Ti4O7/g-C3N4 Visible Light Photocatalytic Performance on Hypophosphite Oxidation: Effect of Annealing Temperature

Introduction

Hypophosphite is commonly used as a reducing agent in metallurgy industries especially in the processes of plating and surface finishing thereby generating large amounts of hypophosphite wastewater (Bulasara et al., 2011; Li et al., 2015). The hypophosphite contaminant should be further treated before being discharged into the river or lake, because it may lead to algae growth and cause eutrophication (Wang et al., 2016; Ge et al., 2017). In addition, phosphorus is a non-renewable resource mainly used as a nutrient in agricultural production (Montangero and Belevi, 2007). Due to an increase in the global demand for phosphorus resource, it will be depleted in the next 50–100 years (Takeda et al., 2010; Ye Y. Y. et al., 2017; Ye Z. L. et al., 2017). Therefore, the phosphorus recovery from wastewater is of considerable interest. However, a high solubility constant of the hypophosphite precipitants limits the transformation of hypophosphite into the precipitated products. In contrast, phosphate is easier to be recovered than hypophosphite by precipitation. As such, a highly efficient approach for the pre-oxidation of hypophosphite to phosphate becomes of great importance for phosphorus recovery. Photocatalysis, a nano-enabled technology, has been recognized for its promising applications with the generation of activated radicals such as hydroxyl radicals and superoxide anion radicals, which herein can be applied for hypophosphite oxidation.

Semiconductor photocatalysts have been recognized as a potential strategy to solve some serious challenges of the twenty-first century, such as energy shortage, environmental pollution, and global warming (Lin et al., 2017). TiO₂ photocatalyst has attracted much attention due to its strong oxidizing power, low cost and high chemical stability. However, a large band gap (3.2 eV) of TiO₂ means that it can only absorb ultraviolet light (only about 3–5% of total sunlight), which greatly limits its performance in industrial applications (Teng et al., 2017; Noman et al., 2018). Therefore, it is urgent to develop novel semiconductor photocatalysts that respond to visible light.

Recently, graphitic carbon nitride (g-C₃N₄), constituted by numerous layers of two-dimensional (2D) counterparts, has attracted enormous attention given its advantages such as low cost and visible light driven semiconductor photocatalyst (Huang et al., 2017b; Liu et al., 2017a,b; Tian et al., 2017; Wang et al., 2017). The metal free g-C₃N₄ can work as photocatalyst under visible light irradiation with a suitable band gap of 2.7 eV. In addition, g-C₃N₄ possesses excellently structural stability, which is suitable for chemical modification. Nevertheless, the photocatalytic property of g-C₃N₄ is still limited for its low surface area, low photoabsorption efficiency and high recombination rate of photo-induced electron-hole pairs (Jourshabani et al., 2017; Shao et al., 2017).

Decreasing the recombination of photo-induced carriers during the photocatalytic process would enhance the photocatalytic activity of the as-prepared g-C₃N₄ photocatalyst (Li J. D. et al., 2017). Therefore, some methods, such as porosity engineering, doping with foreign elements, and compositing with other semiconductors to facilitate charge separation, were developed to enhance the photocatalytic performance of g-C₃N₄. For example, the horn-like hollow mesoporous ultrathin g-C₃N₄ tubes were fabricated with high surface area, drastically boosted bulk charge separation, carrier density and surface charge transfer efficiency and showed the remarkably photocatalytic performance for H₂ evolution (Liu et al., 2017a). Meanwhile, the 3D mesoporous g-C₃N₄ established by ultrathin self-doped nanosheets exhibited the superior photocatalytic performance on hydrogen evolution (Tian et al., 2017). Additionally, the porous and thin g-C₃N₄ nanosheets, prepared via a novel thiourea-assisted melamine-precursor hydrothermal pre-treatment followed by a traditional thermal polymerization method, profoundly enhanced visible-light photocatalytic performance on H₂ evolution and NO removal from the gaseous phase (Huang et al., 2017b). Moreover, the Cl intercalated mesoporous g-C₃N₄ showed outstanding photocatalytic performance for water splitting into H₂, CO₂ reduction, liquid and air contaminants removal (Liu et al., 2017b).

Substoichiometric titanium oxides, known as Magnéli phases (Sun et al., 2016), comprise a series of compounds with the generic formula Ti_nO_2n−1 (4 ≤ n ≤ 10) (Kolbrecka and Przyluski, 1994; Guo et al., 2016). Among these compounds, Ti₄O₇ possesses high electrical conductivity, thermal stability, and corrosion resistance in harsh conditions (Oturan et al., 2017). Therefore, Ti₄O₇ is wildly used as catalyst coated material (Li et al., 2010), wastewater treatment material (You et al., 2016), support material in fuel cell (Chisaka et al., 2016), and additive to positive materials in batteries (Cao et al., 2017). However, it was also reported that pure Ti₄O₇ as the photocatalyst had limited photocatalytic activity with the band gap of 2.9 eV (Maragatha et al., 2017). Herein, coupling Ti₄O₇ and intrinsic g-C₃N₄ to construct the well-matched Ti₄O₇/g-C₃N₄ heterojunction would be an alternative novel pathway to address the intrinsic drawbacks of g-C₃N₄ for photocatalytic applications.

In this study, novel Ti₄O₇/g-C₃N₄ photocatalysts were synthesized at two different temperatures (100 and 160°C) and their performance in photocatalytic oxidation of hypophosphite under visible light irradiation and the corresponding mechanism were compared and investigated. Ti₄O₇/g-C₃N₄ photocatalysts exhibited an enhanced photocatalytic activity for hypophosphite oxidation under visible light irradiation and the annealing temperature significantly affected the photocatalytic performance. The separation mechanism of photogenerated electrons and holes of the photocatalysts was investigated by UV-Vis diffuse reflectance spectra, photoluminescence emission spectra, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The activated species generated in the photocatalytic process were measured by electron spin resonance (ESR). The enhanced photocatalytic performance could be ascribed to the efficient charge separation and transfer across the heterojunction interface and the enhanced photoabsorption efficiency. Our work demonstrated that the rational design and construction of isotype heterojunction was an effective strategy for the development of efficient photocatalysts under visible light irradiation.

Materials and Methods

Chemicals

Melamine (C₃H₆N₆), urea [CO(NH₂)₂], sub-stoichiometric titanium oxide (Ti₄O₇), sodium hypophosphite (NaH₂PO₂), sodium sulfate (Na₂SO₄), isopropanol [IPA, (CH₃)₂CHOH], ethylenediaminetetraacetic acid disodium salt (C₁₀H₁₄N₂Na₂O₈), sodium hydroxide (NaOH), and sulfuric acid (H₂SO₄) were provided by Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). 5,5-dimethylpyrroline-N-oxide (DMPO) was bought from Dojindo Co., Ltd. (Shanghai, China). The entire chemical reagents were analytical grade and all solutions were prepared using Milli-Q water (Millipore, 18.2 MΩ cm).

Synthesis of g-C₃N₄ Materials

The g-C₃N₄ materials were prepared using a liquid-based growth method (Sun et al., 2018). In a typical process, the mixture of melamine and urea (molar ratio = 1:1) was dissolved with 50 mL deionized water and then vigorously stirred for 1 h at room temperature. After that, the mixture suspension was centrifuged at 7,500 r/min for 15 min, and then dried at 60°C for 24 h under the vacuum to obtain the white powder. After that, the prepared white powders were further grinded into smaller powders in a mortar and placed in a muffle furnace. The powders were then annealed at 550°C in a muffle furnace for 4 h in static air at a ramp rate of 2.5°C min⁻¹. The resulting yellow products were collected for further usage.

Preparation of Ti₄O₇/g-C₃N₄ Photocatalysts

The preparation procedure of Ti₄O₇/g-C₃N₄ photocatalysts was shown below: g-C₃N₄ powder (2.0 g) and Ti₄O₇ (1.0 g) were dispersed into 100 mL NaOH (0.1 mol/L) by ultrasonication for 0.5 h. Subsequently, the mixed liquor was transferred to reaction still and then annealed in different temperatures (100 and 160°C) for 20 h. After that, the obtained precipitates were collected by centrifugation and washed with distilled water, and then dried at 60°C for 12 h. Samples fabricated at different annealing temperatures were noted as 100-Ti₄O₇/g-C₃N₄ (fabricated at 100°C) and 160-Ti₄O₇/g-C₃N₄ (fabricated at 160°C), respectively.

Samples Characterization

The concentration of NaH₂PO₂ was measured by ion chromatography using a 732 IC detector (McDowell et al., 2004). The crystal phase composition and fineness of the samples were analyzed by X-ray diffraction (XRD) with Cu Kα radiation in the scanning range of 2θ = 5–80° (model D/max RA, Rigaku Co., Japan). The surface morphology of the as-developed samples was examined by scanning electron microscopy (SEM) (JEOL JSM-6701F). The valence state of the deposition was measured by X-ray photoelectron spectra (XPS) (PHI-5300/ESCA, ULVAC-PHI, INC). The UV–vis diffuse reflection spectra (UV–vis DRS) of the samples was obtained by a UV–vis spectrophotometer (UV-2450, Shimadzu, Japan). Electrochemical properties of the Ti₄O₇/g-C₃N₄ and g-C₃N₄ photocatalysts, including photocurrents (PC), CV, and EIS, were measured on a CHI 660B electrochemical system. Electron spin resonance (ESR) (ESRA-300, Bruker, Germany) signals were recorded by the probe molecular 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) to identify the radicals generated under visible light irradiation (λ > 420 nm) (Tian et al., 2015).

Evaluation of Photocatalytic Activity

The photocatalytic activities of as-synthesized samples were evaluated by the oxidation of hypophosphite in aqueous solution under visible light irradiation. For hypophosphite oxidation, the light source was a 35 W metal-halide lamp (Philips) with a 420 nm UV-cutoff filter. The lamps were located 12 cm away from the surface of reaction solution (about 5 mW cm⁻²). In each experiment, photocatalyst (10 mg) was dispersed in hypophosphite (100 ml, 100 mg L⁻¹) aqueous solution. Before irradiation, the solution was continuously stirred in the dark for 2 h to reach adsorption-desorption equilibrium between the hypophosphite and the photocatalyst. During the photocatalytic reaction, the solutions were kept magnetically stirring, and 4 ml mixture was collected at 1 h intervals followed by centrifugation (10,000 rpm, 5 min) to remove the photocatalyst.

Radicals Quencher Experiment for Photocatalysis

In order to identify the contributions of the radicals generated in the photocatalytic oxidation process, IPA and N₂ purging were applied with IPA acting as the ·OH radicals quencher and N₂ purging reducing the superoxide ·O₂⁻ radicals (Yang et al., 2016). Adding different radical scavengers into reaction solutions would affect the photocatalytic performance. As such, the contributions of ·OH radicals and ·O₂⁻ radicals on photocatalytic oxidation of hypophosphite under visible light irradiation can be evaluated based on the change of photocatalytic oxidation efficiency of hypophosphite with and without IPA (1 mM) and N₂ purging (continuous purging).

Results and Discussion

Structure and Morphology Analyses

The crystal structures of g-C₃N₄, Ti₄O₇, and 160-Ti₄O₇/g-C₃N₄ photocatalysts were characterized by XRD. As shown in Figure 1a, two pronounced diffraction peaks in pure g-C₃N₄ were observed at 13.20° and 27.60°, respectively. The peak at 13.20° was corresponded to (1 0 0) plane of tri-s-triazine units (Zhang et al., 2012a). The peak at 27.60°indexed as (0 0 2) peak was due to the interlayer-stacking of aromatic systems as in graphite (Zhang et al., 2012b). The characteristic peaks of Ti₄O₇ were matched well with the standard card (JCPDF 50-0787). The main diffraction peaks of 160-Ti₄O₇/g-C₃N₄ photocatalyst did not change obviously, indicating that the fabrication process did not destroy the main structure of both counterparts. The microstructure of the 160-Ti₄O₇/g-C₃N₄ photocatalyst was shown in Figure 1b. It was mainly composed of spheroidal crystals, and the shape of synthesized 160-Ti₄O₇/g-C₃N₄ photocatalyst was relatively uniform.

FIGURE 1

Figure 1. Structure and morphology analyses. (a) XRD patterns of the g-C₃N₄, Ti₄O₇ and 160-Ti₄O₇/g-C₃N₄ photocatalysts; (b) SEM image of the 160-Ti₄O₇/g-C₃N₄ photocatalyst.

XPS measurements were carried out to investigate the compositions and elemental chemical states of the samples. As shown in Figure 2, the XPS spectra revealed that the elements of C, N, Ti and O existed on the surface of 160-Ti₄O₇/g-C₃N₄ photocatalyst. The corresponding high resolution spectra of C 1s, N 1s, Ti 2p, and O 1s were also analyzed. The XPS spectra of C 1s core level for 160-Ti₄O₇/g-C₃N₄ photocatalyst was shown in Figure 2A that it could be divided into two components including the standard reference carbon (284.8 eV) and the sp² bonded C in N = C(–N)₂ (288.3 eV) (Jo and Natarajan, 2015). The N 1s spectra of Ti₄O₇/g-C₃N₄ could be divided into four peaks as shown in Figure 2B. The main peak at 398.7 eV was assigned to sp² nitrogen (C = N–C) involved in triazine rings, and the peak at 399.8 eV originated from the tertiary nitrogen bonded to carbon atoms in the form of N–(C)₃ (Wu et al., 2013). The peak at 401.3 eV was ascribed to amino functions (C–N–H). Another peak centered at 404.4 eV was associated to charging effects or positive charge localization in heterocycles (Gao et al., 2014). These assignments of C 1s and N 1s were agreed well with the XPS results of g-C₃N₄ reported previously. Ti₄O₇ is a mixed-valence compound with two evenly occupied Ti⁴⁺ (3d⁰) and Ti³⁺ (3d¹) configurations. As shown in Figure 2C, two broad peaks at about 458.6 and 464.7 eV were observed, corresponding to the characteristic Ti 2p_1/2 and Ti 2p_3/2 peaks of Ti⁴⁺, respectively. Additionally, two peaks at 457.97 and 463.8 eV corresponding to Ti³⁺ also appeared, as reported elsewhere (Zeng et al., 2017). The O 1 s spectra of Ti₄O₇/g-C₃N₄ were shown in Figure 2D. The peak with binding energy of 533.5 eV was assigned to the C–O functional groups, and the peaks centered at the binding energies of 531.8 and 529.7 eV were ascribed to the OH–Ti and O–Ti bonds (Li Z. Q. et al., 2017). These results confirmed the presence of Ti₄O₇ on the g-C₃N₄ surface with covalent bonds.

FIGURE 2

Figure 2. XPS spectra of the 160-Ti₄O₇/g-C₃N₄ photocatalyst. (A) C 1 s; (B) N 1 s; (C) Ti 2 p; (D) O 1 s.

Photocatalytic Activity Analysis

The photocatalytic oxidation of hypophosphite over various samples was analyzed. As shown in Figure 3A, the blank experiment indicated that the concentration of hypophosphite was stable under visible light irradiation (λ > 420 nm) if there was no photocatalyst presented. Pure g-C₃N₄ showed weak photocatalytic activity with the oxidation efficiency of only 10% possibly owing to the rapid recombination of photo-generated charge carriers and low charge transfer ability (Shi et al., 2017). The 160-Ti₄O₇/g-C₃N₄ photocatalyst had the highest photocatalytic activity with the oxidation efficiency of 81% compared with the pure g-C₃N₄ and 100-Ti₄O₇/g-C₃N₄ photocatalysts.

FIGURE 3

Figure 3. Photocatalytic activity of different photocatalysts. (A) The oxidation efficiency of hypophosphite; (B) The comparison of oxidation rate constant k.

The photocatalytic oxidation kinetic of the prepared samples was fitted by a pseudo-first-order model, which was depicted by the following Equation (1) (Lu et al., 2018):

$\begin{array}{cc}\text{ln}(C_0/C)=k\text{t}& (1)\end{array}$

Where C₀ and C are the hypophosphite concentrations in solution at times 0 and t, respectively, and k is the first-order rate constant. As shown in Figure 3B, the 160-Ti₄O₇/g-C₃N₄ photocatalyst showed the highest photocatalytic oxidation rate of 0.467 h⁻¹, which was 2.5 and 26 times higher than that of 100-Ti₄O₇/g-C₃N₄ and pure g-C₃N₄, respectively. Therefore, the results showed that the 160-Ti₄O₇/g-C₃N₄ photocatalyst exhibited an excellent activity in photocatalytic oxidation of hypophosphite under visible light irritation.

The enhanced photocatalytic activity of Ti₄O₇/g-C₃N₄ and the effect of annealing temperature on Ti₄O₇/g-C₃N₄ photocatalytic activity were further investigated and explained from the perspectives of photoabsorption efficiency, band gap, separation, transformation, and recombination processes of photogenerated carriers in the following sections.

Optical Properties Analysis

The optical properties of Ti₄O₇/g-C₃N₄ and pure g-C₃N₄ photocatalysts were evaluated by UV–vis diffuse reflectance spectra. As shown in Figure 4A, the photoabsorption efficiency of Ti₄O₇/g-C₃N₄ was remarkably enhanced compared with the pure g-C₃N₄. The pure g-C₃N₄ held an absorption edge of around 430 nm while the Ti₄O₇/g-C₃N₄ photocatalysts showed a distinct red-shift, indicating that the Ti₄O₇/g-C₃N₄ photocatalysts were more efficient in light harvesting under visible light irradiation. The enhanced photoabsorption efficiency of Ti₄O₇/g-C₃N₄ was due to the narrowed band gap. The band energy gap of the photocatalysts was determined from the formula αhν = A (hν – E_g)^n/2 (Eg, α, h, ν, and A indicate the band gap, optical absorption coefficient, Plank's constant, photonic frequency and a proportionality constant, respectively) (Huang et al., 2017b; Liu et al., 2017a,b; Tian et al., 2017). The band gap calculated from the plot of absorption^1/2 vs. energy was 2.70, 2.32, and 2.13 eV for g-C₃N₄, 100-Ti₄O₇/g-C₃N₄, and 160-Ti₄O₇/g-C₃N₄, respectively.

FIGURE 4

Figure 4. Optical properties of different photocatalysts. (A) UV–vis DRS spectra of the photocatalysts; (B) PL spectra of the photocatalysts.

Photoluminescence (PL) spectra was used to investigate the separation, transformation and recombination processes of photogenerated carriers. The band–band PL spectrum can directly reflect the separation performance of photo-induced charge carriers, viz. the stronger of the band-band PL signal, the higher of the recombination rate of photo-induced carriers. The PL spectroscopy of the photocatalysts was shown in Figure 4B. All photocatalysts exhibited a broad emission peak centered at around 460 nm, which was mainly caused by the recombination of photogenerated electrons and holes produced by g-C₃N₄ (Shi et al., 2017). The PL emission intensity was highest for the pure g-C₃N₄, while the intensity was significantly lowered for Ti₄O₇/g-C₃N₄. This indicated that the charge carrier recombination was effectively suppressed for the Ti₄O₇/g-C₃N₄ photocatalysts. It is well known that the noble metals, such as Ti₄O₇, are good conductors with excellent electric properties. After formation of noble metals-semiconductors heterostructures, the photogenerated electrons of semiconductors could transfer through these noble metals rapidly and the lifetime of these electrons and holes are prolonged (Cui et al., 2017). In addition, the PL intensity of 160-Ti₄O₇/g-C₃N₄ was much lower than that of 100-Ti₄O₇/g-C₃N₄, indicating that 160-Ti₄O₇/g-C₃N₄ had the effectively decreased charge carrier recombination compared with 100-Ti₄O₇/g-C₃N₄. The higher annealing temperature could etch and tailor g-C₃N₄ with the possibly smaller and thinner nanosheet structure of 160-Ti₄O₇/g-C₃N₄ compared with 100-Ti₄O₇/g-C₃N₄ and thus shortened the distance between the photogenerated electrons and the heterostructure surface, which suppressed the recombination probability of photo-generated electron–hole pairs with the photogenerated electrons rapidly transferring through Ti₄O₇ (Dong et al., 2015).

Electrochemical Properties Analysis

The PC responses of g-C₃N₄, 100-Ti₄O₇/g-C₃N₄, and 160-Ti₄O₇/g-C₃N₄ photocatalysts under visible light irradiation were evaluated to further offer information about the separation and transformation efficiency of photogenerated electrons and holes. As shown in Figure 5A, the transient PC responses of all photocatalysts at light on and light off were reversible and stable, and the PC density of 160-Ti₄O₇/g-C₃N₄ was much higher (0.30 μA cm⁻²) than that of 100-Ti₄O₇/g-C₃N₄ (0.23 μA cm⁻²) and g-C₃N₄ (0.10 μA cm⁻²). This indicated that the Ti₄O₇/g-C₃N₄ heterostructures promoted the separation of photogenerated charge carriers (Kang et al., 2016), in line with the PL spectra results as shown in Figure 4B. Additionally, this was also supported by the CV test. As shown in Figure 5B, a reduction peak at about −0.18 V was observed in g-C₃N₄, 100-Ti₄O₇/g-C₃N₄, and 160-Ti₄O₇/g-C₃N₄, but 160-Ti₄O₇/g-C₃N₄ photocatalyst possessed much higher reduction current, which indicated faster electron transfer in 160-Ti₄O₇/g-C₃N₄ photocatalyst (Samanta and Srivastava, 2017).

FIGURE 5

Figure 5. Electrochemical properties analysis of the 160-Ti₄O₇/g-C₃N₄ photocatalyst. (A) photocurrent density of different photocatalysts; (B) cyclic voltammogram scan of different photocatalysts.

Electrochemical impedance spectroscopy was used to investigate the photogenerated charge separation process on the photocatalysts. The radius of the circular arc reflected the resistance of the interfacial charge transfer and separation efficiency of the electron-hole pairs (Leng et al., 2005; Liang and Zhu, 2016). As shown in Figure 6, the arc radius decreased gradually when Ti₄O₇ was doped onto the g-C₃N₄ photocatalyst. This meant that the photogenerated charge separation process occurred more easily on Ti₄O₇/g-C₃N₄ compared with the pure g-C₃N₄ because of the decreased energy barrier that the electrode reaction needed to overcome. Moreover, the arc radius of 160-Ti₄O₇/g-C₃N₄ was smaller than that of 100-Ti₄O₇/g-C₃N₄, meaning that the separation of the photogenerated electron–hole pairs was more effective and the interfacial charge transfer of the electron donor/electron acceptor was faster on the 160-Ti₄O₇/g-C₃N₄ photocatalyst.

FIGURE 6

Figure 6. EIS Nyquist plots of the g-C₃N₄, 100-Ti₄O₇/g-C₃N₄, and 160-Ti₄O₇/g-C₃N₄ photocatalysts under visible light irradiation.

Catalyst Stability Analysis

The stability was another vital consideration for an excellent photocatalyst. To evaluate the stability of the as-prepared 160-Ti₄O₇/g-C₃N₄ photocatalyst, the repetitive experiments of photocatalytic oxidation of hypophosphite were carried out. As shown in Figure 7, the oxidation efficiency of hypophosphite in the four-round continuous reaction tests using 160-Ti₄O₇/g-C₃N₄ photocatalyst was 75, 81, 80, and 84%, respectively. The repetitive experiments results showed that the fabricated Ti₄O₇/g-C₃N₄ photocatalysts had a stable structure possibly with the strong binding force. The N-H groups or conjugated structures in g-C₃N₄ could tightly bond with Ti⁴⁺ (3d⁰) and Ti³⁺ (3d¹) in Ti₄O₇, which effectively reduced the dissolution of bulk g-C₃N₄ material during the photocatalytic process.

FIGURE 7

Figure 7. The stability analysis of the 160-Ti₄O₇/g-C₃N₄ photocatalyst under repetitive experiments.

Proposed Mechanism

To clarify the reaction mechanism of photocatalytic oxidation of hypophosphite, the ROS generated under visible light irradiation were analyzed by ESR technique (with DMPO). As shown in Figure 8A, no ESR signals were observed in the dark while ·OH was observed under visible light irradiation with four peaks with intensities of 1:2:2:1 attributing to DMPO-·OH generated via a hole oxidative process on H₂O and/or OH⁻ (Tu et al., 2017). Moreover, ·O₂⁻ was also observed under visible light irradiation with a four-line spectrum with the relative intensities of 1:1:1:1 assigned to DMPO-·O₂⁻ adduct derived from O₂ reduction by electrons (Huang et al., 2015), however, no ESR signals were observed in the dark as shown in Figure 8B. Therefore, both ·OH and ·O₂⁻ would contribute to the photocatalytic oxidation of hypophosphite and their contributions were further investigated with and without radical scavengers. Isopropanol (IPA) and N₂ purging were applied with IPA acting as the ·OH radicals quencher and N₂ purging reducing the superoxide ·O₂⁻ radicals (Yang et al., 2016). As shown in Figure 8C, the photocatalytic oxidation efficiency of hypophosphite decreased from 83% (without radical scavengers) to 42% (with IPA) and even 27% (with N₂ purging). These results confirmed that both ·OH and O₂·⁻ radicals were the major active radical species for hypophosphite oxidation in the photocatalytic process with O₂·⁻ accounting for a more significant contribution. Note that the photocatalytic oxidation efficiency of hypophosphite did not decrease to zero with the lowest efficiency of 27% in the presence of N₂ purging, indicating that holes and some other radicals may also contribute to the photocatalytic oxidation process to some extent. It was reported that reactive oxygen species (ROS), such as superoxide (·O₂⁻), hydroxyl radicals (·OH), singlet oxygen (¹O₂), peroxyl (RO₂·), and alkoxyl (RO-) as well as hypochlorous acid (HOCl) are basically produced in the photocatalytic process (Huang et al., 2017a), which may also contribute to the photocatalytic oxidation of hypophosphite in this case.

FIGURE 8

Figure 8. Radials analyses. (A) DMPO spin-trapping ESR spectra for ·OH radials analysis; (B) DMPO spin-trapping ESR spectra for ·O₂⁻ radials analysis; (C) Effect of scavengers on the photocatalytic oxidation process.

According to the above results and those reported in the literature, the possible photocatalytic mechanism of Ti₄O₇/g-C₃N₄ on hypophosphite oxidation was illustrated in Figure 9. The possible photocatalytic processes were as follows:

$\begin{array}{cc}{\text{Ti}}_4{\text{O}}_7/\text{g}-{\text{C}}_3{\text{N}}_4+hv\to \text{g}-{\text{C}}_3{\text{N}}_4({\text{e}}^{-}+{\text{h}}^{+})& (2)\end{array}$

$\begin{array}{cc}\text{g}-{\text{C}}_3{\text{N}}_4({\text{e}}^{-})+{\text{Ti}}_4{\text{O}}_7\to {\text{Ti}}_4{\text{O}}_7({\text{e}}^{-})& (3)\end{array}$

$\begin{array}{cc}{\text{Ti}}_4{\text{O}}_7+hv\to {\text{Ti}}_4{\text{O}}_7({\text{e}}^{-})& (4)\end{array}$

$\begin{array}{cc}{\text{Ti}}_4{\text{O}}_7({\text{e}}^{-})+{\text{O}}_2\to ·{\text{O}}_2^{-}& (5)\end{array}$

$\begin{array}{cc}·{\text{O}}_2^{-}+{\text{PO}}_2^{3-}\to {\text{PO}}_4^{3-}& (6)\end{array}$

$\begin{array}{cc}\text{g}-{\text{C}}_3{\text{N}}_4({\text{h}}^{+})+{\text{OH}}^{-}\to ·\text{OH}& (7)\end{array}$

$\begin{array}{cc}·\text{OH}+{\text{PO}}_2^{3-}\to {\text{PO}}_4^{3-}& (8)\end{array}$

$\begin{array}{cc}\text{g}-{\text{C}}_3{\text{N}}_4({\text{h}}^{+})+{\text{PO}}_2^{3-}\to {\text{ PO}}_4^{3-}& (9)\end{array}$

Firstly, the electrons (e⁻) in valence band of g-C₃N₄ under visible light irradiation could be excited to the conduction band, leaving the holes (h⁺) in valence band of g-C₃N₄ (Equation 2). Then the photogenerated electrons in the conduction band of g-C₃N₄ would continually transfer to Ti₄O₇ until the same Fermi levels reached (Equation 3). Thus, the photogenerated electrons and holes were located at Ti₄O₇ and g-C₃N₄, respectively, leading to the effective separation of the photoinduced charge carriers. Furthermore, the electrons might be also generated from the Ti₄O₇ particles (Equation 4) with the product easily reacting with the adsorbed oxygen molecules to produce ·${\text{O}}_2^{-}$ (Equation 5) followed by the oxidation of hypophosphite to phosphate (Equation 6). Meanwhile, the photogenerated holes as the strong oxidants could oxidize OH⁻ to ·OH radicals, and then the hypophosphite was directly oxidized to phosphate (Equations 7 and 8). In addition, some photogenerated holes could directly oxidize hypophosphite to phosphate (Equation 9).

FIGURE 9

Figure 9. The mechanism of the hypophosphite oxidation over the Ti₄O₇/g-C₃N₄ photocatalyst under visible light irradiation.

Conclusion

The enhancement of Ti₄O₇/g-C₃N₄ visible light photocatalytic performance on hypophosphite oxidation and the effect of annealing temperature and the corresponding mechanism were investigated in this study. 160-Ti₄O₇/g-C₃N₄ (fabricated at 160°C) photocatalyst showed the highest oxidation efficiency of hypophosphite of 81% and the highest photocatalytic oxidation rate of 0.467 h⁻¹ comparing with 100-Ti₄O₇/g-C₃N₄ (fabricated at 100°C) and pure g-C₃N₄. The enhanced photocatalytic performance of 160-Ti₄O₇/g-C₃N₄ could be ascribed to the effective charge separation and enhanced photoabsorption efficiency. Additionally, ESR results showed that hydroxyl radicals and superoxide anion radicals were mainly responsible to the oxidation of hypophosphite with O₂·⁻ accounting for a more significant contribution. Moreover, Ti₄O₇/g-C₃N₄ photocatalysts showed the remarkable stability in the repetitive experiments. Our work demonstrated that the rational design and construction of isotype heterojunction was an effective strategy to develop the efficient photocatalysts under visible light irradiation.

Author Contributions

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

Funding

This work was supported by National Natural Science Foundation of China (No. 51608086); Science and Technology Project from Chongqing (CSTC2015JCYJA20027); Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1501103); Chongqing Key Laboratory of Environmental Materials and Remediation Technology, Chongqing University of Arts and Sciences (CKE1407); Natural Science Foundation of Yongchuan, Chongqing (Ycstc, 2014ac4001); Scientific Research Foundation of Chongqing University of Arts and Sciences (R2014CH08).

Conflict of Interest Statement

Author LY was employed by company of Heibei Yinfa Meifute Environmental Engineering Co., Ltd.

The other authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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