Synthesis and photocatalytic properties of visible-light-responsive, three-dimensional, flower-like La–TiO₂/g-C₃N₄ heterojunction composites

Abstract

We prepared a new three-dimensional, flower-like La–TiO₂/g-C₃N₄ (LaTiCN) heterojunction photocatalyst using a solvothermal method. Analysis and characterization were performed by conducting scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform-infrared spectroscopy, ultraviolet-visible spectrophotometry, and nitrogen adsorption and desorption. The prepared g-C₃N₄ nanosheets could reach 100 nm in size and covered the TiO₂ surface. A tightly bound interface formed between the g-C₃N₄ and TiO₂, speeding up the effective transfer of photo-induced electrons. In addition, the incorporation of La³⁺ reduced the electron–hole recombination efficiency. Consequently, the prepared La–TiO₂/g-C₃N₄ composite material exhibited better visible-light catalytic activity than pure TiO₂.

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1. Introduction

Photocatalytic degradation of organic pollutants and hydrogen production from water decomposition can improve the utilization of solar energy. Therefore, photocatalysis is considered to be an ideal green technology for solving the energy crisis and reducing environmental pollution.^1,2 In recent decades, TiO₂ and g-C₃N₄ have been widely used in photocatalysis due to their non-toxicity, chemical stability and photocatalytic activity.^3–5 The close interface formed between the components of a composite semiconductor material can effectively promote the transport of photogenerated carriers, increasing the photocatalytic activity relative to those of pure semiconductors.^6–8 Therefore, the development of novel TiO₂/g-C₃N₄ heterojunction photocatalysts with excellent catalytic performance has become a topic of significant research interest.

Currently, most researchers use high-temperature calcination methods^9–11 or solvothermal methods^12–14 to recombine TiO₂ and g-C₃N₄. For instance, Chen et al.¹⁰ heated modified TiO₂ particles and protonated g-C₃N₄ particles at 70 °C to prepare a TiO₂/Ag/g-C₃N₄ composite microsphere with a particle size of 2–3 μm. Li et al.⁹ prepared a TiO₂/g-C₃N₄ composite hollow sphere via a solvothermal method. In these TiO₂/g-C₃N₄ composite materials, g-C₃N₄ is always uniformly attached to the TiO₂ particle surface and the composite particles exhibit a uniform monodisperse relationship, but the particle size is within the micron range. Through the high-temperature calcination of a TiO₂ hollow sphere with a particle size of 50–100 nm and urea, Zou et al.¹⁵ obtained a TiO₂/g-C₃N₄ composite material. However, TiO₂ microspherical particles can easily disperse in g-C₃N₄ particles, mainly because it is difficult to control the bulk-g-C₃N₄ coating on the surfaces of nanosized TiO₂ particles. To control this coating effectively and obtain a well-dispersed TiO₂/g-C₃N₄ composite material, the key is to prepare nanosized g-C₃N₄ lamellae.

Adding metal ions can enable catalytic materials to absorb visible light more effectively.^16–18 It has been found that La is rich in electronic energy levels and that the introduction of electronic energy levels can reduce the energy gaps of catalytic materials and further increase the visible light absorption.^19–21 Rong et al.²² reported that a g-C₃N₄ photocatalyst doped with La exhibited higher photocatalytic activity in the degradation of organic pollutants. Khalid et al.²³ reported that a TiO₂-graphene composite material doped with La resulted in remarkably slower electron–hole pair recombination than pure TiO₂.

The main purposes of this study were to prepare a nanosized TiO₂/g-C₃N₄ composite material and further improve the photocatalytic activity of the TiO₂/g-C₃N₄ composite material by incorporating La³⁺. First, we prepared a g-C₃N₄ nanosheet with plenty of nanopores via thermal spalling. Then, we achieved molecular self-assembly by employing a one-step solvothermal method and successfully prepared a La (x%)–TiO₂/g-C₃N₄ composite material with a three-dimensional, flower-like structure. During the molecular self-assembly process, the formed TiO₂ crystals acted as scissors, and the g-C₃N₄ nanosheet was cut into increasingly smaller slices. Actually, the g-C₃N₄ can also loaded on many other semiconductors with matched energy band structure to construct high-efficient heterojunctions, and thus providing a new idea for fabricating g-C₃N₄ based composite photocatalytic materials.

2. Experimental

2.1 Materials and chemicals

Melamine (C₃H₆N₆) was used to synthesise the g-C₃N₄ nanosheets. Tetrabutyl titanate (Ti(OC₄H₉)₄) was utilized as the titanium precursor in the synthesis of nanocrystalline TiO₂. Lanthanum nitrate (La(NO)₃·6H₂O) was employed as the metal precursor for doping TiO₂. These chemicals, together with hydrofluoric acid (HF, 40 wt%), ethanol (CH₃CH₂OH), sodium bicarbonate (NaHCO₃), tert-butyl alcohol (C₄H₁₀O), r-benzoquinone (C₆H₄O₂), and RhB (C₂₈H₃₁N₂O₃Cl), were of analytical grade and purchased from Sigma Aldrich Co. All of the reagents were of analytical grade and were used as obtained without further purification. Distilled water was employed in all of the experiments requiring water.

2.2 g-C₃N₄ nanosheet preparation

Yellow bulk g-C₃N₄ was obtained through the calcination of 5.0 g of melamine in a muffle furnace at a heating rate of 2 °C min⁻¹ at 550 °C for 4 h. After that, which was further oxidized and peeled off. The obtained light yellow powder was tiled in a square porcelain boat and calcined at a heating rate of 5 °C min⁻¹ at 530 °C for 2 h to obtain a white g-C₃N₄ nanosheet.

2.3 La–TiO₂/g-C₃N₄ hybrid preparation

First, 0.6 g of g-C₃N₄ nanosheet powder was added to 30 mL of anhydrous ethanol and ultrasonically dispersed for 2 h. Then, 3.0 mL of Ti(OC₄H₉)₄ was added to 20 mL of absolute ethanol to obtain a pale yellow transparent solution. La(NO)₃·6H₂O was added stoichiometrically to dissolve the mixture completely into a transparent mixed solution and stirred for 30 min. The g-C₃N₄ nanosheet dispersion was added to the Ti(OC₄H₉)₄ mixed solution and stirred for 30 min. Next, 1.5 mL of HF was pipetted dropwise into the resulting mixed solution and stirred for 15 min. The solution was then transferred to a 100 mL Teflon-lined high-value kettle and maintained at 160 °C for 20 h. It was subsequently cooled to room temperature, and the powder was washed several times with absolute ethanol and deionized water. The resulting sample was placed in a vacuum oven at 60 °C for 4 h. The atomic ratios of La to TiO₂ were 0.5%, 1%, and 2%. For comparison, the same method was used to prepare pure TiO₂.

2.4 Photocatalytic activity evaluation

The photocatalytic activity of La (x%)–TiO₂/g-C₃N₄ (La_xTiCN) in the degradation of RhB in aqueous solution was tested under visible light irradiation. A 500 W Xe lamp was employed as the light source, and the visible wavelength was adjusted using a 420 nm filter. For the experiments, 50 mg of La_xTiCN and 50 mL of RhB solution (5 mg L⁻¹) were added to the tube reactor. The suspension was magnetically stirred in the dark for 30 min prior to each assay so that it reached adsorption equilibrium prior to irradiation. The samples were taken from the reactor at regular time intervals during irradiation and centrifuged using a high-speed centrifuge (RG-TGL-16C, Ruijiang Co.) to remove the photocatalyst. The samples were collected by centrifugation every 15 min to measure the RhB degradation by performing ultraviolet-visible (UV-vis) spectrophotometry, using the absorbance at 552.0 nm as the representative peak to determine the RhB concentration.

3. Results and discussion

3.1 Characterisation of synthesized catalysts

Fig. 1 shows the X-ray diffraction (XRD) patterns of TiO₂, g-C₃N₄, TiO₂/g-C₃N₄ (TiCN), and La_xTiCN. Evidently, the TiO₂ in all of the samples was of the anatase phase. The diffraction peaks at 25.3°, 38.0°, 48.0°, 54.1°, 62.6°, 69.6°, and 75.2° respectively correspond to the (101), (004), (200), (211), (204), (220), and (215) crystal faces of anatase (JCPDS-21-1272). Meanwhile, the peaks at 13.2° and 27.7° respectively correspond to the (100) and (002) crystal faces of g-C₃N₄ (JCPDS no. 87-1526). In the composite samples of TiO₂/g-C₃N₄ and La_xTiCN, the characteristic diffraction peaks of TiO₂ and g-C₃N₄ are observable, indicating that TiO₂ and g-C₃N₄ constituted a composite structure.²⁴ In addition, as compared with TiO₂/g-C₃N₄, the diffraction peaks of La (x%)–TiO₂/g-C₃N₄ composite samples are not shifted higher or lower, indicating that La³⁺ did not enter the TiO₂ lattice, because the ionic radius of La³⁺ (0.115 nm) is much larger than that of Ti⁴⁺ (0.064 nm), and La³⁺ cannot replace Ti⁴⁺ in a TiO₂ lattice and form lattice distortion,²⁵ which means that La³⁺ ions may exist in the form of La₂O₃ or LaF₃. However, the relevant information about La₂O₃ and LaF₃ is not observable, because the contents of La₂O₃ and LaF₃ were relatively small. The lattice parameters of the samples were calculated from the XRD results, which are summarized in Table 1. Compared with the TiCN, the lattice parameters of La₁TiCN were almost unchanged (a little decrease), which is consistent with the above results.

Fig. 1 XRD patterns of TiO₂, g-C₃N₄, TiCN, and La_xTiCN composites with various La contents.

Table 1 The lattice parameters and the crystal size of the samples

Samples	a = b (nm)	c (nm)	Crystal size (nm)
g-C3N4	0.6412	0.2448	6.72
TiO2	0.3786	0.9526	8.57
TiCN	0.3796	0.9512	8.52
La1TiCN	0.3782	0.9508	7.66

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To demonstrate the formation of the La_xTiCN sample heterostructure, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses were conducted. Fig. 2a is the SEM diagram of pure TiO₂. The prepared TiO₂ exhibits a three-dimensional, flower-like structure, which was formed by the staggered accumulation of 500 nm lamellae. Fig. 2b presents the TEM and SEM diagrams of g-C₃N₄ that were obtained after secondary thermal spalling, displaying a structure with numerous pores. The size of the observed g-C₃N₄ nanosheet is about 100 nm, enabling the g-C₃N₄ to cover the TiO₂ lamellae. Fig. 2c and d are SEM diagrams of the La₁TiCN composite material prepared by using the one-step solvothermal method. All of the sample particles exhibit three-dimensional, flower-like structures of the same size. Compared with pure TiO₂, the lamella surface of the flower-like structure of the La₁TiCN is coarser and the accumulation between lamellae is denser, which is probably due to the existence of the g-C₃N₄ nanosheet, which provided more attachment points for the formation of TiO₂ nanoparticles and enhanced the aggregation effect of the TiO₂ nanoparticles during self-assembly. Fig. 2f is a TEM diagram of the La₁TiCN composite material, from which it is clear that g-C₃N₄ nanosheet covers the surface of the TiO₂ lamellae. The element mapping in Fig. 3 further proves the existence of elemental Ti, O, C, N, and La. It is evident that the Ti and O distribution diagrams can outline the basic shape of the three-dimensional, flower-like structure; the distributions of C and N are basically consistent with the material structure in Fig. 3d and e; and La is sparsely and evenly dispersed over the particle surface. The EDX spectrum confirms the presence of C, N, O, Ti, and La elements in La₁TiCN and the percentage of La is 1 wt% in Fig. 3g.

Fig. 2 SEM images of (a) TiO₂, (b) the g-C₃N₄ nanosheet, and (c) and (d) La₁TiCN. (e) and (f) TEM images of La₁TiCN.

Fig. 3 (a) TEM image of the La₁TiCN sample. Corresponding energy-dispersive X-ray spectroscopy elemental mapping images of (b) Ti, (c) O, (d) C, (e) N, (f) La, and (g) EDX plots.

The amount of g-C₃N₄ in the sample was determined by TG analysis. Fig. 4 shows a comparison of TG curves for g-C₃N₄, TiCN, and La₁TiCN samples. Pure g-C₃N₄ has a large weight loss in the range of 500–700 °C. While TiCN and La₁TiCN lose weight rapidly in the same temperature range, which can be attributed to the combustion of g-C₃N₄. Therefore, the actual amount of g-C₃N₄ in the TiCN and La₁TiCN₄ samples were about 12.77 wt% estimating from the weight loss.

Fig. 4 TGA curves of g-C₃N₄, TiCN, and LaTiCN.

To determine the structural parameters of the synthesized samples, the N₂ adsorption–desorption isotherms were measured in Fig. 5. The specific surface area of pure g-C₃N₄ is 126.5 m² g⁻¹, which mainly depends on the porous structure of g-C₃N₄. Compared with pure TiO₂ (46.53 m² g⁻¹), the composite TiCN (62.75 m² g⁻¹), which is mainly produced by the deposition of the porous g-C₃N₄ structure onto the TiO₂ surface, provides a larger surface area. The surface area of La₁TiCN is 72.77 m² g⁻¹, indicating that La³⁺ can increase the surface area of the catalyst and provide more reactive centres. Fig. 4b presents the pore size distribution curves of the samples. The pure g-C₃N₄ pore size is 3.4 nm, while the TiO₂ pore size is 5–12 nm. The main pore size of t La₁TiCN presents a layered void composed of 3.8 nm (small pores) and 7.0 nm (larger mesopores). The existence of the layered void structure is beneficial to multi-light scattering/reflection, which greatly enhances the excitation.²⁶ In addition, a hierarchical porosity composed of mesopores facilitates fast mass transport, resulting in good electrode performance.²⁷ The similar effects from mesopores and a hierarchical porosity of the as-prepared La_xTiCN increase the photocatalytic activity.

Fig. 5 (a) N₂ adsorption–desorption isotherms of TiO₂, g-C₃N₄, and La₁TiCN composites with various La contents and (b) pore size distributions of the corresponding samples.

The compositions and combination modes of the TiO₂, g-C₃N₄, TiCN, and La₁TiCN samples were analysed by performing Fourier transform-infrared (FT-IR) spectroscopy. The results are shown in Fig. 6. For pure TiO₂, a strong absorption peak is evident at 500–700 cm⁻¹, corresponding to the stretching vibrations of the Ti–O and Ti–O–Ti bands, and the two strong peaks at 3350–3550 cm⁻¹ and 1645 cm⁻¹ are attributable to the water and hydroxy absorbed by the TiO₂ surface. In the pure g-C₃N₄, the absorption peaks at 1239 cm⁻¹, 1317 cm⁻¹, 1407 cm⁻¹, and 1571 cm⁻¹ are attributable to the stretching vibrations of the C–N hybridized by sp³; the peak at 1637 cm⁻¹ is attributable to the stretching vibrations of C=N hybridized by sp²; and the peak at 808 cm⁻¹ corresponds to the out-of-plane bending vibrations of the 3-S-triazine units. For the TiCN and La_xTiCN composite samples, the main characteristic absorption peaks of TiO₂ and g-C₃N₄ are observable, indicating that the composite heterojunction structure of g-C₃N₄/TiO₂ formed.²⁸ This finding is consistent with the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results. Notably, compared with the patterns of the characteristic peaks of the pure g-C₃N₄ nanosheet, those of the g-C₃N₄ in the La_xTiCN composite sample are sharper, because the La_xTiCN composite sample had smaller g-C₃N₄ lamellae than the pure g-C₃N₄ nanosheet, causing the atoms in the g-C₃N₄ neighbouring 3-S-triazine ring to separate from each other and weakening the interactions among the atoms. Therefore, the atoms in g-C₃N₄ in the La_xTiCN samples were under less constraining force, so the vibrations were stronger.²⁹

Fig. 6 FT-IR spectroscopy patterns of TiO₂, g-C₃N₄, TiCN, and La_xTiCN composites with various La contents.

Based on the analysis of the above experimental results, the formation mechanism of the La_xTiCN heterojunction catalyst was determined and is depicted in Fig. 7. First, the Ti precursor micromolecule infiltrated the gaps among the g-C₃N₄ lamellae. Then, the TiO₂ crystal grew on the lamella surface of g-C₃N₄ and formed a sheet structure. The g-C₃N₄ was divided into increasingly smaller lamellae, and La³⁺ was deposited on the crystal surface. Finally, the obtained composite lamellae formed a La (x%)–TiCN heterojunction catalyst with a three-dimensional, flower-like structure in a hydrotherm through self-assembly. During the formation of the samples, the generation of small g-C₃N₄ through cutting during the formation of the TiO₂ crystal and through the formation of smaller g-C₃N₄ particles was the key to the formation of the heterojunction structure.³⁰

Fig. 7 Schematics illustrating La_xTiCN formation.

The chemical compositions of the prepared La_xTiCN samples and the chemical states of all of the elements were analysed by performing XPS. Fig. 8a shows the full XPS results for the La₁TiCN sample, where the spectral peaks of C 1s, N 1s, Ti 2p, O 1s, and La 3d are easily observable, indicating that the sample was mainly composed of C, N, Ti, O, and La. Notably, a weak F 1s peak is evident at 684.3 eV, which is probably attributable to the fluorination caused by the introduction of HF into the preparation process.³¹

Fig. 8 (a) Overview of the XPS results and individual high-resolution XPS results for (b) Ti 2p, (c) O 1s, (d) C 1s, (e) N 1s, and (f) La 3d of as-prepared La₁TiCN.

Fig. 8b presents the XPS results for Ti 2p, exhibiting two peaks at 459.4 eV and 465.1 eV, which respectively correspond to the Ti 2p_3/2 and Ti 2p_1/2 orbitals. Fig. 8c shows the XPS results for O 1s. The peak at 530.3 eV corresponds to Ti–O in the TiO₂ lattice, and that at 532.1 eV is attributable to the hydroxy or adsorbed water molecules on the sample surface. Fig. 8d presents the XPS results for C 1s, where there are two characteristic peaks at 284.8 eV and 288.3 eV, which are attributable to the C–C bands hybridized by sp³ in g-C₃N₄ and the N–C=N bands hybridized by sp², respectively. Three peaks can be fitted from the XPS results for N 1s in Fig. 8e: C=N–C at 398.8 eV, N–(C)₃ at 399.6 eV, and C–N–H at 401.1 eV. In addition, no significant Ti–C (C 1s 283.0 eV)³² or Ti–N (N 1s 399.3 eV)^33,34 signal is observable, and the binding energy of Ti 2p_3/2 does not shift lower,³⁴ indicating that atomic C and N were not incorporated into the TiO₂ lattice. An additional explanation is that the g-C₃N₄ particles in the compound were only deposited on the surface of the TiO₂, forming a heterojunction structure.

Fig. 8f presents the XPS results for La 3d, showing peaks at 837.1 eV and 854.3 eV, which respectively correspond to La 3d_5/2 and La 3d_3/2, as well as satellite peaks at 841.5 eV and 857.6 eV, which were respectively split by La 3d_5/2 and La 3d_3/2. According to the literature, the characteristic peaks of La₂O₃ are located at 834.9 eV and 851.8 eV,³⁵ indicating that La³⁺ in the La_xTiCN sample and elemental O with stronger electronegativity formed Ti–O–La, and the binding energy shifted higher.³⁶ These findings agree with the XRD results.

UV-vis diffuse reflectance spectrometry was then conducted to measure the absorbances of the samples. Fig. 9a shows the UV-vis diffuse reflection spectra of the g-C₃N₄, TiO₂, TiCN, and La_xTiCN composite catalysts. The absorption edge wavelengths of the pure TiO₂ sample and g-C₃N₄ are about 390 nm and 460 nm, respectively. Compared with the absorption edge of the pure TiO₂ particles, that of the TiCN composite sample is shifted towards longer wavelengths and displays strong absorption in the 380–430 nm range, because the interface matching effect between TiO₂ and g-C₃N₄ changed the optical properties of TiO₂.³⁷ When doped with La³⁺, La_0.5TiCN and La₁TiCN display stronger visible light absorption in the 380–450 nm range. La₁TiCN exhibits the best visible light absorption, while that of La_0.5TiCN is slightly weaker, probably because the La³⁺ concentration was too low and could not provide more electronic row traps.³⁸ Meanwhile, for the La₂TiCN sample, the UV and visible light absorption are both significantly lowered, because an excessively high La³⁺ concentration can reduce the light transmittance of the catalysts and light absorption efficiency.³⁹

Fig. 9 (a) UV-vis diffuse reflection spectra and (b) plots of (αhv)² versus energy hv for the catalysts.

The band-gap energies of the samples were estimated by performing Kubelka–Munk conversion, and the results are shown in Fig. 8b. The band-gap energies of pure TiO₂, g-C₃N₄, TiCN, and La₁TiCN are respectively 3.00 eV, 2.50 eV, 2.78 eV, and 2.45 eV. The La₁TiCN sample has a lower band-gap energy, probably because the introduction of La³⁺ led to the formation of a new impurity energy level in the TiO₂ band gap, narrowing its band gap and extending its light absorption range.⁴⁰

3.2 Photocatalytic activity and photodegradation mechanism

Under visible light irradiation, the photocatalytic efficiencies of the various samples were determined based on the RhB degradation, with the results shown in Fig. 10a. The photocatalytic activities of the various samples were further compared by using the equation ln(C₀/C) = kt, where C₀ is the initial RhB concentration, C is the instantaneous RhB concentration at time t, and k is the kinetic constant.

Fig. 10 (a) Photocatalytic degradation of RhB under visible light irradiation, (b) linear transforms – ln(C/C₀) of the kinetic curves of RhB degradation, (c) apparent pseudo-first-order rate constant k_app with different catalysts, (d) recycling of the La₁TiCN in the removal of RhB dyes, and (e) scavenger test results obtained using La₁TiCN.

The first-order kinetic model of RhB degradation was obtained, with the results presented in Fig. 10b and the calculated k values given in Fig. 10c. Compared with the photocatalytic efficiencies of pure TiO₂ and g-C₃N₄, those of all of the La_xTiCN composite samples are increasingly enhanced, mainly because the large and dense hetero interface formed between TiO₂ and g-C₃N₄ could speed up the efficient electron–hole shifting. In addition, along with continuously increasing La³⁺, the photocatalytic efficiencies of the La_0.5TiCN and La₁TiCN samples are increasingly enhanced. The photocatalytic efficiency of La₁TiCN is 95.2%, and the kinetic constant is 0.0325 min⁻¹, which is 2.1 and 10 times those of pure TiO₂ and g-C₃N₄, respectively, and 1.63 times that of La_xTiCN. Incorporating La³⁺ can firstly provide more electron traps and improve the electron–hole separation efficiency in La₁TiCN. In addition, incorporating La³⁺ can increase the specific surface area of TiCN, with the strongest photocatalytic effect. Notably, the photocatalytic effect of La₂TiCN is reduced, because excessive La³⁺ can serve as the recombination centre of electron–holes, reducing the photocatalytic activity of La₂TiCN. For evaluating the recyclability of the flower-like La_xTiCN heterojunction composites, cycling experiments were performed by washing the composite with ethanol for several times. As shown in Fig. 10d, the photocatalytic efficiency of RhB decreases from 95.2 to 88.3% for La₁TiCN after five cycles. This fact demonstrates that the obtained flower-like La_xTiCN heterojunction composites have good stability under visible-light irradiation and are less photo-corroded during the photocatalytic oxidation, which can be applied as a new option for dye wastewater treatment.

To determine the reactive species during a light-catalysed reaction, it is possible to eliminate holes h⁺, superoxide radicals ·O²⁻, and hydroxyl radicals ·OH by adding NaHCO₃, BQ, and TBA. The obtained results are shown in Fig. 9d. It is evident that adding NaHCO₃ and BQ significantly inhibited the light degradation of RhB in the La₁TiCN sample, indicating that under visible light irradiation, O²⁻ and h⁺ are the main active substances in RhB photolysis. The equations E_VB = χ − E_e + 0.5BG and E_CB = E_VB − BG were then employed to calculate the VB and CB potentials, E_VB and E_CB, respectively, where BG is the band gap, χ is the absolute electronegativity of the semiconductor, and E_e is the energy of free electrons based on the hydrogen scale and has a value of 4.50 eV.⁴¹ For TiO₂ and g-C₃N₄, χ is 5.81 and 4.72, respectively.

The VB and CB edge electric potentials of TiO₂ and g-C₃N₄ were calculated. The VB and CB edge potentials of TiO₂ were determined to be +2.83 eV and −0.17 eV, respectively, and those of g-C₃N₄ were found to be +1.47 eV and −1.03 eV, respectively.

Under visible light irradiation, the electrons from the VB of TiO₂ and g-C₃N₄ were respectively excitated into the CB and left positively charged holes in the VB. Since the CB of g-C₃N₄ (−1.03 eV) has a potential lower than that of TiO₂ (−0.17 eV), the excitated electrons on the g-C₃N₄ surface can easily shift to the TiO₂ surface through the hetero interface.⁴² Finally, these electrons can be captured by the La³⁺ with free orbitals.⁴³ The electrons accumulated on the La³⁺ surface can easily be captured by the O₂ dissolved in solution to generate superoxide ·O²⁻, and the generated ·O²⁻ can subsequently be used to degrade RhB. Similarly, the holes in the VB of TiO₂ can easily be transferred to the VB of g-C₃N₄. Compared with ·OH/OH⁻ (1.99 eV vs. NHE) and ·OH/H₂O (2.68 eV vs. NHE),¹¹ the reduction potential of the VB of g-C₃N₄ is even lower (1.65 eV vs. NHE). Therefore, the holes in the VB of g-C₃N₄ can be used to degrade RhB directly. Fig. 11 illustrates the La_xTiCN heterojunction degradation mechanism. The close interface formed by TiO₂ and g-C₃N₄ is a green channel for electron passage, which is beneficial to increase the transfer rate of g-C₃N₄ photoelectrons. The proper amount of La is introduced, the interface barrier of La and TiO₂ becomes higher and the space charge region is narrowed in the system. The generated large electric field promotes the light-induced electron orientation of g-C₃N₄ to the interface between La and TiO₂, and the electron–hole recombination is effectively prevented in this process. Thus, during the photocatalytic reaction, h⁺ and ·O²⁻ are the most important active species, which is consistent with the results of the active component trapping experiment. In addition, the hetero interface formed between TiO₂ and g-C₃N₄ can facilitate charge transfer among the particles, and the incorporation of La³⁺ can help enhance the electron–hole pair separation efficiency in such a three-way catalytic system and further promote the photocatalytic efficiency.

Fig. 11 Proposed visible light photodegradation mechanism of the La₁TiCN hybrid photocatalyst.

4. Conclusion

La_xTiCN heterojunction composites were fabricated successfully using a simple hydrothermal method. The prepared composite catalysts exhibited enhanced photocatalytic activity compared to that of pure TiO₂ under visible light irradiation for RhB degradation. The tightly connected interface between the TiO₂ and g-C₃N₄ contributed to the stable transmission of photo-generated electrons, and La³⁺ could trap electrons, thereby hindering electron–hole recombination. The mutual synergy between the three-way catalytic systems effectively improved the separation efficiency of the electron–hole pairs. The prepared three-dimensional, flower-like La_xTiCN proved capable of exhibiting a strong visible light response and is potential for the removal of environmental pollutants originating from industrial wastewater.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Research Project of Education Ministry of Heilongjiang Province of China (YSTSXK201843, 135209223), the Research Innovation Program for College Graduates of Qiqihar University (YJSCX2017-033X) and the Research Project of the Ministry of Human Resources and Social Security of China (2015).

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