed.Figure 8
Photodegradation efficiency of OTC in function of pH (a) and photostability test on 5TiCN (b).
(a)(b)The stability tests were carried out for the 5TiCN composite (Figure8(b)). The OTC conversion decreased from 93.4% to 89.4% after 5 cycles. This is the promising result in an application view.


## 3.2.2. Photoreduction of CO2

To investigate the dual photocatalytic behavior of N-TiO2-δ/g-C3N4 composites, they were used in the photoreduction of CO2 by H2O in the gaseous phase (Figure 9). Concentration of CH4 (the only product detected) was monitored to analyze the photocatalytic behavior of the synthesized composites. 5TiCN exhibited the highest activity, with 7.0 ppm CH4 concentration, followed by 10TiCN (5.2 ppm CH4), 15TiCN (4.3 ppm CH4), and g-C3N4 (0.9 ppm CH4). No product was detected in the N-TiO2 test. As mentioned in the OTC photooxidation results above, this low photoactivity of N-TiO2 could be explained by the formed melon structure that covered TiO2/N-TiO2 particles and possibly the lower CB potential position of N-TiO2 than the standard reduction potential of CO2/CH4 [55]. The detected CH4 concentration was quite low over g-C3N4. Thus, the test was performed three times, and average value was taken. Hence, the CH4 concentration was 7.8 times higher for 5TiCN (composite exhibiting maximum photocatalysis) than for pristine g-C3N4.Figure 9
CH4 concentration on g-C3N4 and xTiCN composite.There are very few reports on photoreduction using a photocatalyst formed by TiO2 and g-C3N4 [45, 63–69]. CO2 photoreduction in the gaseous phase [45, 63–65] and liquid phase [66–69] has been reported. Gas phase studies, with CH4 and CO products, indicate better photocatalytic activity for the TiO2-g-C3N4-combined photocatalyst, compared to bulk TiO2 and g-C3N4. Zhou et al. reported a 4 times higher CO formation on g-C3N4-N-TiO2 (14.73 μmol) than on P25 (TiO2) [63]. In liquid phase photoreductions, besides CH4 and CO, other oxygenated hydrocarbons (CH3OH, HCOOH, and CH3COOH) are formed. TiO2-g-C3N4-combined photocatalysts exhibit also higher photoactivity compared to single-phase TiO2 or g-C3N4. Badiei et al. reported an 11.3 μmol·g-1·h-1 CH3OH formation for g-C3N4@TiO2, which was 5 and 10 times higher than those for g-C3N4 and P-25 (TiO2), respectively [66]. Lu et al. reported a 283.9 μmol·h-1·g-1 CO formation for 2D g-C3N4/TiO2, which was 292.2, 6.8, and 5.7 times higher than those for TiO2, bulk g-C3N4, and mechanically mixed TiO2/g-C3N4, respectively [67]. These results cannot be compared due to different experimental conditions. However, all studies indicate higher photoactivity using TiO2-g-C3N4-combined photocatalysts. In this study, CH4 production on 5TiCN was 7.8 times higher than that on bulk g-C3N4.


## 3.2.3. Photocatalytic Mechanism

An outstanding photocatalytic activity was exhibited by thexTiCN composites during OTC photodegradation and CO2 photoreduction as compared to the pristine g-C3N4 and N-TiO2. Hence, the coupling mechanism of g-C3N4 and TiO2 is worth investigating. From the UV-vis DR spectrum, the semiconductor conduction/valance edge energies (ECB/EVB) can be calculated using the electronegativity theory [70]. According to this theory, we have the following empirical formulas:
(2)ECB=X−Ee−0.5Eg,EVB=ECB+Eg,where X, Ee, and Eg are the semiconductor electronegativity (5.81 eV for TiO2 [71] and 4.72 eV for g-C3N4 [30]), free electron energy corresponding to the hydrogen scale (4.5 eV), and semiconductor bandgap energy, respectively. As determined above from the differential curve of the UV-vis DR spectrum for 5TiCN, the bandgap energies are 3.08 and 2.85 eV, respectively, for the N-TiO2-δ and g-C3N4 constituents. Using equation (2), ECB/EVB is -0.23 V/+2.85 V for N-TiO2-δ and -1.21 V/+1.64 V for g-C3N4 (-0.14/+2.76 for separately synthesized N-TiO2). With these calculated Eg and ECB/EVB values, the energy band diagram before the coupling of the N-TiO2-δ and g-C3N4 phases is presented in Figure 10(a). As thoroughly discussed by Yang [70], when coupling two semiconductors, the built-in electric field formation and photoexcited carrier transfer occur by different routes depending on the semiconductor types, their Fermi level, and CB/VB potential positions. In xTiCN composites, N-TiO2 and g-C3N4 behave as n-type semiconductors [72, 73], and thus, their Fermi level is near the top conduction edge energy. As the N-TiO2-δ Fermi level is higher than that of g-C3N4, spontaneous electron diffusion from N-TiO2-δ to g-C3N4 occurs during their coupling and generates a built-in electric field with positive charge accumulation in the g-C3N4 interface zone and a negative one in the N-TiO2-δ interface zone. This diffusion is gradually suppressed by the built-in electric field itself, and finally, a thermal equilibrium state is established in the N-TiO2-δ/g-C3N4 heterojunction [70, 74]. On irradiating the composite, photogenerated electron (e-) and hole (h+) pairs formed on the CB and VB of each constituent. Subsequently, the built-in electric field promoted the photogenerated electrons on the CB of N-TiO2-δ to the VB of g-C3N4. This transfer is displayed in Figures 10(b) and 10(c) and is well-known as the Z-scheme mechanism. In this system, the charge transfer between two phases caused the separation of photogenerated electrons and holes into different zones—electrons in the CB of g-C3N4 and holes in the VB of TiO2—preventing their recombination or, simply put, prolonging their lifetime. Notably, this type of charge transfer enriched the photogenerated electrons on the negative CB of g-C3N4 and photogenerated holes on the positive VB of TiO2, thereby strengthening the redox property of the composite. Additionally, a low bandgap of g-C3N4 broadened the light absorption region and improved the light utilization efficiency. All these factors contributed to the photocatalytic enhancement of N-TiO2-δ/g-C3N4 composites.Figure 10
Schematic illustrations of the energy band structures of N-TiO2-δ and g-C3N4 before contact (a), N-TiO2-δ/g-C3N4 heterojunction composite for OTC photooxidation mechanism (b), and CO2 photoreduction mechanism (c).
(a)(b)(c)Note that the type-II andZ-scheme mechanisms entail a similar photocatalyst composite structure. However, in the type-II mechanism, reduction takes place on the CB of the semiconductor with a less negative CB potential. Hence, for the xTiCN composite, if the type-II mechanism had occurred, the CO2 would have been reduced on the CB of the N-TiO2-δ constituent. This is unlikely as the reduction potential of the N-TiO2-δ constituent (-0.23 V) is less negative than that of the CO2/CH4 (-0.24 V) [14]. Nevertheless, the obtained experimental results show a remarkably higher CH4 content formation than that of bare g-C3N4. This evidence suggests that the xTiCN photocatalyst composites entail the Z-scheme mechanism. It is also found that CO2 photoreduction could not take place on separately synthesized N-TiO2 as its CD potential position (-0.14 V) is less negative than that of CO2/CH4 (-0.24 V).The photogenerated electron-hole pair transfers are summarized as follows:N‐TiO2‐δ+hν⟶e−N‐TiO2‐δCB+h+N‐TiO2‐δVBg‐C3N4+hν⟶e−g‐C3N4CB+h+g‐C3N4VBe−N‐TiO2‐δCB⟶h+g‐C3N4VBchargetransferThen, for oxidation of OTC,e−g‐C3N4CB+O2⟶O⋅2h+N‐TiO2‐δVB+OTC⟶byproduct+CO2+H2Oor for reduction of CO2,8e−g‐C3N4CB+CO2+8H+⟶CH4+2H2O8h+TiO2‐δVB+8H2O⟶8H++8⋅OH8⋅OH⟶4H2O+2O2


## 4. Conclusions

Here,x% (wt) N-TiO2-δ/g-C3N4 composites were synthesized, and their textural and structural properties were analyzed by XRD, FTIR, UV-DRS, TEM, and XPS. All composites showed better activity than bulk g-C3N4 towards OTC photodegradation and CO2 photoreduction. OTC photodegradation yields were higher for the composites (93%, 97%, and 92%, on 5TiCN, 10TiCN, and 15TiCN, respectively) than for bulk g-C3N4 (86%). Among these catalysts, the 10TiCN showed the highest rate constant of 0.647 h-1. In CO2 photoreduction, CH4 was the only product detected. CH4 concentrations of 0.9, 7.0, 5.2, and 4.3 ppm were detected using bulk g-C3N4, 5TiCN, 10TiCN, and 15TiCN, respectively. CH4 formation of 5TiCN was 7.8 times higher than that of bulk g-C3N4. Enhanced composite photoactivities were attributed to their Z-scheme mechanism. With this structure, charge transfer between N-TiO2-δ (CB) and g-C3N4 (VB) occurred, leading to recombination prevention of photogenerated electron-hole pairs and stronger redox abilities. The interesting obtained result above indicates the promising novel dual photocatalysts. Tuning composite morphology (e.g., specific surface area and porosity) and contact surface area between components (e.g., good dispersion phase and core-shell system) could enhance photoactivity of x% (wt) N-TiO2-δ/g-C3N4 further.



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