Facile Synthesis with TiO2 Xerogel and Urea Enhanced Aniline Aerofloat Degradation Performance of Direct Z-Scheme Heterojunction TiO2/g-C3N4 Composite.

Different TiO2/g-C3N4 (TCN) composites were synthesized by a simple pyrolysis method with TiO2 xerogel and urea. The structure and physicochemical properties of TCN were characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope, ultraviolet-visible diffuse reflectance spectrum, X-ray photoelectron spectroscopy, N2-adsorption isotherms and electrochemical impedance spectroscopy. Aniline Aerofloat was chosen as a typical degradation-resistant contaminant to investigate the photodegradation activity of TCN under UV irradiation. The results indicated that TCN had higher light absorption intensity, larger specific surface area and smaller particle size compared to pure TiO2. Furthermore, TCN had great recycling photocatalytic stability for the photodegradation of Aniline Aerofloat. The photocatalytic activity depends on the synergistic reaction between holes (h+) and hydroxyl radicals (·OH). Meanwhile, the direct Z-scheme heterojunction structure of TiO2 and g-C3N4 postpones the recombination of h+ and electrons to enhance UV-light photocatalytic activity.

1. Introduction

Aniline Aerofloat (dianilinodithiophosphoric acid, AAF, (C6H5NH)2PSSH) has been used frequently as an efficient flotation collecting agent for lead-zinc sulfide ore in China [1]. It was discharged with flotation wastewater into nature although it is highly toxic and contains dithiophosphate and aniline groups [2,3,4]. Moreover, AAF has been hard to degrade thoroughly so far. Some degradation methods have been researched such as sodium hypochlorite oxidation [5], vacuum ultraviolet/ozone [6], chelate precipitation [7], and so on. Thereinto, photocatalytic degradation has great potential to degrade AAF quickly, easily, and effectively.

In the photocatalytic degradation of organic pollution, titanium dioxide (TiO2) has been researched extensively in dyes, endocrine disrupters, pesticides, etc., as a photocatalyst [8,9,10]. It deserves more attention even as a commercialized attribute to safety, non-toxicity, stability, high-performance, and low cost [11,12,13]. However, TiO2 was limited in practical applications due to its wide bandgap (3.2 eV) and the rapid recombination of electron (e−) and hole (h+). The construction of TiO2/semiconductor heterojunction is an effective method in many ways. Graphene carbon nitride (g-C3N4) has a narrow bandgap (2.7 eV) with an electronic structure, excellent visible light responding and is easy to prepare in the meantime [14,15]. Therefore, g-C3N4 was identified as an appropriate and complementary semiconductor to composite TiO2/semiconductor heterojunction [16,17,18]. For example, Tan et al. prepared Ti3+ self-doped TiO2/g-C3N4 heterojunctions via solid-state chemical reduction, and the material showed better degradation of phenol and production of hydrogen [19]. Wei et al. synthesized mesoporous brookite/anatase TiO2/g-C3N4 heterojunction hollow microspheres that had a more negative conduction band and a wider light absorption range [20]. Mesoporous TiO2/g-C3N4 composite was fabricated by Zhang et al. had about 50 times the removal rate of enrofloxacin than pure g-C3N4, which attribute to the O–Ti–N bond interface heterojunction between TiO2 and g-C3N4 separated and transferred charge carriers efficiently [21]. To enhance TiO2 light response, a uniform mesopore TiO2/g-C3N4 composite with core/shell heterojunction structure was prepared by Xia et al., and the uniform mesoporous g-C3N4 nanosheets were used to decorate TiO2 spheres [22].

The TiO2/g-C3N4 composite was studied deeply by lots of specialists and multitudinous synthesis methods and raw materials were exploited. In the synthesis process of TiO2/g-C3N4, there are many ways to prepare such as adding g-C3N4 powder in a precursor solution of TiO2, depositing TiCl4 precursor onto the surface of the g-C3N4 powder, mixing TiO2 powder with melamine in solution, and so on [23,24,25,26].

To our knowledge, even though so many methods were adopted to prepare TiO2/g-C3N4 composite, TiO2 xerogel has not been directly mixed with urea as raw materials to synthesize the composite. TiO2 xerogel was used rather than commercial TiO2 (P25), which could compose better with g-C3N4 during TiO2 catalyst forming. In this work, TiO2/g-C3N4 composite was synthesized by simple pyrolysis with urea and TiO2 xerogel, and AAF was selected as the potential pollutant to detect the photocatalytic activities of the prepared composite catalyst. Synthesis conditions including the mass ratio of TiO2 to C3N4, calcination temperature, calcination time, and heating rate were studied detailed. X-ray diffraction, scanning electron microscope, transmission electron microscope, ultraviolet-visible diffuse reflectance spectrum, X-ray photoelectron spectroscopy N2-adsorption isotherms and electrochemical impedance spectroscopy were all characterized to analyze the heterojunction structure and physicochemical properties of TiO2/g-C3N4. Furthermore, the stability and mechanism of TiO2/g-C3N4 were also deduced.

2. Materials and Methods

2.1. Materials

Aniline Aerofloat (AAF, (C6H5NH)2PSSH, 99.5%): Hunan Mingzhu Flotation Reagents Limited Company, Hunan, China. Ethylene glycol (CH3CH2OH, 99.7%): Tianjin Guangfu Science and Technology Development Co., Ltd., Tianjin, China. Glacial acetic acid (CH3COOH, 99.5%): Xilong Scientific Co., Ltd., Guangdong, China. Ammonium oxalate ((NH4)C2O4, 99.8%): Damao Chemical Reagent Factory, Tianjin, China. Methanol (CH3OH, 99.9%), tetrabutyl titanate (Ti(OC4H9)4, 98.0%), isopropanol (C3H8O, 99.7%) and urea ((NH2)2CO, 99.0%): Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The reagents above are all used without further purification.

2.2. Synthesis of Photocatalysts

2.2.1. Synthesis of TiO2 Xerogel

The TiO2 xerogel was prepared by the sol-gel method [27,28]. In preparing solutions A and B, respectively, solution A was the mixture consisting of 10 mL ethyl alcohol and 2 mL glacial acetic acid; solution B could be obtained when the 10 mL tetrabutyl titanate was dripped into 15 mL ethyl alcohol slowly at a speed of 0.1 mL·s−1 with magnetic stirring. The next step is adding solution A to solution B at a speed of 0.1 mL·s−1, the transparent colloid was obtained after 30 min. The transparent colloid was naturally aged for 48 h and dried in the oven at 90 °C for 24 h. Finally, xerogel powder was collected after ground.

2.2.2. Synthesis of TiO2/g-C3N4

According to previous reports [29,30,31,32], TiO2/g-C3N4 composite photocatalysts were prepared with a simple pyrolysis process. Urea (20 g) and TiO2 xerogel powder (2, 1, 0.67, 0.5, 0.4 g) were mixed and vibrated for 30 min with ultrasonic (KQ5200E, 40 KHz, Kunshan Ultrasonic Instruments Co., Ltd., Kunshan, China). The solid mixture was placed into the ceramic crucible with a cover and calcined at different temperatures for several hours at a heating rate with the muffle furnace. The details are illustrated in Scheme 1. The resultant powder which was ground and collected after cooling to an ambient temperature was TiO2/g-C3N4 composite denoted as TCN. TiO2/g-C3N4 composite with a doping mass ratio of x and a heating rate of 5 °C·min−1, 550 °C for 6 h is denoted as TCN-Ux. TiO2/g-C3N4 composite with a doping mass ratio of 20 and a heating rate of 10 °C·min−1, y °C for 6 h is denoted as TCN-Ty. TiO2/g-C3N4 composite with a doping mass ratio of 20 and a heating rate of z °C·min−1, 550 °C for 6 h is denoted as TCN-Rz. TiO2/g-C3N4 composite with a doping mass ratio of 20 and a heating rate of 10 °C·min−1, 550 °C for n h is denoted as TCN-Hn (x = murea·mxerogel−1 = 10, 20, 30, 40, 50; y = 450, 500, 550, 600, 650; z = 5, 10, 15, 20, 25; n = 2, 3, 4, 5, 6, 7).

Scheme 1

The synthesis of simple TiO2/g-C3N4 by pyrolysis process.

2.3. Characterization

X’TRA rotating anode powder X-ray diffraction (XRD, PANalytical B.V., X’Pert powder) with Cu Kα radiation (λ = 0.154056 nm) in the diffraction angle range of 2θ = 10~90° with 0.02° interval. The sample’s phase and composition were identified by it. The morphology of TCN was exhibited by scanning electron microscope (FE SEM, Tescan, Brno, Czech Republic, MIRA3 LMH). Crystal and element imaging of samples were shown via transmission electron microscope (TEM, FEI, Tecnai Hillsboro, USA, G2-20) connected with a CCD camera from Gatan Company (Pleasanton, CA, USA) and X-ray energy dispersive spectrometer from EDAX Company. Ultraviolet-visible diffuse reflectance spectrum (UV-Vis DRS, Shimadzu, Kyoto, Japan, UV-2600PC) was employed in the range of 200~800 nm with 1 nm interval to obtain the optical absorption spectra of materials, and BaSO4 was used as the reference. The Brunauer–Emmett–Teller (BET) surface area and Barrett–Joyner–Halenda (BJH) pore width were analyzed by Micromeritics ASAP 2460 nitrogen adsorption apparatus. X-ray photoelectron spectroscopy (XPS, Thermo, Waltham, MA, USA, ESCALAB 250Xi) spectra were operated with a monochromatic Al Kα radiation source (1486.6 eV) and the binding energies were adjusted with C1s (284.8 eV). Electrochemical impedance spectroscopy (EIS) was analyzed by three-electrode cell electrochemical workstation (CHI-660E, Chinstruments, Shanghai, China) with Pt and saturated calomel electrode as the counter and reference electrode, and 0.1 mol·L−1 Na2SO4 aqueous solution as electrolyte. The sample (10 mg) was dispersed in 1 mL ethylalcohol with ultrasonic for 30 min, and then the mixture was dispersed uniformly on the indium tin oxide (ITO) glass as a working electrode within a 1.0 cm2 area.

2.4. Photocatalytic Degradation and Analysis

The photocatalytic activity of TCN was explored by degradation of AAF under ultraviolet light provided by a 300 W mercury lamp. All photocatalysis experiments were implemented with the rotary photochemical reaction instrument (XPA-7, Xujiang Electromechanical Plant, Nanjing, China). The catalyst of TCN (30 mg) was dispersed in the AAF solution (50 mL, 100 mg·L−1) contained in the quartz tube. The first 30 min of the reaction were in the dark to reach absorption equilibrium, and then they were illuminated for 180 min. After every given time interval, 5 mL of solution was obtained and filtered through a 0.45 μm water filtration membrane to be analyzed by high-performance liquid chromatography (HPLC, 1260 infinity, Agilent, Santa Clara, CA, USA).

The concentration of AAF was tested by HPLC, equipped with eclipse plus C18 (3.5 μm, 4.6 mm × 150 mm) and the temperature of the chromatograph column was 30 °C, The measured wavelength was set as 230 nm according to the full wavelength scanning by ultraviolet-visible spectrophotometer (T6 New century, Beijing Persee General Co., Ltd., Beijing, China). The mobile phase consists of water and methanol (v/v = 60:40) and flows at 1 mL·min−1. The degradation efficiency was calculated by Ct/C0, in which C0 and Ct were the concentration of AAF at the beginning and a certain time.

At the same time, the catalyst was reused 4 times under the same conditions to investigate the stability and the prospects for practicality in industrial. Reactive active species hole (h+) and hydroxyl radical (·OH) in the photocatalytic degradation process were analyzed by quenching agent ammonium oxalate and isopropanol experiments.

3. Results and Discussion

3.1. Morphology Analysis

SEM images of pure TiO2, g-C3N4, TCN-U20 and TCN-Ty shown in Figure 1a–c and Figure S1 provided that TiO2 and g-C3N4 were composed successfully. As we can see, the TCN composite combined with the structure of the lamellae g-C3N4 stack on bulk TiO2. The thickness and density of g-C3N4 lamellae increase, respectively, which made the poriness and active positions more with the doping ratio and calcination temperature higher. Nevertheless, overmuch g-C3N4 lamellae would impede UV-light absorption of TiO2 which was covered and thus weaken the photocatalytic activity of CNT. Meanwhile, the TEM image of TCN-U20 shown in Figure 1d suggested the lattice spaces of 0.350 nm and 0.320 nm correspond to the plane (101) of TiO2 and the plane (002) of the g-C3N4 [33,34]. The results of mapping images and EDS spectrum in Figure 1e showed the percent of C, N, O and Ti. The atom percent of C and N is approximately 0.73~0.89 conforming to 0.75 that the ratio of C and N in g-C3N4. Because of the oxygen in the air, the atom percent of Ti and O is approximately 0.30~0.31, which is far less than 0.5 that the ratio of Ti and O in TiO2. The results suggested doping of g-C3N4 is successful and the formation of the heterojunction between TiO2 and g-C3N4.

Figure 1

SEM images of (a) pure TiO2, (b) g-C3N4, and (c) TCN-U20; (d) TEM image, (e) mapping images of TCN-U20.

3.2. XRD Analysis

The crystal phases of TCN, pure g-C3N4 and TiO2 are exhibited in Figure 2a and Figure S2. Diffraction peaks at 25.3°, 37.8° and 48.0° were pointed to (101), (004) and (200) of anatase TiO2 (JCPDS 21-1272). Peaks at 27.4°, 36.1°, 41.2° and 54.3° are pointed to (110), (101), (111) and (211) of rutile TiO2 (JCPDS 21-1276), and peaks at 12.9° and 27.7° correspond to (100) in the in-plane repetitive unit of tri-s-triazine and (002) attributed to interlayer stacking of g-C3N4 (JCPDS 87-1526) [35,36,37]. Almost all TCN samples were mixed with the rutile phase and anatase phase of TiO2. Observing all XRD patterns of TCN, it is indistinguishable if the peak at 27.7° belongs to the phase of g-C3N4 due to low loading and overlapped peaks [38]. The calculated crystalline phase ratio and cell parameters of TCN are shown in Table 1 and Table S1. The growth of the TiO2 rutile phase was suppressed with the doping of g-C3N4, and the ratio was lowest when the doping ratio was 20. Similarly, the other results can be seen in Table S1. The growth of the TiO2 rutile phase was also suppressed with the heating rate, and the ratio was lowest when the heating rate was 10, and the growths of the TiO2 rutile phase were both always promoted with the temperature and heat preservation hour. Suggesting the changes of diffraction peaks were caused by the transformation between TiO2 anatase and rutile and the growth of crystal particles. Excellent mixed ratios of TiO2 anatase and rutile when the weight percent of the anatase phase was 76.8 or 73.8 and the rutile phase was 23.2 or 26.2 demonstrated excellent photocatalytic activity [39,40]. The change in lattice size was caused by the formation of a heterojunction between g-C3N4 and TiO2 [33]. Obviously, the crystal phase ratio of TiO2 anatase and rutile changed with TCN preparation conditions. To clear the relationship between TCN degradation efficiency of AAF and the ratio of TiO2 anatase, the degradation efficiency plots were fitted with a curve as shown in Figure 2b. The fittted curve was obtained by the power function with six and the correlation coefficient R-square is 0.618. So, the change of crystal phase ratio not only decided cell size but also affected the photocatalytic activity significantly [41].

Figure 2

(a) XRD patterns of pure TiO2, g-C3N4 and TCN-Ux and (b) fitted curve between photocatalytic degradation efficiency of AAF and ratio of TiO2 anatase.

Table 1

Crystalline phase ratio and cell parameters of pure TiO2 and TCN-Ux.

Sample	Anatase/wt%	Rutile/wt%	a = b/nm	c/nm
Pure TiO2	5.6	94.4	1.42442	2.08097
10:1	30.3	69.7	1.30545	1.99599
20:1	76.8	23.2	0.80742	1.74158
30:1	23.7	76.3	1.05978	2.33788
40:1	26.8	73.2	1.19316	2.29493
50:1	48.9	51.1	1.10269	2.11718

3.3. UV-Vis DRS Analysis

The ultraviolet light absorption intensity of all TCN samples was much stronger than pure g-C3N4 and TiO2 as shown in Figure 3a and Figure S3a–c suggested the doped of g-C3N4 enhanced the UV-light absorption intensity significantly. Furthermore, the bandgap was calculated by the Kubelka–Munk function Equation (1) as follows: where α, h, ν, A, and Eg was absorption coefficient, Planck constant, light frequency, a constant, and optical bandgap energy, respectively. The bandgap calculation spectra shown in Figure 3b and Figure S3d–f and the results indicated that different preparation conditions influenced the bandgaps to different degrees. Combined with the XRD results, it can be inferred that the change in UV-light absorption intensity and bandgap were related to the formation of the TiO2/g-C3N4 heterojunction and TiO2 phase transformation between anatase and rutile [42,43,44].

Figure 3

UV-Vis DRS spectra (a) and bandgap calculation spectra (b) of pure TiO2, g-C3N4 and TCN-Ux.

3.4. SBET and Porosity Analysis

N2 adsorption-desorption isotherms and pore size distribution of TCN-Ux and pure TiO2 showed in Figure 4. The BET fitted curve and specific surface area data of TCN-Ux are exhibited in Figure S4, Table 2 and Table S2. Compared with pure TiO2, SBET of TCN increased 3–7 times after g-C3N4 doping. All samples showed the typical IV isotherm with H1 hysteresis loops reflecting the existence of homogeneous mesoporous. The hysteresis loops were caused by capillary condensation in the relative pressure (P/P0) range of about 0.5–1.0. The results suggested that TCN has a bigger specific surface area and smaller pore size than pure TiO2. TCN performed high photocatalytic activity demonstrated by a bigger specific surface area, more abundant mesoporous structure, and more adsorption sites [39]. The increase of SBET was attributed to covered g-C3N4 increased with doping ratio, and growth of TiO2 anatase with heating rate sped within range of 10 °C/min. The decrease of SBET was due to the growth of TiO2 rutile with a heating rate speed from 10 to 25 °C/min, calcination temperature raised from 450 to 650 °C, and heating preservation hour increasing from 2 to 7 h. The results were consistent with the XRD and SEM characterization of the samples.

Figure 4

N2 adsorption-desorption isotherms (a) and pore size distribution (b) of pure TiO2, g-C3N4 and TCN-Ux.

Table 2

Specific surface area of pure TiO2 and UxT550R5H6-TCN.

Sample	g-C3N4	Pure TiO2	10:1	20:1	30:1	40:1	50:1
Surface area(m2·g−1)	121.0	2.5	7.3	9.7	11.7	13.1	18.1

3.5. Photodegradation Experiment

The photocatalytic degradation of AAF by TCN (Figure 5) was always higher than by commercial TiO2 (P25) and pure TiO2 prepared, which might be attributed to the formation of the TiO2/g-C3N4 heterojunction [29,44,45,46,47]. The TCN samples with a 20 doping ratio, 10 °C·min−1, 550 °C, 6 h in the same three other conditions showed the optimum photocatalytic degradation efficiency.

Figure 5

Activity of (a) TCN-Ux, (b) TCN-Rz, (c) TCN-Ty, (d) TCN-Hn on the photocatalytic degradation of AAF.

The performance of photocatalytic degradation could be attributed to these reasons as results of characterization showed: the bigger specific surface area and more active positions with the doping ratio of g-C3N4 compared with pure TiO2, transforming from urea to g-C3N4 smoother with the heating rate, completeness, and crystallinity of TiO2/g-C3N4 composite better with the calcination temperature and heating preservation hour. However, the excessive g-C3N4 would obstruct the UV-light absorption to TiO2 with the further increase of doping ratio. The transformation process was too fast to attach uniformly to the TiO2 surface resulting in stacking, and TCN got heavily agglomerated with the further rise in calcination temperature. Moreover, a suitable ratio of TiO2 rutile phases and anatase phases is a vital contribution to promoting photocatalytic degradation efficiency [40,41,48]. On the whole, the calcination temperature has the greatest influence on the photocatalytic activity of aniline aerofloat degradation, followed by the doping ratio, heat preservation hour and heating rate have the least influence in a certain range.

The stability of the photocatalyst is an important index to estimate the photocatalytic activity so it is significant to study the recycling of the TiO2/g-C3N4 composite. At room temperature, TCN-T550 (30 mg) was added to a 50 mL AAF solution (100 mg·L−1). The catalyst used was collected and reused in the next degradation experiment. The results of reusing catalyst photocatalytic activity were showed in Figure 6. After four times of recycling, the TiO2/g-C3N4 composite photocatalytic degradation efficiency of AAF was always steadily high, indicating that the TiO2/g-C3N4 composite is a kind of potential photocatalyst for practical engineering application.

Figure 6

Activity of recycling test of TCN-T550 for photocatalytic degradation of AAF.

3.6. XPS Analysis

The surface chemical compositions of TCN-T550 were tested by XPS and the results shown in Figure 7 show survey, C1s, N1s, O1s and Ti2p scans. The peaks at 284.6 eV, 285.3 eV, and 288.6 eV shown in Figure 7b correspond to the sp2 C–C, C=, N–C=N bonds [49]. Meanwhile, the binding energies of 399.3 eV and 400.6 eV shown in Figure 7c correspond to the sp3 N-(C)3 and interstitial N, which indicated the existence of g-C3N4 and composite structure [8,50]. Binding energies in 529.8 eV and 530.6 eV shown in Figure 7d corresponds to Ti–O band and surface absorbed –OH [44], and binding energies in 458.6 eV and 464.3 eV shown in Figure 7e were attributed to Ti 2p3/2 and Ti 2p1/2 [51], which directly testified TiO2. Furthermore, there no peaks of C–O, N–O, Ti–C, or Ti–N observed indicating that TiO2 and g-C3N4 were composed with molecule doping rather than lattice replace.

Figure 7

XPS of survey (a), C1s (b), N1s (c), O1s (d) and Ti2p (e) scans of TCN-T550.

3.7. Electrochemical Analysis

The electrochemical impedance spectroscopy (EIS) of pure TiO2 and TCN-T550 shown in Figure 8 indicated that e−-h+ pair separation-effective of TCN-T550 is more efficient than TiO2 [49].

Figure 8

The EIS of pure TiO2 and TCN-T550.

3.8. Mechanism of Composite

3.8.1. Reactive Active Species of Photocatalyst

Research of photocatalysts’ reactive active species in the photocatalytic degradation process is the fundamental research of photocatalytic mechanism. As we all know, ammonium oxalate is the quenching agent of hole (h+), and isopropanol is the quenching agent of hydroxyl radical (·OH). The results are shown in Figure 9.

Figure 9

Effect of TCN-T550 for photocatalytic of AAF degradation with different concentration quenching agents: (a) ammonium oxalate, (b) isopropanol.

The photocatalytic degradation of AAF efficiency declined gradually with the increasing concentration of ammonium oxalate and isopropanol shown in Figure 9. The result illustrates the oxidation action of h+ and ·OH in the photocatalytic degradation process. Meanwhile, the effect of ammonium oxalate and isopropanol on the photocatalytic degradation efficiency is similarly judged from the degradation efficiency decrement. In conclusion, the h+ and ·OH, which were both photocatalysts reactive active species in the photocatalytic degradation process, react synergistically in the photocatalytic reaction system.

3.8.2. Mechanism Conjecture of Photodegradation

The bandgap energy of TiO2 and g-C3N4 were 3.04 eV and 2.98 eV, which is based on UV-Vis DRS results. The valence band (VB) and conduction band (CB) edge potentials were calculated by the following Equations (2) and (3) [52]: where EVB, ECB, χ, Ee and Eg were the VB edge potential, the CB edge potential, the geometric mean of Mulliken electronegativity [53], and the energy of electrons on the hydrogen scale (~4.5 eV, vs. NHE) and optical bandgap energy. The value of χ of TiO2 and g-C3N4 were 5.81 eV [54] and 4.73 eV [55]. The EVB of TiO2 and g-C3N4 were 2.83 eV and 1.72 eV, and the ECB of TiO2 and g-C3N4 were calculated as –0.21 eV and −1.26 eV, respectively. The VB potential of TiO2 (2.83 eV) is more positive than H2O/·OH (+1.99 eV vs. NHE), and the CB potential of g-C3N4 is more negative than O2/·O2− (−0.33 eV vs. NHE) [56], so there was an oxidation reaction in the VB of TiO2 and reduction reaction in the CB of g-C3N4. This result and the results of photocatalysts reactive active species confirmed each other. TiO2 and g-C3N4 might form type-II or direct Z-scheme heterojunction. If it belongs to type-II heterojunction, e− on the CB of g-C3N4 would transfer to the CB of TiO2 and h+ on the VB of TiO2 would transfer to the VB of g-C3N4. However, the VB potential of g-C3N4 (1.72 eV) is more negative than H2O/·OH (+1.99 eV vs. NHE) and cannot produce ·OH which was the active species. The conclusion is contradictory to the result of the quenching experiment. So, the possible photodegradation mechanism process was conjectured as a direct Z-scheme heterojunction shown in Figure 10 [57] and following Equations (4)–(8):TiO TiO TiO g-C

Figure 10

The possible photodegradation mechanism process of TiO2/g-C3N4.

Under light irradiation excitation, e− in transfers from CB to VB of TiO2 and g-C3N4 severally, and there is h+ left in CB. Meanwhile, e− of TiO2 combines with h+ of g-C3N4 on the VB of g-C3N4. Next h+ of TiO2 oxidizes the H2O molecule into ·OH and H· and e− of g-C3N4 reduces the O2 molecule into ·O2−. Then, ·OH, h+ and ·O2−, which all have oxidizability, can oxidize the Aniline Aerofloat molecule into degraded products.

4. Conclusions

TiO2/g-C3N4 composites with different doping ratios, heating rates, calcination temperatures and heating preservation hours were synthesized successfully by a simple pyrolysis method with the raw material of TiO2 xerogel and urea. TCN samples with a 20 doping ratio, 10 °C·min−1, 550 °C, 6 h in the same three other conditions showed the optimum photocatalytic degradation efficiency. The excellent activity was attributed to the appropriate crystal phase ratio of TiO2 rutile phases and anatase phases, high UV-light absorption intensity, big specific surface area, and formation of the TiO2/g-C3N4 heterojunction. In addition, the TiO2/g-C3N4 heterojunction could postpone the recombination of h+ and e− to extend the reaction time and enhance the photodegradation efficiency. Furthermore, TiO2/g-C3N4 composites have great photodegradation stability for AAF after four times.