Hydrothermal synthesis of novel 1-aminoperylene diimide/TiO₂/MoS₂ composite with enhanced photocatalytic activity

Abstract

1-aminoperylene diimide/TiO₂/MoS₂ composite (NH₂-PDI/TiO₂/MoS₂) with ordered structure was prepared by hydrothermal synthesis method. The composite was characterized by XRD, SEM, FTIR, XPS, BET, DRS, PL, EIS, Raman, photocurrent, and Mott-Schottky plots spectroscopy. The potential positions of the conduction and valence bands, and the band gap energy of the semiconductors were estimated. The composite exhibited higher photocatalytic activity compared with the mono-component systems. The apparent rate constants (k) were determined as 0.00616, 0.00352, 0.00738, 0.00517, 0.00752, and 0.00806 min⁻¹ for TiO₂, NH₂-PDI, NH₂-PDI/TiO₂, MoS₂, MoS₂/TiO₂, and NH₂-PDI/TiO₂/MoS₂, respectively. The detection of radical scavengers confirmed that superoxide radicals, photogenerated holes, and photogenerated electrons were the main active substances for MB degradation. Between type II- heterojunction mechanism and Z-scheme mechanism, the latter could explain the enhanced photocatalytic activity of the composite better. The Z-scheme mechanism accumulates more electrons at CB level of NH₂-PDI and hence generates more super oxide radicals.

Introduction

Water pollution is an important issue of concern in many countries. Many dyes in textile printing or dyeing wastewater contain aromatic compounds, which are chemically stable and harmful to human health¹. Because of the adverse environmental impacts of these refractory organic compounds, it is necessary to develop new methods to degrade them. Photocatalysis is a promising technique for photodegradation of hazardous chemicals in wastewater². Titanium dioxide (TiO₂) and TiO₂ based nanostructures are being used for photocatalytic degradation of organic pollutants to protect environment³. With the advantages of low cost, large specific surface area, high oxidizing power, and good chemical stability, TiO₂ has become one of the most promising candidates for photocatalysis⁴. However, the catalytic performance of TiO₂ is severely limited by the large band gap (3.2 eV), low photon utilization rate of solar energy (about 5%), and high recombination rate for photogenerated electron–hole pairs^5,6. On the other hand, the industrial treatment of wastewater containing various organic pollutants using TiO₂-photocatalyst is not common due to low efficiency of photodegradation⁷. To solve this problem, several methods have been used to improve the photocatalytic efficiency of TiO₂. It has been reported that in a suitable heterostructured system, the presence of heterojunction changes the energy band positions and their inclination on the interface to accelerate the migration of photogenerated charge carriers, and eventually enhance the efficiency of photocatalyst⁸.

Perylenetetracarboxylic acid diimides (PDIs) are cheap organic dyes with high photothermal stability and strong absorption in the visible region^9,10. As a typical n-type semiconductor, PDIs have high electron mobility and electron affinity [LUMO] due to its strong π–π stacking between the conjugated π bonds^11,12. PDIs materials have been used in photocatalysis because it can improve the light absorption and decrease the photo-generated electron–hole recombination¹³. PDIs have been introduced into TiO₂, ZnO, and other photocatalysts to improve the photocatalytic performance^14,15. However, such composite materials still have some defects such as quick recombination of photoexcited electron–hole pairs and photoetching, which lead to low efficiency of visible light driven photodegradation¹⁶. Therefore, it is necessary to further improve their photocatalytic activities and stabilities.

Molybdenum disulfide (MoS₂) is a rapidly rising two-dimensional layered material, which has aroused enormous scientific interest in developing new MoS₂-based materials for rich potential applications¹⁷. MoS₂ with different structures such as nanoribbon and nanosheet has been successfully synthesized^18,19. It has unique physical properties that distinguish it from other materials and has been used in many applications such as lubricant, photovoltaic, and photocatalysis^20,21. Hu et al. synthesized bulk, nano-slice, and nano-ball MoS₂ by precipitation method, and found that the photocatalytic efficiency of nano-slice MoS₂ towards methyl orange (90%) was higher than others²². James et al. synthesized MoS₂ nanoparticles through thermally decomposing method, and the material was used for photocatalytic degradation of methylene blue (MB) with an efficiency of 30%²³. Xu et al. prepared flower-like MoS₂ nanopowders through hydrothermal method for photocatalytic degradation of Rhodamine B (RhB) and an efficiency of 15% was achieved²⁴. In addition, the good conductivity of monolayer MoS₂ can efficiently separate the electron–hole pairs and enhance the photocatalytic activity. Hence, MoS₂ can be used in TiO₂ photocatalyst to improve its photocatalytic performance. For instance, Behzad Pourabbas et al. reported the photo-oxidation of phenol by MoS₂/TiO₂ hybrids. Suddhasatwa Basu et al. demonstrated the ability to degrade RhB by MoS₂/TiO₂ nanocomposite²⁵. Giri et al. reported the photo-oxidation of RhB by MoS₂/TiO₂ hybrids²⁶. Du et al. reported a MoS₂/CdS/TiO₂ photocatalyst which has high photocatalytic activity and stability²⁷. This technique produces powerful non-selective oxidants, such as superoxide radicals and hydroxyl radicals, which can degrade and mineralize a wide variety of pollutants²⁸. The produced electrons-holes would recombine in pure TiO₂, which limited its application in the photodegradation. By preventing electron/hole recombination and semiconductors aggregation, multi-component support can improve the photoreactivity of the obtained composite²⁹.

In this work, a novel ternary photocatalyst: NH₂-PDI/TiO₂/MoS₂ was synthesized by hydrothermal method (Scheme 1). The CB/VB (conduction band/valence band) of TiO₂, NH₂-PDI, and MoS₂ are evaluated to be − 1.26/1.72 eV, − 1.56/0.8 eV, and -0.75/1.00 eV, respectively. These matched potentials are suitable for rapid charge separation between these semiconductors. The composite showed enhanced activity in the photodegradation of Methylene Blue (MB) and amoxicillin (AMX).

Scheme 1

Molecular structure of 1-aminoperylene diimide (NH₂-PDI).

Experimental

Materials and methods

All other chemicals were analytically pure, purchased from commercial sources and used without further purification. Distilled water was used throughout the experiment. FT-IR spectrum has been studied by a Bruker Tensor-27 spectrophotometer. The ¹H NMR and mass spectrum of NH₂-PDI were measured on a Bruker Advance 400 spectrometer and a Bruker Maxis UHR-TOF mass spectrometer, respectively. Scanning electron microscopy (SEM) images were obtained on a Sigma, Zeiss microscope equipped with an energy dispersive X-ray (EDX) spectrometer. The specific surface area of the catalyst has been analyzed using nitrogen adsorption at 77 K applying the Brunauer–Emmett–Teller (BET) method using a micrometrics ASAP 2020 V3.00 H. X-ray diffraction (XRD) measurements have been performed using a Rigaku R-AXIS RAPID X-ray diffractometer. Crystallinity and phase composition of the as-synthesized composite have been confirmed from the Lab Ram HR800, Jobin Yvon micro-Raman measurement. The ultraviolet–visible diffuse reflectance absorption spectra (DRS) have been measured on a Shimadzu model UV–Vis diffuse reflectance spectrometer. X-ray photoelectron spectroscopy (XPS) has been performed on a KRATOS model XSAM800 instrument. Fluorescence measurements were performed at a Hitachi FL-4500 spectrometer with 290 nm excitation wavelength at room temperature. The Mott-Schottky plots, photocurrent, and electrochemical impedance spectroscopy (EIS) were measured on a CHI760E electrochemical workstation using a three-electrode system. Platinum wire as counter electrode, ITO deposited by photocatalyst was used as working electrodes (50 mg of photocatalyst was dispersed in 2 mL of ethanol and grounded for 20 min, then it was uniformly dropped onto the FTO glass and stay still overnight to dry off the ethanol.), saturated calomel as reference electrode, and 0.1 M Na₂SO₄ aqueous solution was used as electrolyte. The EIS was performed at an open circuit potential at a frequency of 0–10,000 Hz, and the photoelectric response of the sample was measured at 0.0 V. Photoelectrochemical properties were measured with a 300 W xenon lamp as the light source.

Synthesis and characterization of NH₂-PDI

The compound NH₂-PDI was synthesized according to literature method³⁰. The synthetic route along with the characterization data of NH₂-PDI (Fig. S-1) are reported in the Supplementary Methods.

Preparation of TiO₂ and NH₂-PDI/TiO₂

The NH₂-PDI/TiO₂ composite was prepared by the hydrothermal synthesis method. First, tetrabutyl titanate (10.0 mL) was dissolved in anhydrous ethanol (20.0 mL). Then, 5.0 mL of distilled water was slowly added under vigorous stirring. Later, NH₂-PDI (0.01 g) was dissolved in dichloromethane (5.0 mL) and added to the solution under sonication. The resulting mixture was stirred for 12 h and then transfer to the hydrothermal kettle for 3 h at 200 °C. The precipitate was filtered, washed thoroughly with distilled water, and dried in an air oven at 100 °C for 4 h. This catalyst contained 0.1 wt% of NH₂-PDI. Pure TiO₂ was prepared by the same method.

Preparation of MoS₂

MoS₂ was synthesized by hydrothermal route using Na₂MoO₄·2H₂O and NH₂CSNH₂ as Mo and S sources, respectively. Briefly, Na₂MoO₄·2H₂O (2.0 g) and NH₂CSNH₂ (8.0 g) was dissolved in 60 mL of water under vigorous stirring for 30 min. The mixture was kept in a hydrothermal kettle at 200 °C for 24 h. The resulting black precipitate was washed several times with ethanol and water to further remove impurities and contaminants followed by a centrifugation and drying process at 60 °C for 6 h to obtain MoS₂ nanoarchitectures.

Preparation of MoS₂/TiO₂ and NH₂-PDI/TiO₂/MoS₂

The NH₂-PDI/TiO₂/MoS₂ composite was fabricated by loading MoS₂ onto the preformed NH₂-PDI/TiO₂ solid solutions. Hydrothermal synthesis of NH₂-PDI/TiO₂/MoS₂ composite was prepared as follows: 3.0 g NH₂-PDI/TiO₂ and 0.06 g of MoS₂ were dispersed in 5 mL water. The resulting mixture was stirred for 24 h and then transferred to a hydrothermal kettle at 200 °C for 4 h. The prepared composite material was filtered, washed thoroughly with distilled water, and dried in an air oven at 100 °C for 4 h. NH₂-PDI/TiO₂/MoS₂ was formed as a gray powder. This catalyst contained 0.1 wt% NH₂-PDI and 2.0 wt% MoS₂. MoS₂/TiO₂, NH₂-PDI/TiO₂/1%MoS₂, and NH₂-PDI/TiO₂/3%MoS₂ were prepared using the same procedure.

Photocatalytic activity tests

The photocatalytic experiments were carried out in a photochemical reactor (PhchemIII, Beijing China NBeT) consisting of a 500 W xenon lamp (XE-JY500). The reaction chamber is equipped with 50 mL capacity reaction glass tubes and a magnetic stirrer. The specially designed reflector was made of highly polished aluminum and a built-in cooling fan. The light exposure length is 230 mm.

Methylene Blue (MB) and amoxicillin (AMX) were selected to test the photodegradation activity of the prepared photocatalysts. First, 50 mg of synthesized sample was suspended in 50 ml of 10 mg/L MB aqueous solution. The visible light source was obtained by a 500 W xenon lamp. Before irradiation, the suspension solutions were stirred magnetically for 120 min in the dark to achieve adsorption–desorption equilibrium. The samples were withdrawn at given time intervals and the photocatalyst was removed by centrifugation. A UV–vis spectrophotometer at 664 nm and 198 nm was used to measure the absorbance of MB and AMX solutions, respectivly.

Results and discussion

Figure 1 shows SEM images of TiO₂, NH₂-PDI, NH₂-PDI/TiO₂, MoS₂, MoS₂/TiO_2, and NH₂-PDI/TiO₂/MoS₂. Figure 1(a) shows that TiO₂ forms uniform elongated spherical particles with a length of about 300 nm and a width of about 200 nm. As can be seen from Fig. 1(b), NH₂-PDI forms nanorods structure. The average width is 1 μm, and the length is in the range of 1–3 μm. Figure 1(c) depicts that NH₂-PDI/TiO₂ particles formed an agglomerated spherical structure. As shown in Fig. 1d, the average diameter of MoS₂ nanoflowers is 0.5–1 μm. These nanoflowers have massive petals on the surfaces, which are free to gather closely. Figure 1(e) and (f) shows the morphology of the synthesized MoS₂/TiO₂ heterostructure characterized by SEM. It can be seen that MoS₂ and TiO₂ mixed together. In Fig. 1(h) and (i), a peculiar morphology of NH₂-PDI/TiO₂/MoS₂ is shown. The NH₂-PDI and MoS₂ mixed with TiO₂, and they can provide more active sites and mass charge transport pathways in the catalytic system. The color of MoS₂/TiO₂ and NH₂-PDI/TiO₂/MoS₂ are shown in Fig. 1(g) and (j), respectively. MoS₂/TiO₂ was light grey while NH₂-PDI/TiO₂/MoS₂ sample showed a red grey color.

Figure 1

SEM images of TiO₂ (a), NH₂-PDI (b), NH₂-PDI/TiO₂ (c), MoS₂ (d), MoS₂/TiO₂ (e,f), NH₂-PDI/TiO₂/MoS₂ (h,i), and the appearance of MoS₂/TiO₂ (g), NH₂-PDI/TiO₂/MoS₂ (j).

The composition of the NH₂-PDI/TiO₂/MoS₂ was further investigated by an EDX attached to SEM (Fig. 2). Figure 2(b)–(h) shows the element mappings in a selected area of the composite (Fig. 2a). The homogeneous distributions of the elements Mo, S, C, N, Ti, and O can be clearly seen from the graphs. As shown in Fig. 2(i) and Fig. S-2, EDX analysis reveals that the composite contains Ti and O (in the case of TiO₂), Mo and S (in the case of MoS₂), or C and N (in the case of NH₂-PDI). The atomic ratio of Ti, Mo and C equals to 25.2: 1: 1.3, meaning that the molar ratio of TiO₂, MoS₂, and NH₂-PDI is about 95.3: 1.89: 0.68. The high proportion of C is due to the elemental carbon and carbonate species adsorbed on the TiO₂ surface.

Figure 2

(a) SEM image of NH₂-PDI/TiO₂/MoS₂ photocatalyst; Element mappings of Mo, S, C, N, Ti, and O elements (b), C (c), N (d), O (e), Mo (f), S (g), and Ti (h) in the NH₂-PDI/TiO₂/MoS₂ photocatalyst; (i) EDX spectrum of NH₂-PDI/TiO₂/MoS₂ photocatalyst.

XRD measurements were performed to determine the crystalline structures of TiO₂, NH₂-PDI, NH₂-PDI/TiO₂, MoS₂, MoS₂/TiO₂, and NH₂-PDI/TiO₂/MoS₂ composites (Fig. 3). The crystal phase of TiO₂ was consistent with anatase phase TiO₂ (JCPDS NO. 71–1167) (Fig. 3a). Diffraction peaks appeared at 2θ = 25.3°, 37.7°, 48.0°, 53.8°, 55.0°, 62.6°, 58.6°, 70.6°, and 75.2°, corresponding to (101), (311), (200), (105), (211), (204), (116), (220), and (241) planes of TiO₂, respectively³¹. The NH₂-PDI appeared in the corresponding d-spacing as 10.35 Å, 9.22 Å, 5.24 Å, and 3.43 Å (Fig. 3b). The peak at 2θ = 25.74° (d spacing 3.43 Å) can be attributed to the π-π stacking of the adjacent NH₂-PDI, since the distance of π-π stacking between the perylene nuclei was about 3.5 ų². It was well known that PDIs exhibit a typical sharp diffraction peak at about 8–9°, which was centered at 9.57° (d spacing 9.22 Å)³³. Additionally, X-ray powder diffraction measurements on NH₂-PDI showed that the first diffraction peak appeared at 2θ = 8.53° (d spacing 10.35 Å) and the second-order diffraction peak appeared at 2θ = 16.85° (d spacing 5.24 Å). The first diffraction peak was assigned as (001) and the second peak was assigned as (002) α-form crystal diffraction, which has been reported by Miyata³⁴. The multiple orders of reflection indicated that the self-assembled structures of NH₂-PDI are well-ordered and layered microstructures. For the pure 3D flower-like spherical MoS₂ nanostructure, the peaks correspond to (002), (100), (103), and (110) diffraction planes of 2H-MoS₂ (JCPDS: 37-1492), which is consistent with previous studies (Fig. 3d)³⁵. For MoS₂/TiO₂, the addition of MoS₂ did not change the diffraction peak positions of TiO₂ obviously, indicating that MoS₂ was not incorporated into the TiO₂ lattice. Obviously, both NH₂-PDI/TiO₂ and NH₂-PDI/TiO₂/MoS₂ formed ordered structures. Comparison of the XRD patterns in Fig. 3a,c,e,f confirms that the XRD patterns of the NH₂-PDI/TiO₂, MoS₂/TiO₂, and NH₂-PDI/TiO₂/MoS₂ nanoparticles have good consistency with the XRD data of anatase phase TiO₂ (JCPDS NO. 71–1167) at 2θ degrees of 25.3°, 37.7°, 48.0°, 53.8°, 55.0°, 62.6°, 58.6°, 70.6°, and 75.2°³⁶. Compare with pure TiO_2, the crystallization properties of the two composites were slightly weakened, which should be due to the binding of NH₂-PDI (Fig. 3c,f). Since the NH₂-PDI/TiO₂/MoS₂ composite was loaded with very little NH₂-PDI and MoS₂, the characteristic peaks of NH₂-PDI and MoS₂ did not appear in its XRD patterns (Fig. 3f). Additionally, compared with pure TiO₂, NH₂-PDI/TiO₂/MoS₂ exhibited a widened peak width. This may be caused by the formation of heterojunction between TiO₂, NH₂-PDI and MoS₂, and the heterojunction lead to lattice distortion of TiO₂. The average particle sizes of TiO₂, NH₂-PDI/TiO₂, MoS₂/TiO₂, and NH₂-PDI/TiO₂/MoS₂ calculated by Scherrer’s equation were 10.2 nm, 12.5 nm, 11.3 nm, and 13.6 nm, respectively³⁷.

Figure 3

XRD patterns of TiO₂ (a), NH₂-PDI (b), NH₂-PDI/TiO₂ (c), MoS₂ (d), MoS₂/TiO₂ (e), NH₂-PDI/TiO₂/MoS₂ (f).

Raman spectroscopy was applied to further check the phase and formation of the NH₂-PDI/TiO₂/MoS₂ nanocomposite. As shown in Fig. S-3, the pristine TiO₂ nanoparticles exhibit five peaks at 145.1 ± 0.2, 196.6 ± 0.1, 396.1 ± 0.2, 513.7 ± 0.2, and 638.1 ± 0.1 cm⁻¹, which belong to the E_g, E_g, B_1g, A_1g, and E_g anatase tetragonal vibration modes of TiO₂, respectively³⁸. The E_g, B_1g, and A_1g peaks correspond to symmetric stretching, symmetric bending, and antisymmetric bending vibrations of O-Ti–O, respectively. The results demonstrate the existence of typical anatase TiO₂ phase, and it is consistent with the XRD results. Characteristic peaks of MoS₂ (A_1g and E¹_2g modes) were observed in the Raman spectra of NH₂-PDI/TiO₂/MoS₂ photocatalyst, indicating that MoS₂ was loaded on the surface of TiO₂³⁹. Compared with pure TiO₂, NH₂-PDI/TiO₂/MoS₂ photocatalyst exhibited red shifts of E_g, E_g, A_1g, and E_g modes (145.6 ± 0.3, 197.8 ± 0.1, 514.1 ± 0.1, and 638.4 ± 0.1 cm⁻¹). This indicates that there are strong intimate interactions between TiO₂, MoS₂, and NH₂-PDI.

FT-IR measurements revealed the connection between TiO₂, NH₂-PDI and MoS₂ in NH₂-PDI/TiO₂/MoS₂ nanocomposite (Fig. S-4). As shown in Fig. S-4a, for pure TiO₂, the vibration modes observed at 617 cm⁻¹ and 749 cm⁻¹ are due to the O–Ti–O bending and Ti–O stretching vibrations, respectively. For MoS₂, the vibration mode at 1389 cm⁻¹ is assigned to Mo–S vibration, while the vibration mode at 1550 cm⁻¹ may be S–O asymmetric stretching (Fig. S-4d)⁴⁰. In MoS₂/TiO₂ and NH₂-PDI/TiO₂/MoS₂, all characteristic vibration modes were present but with very low Mo–S and S–O vibration modes, which indicates that the MoS₂/TiO₂ and NH₂-PDI/TiO₂/MoS₂ contain very little amount of MoS₂ (Fig. S-4e, f). Remarkably, the bending vibration of C-H at 2926–2817 cm⁻¹ observed in NH₂-PDI shifted to 2968–2875 cm⁻¹ in NH₂-PDI/TiO₂ and to 2992–2889 cm⁻¹ in NH₂-PDI/TiO₂/MoS₂, which may be an indicative of the binding between NH₂-PDI and TiO₂ or MoS₂. In addition, the stretching vibration of C = C in NH₂-PDI at 1645 cm⁻¹ shifted to 1639 cm⁻¹ in NH₂-PDI/TiO₂ and to 1627 cm⁻¹ in NH₂-PDI/TiO₂/MoS₂ nanocomposite. Compared with NH₂-PDI, the C=C absorption peak of NH₂-PDI/TiO₂/MoS₂ was blue shifted, indicating that the π-π interaction within NH₂-PDI decreased. The absorption peak around 3300 cm⁻¹ was caused by the stretching vibration of O–H bond, which was related to the atmospheric water adsorbed on the catalyst surface. As shown in Fig. 4, the Ti…O bond and S…H bond can be formed when NH₂-PDI meets with TiO₂ and MoS₂. After the addition of deionized water, NH₂-PDI self-assembled through intermolecular π–π stacking interactions. For NH₂-PDI, dichloromethane is a “good” solvent, while water is a “poor” solvent. When the “poor” solvent was added, the solubility of originally dissolved NH₂-PDI was limited, and NH₂-PDI self-assembled through non-covalent interactions and precipitated into solid.

Figure 4

Graphical representation of the investigated adsorption mode of NH₂-PDI onto the TiO₂ and MoS₂ surface.

The chemical composition and valence state of NH₂-PDI/TiO₂/MoS₂ sample were studied by XPS (Fig. 5). According to Fig. 5a, NH₂-PDI/TiO₂/MoS₂ composite contains Ti, O, C, Mo, and S elements. For NH₂-PDI/TiO₂/MoS₂ nanocomposite, the positions of Ti2p1/2 and Ti2p3/2 were observed at 464.9 eV and 459.1 eV, respectively. While for pure TiO₂ nanoparticles, Ti2p1/2 and Ti2p3/2 peaks were observed at 464.3 eV and 458.5 eV, respectively (Fig. 5b)⁴¹. The O1s peak positions of NH₂-PDI/TiO₂/MoS₂ and pure TiO₂ were observed at 530.2 eV and 529.8 eV, respectively (Fig. 5c). These results indicate that after adding NH₂-PDI and MoS₂ to TiO₂ nanoparticles, Ti2p peaks move to a higher energy by 0.6 eV than pure TiO₂. Also, the peak position of O1s moved 0.4 eV towards high energy. The shifted spectrum implies the presence of more defects or adsorbed hydroxyl groups on the surface of NH₂-PDI/TiO₂/MoS₂⁴². These defect states may serve as shallow donors to enhance charge transfer at the multiple interfaces and thus improve the overall degradation efficiency towards dyes. In NH₂-PDI/TiO₂/MoS₂ sample, The C1s spectra (Fig. 5d) shows the corresponding peak at 284.8 eV, which can be identified as C–C/C=C/C–H functional groups. The positions of the Mo3d5/2 and Mo3d3/2 peaks were observed at 232.4 eV and 228.5 eV, respectively. Similarly, the position of S2p was observed at 161.9 eV⁴³.

Figure 5

XPS spectra of (a) survey spectra in NH₂-PDI/TiO₂/MoS₂, (b) Ti2p in pure TiO₂ and NH₂-PDI/TiO₂/MoS₂, (c) O1s in pure TiO₂ and NH₂-PDI/TiO₂/MoS₂, (d) C1s in NH₂-PDI/TiO₂/MoS₂, (e) Mo3d in NH₂-PDI/TiO₂/MoS₂, and (f) S2p in NH₂-PDI/TiO₂/MoS₂.

The surface areas of TiO₂, NH₂-PDI, NH₂-PDI/TiO₂, MoS₂, MoS₂/TiO₂, and NH₂-PDI/TiO₂/MoS₂ were determined by nitrogen adsorption method. The BET surface area of NH₂-PDI/TiO₂/MoS₂ (39 m²/g) is higher than that of TiO₂ (21 m²/g), NH₂-PDI/TiO₂ (37 m²/g), MoS₂ (23 m²/g), or MoS₂/TiO₂ (26 m²/g). Therefore, it can be speculated that the large surface area of NH₂-PDI/TiO₂/MoS₂ could promote the photocatalytic activity.

The UV–vis diffuse reflection spectra (DRS) and the corresponding Tauc plots of samples are shown in Fig. 6. Pure TiO₂ absorbs only UV light and exhibits absorption below 380 nm. Adding NH₂-PDI and MoS₂ nanoflakes to TiO₂ nanoparticles can result in more light-harvesting in visible region (Fig. 6a). The absorption intensity of NH₂-PDI/TiO₂/MoS₂ in visible light region was obviously enhanced, which could be attributed to the visible light absorption characteristics of NH₂-PDI and MoS₂ nanoflakes. The band gap energy of the samples can be estimated by the formula Eg (eV) = 1240/λ_AE (nm) using the position of the absorption edge (λ_AE)⁴⁴. The downward slopes of the absorption curves are extrapolated to cross the X-axis and the λ_AE-values of TiO₂, MoS₂/TiO₂, NH₂-PDI/TiO₂, and NH₂-PDI/TiO₂/MoS₂ samples were estimated to be at 389 nm, 423 nm, 459 nm, and 467 nm, respectively (Fig. S-5). The Eg values of TiO₂, MoS₂/TiO₂, NH₂-PDI/TiO₂, and NH₂-PDI/TiO₂/MoS₂ samples were estimated to be at 3.18, 2.93, 2.70, and 2.66 eV, respectively.

Figure 6

(a) UV–vis diffuse reflection spectra (DRS) and (b) The corresponding Tauc plot of samples.

The band gaps (Eg, eV) of the samples are also calculated by the Tauc equation and Kubelka–Munk function⁴⁵:

$${[F(R)h\upsilon]^0.5} =A(h\upsilon-Eg)$$

(1)

$$F(R)={(1-R)^{2}}/2R$$

(2)

where, R is the calibrated reflection of samples with BaSO₄ reflection, and F(R) is proportional to the absorption constant. h and ν represent the Planck constant and frequency, while A and Eg represent constant and band gap energy, respectively. The [F(R)hν]^0.5 versus hν is shown in Fig. 6b. Extrapolation of the linear portion at [F(R)hν]^0.5 = 0 provides Eg value of the samples. The band gaps were found to be 2.98, 2.75, 2.47, and 2.27 eV for TiO₂, MoS₂/TiO₂, NH₂-PDI/TiO₂, and NH₂-PDI/TiO₂/MoS₂, respectively. The increase of UV–visible light absorption and decrease of band gap energy enhanced the photodegradation efficiency of NH₂-PDI/TiO₂/MoS₂ towards MB.

The activity diagram of MB degradation by different catalysts in visible light and the first order kinetics curve fitting of MB degradation by different catalysts are shown in Fig. 7. Prior to the illumination, each catalyst was dispersed in the dye solution. After 120 min of vigorous magnetic stirring under dark condition, 51.3 ± 1.5% of MB was absorbed by NH₂-PDI/TiO₂/MoS₂, whereas pure TiO₂ only absorbed 8.2 ± 0.7% of MB. It can be seen that the spongy NH₂-PDI clusters and multi-layer MoS₂ nanoflowers decorated TiO₂ exhibited extremely high adsorption efficiency under dark condition. Both MoS₂ and TiO₂/MoS₂ have strong adsorption capacity, which may be due to the flower-like MoS₂ with negative surface charge promoted the adsorption of cationic dye MB. It can be noted that after 120 min of irradiation, the degradation rate of dye was 99.0 ± 1.0% for NH₂-PDI/TiO₂/MoS₂, while the corresponding data was 4.0 ± 0.3%, 31.3 ± 1.5%, 71.7 ± 1.6%, 78.7 ± 1.3%, and 87.6 ± 1.5% for NH₂-PDI, TiO₂, NH₂-PDI/TiO₂, MoS₂, and MoS₂/TiO₂, respectively (Fig. 7A). Though TiO₂ has no visible light absorption, it has been found to be solar active. This may be due to the dye sensitization mechanism of MB to TiO₂. For better comparison, NH₂-PDI/TiO₂/1%MoS₂ or NH₂-PDI/TiO₂/3%MoS₂ photocatalysts was used for dye degradation under identical conditions. As a result, 97.3 ± 0.6% or 94.0 ± 1.1% of dye were degraded respectively under visible light irradiation in 120 min. The results show that 0.1%NH₂-PDI/TiO₂/2%MoS₂ had higher efficiency in MB degradation than other catalysts. The kinetics plot shows that ln (C₀/C) has linear relationships with reaction time, indicating that the photodegradation of MB follows first-order kinetics (Fig. 7B). The apparent rate constants (k) were determined as 0.00616, 0.00352, 0.00738, 0.00517, 0.00752, and 0.00806 min⁻¹ for TiO₂, NH₂-PDI, NH₂-PDI/TiO₂, MoS₂, MoS₂/TiO₂, and NH₂-PDI/TiO₂/MoS₂, respectively. These results confirmed the high photocatalytic activity of NH₂-PDI/TiO₂/MoS₂. Fig. S-6 shows the decolorization of MB dye (0.01 g/L) within 90 min in the presence of NH₂-PDI/TiO₂/MoS₂ catalyst. The decrease of absorption spectrum indicates decolorization of the dye under the applied conditions. There were no additional peaks in the UV–Vis spectrum, indicating that the dye was completely degraded. It is reported that the products of the decolorization process are H₂O, CO₂, NO₂, and SO₂⁴⁶, and the mechanism of MB degradation is shown in Fig. S-7.

Figure 7

(A) The activity diagram of MB degradation by different catalysts in visible light, (B) The first order kinetics curve fitting of MB degradation by different catalysts. (a) TiO₂, (b) NH₂-PDI, (c) NH₂-PDI/TiO₂, (d) MoS₂, (e) MoS₂/TiO₂, (f) NH₂-PDI/TiO₂/2%MoS₂, (g) NH₂-PDI/TiO₂/1%MoS₂, and (h) NH₂-PDI/TiO₂/3%MoS₂. [MB] = 0.01 g/L, pH = 7, catalyst suspended = 1 g/L.

Chemical oxygen demand (COD) is an indicator that reflecting the required oxygen for the oxidation of organic matters present in solution⁴⁷. A smaller COD value means a lower pollution of the sample. The mineralization degree of MB was calculated by COD result, and the kinetic results shown that the k value of NH₂-PDI/TiO₂/MoS₂ was 0.0076 min⁻¹ (Fig. S-8). The value correspond to t_1/2 values of 85.9 and 91.2 min (based on t_1/2 = 0.693/k) for the degradation and mineralization of MB molecules, respectively⁴⁸. These confirmed that the photodegradation of MB was 1.06 times faster than its mineralization. The results confirmed a relatively smaller mineralization extent for MB molecules than its photodegradation extent.

The photonic efficiency of MB dye degradation by NH₂-PDI/TiO₂/MoS₂ under optimum condition was calculated using a reported method⁴⁹. The quantum yield of reaction (Ø) is defined as follows (Eq. (1)):

$$\O =\frac{\mathrm{Number \,of\, molecules\, decomposed }}{\mathrm{Number\, of\, photons\, of\, light\, absorbed}}$$

The photo-degradation rate constants (k’) of MB dyes under monochromatic light source can also be used to calculate the reaction quantum yield (Eq. (2)).

$$\O =\frac{k^{\prime}}{303I\varepsilon l}$$

where Ø is the reaction of quantum yield. I is the light intensity range in 400–800 nm (2.312 × 10^–3 Einstein). ɛ is the molar absorptivity of MB at 664 nm (25,440 cm^–1 M^–1). l is the path length of reaction tube, which is 0.23 m for 50 mL solution. The degradation quantum yields obtained with TiO₂, NH₂-PDI, NH₂-PDI/TiO₂, MoS₂, MoS₂/TiO₂, and NH₂-PDI/TiO₂/MoS₂ were 0.000152, 0.000869, 0.000182, 0.000128 and 0.000186, respectively. These results indicate that the quantum yield of NH₂-PDI/TiO₂/MoS₂ catalyzes process was higher than that of other processes.

During the process of MB degradation, a large amount of MB was adsorbed on the surface of catalysts. The visible light driven catalysis of the NH₂-PDI/TiO₂/MoS₂ may be enhanced due to the effect of dye sensitization. Therefore, amoxicillin (AMX, a colorless medical antibiotic) was selected to evaluate the photocatalytic activity of NH₂-PDI/TiO₂/MoS₂ nanoparticles under the same conditions. The photocatalytic degradation results of AMX are shown in Fig. S-9. NH₂-PDI/TiO₂/MoS₂ exhibited higher visible photocatalytic activity than NH₂-PDI/TiO₂ or MoS₂/TiO₂. When AMX solution was mixed with NH₂-PDI/TiO₂/MoS₂ in the absence of light, the removal rate of AMX was 48%. When NH₂-PDI/TiO₂ or MoS₂/TiO₂ was used under visible light irradiation for 120 min, the degradation rates of AMX were 56.9% or 71.2%, respectively. Comparatively, NH₂-PDI/TiO₂/MoS₂ exhibited superior photodegradation performance (84.8%). The degradation processes conforms to pseudo-first-order kinetics. The photocatalytic activity of NH₂-PDI/TiO₂/MoS₂ reached a maximum value of 0.00986 min⁻¹, which was about 1.67 times and 1.79 times higher than NH₂-PDI/TiO₂ and MoS₂/TiO₂ (0.00590 min⁻¹ and 0.00551 min⁻¹), respectively. The result indicates that NH₂-PDI/TiO₂/MoS₂ has strong visible-light driven photocatalytic activity.

The reusability and stability of a composite photocatalyst are important factor affecting its application (Fig. S-10). The NH₂-PDI/TiO₂/MoS₂ was used for the degradation of different solutions of MB for six consecutive experiments under the same reaction conditions. After each experiment, the catalyst was recovered by centrifugation and dried in an air oven. The degradation efficiency of NH₂-PDI/TiO₂/MoS₂ reduced from 99 ± 0.9% to 89 ± 2.3% after six cycles of reuse, which was probably due to a small loss of catalyst during recycle (Fig. S-10a). NH₂-PDI/TiO₂/MoS₂ was prepared by hydrothermal synthesis method, which made the contact between three monomers more compacted and reduced the dissolution of MoS₂ or NH₂-PDI molecule. Therefore, the composite catalyst exhibits high stability in the degradation process. In addition, the composite material packaged TiO₂ by NH₂-PDI and MoS₂. The contact area between the three semiconductors increases due to the point-to-surface contact. Due to the close contact interface, the interactions between the three semiconductors would also be more intense. The π-conjugated electrons in NH₂-PDI can be quickly transferred from the interior of the semiconductor to the surface, and then to the surface of TiO₂ or MoS₂. This reduces electrons and holes recombination and improves the photocatalytic activity. Fig. S-10b shows the FT-IR spectra of the NH₂-PDI/TiO₂/MoS₂ photocatalyst before and after six cycles of photocatalytic degradation of MB. It can be seen that the spectrum of the regenerated photocatalyst was basically the same as that of the fresh photocatalyst. There is no spectrum of MB, which proves that MB has been completely degraded rather than adsorbed on the catalyst surface during the illumination process. The result of UV–vis spectra shows that the concentration of organic matter in the washing solution of catalyst was very low. Few intermediate species and NH₂-PDI were detected, indicating that the dissolution of NH₂-PDI in the composite structure was very few and MB was mineralized to CO₂. These results show that the NH₂-PDI/TiO₂/MoS₂ catalyst is stable and reusable.

The photoluminescence (PL) spectrum was used to analyze the recombination of electron hole pairs. In general, a higher peak indicates a more rapid charge recombination. As shown in Fig. S-11, the peak at 399 nm is attributed to a direct transition from the conduction band to the valence band. Moreover, the luminescence peaks at 440, 451, and 469 nm are caused by inter-band transitions, and the peaks at 483 and 492 nm are caused by intra-band transitions within the energy level traps or surface states⁵⁰. The peak intensity was arranged as follow: TiO₂ > NH₂-PDI/TiO₂ > MoS₂/TiO₂ > NH₂-PDI/TiO₂/MoS₂. Especially, the peak intensity of NH₂-PDI/TiO₂/MoS₂ composite was much weaker than that of the pure TiO₂. The –C=O…H hydrogen bond between the -OH of TiO₂ and –C=O of NH₂-PDI, and the S…H bond between the -S of MoS₂ and -NH₂ of NH₂-PDI acted as short and fast channels for migrating the photogenerated charge carriers from NH₂-PDI to TiO₂ and MoS₂. Based on the above results, the introduction of NH₂-PDI and MoS₂ can greatly accelerate the charge transfer process.

The mechanism of enhanced photocatalytic activity of NH₂-PDI/TiO₂/MoS₂ composite was further studied by photoelectrochemistry method. The separation of photogenerated electrons and holes plays an important role in the photocatalytic decomposition of organic pollutants, which can be evaluated by transient photocurrent responses (I-t) and electrochemical impedance spectra (EIS) (Fig. 8). Figure 8a shows the I-t curves of TiO₂, NH₂-PDI, NH₂-PDI/TiO₂, MoS₂, MoS₂/TiO₂, and NH₂-PDI/TiO₂/MoS₂ under several intermittent visible light irradiation cycles. It can be seen that the photocurrent of all samples exhibited good repeatability when the light was turned on and off. The light current response of pure TiO₂ and NH₂-PDI were very weak (only 0.119 and 0.035 μA cm⁻², respectively). NH₂-PDI/TiO₂/MoS₂ composite had the largest photocurrent response, and the stable photocurrent was about 1.007 μA cm^-2, which was higher than that of MoS₂ (0.372 μA cm⁻²), MoS₂/TiO₂ (0.522 μA cm⁻²), and NH₂-PDI/TiO₂ (0.846 μA cm⁻²). The enhanced photocurrent response indicates that the separation efficiency of photogenerated carriers and the photocatalytic performance were improved. Figure 8b displays the EIS Nyquist plots of as-prepared samples. The radius of the arc on the EIS spectrum reflects the surface charge transfer resistance and the solid interface delamination resistance. The smaller the semicircle arc is, the easier the charge transfer proceeds⁵¹. Well discussion on EIS spectra and the resulted Bodes' plots has been presented in previous work⁵². Here, the following trend was obtained for the arc radius of the Nyquist plots, confirming that the NH₂-PDI/TiO₂/MoS₂ composite have better charge transfer capability.

Figure 8

(a) Photocurrent transient responses and (b) EIS spectra of TiO₂, NH₂-PDI, NH₂-PDI/TiO₂, MoS₂, MoS₂/TiO₂, and NH₂-PDI/TiO₂/MoS₂ at a constant potential of 0.4 V with simulated solar light at 300 mW/cm². frequency: 0–10,000 Hz, electrolyte: 0.1 M Na₂SO₄, amplitude: 10 mV.

NH₂-PDI > TiO₂ > MoS₂ > NH₂-PDI/TiO₂ > MoS₂/TiO₂ > NH₂-PDI/TiO₂/MoS₂.

The Mott-Schottky plots were used to estimate the conduction band (CB) of the TiO₂, NH₂-PDI, and MoS₂, which were − 1.26 eV, − 1.61 eV, and − 0.75 eV, respectively (Fig. S-12). Combining with their bandgaps (TiO₂ at 2.98 eV [Fig. 5b], NH₂-PDI at 2.36 eV³⁰, and MoS₂ at 1.75 eV²⁶), the CB/VB (conduction band/valence band) of TiO₂, NH₂-PDI, and MoS₂ were evaluated to be − 1.26/1.72 eV, − 1.56/0.8 eV, and − 0.75/1.00 eV, respectively. It can be found that the CB position of singlet oxygen (·O₂^-) (− 0.18 eV) is shallower than that of NH₂-PDI, TiO₂, and MoS₂. This result indicates that ·O₂^- may be the main active substance in NH₂-PDI/TiO₂/MoS₂ composite for photocatalytic degradation of MB. The VB positions of MoS₂ was shallower than that of hydroxyl radical (·OH) (2.40 eV)⁵³, so it is possible that the ·OH cannot be generated by NH₂-PDI/TiO₂/MoS₂.

The active species in the photocatalytic process was studied by free radical capture experiment⁵³. As can be seen from Fig. S-13, with the addition of p-BQ scavenger, the degradation rate decreased from 99.9% to 33%, indicating that ·O₂⁻ is one of the main active species in the photocatalytic degradation process. In the presence of EDTA-2Na, the degradation efficiency was significantly reduced, indicating that h⁺ is also one of the major active radicals. After the addition of IPA, the degradation activity of the catalyst did not decrease, indicating that ·OH is not the main active species in MB degradation.

Based on the results above, two mechanisms including ‘type II- heterojunction’ and ‘Z-scheme’ were suggested to illustrate the photodegradation pathways occurred in this work (Fig. 9)⁵⁴. In ‘type II- heterojunction’ mechanism (Fig. 9a), both the NH₂-PDI and MoS₂ semiconductors can be excited by the arrived photons under visible light irradiation, resulting in production of e^-/h⁺ pairs in both semiconductors. As the CB and VB of NH₂-PDI are located at higher energy levels than those of TiO₂ and MoS₂, the photogenerated electrons in the CB of NH₂-PDI can transfer to the CB of TiO₂, and then to the CB of MoS₂, while holes travel in the opposite direction in the VB. These charge carriers’ transfer processes result in the accumulation of the photogenerated electron in the CB level of MoS₂ and the photogenerated holes in the VB level of NH₂-PDI. While in the ‘Z-scheme’ mechanism (Fig. 9b), the photogenerated electrons at the CB level of MoS₂ can migrate to the VB level of NH₂-PDI based on the standard potentials, and thus, the ‘Z-scheme’ pathway accumulates the photogenerated electrons at the CB level of NH₂-PDI and the photogenerated holes at the VB level of MoS₂. The accumulated electrons at the CB level of NH₂-PDI have the potential of − 1.61 eV, which is more negative than the potential of MoS₂ (-0.75 eV). Therefore, the accumulated electrons at the CB level of NH₂-PDI can effectively reduce dissolved oxygen and generate enough ·O₂⁻ as an efficient reactive species for the degradation of MB. These holes are also stronger oxidizing agents than the accumulated holes in the VB level of NH₂-PDI (accumulated in type II mechanism) that can directly oxidize MB molecules. Therefore, the ‘Z-scheme’ mechanism is a better illustration for the enhanced photocatalytic activity of NH₂-PDI/TiO₂/MoS₂ composite in MB degradation than the ‘type II-heterojunction’ mechanism. Nevertheless, in both mechanisms, TiO₂ facilitates the electron transfer between the conduction bands, reduces photoinduced charge carrier recombination and increases photocatalytic activity.

Figure 9

Schematic illustration of the photocatalytic mechanism over the NH₂-PDI/TiO₂/MoS₂ heterostructure. (a) Type II-Heterojunction and (b) Z-scheme pathways.

Conclusions

In conclusion, NH₂-PDI/TiO₂/MoS₂ ternary composite was successfully synthesized by hydrothermal synthesis method. Morphological and chemical features of TiO₂, NH₂-PDI, NH₂-PDI/TiO₂, MoS₂, MoS₂/TiO₂, and NH₂-PDI/TiO₂/MoS₂ composites were examined by SEM, XRD, FTIR, XPS, and DRS. SEM results demonstrate that NH₂-PDI nanorods and MoS₂ nanoflowers mixed with TiO₂, which can provide more active sites and mass charge transport pathways in the catalytic system. The photoelectrochemical measurement results show that the photocurrent performance of the ternary composite catalyst was superior to that of binary composites or pure NH₂-PDI, MoS₂, or TiO₂. The NH₂-PDI/TiO₂/MoS₂ exhibits excellent photocatalytic activity and high stability. The ‘Z-scheme’ accumulates more electrons at the CB level of NH₂-PDI and photogenerated holes at the VB level of MoS₂. The accumulated electrons can effectively reduce dissolved oxygen and generate enough ·O₂⁻ as an efficient reactive species for the degradation of MB. These holes are also strong oxidizing agents that can directly oxidize MB molecules. TiO₂ could reduce photoinduced charge carrier recombination and increases photocatalytic activity. This study highlights the potential application of organic and inorganic photocatalysts, and provides a feasible strategy for the photodegradation of dyes.