Ball-milling combined calcination synthesis of In₂O₃/C₃N₄ for high photocatalytic activity under visible light irradiation

Reagents were of analytical reagent (AR) grade, which were purchased from sinopharm company sources. Distilled water was used in the whole experiment. Melamine was used as precursor to synthesize C₃N₄ powders in a muffle furnace. Typically, 5 g melamine was put in an alumina crucible with a cover, and first heated to 500 °C at a rate of 3.3 °C /min and held for 2h, subsequently the sample was kept at 550 °C for another 2 h. The bulk C₃N₄ was grounded into powder. The obtained sample was labeled as C₃N₄.

In₂O₃ powder was synthesized by heating In(OH)₃ in a muffle furnace. Typically, 3 g In(OH)₃ was put in an alumina crucible with a cover, and then heated to 500 °C (kept for 2h).

Synthesis of In₂O₃/C₃N₄ composites was as follows: C₃N₄ (1.000 g) and different amount of In(OH)₃ were added into a mortar and grounded for 5h with 5 ml alcohol. Then the mixed powder was dried at 65 °C for 5 h. Subsequently the mixed powder was placed in an alumina crucible with a cover, which was heated to 440 °C (kept for 2 h) and then kept at 390 °C for another 12 h. In₂O₃/C₃N₄ photocatalysts with 0.100 g, 0.200 g and 0.300 g In(OH)₃ were synthesized and according to the results of TG, they were named as In₂O₃/C₃N₄ (9.6%), In₂O₃/C₃N₄ (17.3%) and In₂O₃/C₃N₄ (41.2%), respectively.

The thermogravimetric analysis (TG, STA 449 C, Germany) was performed using a thermal analyzer at a rate of 10 °C /min from 25 °C to 800 °C in the air. X-ray powder diffraction (XRD, XRD-6100, Japan) was performed in a parallel mode (7°/min, 2θ range from 10° to 80°) using a Shimadzu XRD-6000 X-ray diffractometer (Cu K source). Field emission scanning electron microscopy (SEM, JSM-6010 PLUS/LA, Japan) was used to examine the surface condition of the samples. Transmission electron microscopy (TEM, JSM-2010F, Japan) was carried out with an acceleration voltage of 200 kV to investigate the fine morphology. The element chemical valence on the surface of In₂O₃, C₃N₄ and In₂O₃/C₃ N₄ (17.3%) were investigated by X-ray photoelectron spectroscopy (XPS) for chemical analysis on a photo-electron spectrometer (ESCA Lab MKII X-ray) using the Mg Kα radiation. Fourier transform infrared spectra (FT-IR, Nicolet Nexus 470, America) was recorded on a Fourier transform infrared spectrometer at room temperature. UV–Vis diffuse reflectance spectra (DRS, UV-3600Plus, Japan) were detected on an UV–Vis spectropho-tometer and BaSO₄ was used as background. The degradation of MO in aqueous solutions was tracked using an UV–Vis spectroscopy (Cary 8454, America).

Photocurrent tests were analyzed with an electro-chemical analyzer (CHI760 D, CHI Shanghai, Inc.). The detection was operated in a conventional three-electrode unit with counter electrode, working electrode and reference. A Pt wire and Hg/Hg₂Cl₂ (in saturated KCl) were used as the counter electrode and reference electrode, respectively. The working electrodes were the as-prepared samples (0.1 mg) having an active area of 0.5 cm². 0.1 M Na₂SO₄ was used as the electrolyte and irradiation proceeded with a LED lamp (30 W).

EIS measurements were performed in a 0.1 M KCl solution containing 5 mM Fe(CN)₆ ³⁻/(Fe(CN)₆ ⁴⁻ with a frequency range from 0.01 to 100 kHz at 0.24 V, and the amplitude of the applied sine wave potential in each case was 5 mV. The modified electrode was prepared in a simple method as follows: 5 mg g-C₃N₄ was dispersed 1 mL distilled water to make a g-C₃N₄ homogeneous suspension. Then, 20 μL of the slurry was dripped on the ITO glass with a 1 cm × 0.5 cm area and dried at 60 °C for 8 h (denoted as g-C₃N₄/ITO). In₂O₃/C₃N₄/ITO with the same quantities was prepared with the same procedure.

The photodegradation of MO was conducted in Pyrex glass vessel with a 300 W xenon lamp as light source (≥400 nm). Typically, 0.10 g of the as-prepared catalyst was dispersed in 100 ml MO solution (15mg L⁻¹) under ultrasonication for 10 min. Then, the mixture was stirred in a dark environment for 0.5 h to get an adsorption–desorption equilibrium. Moreover, 4 ml solution was taken per 30 min and then centrifuged to remove the photocatalyst particles. The absorbance of MO in aqueous solutions was tracked by UV–Vis spectroscopy at 464 nm.

The contents of In₂O₃ in In₂O₃/C₃N₄ composite material can be obtained by TG analysis and the curves were shown in Fig. 1. The mass of the pure In₂O₃ had no change within the range from 25 °C to 800 °C and In₂O₃ had good thermal stability. The weight of pure C₃N₄ decreased rapidly from 550 °C to 740 °C, indicating the thermal decomposition of C₃N₄ to generate small molecule volatile substances. The weight of In₂O₃/C₃N₄ composites decreased rapidly between 500 °C and 720 °C, corresponding to the system of C₃N₄ thermal decomposition. For In₂O₃/C₃N₄ composites, decomposition temperature of C₃N₄ was lower than that of pure C₃N₄. The result shows that it was easier to make C₃N₄ thermal decomposition due to the existence of In₂O₃, which was a oxidation catalyst that can absorb and support reactive oxygen in the air [31], and then oxidize the g-C₃N₄ at a relatively low temperature.

XRD was applied to study the phase structures of C₃N₄, In₂O₃, and In₂O₃/C₃N₄ composites (Fig. 2). For C₃N₄, there were two apparent diffraction peaks at 27.3° and 13.1°, which were correspond to the typical peak of aromatic systems as the (002) peak for graphitic materials due to interlayer stacking, and the interplanar separation as the (100) peak, respectively [32]. For In₂O₃, six distinct diffraction peaks at 21.5°, 30.6°, 35.6°, 45.4°, 50.9°, and 60.8° can be observed, which were assigned to the (211), (222), (400), (431), (440), and (622) crystal planes (JCPDS No.71-2194), respectively [30]. In XRD patterns, In₂O₃/C₃N₄ composites exhibited the typical peaks of In₂O₃ and C₃N₄.

The morphology and structure of In₂O₃, C₃N₄ and In₂O₃/C₃ N₄ (17.3%) composite were analyzed by TEM (Fig. 3). The size of In₂O₃ nanoparticles ranged from 50 nm to 200 nm (Fig. 3A) and C₃N₄ displayed lamellar structure (Fig. 3B). For In₂O₃/C₃N₄ (17.3%) composite (Fig. 3C), the grey area can be assigned to C₃N₄ and black particles can be assigned to In₂O₃ nanoparticles. In order to further study the construction of In₂O₃/C₃ N₄ (17.3%) composites, the enlarged TEM image (Fig. 3D) was obtained from the black box of Fig. 3C. It can be observed that In₂O₃ nanoparticles tightly attached on the surface and at edge of C₃N₄. Although the samples had been sonicated for 3 h before TEM, In₂O₃ nanoparticles were not peeled from the surface of C₃N₄, suggesting that there was a strong interaction between the In₂O₃ and C₃N₄. According to TEM analysis results, the heterojunction structure formed between In₂O₃ and C₃N₄, which played an important role in the charge transfer of the compound semiconductor materials. The morphologies of the In₂O₃/C₃N₄ (17.3%) composite were observed by SEM (Fig. 3E), in which In₂O₃/C₃N₄ (17.3%) composite showed sheet-shaped structure. The chemical composition of In₂O₃/C₃N₄ (17.3%) composite was further conformed by EDS analysis. As shown in Fig. 3F, In, O, C and N elements can be detected in this spectrum.

The element chemical valence on the surface of In₂O₃, C₃N₄ and In₂O₃/C₃N₄ (17.3%) composite can be obtained by XPS. In the full spectrum, it can be observed that In₂O₃/C₃ N₄ (17.3%) composite contained In, O, C and N (Fig. 4A). High-resolution XPS analyses of In3d, O1s, C1s and N1s were used to determine the valence state of each element in In₂O₃, C₃N₄ and In₂O₃/C₃N₄ (17.3%) composite (Fig. 4B, C, D, E). The peak of In3d binding energies at 444.4 eV and 452.0 eV (Fig. 4B) can be ascribed to the In 3d_3/2 and In 3d_5/2, which was consistent with the spectra of In₂O₃ reported on literatures [33], and it was the characteristic binding energies of In³⁺. Whereas, the peaks of the In 3d_3/2 and In 3d_5/2 in the In₂O₃/C₃ N₄ (17.3%) composite were at 444.9 eV and 452.5 eV, which both moved to a higher energy direction. The interaction between In₂O₃ and C₃N₄ may result in such a shift. In Fig. 4C, for In₂O₃, O1s peak centered at 530.0 eV and 532.3 eV, which was ascribed to the lattice oxygen and the surface hydroxyl oxygen of In₂O₃, respectively [34]. For In₂O₃/C₃ N₄ (17.3%) composite, the O1s peak was much weaker than that of In₂O₃. It may be attributed to the existence of C₃N₄. For C₃N₄, the peak of C1s at 288.2 eV was contributed to a C–N coordination (Fig. 4D). Another peak at 284.7 eV was corresponding to surface carbon [9]. The C1s peak of In₂O₃/C₃ N₄ (17.3%) composite showed a slight shift compared with pure C₃N₄, which was attributed to the interaction between In₂O₃ and C₃N₄. For both C₃N₄ and In₂O₃/C₃ N₄ (17.3%) composite, N1s (peaked) peak at 398.8 eV corresponded to sp² -hybridized nitrogen (C = N-C) (Fig. 4E), indicating the existence of carbon nitride [30].

FT-IR spectra of In₂O₃, C₃N₄ and In₂O₃/C₃N₄ composites were recorded in Fig. 5. For C₃N₄,

the absorption peaks of 1240 cm⁻¹, 1321 cm⁻¹, 1558 cm⁻¹, and 1635 cm⁻¹ were assigned to the characteristic stretching modes of the C-N heterocycle [35] and the typical peak at 806 cm⁻¹ was assigned to triazine units [36]. For In₂O₃, a characteristic absorption peak at 562 cm⁻¹ was observed, which was assigned to the presence of In-O phonon vibration [37]. For In₂O₃/C₃N₄ composites, both the bands of C₃N₄ and In₂O₃ can be observed and the characteristic absorption peak of In₂O₃ became more and more obvious with the increasing content of In₂O₃. Therefore, on the basis of XRD, XPS and FT-IR analysis, the presence of In₂O₃ and C₃N₄ species was confirmed in composites.

Figure 6 showed the UV–Vis DRS of In₂O₃, C₃N₄ and In₂O₃/C₃N₄ composites. The absorption band edge was about 440 nm and 460 nm for In₂O₃ and C₃N₄ [38, 39], respectively. Compared with In₂O₃, the absorption bands edge of In₂O₃/C₃N₄ composites red-shifted and showed clear visible light response. The red-shifting of absorption band edge meant that the photocatalysis can produce more electron and holes under the stimulation of the visible light, which was beneficial to enhance the catalytic activity. The forbidden band width of semiconductor photocatalytic materials can be presented as followed formula:

where α is absorption coefficient, h is Planck constant, ν is frequency of light, A is semiconductor material constants, Eg is forbidden band width and n is constant (usually direct bandwidth material n = 1/2, indirect bandwidth material n = 2 [40]). In₂O₃ belongs to direct bandwidth material and C₃N₄ belongs to indirect bandwidth material, so the value of n take 1/2 and 2, respectively. The E_g of C₃N₄ and In₂O₃ can be estimated form the forbidden band width spectra of In₂O₃ and C₃N₄ (Fig. 6b), which was drew by using (αhν)^1/n as ordinate and hν as abscissa. The intersection point of the tangent to the curve and abscissa is the Eg of material. The estimated forbidden band width of In₂O₃ and C₃N₄ were 2.78 eV and 2.70 eV. The result was in agreement with the literature [41, 42].

The organic pollutant MO was chosen to evaluate the photocatalytic performance of In₂O₃/C₃N₄ composites, and the result was shown in Fig. 7. The photocatalytic performance of In₂O₃ and C₃N₄ were comparatively low, only 12.1% and 20.8% MO were degraded when the irradiation time lasted for 3 h (Fig. 7A), respectively. However, most of the In₂O₃/C₃N₄ composites displayed much higher photocatalytic activity than C₃N₄ and In₂O₃. Upon increasing the content of In₂O_3, the photocatalytic activity first increased and then decreased. In₂O₃/C₃ N₄ (17.3%) composite showed the optimal photocatalytic performance among all composites. The reason may be that when the In₂O₃ content was too high, In₂O₃ nanoparticles had self-agglomeration and reduce the contact interface and heterojunction structure between In₂O₃ and C₃N₄. It leads to the decrease of electrons and holes separation efficiency, which weakens the photocatalytic activity. The removal rate of MO was about 61% for In₂O₃/C₃ N₄ (17.3%) after 3 h. The temporal evolution of the absorption spectra of MO catalyzed by In₂O₃, C₃N₄ and In₂O₃/C₃ N₄ (17.3%) composites is illustrated in Fig. 7B, C, D. For C₃N₄ or In₂O₃, the MO degradation was quite slow and the MO could not be distinctly degraded after 3 h (Fig. 7B, C). However, the absorption peak at 464 nm decreased obviously within 3 h (Fig. 7D) for In₂O₃/C₃ N₄ (17.3%) composite, which indicated that In₂O₃/C₃ N₄ (17.3%) sample exhibited high photocatalytic activity. Therefore, the photocatalytic activity of both C₃N₄ and In₂O₃ were lower than In₂O₃/C₃ N₄ (17.3%) sample under the experimental conditions.

The photoproduction electron holes efficiency of In₂O₃, C₃N₄ and In₂O₃/C₃ N₄ (17.3%) composite can be performed through the transient photocurrent response experiment. As shown in Fig. 8A, the samples can rapidly generate photocurrent when the light is on, suggesting that there were charge carriers in samples. The photocurrent of In₂O₃/C₃N₄ (17.3%) composite was higher than that of In₂O₃ and C₃N₄. The photocurrent response to In₂O₃/C₃ N₄ (17.3%) composite was 11.3 times and 183.1 times as that of pure In₂O₃ and C₃N₄, respectively. The result showed that In₂O₃/C₃ N₄ (17.3%) composite had low electron–hole recombination rate and also confirmed that the introduction to In₂O₃ in C₃N₄ can effectively improve the electron–hole separation efficiency to enhance the photocatalytic activity. According to this result, the formation of heterojunction was beneficial to the charge separation and enhancement of photocatalytic activity [43].

EIS measurement was also employed to investigate the charge transfer and the separation efficiency between the photogenerated electrons and holes. From Fig. 8B, it could be observed that the diameter of the Nquist circle of In₂O₃/C₃ N₄ (17.3%) composite was smaller than that of C₃N₄. It  indicated that In₂O₃/C₃N₄ (17.3%) composite had lower resistance than that of C₃N₄. This result confirmed that the introduction In₂O₃ into C₃N₄ can enhance the separation and transfer efficiency of photogenerated electron–hole pairs [44], which was benefit to the enhancement of photocatalytic activity.

Except photocatalytic efficiency, another important evaluation criterion of catalyst is the stability. The cyclic photocatalytic degradation of MO was carried out to investigate the recyclability of In₂O₃/C₃N₄ and the result was shown in Fig. 9. The photocatalytic activity of In₂O₃/C₃ N₄ (17.3%) showed a slight loss after 3 recycles, which was ascribed to the loss of photocatalyst.

Photoluminescence (PL) emission spectra of C₃N₄ and In₂O₃/C₃ N₄ (17.3%) composite was shown in Fig. 10 to study the efficiency of charge transfer and electron–hole pairs separation. For the pure C₃N₄, an emission peak at about 460 nm was attributed to the band gap energy of C₃N₄ (2.7 eV) [45]. In addition, the peak of In₂O₃/C₃ N₄ (17.3%) composite was weaker than that of pure C₃N₄, suggesting the efficient inhibition of the recombination between charge carriers. The decrease of electron–hole recombination enhanced the photocatalytic activity for degradation of pollution.

In order to investigate the mechanism, reactive species trapping experiments were carried out to investigate the main active oxygen species during the degradation process. In₂O₃/C₃ N₄ (17.3%) was selected for testing. As shown in Fig. 11, when N₂ was used to scavenger ·O₂ ⁻, the photocatalytic activity decreased a lot. This phenomenon indicated that ·O₂ ⁻ played an important role in the degradation process. The scavenger triethanolamine (TEOA) was chosen for catching holes (h⁺) and the photocatalytic activity was significantly decreased, which meant that h⁺ also played a role in the degradation. However, the photocatalytic performance did not change with the addition of isopropanol (IPA) as the scavenger for ·OH. The result suggested that ·OH was not the major oxidative species in the photocatalytic reaction. In conclusion, the reactive species were ·O₂ ⁻ and h⁺ during the photo-degradation process.

The high efficiency of charge separation was an important factor for the significant enhancement of photocatalytic activity. However, effectively separation of electron and holes depended on whether the two kinds of semiconductor materials had suitable position of band gaps. It was conducive to the separation of electrons and holes due to the suitable band gap structure of two kinds of semiconductor materials [44]. According to the structure characteristics and the photocatalytic performance tests of C₃N₄/In₂O₃ composite, a possible mechanism was presented and showed in Fig. 12. For semiconductors, the conduction bands and valence bands can be expressed as following equation:

where χ is absolute electronegativity of the semiconductor (χ is 5.28 eV for In₂O₃ [46]), E^C is energy of free electrons on the hydrogen scale (−4.5 eV [47]), E_g is band gap of the semiconductor. According to Fig. 6B, the band gaps of C₃N₄ and In₂O₃ were 2.70 eV and 2.68 eV, respectively. The calculated top VB and bottom CB of In₂O₃ were 2.12 eV and −0.56 eV, respectively. For C₃N₄, the top VB and the bottom CB were 1.57 eV and −1.13 eV, respectively [44]. Upon irradiated by visible light (>400 nm), both In₂O₃ and C₃N₄ can be activated in the composites. Since CB of C₃N₄ (−1.13 eV) was lower than that of In₂O₃ (−0.56 eV), the excited electrons at the CB of C₃N₄ crystallites could transfer to the CB of In₂O₃ crystallites and the photogenerated holes on In₂O₃ could move to C₃N₄ via the hybrid interface. Thus, this suppressed the charge recombination easily, which resulted in the enhancement of photocatalytic performance.