Microwave Synthesis of Visible-Light-Activated g-C3N4/TiO2 Photocatalysts

Abstract

The preparation of visible-light-driven photocatalysts has become highly appealing for environmental remediation through simple, fast and green chemical methods. The current study reports the synthesis and characterization of graphitic carbon nitride/titanium dioxide (g-C₃N₄/TiO₂) heterostructures through a fast (1 h) and simple microwave-assisted approach. Different g-C₃N₄ amounts mixed with TiO₂ (15, 30 and 45 wt. %) were investigated for the photocatalytic degradation of a recalcitrant azo dye (methyl orange (MO)) under solar simulating light. X-ray diffraction (XRD) revealed the anatase TiO₂ phase for the pure material and all heterostructures produced. Scanning electron microscopy (SEM) showed that by increasing the amount of g-C₃N₄ in the synthesis, large TiO₂ aggregates composed of irregularly shaped particles were disintegrated and resulted in smaller ones, composing a film that covered the g-C₃N₄ nanosheets. Scanning transmission electron microscopy (STEM) analyses confirmed the existence of an effective interface between a g-C₃N₄ nanosheet and a TiO₂ nanocrystal. X-ray photoelectron spectroscopy (XPS) evidenced no chemical alterations to both g-C₃N₄ and TiO₂ at the heterostructure. The visible-light absorption shift was indicated by the red shift in the absorption onset through the ultraviolet-visible (UV-VIS) absorption spectra. The 30 wt. % of g-C₃N₄/TiO₂ heterostructure showed the best photocatalytic performance, with a MO dye degradation of 85% in 4 h, corresponding to an enhanced efficiency of almost 2 and 10 times greater than that of pure TiO₂ and g-C₃N₄ nanosheets, respectively. Superoxide radical species were found to be the most active radical species in the MO photodegradation process. The creation of a type-II heterostructure is highly suggested due to the negligible participation of hydroxyl radical species in the photodegradation process. The superior photocatalytic activity was attributed to the synergy of g-C₃N₄ and TiO₂ materials.

Keywords: g-C3N4/TiO2; heterostructures; microwave synthesis; photocatalysis; pollutant degradation.

Figure 1

Schematic diagram for the microwave synthesis of g-C₃N₄/TiO₂ heterostructures. The schematic in the orange box represents the first steps for the synthesis of g-C₃N₄/TiO₂ heterostructures, while the schematic in the green box represents the last steps for the synthesis of g-C₃N₄/TiO₂ heterostructures.

Figure 2

XRD diffractograms of the pure TiO₂, pure g-C₃N₄ and heterostructures composed of TiO₂ with different weight loading percentages of g-C₃N₄ (15-GCN-T, 30-GCN-T and 45-GCN-T). The simulated brookite, rutile and anatase TiO₂ are also shown for comparison. Orange arrows indicate the diffraction maximum at 27º likely associated with the presence of graphitic carbon nitride.

Figure 3

SEM images of the produced materials (a) TiO₂, (c) g-C₃N₄, (e) 15-GCN-T, (g) 30-GCT and (i) 45-GCN-T. The respective high-magnification SEM images are shown in (b,d,f,h,j). (b) also displays an amplified SEM image of a hollow TiO₂ sphere.

Figure 4

(a) Secondary electron (SE) STEM image of the 30-GCN-T material, (b) Bright-field (BF) STEM image of the same area of (a), (c) HAADF-STEM image of the area in (a,d) Bright-field TEM image of the area in (a). The insets in (d) depict the electron diffraction pattern of TiO₂ nanoparticles with the anatase phase and the particle size distribution of the TiO₂ nanoparticles measured by TEM analyses. (e,f) Magnified SE-STEM and HAADF-STEM images of the area analyzed in (a), respectively. (g) SE-STEM and (h) HAADF-STEM image of a TiO₂ nanocrystal attached to the g-C₃N₄ sheet, and (i) atomic-resolution HAADF-STEM image of the area in (g,h), where the interface between the TiO₂ nanocrystal and the g-C₃N₄ sheet is clear. The inset in (i) shows the FFT image of the area indicated as A (white square).

Figure 5

(a,b) Atomic-resolution HAADF-STEM images of two distinct TiO₂ nanocrystals. The insets in (a,b) show the FFT images of areas indicated as B and C, respectively (black squares).

Figure 6

Artificially colored (mixed) SE-STEM image of the 30-GCN-T material (a), together with the corresponding EDS maps of C (b), N (c), O (d) and Ti (e).

Figure 7

(a) AFM image of a g-C₃N₄ nanosheet and (b) corresponding average height values measured between the red lines.

Figure 8

(a) XPS survey spectra of TiO₂, g-C₃N₄ and 30-GCN-T materials. (b) Deconvolution of XPS C 1s of TiO_2, g-C₃N₄ and 30-GCN-T spectra. (c) Deconvolution of XPS O 1s spectra of TiO_2, g-C₃N₄ and 30-GCN-T. (d) XPS Ti 2p spectra of TiO₂ and 30-GCN-T. (e) Deconvolution of XPS N 1s spectra of g-C₃N₄ and 30-GCN-T.

Figure 9

(a) RT absorption spectra of TiO₂, g-C₃N₄ and 30-GCN-T nanopowders. The inset shows the Kubelka–Munk plots (from DRS spectra of TiO₂ and g-C₃N₄ nanopowders). For the determination of the optical band gap values, TiO₂ and g-C₃N₄ materials were considered as indirect band gap semiconductors. (b) RT PL spectra of TiO₂, g-C₃N₄ and 30-GCN-T nanostructures from 400 to 600 nm using an excitation wavelength of 350 nm. The inset shows the RT PL spectra (normalized intensity as a function of the wavelength from 428 to 470 nm) of g-C₃N₄ and 30-GCN-T nanostructures.

Figure 10

(a) Degradation curves (C/C₀ as a function of the exposure time) under solar simulating light up to 4 h without photocatalyst (photolysis) and for TiO₂, g-C₃N₄, 15-GCN-T, 30-GCN-T,45-GCN-T photocatalysts. The lines are for eye guidance only. (b) Pseudo-first-order kinetics for MO degradation in the presence of TiO₂, g-C₃N₄, 15-GCN-T, 30-GCN-T and 45-GCN-T photocatalysts. The lines represent the linear fittings of the pseudo-first-order kinetics equation.

Figure 11

(a) MO degradation curves (C/C₀ as a function of the exposure time) in the presence of 30-GCN-T heterostructures up to 4 h under five consecutive cycles. The lines are for eye guidance only. (b) Comparison of the degradation rates of MO under solar simulating light using the 30-GCN-T heterostructure with no scavengers (NS) and 5 mL of water, and in the presence of trapping reagents (EDTA, BQ and IPA).

Figure 12

Mott–Schottky analyses performed at 1 kHz in 0.5 M Na₂SO₄ electrolyte for TiO₂ and g-C₃N₄. The potentials were measured against the Ag/AgCl reference. The flat band potentials (E_fb) can be estimated from the linear portion of the graphs, represented in black and red lines, respectively, for TiO₂ and g-C₃N₄.

Figure 13

Illustrated mechanism of the photocatalytic activity of the 30-GCN-T heterostructure under solar simulating light. The potentials (V) relative to NHE scale are also represented. Black and red dashed lines correspond to the CB/VB potentials of TiO₂ and g-C₃N₄, respectively. Purple dot lines are related to the redox potentials of the common reactive species in photocatalysis. (a) represents a possible Z-scheme photocatalytic degradation mechanism of the 30-GCN-T heterostructure, and (b) represents a possible type-II photocatalytic degradation mechanism of the 30-GCN-T heterostructure.