TiO₂ Nanowires with Doped g-C₃N₄ Nanoparticles for Enhanced H₂ Production and Photodegradation of Pollutants

Abstract

With the rapid consumption of fossil fuels, along with the ever-increasing environmental pollution, it is becoming a top priority to explore efficient photocatalysts for the production of renewable hydrogen and degradation of pollutants. Here, we fabricated a composite of g-C₃N₄/TiO₂ via an in situ growth method under the conditions of high-temperature calcination. In this method, TiO₂ nanowires with a large specific surface area could provide enough space for loading more g-C₃N₄ nanoparticles to obtain C₃N₄/TiO₂ composites. Of note, the g-C₃N₄/TiO₂ composite could effectively photocatalyze both the degradation of several pollutants and production of hydrogen, both of which are essential for environmental governance. Combining multiple characterizations and experiments, we found that the heterojunction constructed by the TiO₂ and g-C₃N₄ could increase the photocatalytic ability of materials by prompting the separation of photogenerated carriers. Furthermore, the photocatalytic mechanism of the g-C₃N₄/TiO₂ composite was also clarified in detail.

1. Introduction

The vigorous development of various industries drives rapid economic progress but causes excessive energy consumption and environmental pollution [1]. Such problems should not be underestimated; thus, finding efficient technology and sustainable energy has become the top priority. In recent years, hydrogen energy (H₂) has been greatly popular with scientists because of its peculiarities of being renewable and clean. Considerable efforts have been devoted to the development of advanced technologies for harvesting hydrogen, and water splitting catalyzed by semiconductors is widely recognized as a promising approach to producing H₂ [2,3].

TiO₂ is a robust photocatalyst for water splitting for generating hydrogen, photodegradation, dye-sensitized solar cell biosensors, and other fields due to its high photostability, outstanding chemical stability, nontoxicity, and high efficiency [4,5]. However, the applications of TiO₂ are still greatly limited by its intrinsic shortcomings. For example, the wide band gap (3.2 eV) and single crystalline phase can reduce the migration efficiency and lead to a high recombination rate for charges or limit the utilization of solar energy [6,7,8]. Therefore, numerous scientists have devoted themselves to solving these difficulties from different perspectives, such as the auto-doping of TiO₂ [9], noble metal deposition, heterojunction construction, and ion doping [10,11,12,13,14,15,16,17]. Among these methods, the construction of heterojunctions is believed to be one favorable way [18].

Over the past decade, g-C₃N₄ has drawn worldwide attention due to its advantages of high stability, being green, being cheap, and being easy to synthesize, as well as unique electronic structure [19,20]. Furthermore, the morphology of materials also impacts the performance. The surface area of nanowires is dozens of times bigger than that of particles and has a strong absorption capacity. Meanwhile, nanowires can be filled with other nanomaterials to generate nanocomposites for greatly enhancing photocatalytic ability [21].

Herein, we propose a strategy of combining TiO₂ nanowires and g-C₃N₄ nanoparticles for generating a nanocomposite, aiming at improving photocatalytic performance. Considering that the synthetic process should be environmentally friendly and low cost as well as efficient, we used commercial P25 powder and sodium hydroxide (NaOH) as raw materials to prepare TiO₂ nanowires. Under the reaction conditions of high temperature, high pressure, and strong alkalinity, P25 reacted with NaOH to produce titanium, which convolved to form a tubular structure after the Na⁺ was exchanged in the washing process. Simultaneously, hydroxide was dissolved in the water, and oxide precipitated due to the different solubility. At last, g-C₃N₄ nanoparticles were loaded on the TiO₂ nanowires to construct a novel g-C₃N₄/TiO₂ heterojunction via an in situ growth method at a high calcination temperature. Furthermore, the photocatalytic ability of all the materials was examined by the photodegradation of pollutants and photocatalytic hydrogen production.

2. Experimental Section

2.1. Experimental Reagents

Titanium dioxide (TiO₂, P25, nanoscale) and sodium hydroxide (NaOH, AR) were both obtained from Xilong Chemical Co., Ltd (Shantou China). Melamine (C₃N₃(NH₂)₃) was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China).

2.2. Synthesis of TiO₂ Nanowires

The TiO₂ nanowires were prepared according to a previous report with some modifications [22]. Firstly, P25 (0.5 g) was dispersed into a 75 mL NaOH solution (10 mg/L) using an ultrasonic instrument for 30 min, and then was stirred for another 30 min. After that, the well-mixed reactant was poured into a Teflon reactor (100 mL) and reacted at 130 °C for 24 h. After cooling down to room temperature, the precursor was obtained through centrifugation and washed until the pH was 9. Afterwards, the above product was washed and stirred in 100 mL of HNO₃ (0.1 mol/L) for 5 h. After pickling, the solution was purified with deionized water until the pH was 7 and centrifuged to obtain precipitates. The precipitate was dried in a vacuum at 60 °C for 24 h and then calcined at 600 °C for 3 h with a heating rate of 5 °C/min to obtain TiO₂ nanowires.

2.3. The Preparation of g-C₃N₄/TiO₂ Samples

g-C₃N₄ was prepared by calcining the melamine at 500 °C. The as-prepared TiO₂ nanowire powder and 1 g of melamine were evenly ground, and then calcined at 540 °C for 4 h with a heating rate of 3 °C/min. The different mass ratios of g-C₃N₄/TiO₂ composite (such as 20%, 30%, 40%, and 50%) were synthesized by changing the quality of g-C₃N₄ without the other conditions being changed.

2.4. Characterization

The phase and textual properties of the samples were tested by X-ray powder diffraction (XRD). The micromorphology and lattice structure were further analyzed with a scanning electron microscope (SEM) and transmission electron microscope (TEM). The chemical compositions and states were determined by X-ray photoelectron spectroscopy (XPS). Besides, the surface area and porosity were determined on a specific surface area tester. The UV-vis diffuse reflection spectra (UV-vis DRSs) of the samples were acquired with an ultraviolet and visible spectrophotometer with an integrating sphere; BaSO₄ was used as the background material. The photocurrent tests were carried out with an electrochemical workstation (CHI 660D) with a standard three-electrode system.

2.5. The Photocatalytic Ability Tests

2.5.1. The Experiments for Photodegradation

The photodegradation tests of methyl orange and rose red were carried out under a 300 W xenon light with a light filter (λ > 420 nm). Firstly, 50 mg of as-prepared g-C₃N₄/TiO₂ complex was ultrasonically dispersed in a beaker containing the pollutant (50 mL, 10 ppm) and then magnetically stirred in the dark for 60 min to achieve the absorption–desorption equilibrium, labeled C₀. Afterwards, the treated solution was irradiated under the xenon light for 120 min to perform the photodegrade reaction. Then, 3 mL of the pollutant suspension was extracted at the given interval times and then centrifuged and labeled C_t. At last, the change in the concentration of the contaminant was measured with an ultraviolet–visible spectrophotometer.

2.5.2. Photocatalytic H₂ Generation

The photocatalytic hydrogen evolution reaction was performed in a quartz glass at 279.15 K. Meanwhile, the quartz glass was connected with a closed circulation system, and a 300W xenon light served as the light source. Typically, g-C₃N₄/TiO₂ complex (50 mg) was dispersed in the solution, which consisted of methyl alcohol (8 mL) and deionized water (72 mL); then, Ar was pumped into the system to remove the air. Afterwards, the above reactor was illuminated under a 300 W xenon light for 3 h. At intervals of 0.5 h, the generated hydrogen gas was quantified with a gas chromatograph (GC7900).

2.5.3. The Tests for Photocurrent

The photocurrent experiments for the g-C₃N₄/TiO₂ composite were monitored with an electrochemical station (CHI660C) with 0.5 mol/L NaSO₄ solutions as the electrolyte solution. For this, the Ag/AgCl electrode was used as the reference electrode, and the graphite electrode and prepared samples served as the auxiliary electrode and the working electrode, respectively. Typically, the working electrode could be fabricated by the following method: 2 mg samples were evenly dispersed in 1 mL of deionized water. Then, 100 μL of suspension liquid was coated uniformly on tin oxide conductive glass, dried at room temperature, and set aside.

3. Results and Discussion

3.1. Micromorphology and Lattice Structural Characteristics

The micromorphology of the samples was preliminarily studied through the SEM images (Figure 1). Obviously, the shape of TiO₂ was wirelike with a smooth surface and uniform thickness (Figure 1a,b). Meanwhile, it can be seen that the length of the TiO₂ nanowires was inhomogeneous. This was attributed to the fact that P25 reacted with NaOH, forming titanate, which was convolved to a linear structure after sodium ions were exchanged in the washing process. Besides, the reason for the different lengths may also have been the high-temperature calcination. As shown in Figure 1c, we can clearly observe that the g-C₃N₄ particles stuck tightly on the TiO₂ nanowires. The longer TiO₂ nanowires were able to provide sufficient supporting space for the in situ growth of g-C₃N₄, which made it possible to load g-C₃N₄ on the TiO₂.

In addition, we used a transmission electron microscope (TEM) to further view the morphology, particle size, and lattice structure of the samples. Similarly, the morphology (Figure 2a,b) of the TiO₂ and g-C₃N₄ was nanowires and particles, respectively. These was consistent with the SEM images. The lattice fringes at a distance of 0.352 nm (Figure 2c,d) corresponded to the (101) plane of TiO₂ [22,23,24], while the lattice fringes at 0.326 nm (Figure 2d) could be assigned to the crystal plane of g-C₃N₄ [25,26]. Therefore, it could be identified that the g-C₃N₄/TiO₂ was successfully synthesized. In addition, the crossed and overlapped lattice fringes of the partial g-C₃N₄ and TiO₂ reflected the idea that the coupling of g-C₃N₄ and TiO₂ was not simple surface contact but the presence of chemical bonding. During the reaction process, Ti(OH)₄ particles of the TiO₂ nanowires were in contact with the triazine structure in the melamine, and then, the precursor was polymerized with high-temperature calcination. Therefore, TiO₂ nanowires closely contacted g-C₃N₄ via chemical bonds, thereby generating a heterojunction.

3.2. Crystal Phase and Textural Characteristics

The crystal form and crystallinity of the samples were analyzed by obtaining the X-ray diffraction (XRD) patterns. The XRD patterns of the materials displayed five strong diffraction peaks located at 25.2°, 37.7°, 47.7°, 53.9°, and 54.5° (Figure 3), which corresponded to the (101), (004), (200), (105), and (211) crystal planes of TiO₂ (ICPDS card No.21-1272), respectively [27]. This result indicates the as-prepared TiO₂ was anatase phase. For g-C₃N₄, the characteristic peaks at 13.1° and 27.4° were consistent with the standard card for g-C₃N₄ [28]. However, no distinct characteristic peak of g-C₃N₄ was observed on the XRD patterns of the composite, probably due to the low content of g-C₃N₄ [29].

In general, the pore characters and specific surface areas of materials have great influences on the photocatalytic performance of the materials. The N₂ absorption–desorption isotherms and pore diameter distribution are shown in Figure 4. As shown in Figure 4a,b, the lack of hysteresis loops and lack of obvious pore size distribution peaks in the TiO₂ curve indicate that there was no large pore structure or mesoporous structure [30]. With an increase in g-C₃N₄ loading content, we found that curves typical of 20% g-C₃N₄/TiO₂, 30% g-C₃N₄/TiO₂, 40% g-C₃N₄/TiO₂, and 50% g-C₃N₄/TiO₂ belonged to the type-IV absorption isotherm, with a H₃ hysteresis loop at higher pressures (0.8–1.0), which clarified the presence of mesoporous structure in the samples [31,32]. The formation of the pore was on account of the accumulation of g-C₃N₄ nanoparticles on the surface of the TiO₂ nanowires, and the g-C₃N₄ changed the surface roughness of the nanowires’ structure. As shown in

Table 1. The specific surface area of samples.

Samples	Specific Surface Area (m2/g)	Pore Volume (cm3/g)
TiO2	11.97	10.07
20% g-C3N4/TiO2	66.18	10.53
30% g-C3N4/TiO2	80.69	10.51
40% g-C3N4/TiO2	131.18	10.57
50% g-C3N4/TiO2	72.87	10.48

, with the loading of g-C₃N₄ increasing from 20% to 50%, the specific surface areas of the samples firstly increased and then decreased, but the pore volume almost remained unchanged. This is because the excessive loading of g-C₃N₄ led to the aggregation of the g-C₃N₄ nanoparticles and thus reduced the specific surface area of the TiO₂ nanowires [20].

3.3. Chemical State and Band Gap Analysis

Typically, XPS spectra are a powerful method for investigating the electronic structures of different elements in nanocomposites. The characteristic peaks of Ti, O, and C were detected in the survey of the g-C₃N₄/TiO₂ composite (Figure 5a), which suggested the successful synthesis of the nanocomposite containing g-C₃N₄ and TiO₂. The XPS spectra of C 1s (Figure 5b) displayed three peaks located at 284.6, 286.27, and 288.74 eV, corresponding to C-C, C-OH, and C=O (and COO) bonding, respectively [20,33]. In the case of O 1s (Figure 5c), the binding energies of 529.72, 530.72, and 532.05 eV were attributed to Ti-O, H-O, and C-O, respectively [34]. The XPS spectra of Ti 2p displayed two peaks for Ti⁴⁺ (Figure 5d), indicating that the Ti species were in the form of TiO₂ [27]. After peak splitting fitting, the broad N 1s peak was divided into three contributions (Figure 5e), namely, C=N-C (398.71 eV), N-C₃ (399.70 eV), and C-N-H (400.54 eV) [19].

The optical performance of the photocatalysts is shown in Figure 6a. Obviously, the absorption edge of the 40% g-C₃N₄/TiO₂ composite showed a red shift compared with pure TiO₂, which would be of benefit for enhancing the utilization of solar energy for improving the photocatalytic ability of materials. The band gap value (Figure 6b) of materials can be acquired by plotting (αhν)^1/2 against hv and then extending the tangent line to intersect the coordinate axis [29]. The band gap value of 40% g-C₃N₄/TiO₂ was lower than that of TiO₂ and other materials (Figure S3 in Supporting information), which was of benefit for the photocatalytic reaction.

3.4. Photocatalytic Ability Analysis

During the experimental process, rose red and methyl orange were regarded as the target pollutants for evaluating the photodegradation performance of materials under ultraviolet visible light. As shown in Figure 7a, g-C₃N₄ is almost inactive for the photocatalytic degradation of methyl orange. Although the TiO₂ is active for the degradation of methyl orange, the performance is very poor, with a 70% removal efficiency within 120 min of irradiation time. Impressively, the optimal proportional complex (40% g-C₃N₄/TiO₂) can completely photodegrade the methyl orange in 60 min. Moreover, the efficiency of methyl orange removal by 40% g-C₃N₄/TiO₂ in 20 min was as high as that by TiO₂ in 120 min. Thus, it can be deduced that the heterojunction in the composite has a positive effect on the photocatalytic performance. Furthermore, the loading content of g-C₃N₄ also played a key role in the photodegradation performance. Excessive g-C₃N₄ aggregation led to a decrease in the specific surface area and thus limited the reaction. Therefore, we could further deduce that the heterojunction in the complex could inhibit the recombination of carriers facilitating the photocatalytic ability of materials [32].

To further investigate the excellent photodegradation ability of 40% g-C₃N₄/TiO₂ complex, the photodegradation of rose red and methyl orange cycling experiments were also carried out. As shown in Figure 7b, it is obvious that 40% g-C₃N₄/TiO₂ also had a favorable ability to treat rose red. Furthermore, the efficiency of methyl orange degradation by 40% g-C₃N₄/TiO₂ was almost unchanged after five cycling experiments (Figure 7c), indicating that the 40% g-C₃N₄/TiO₂ possessed high stability and reusability. In a word, the above experiments demonstrate that the as-prepared 40%TiO₂/g-C₃N₄ composite has a bright future in applications for wastewater treatment.

The photodegradation principally made use of the photocatalytic oxidation performance of the photocatalysts, whereas the photocatalytic reduction could also play a part in plenty of fields. Therefore, the photocatalytic hydrogen generation experiments were conducted under ultraviolet–visible irradiation, and the hydrogen produced was quantified every 0.5 h. The H₂ production performance of pure g-C₃N₄ and TiO₂ was not satisfactory (Figure 8a,b). After coupling g-C₃N₄ with TiO₂, the quantity and rate of hydrogen generation greatly increased, which can be explained as the presence of a heterojunction in the composite. It effectively prompted the separation of photogenerated carriers, thereby improving the performance of hydrogen evolution. However, when the content of g-C₃N₄ was 50%, the generation rate for H₂ became slow because the excessive loading could block the active sites on the TiO₂.

3.5. Photocurrent Analysis

Usually, the photocurrent response can reflect the transfer and separation of photogenerated charges under irradiation [35]. As depicted in Figure 9, the photocurrent signal of TiO₂ was the weakest, indicating that the recombination of photogenerated charges was serious. However, the photocurrent response increased after g-C₃N₄ was loaded on the surface of the TiO₂. The reason for this phenomenon was that the heterojunction prompted the separation of photogenerated electrons and holes, thereby providing more electrons that participated in hydrogen evolution [20].

3.6. Mechanism of Photocatalysis

Figure 10 shows the mechanism of the photodegradation process (a) and the hydrogen production (b). Under irradiation, g-C₃N₄ and TiO₂ were excited to produce a mass of electrons and holes. According to a previous report [36], the conduction band value of g-C₃N₄ (−1.3eV vs. NHE) is more negative than that of pure TiO₂ (−0.5 eV vs. NHE), while the valence band value of TiO₂ (2.7eV vs. NHE) is more positive than that of g-C₃N₄ particles (1.4eV vs. NHE). Therefore, the partial electrons on the conduction band of g-C₃N₄ transfer to the conduction band of TiO₂, enabling O₂ to change into ·O₂₋. At the same time, the holes in the valence band of TiO₂ were transferred to g-C₃N₄. The OH^- and water were oxidated to hydroxyl radicals. At last, the methyl orange and rose red reacted with these radicals to form CO₂ and H₂O. Thereby, the electrons and holes were effectively separated. However, the electrons played a major role in the process of hydrogen production. The potential of H₂ evolution (0 eV vs. NHE) was more positive than the conduction band value of TiO₂ (−0.5 eV vs. NHE); thus, H⁺ could capture the electrons and change into H₂.

4. Conclusions

In this work, TiO₂ nanowires were firstly fabricated via the simple hydrothermal method, and then, g-C₃N₄ was prepared via thermal polymerization using melamine as the precursor. A novel g-C₃N₄/TiO₂ heterojunction composite was successfully prepared by adjusting the mass ratio of TiO₂ and melamine in the calcination process. After g-C₃N₄ was loaded on the surface of TiO₂, the absorption range for light, the photocurrent response, and the specific surface area increased significantly, which were benefited by the presence of the heterojunction. The heterojunction in the composite could greatly prompt the separation of the photogenerated electrons and holes, thus enhancing the photocatalytic ability of the g-C₃N₄/TiO₂. More importantly, the g-C₃N₄/TiO₂ composite has the ability to both photocatalyze the degradation of different pollutants and produce hydrogen, which means it has broad application prospects. Meanwhile, the as-prepared sample also kept favorable stability after the cycling experiments. Therefore, this study provides a friendly and effective method for treating complex environmental pollutants and energy consumption.