From Pollutant Removal to Renewable Energy: MoS2-Enhanced P25-Graphene Photocatalysts for Malathion Degradation and H2 Evolution

Abstract

The widespread presence of pesticides-especially malathion-in aquatic environments presents a major obstacle to conventional remediation strategies, while the ongoing global energy crisis underscores the urgency of developing renewable energy sources such as hydrogen. In this context, photocatalytic water splitting emerges as a promising approach, though its practical application remains limited by poor charge carrier dynamics and insufficient visible-light utilization. Herein, we report the design and evaluation of a series of TiO₂-based ternary nanocomposites comprising commercial P25 TiO₂, reduced graphene oxide (rGO), and molybdenum disulfide (MoS₂), with MoS₂ loadings ranging from 1% to 10% by weight. The photocatalysts were fabricated via a two-step method: hydrothermal integration of rGO into P25 followed by solution-phase self-assembly of exfoliated MoS₂ nanosheets. The composites were systematically characterized using X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy (DRS), and photoluminescence (PL) spectroscopy. Photocatalytic activity was assessed through two key applications: the degradation of malathion (20 mg/L) under simulated solar irradiation and hydrogen evolution from water in the presence of sacrificial agents. Quantification was performed using UV-Vis spectroscopy, gas chromatography-mass spectrometry (GC-MS), and thermal conductivity detection (GC-TCD). Results showed that the integration of rGO significantly enhanced surface area and charge mobility, while MoS₂ served as an effective co-catalyst, promoting interfacial charge separation and acting as an active site for hydrogen evolution. Nearly complete malathion degradation (~100%) was achieved within two hours, and hydrogen production reached up to 6000 µmol g^-1 h^-1 under optimal MoS₂ loading. Notably, photocatalytic performance declined with higher MoS₂ content due to recombination effects. Overall, this work demonstrates the synergistic enhancement provided by rGO and MoS₂ in a stable P25-based system and underscores the viability of such ternary nanocomposites for addressing both environmental remediation and sustainable energy conversion challenges.

Keywords: MoS2 nanocomposite; malathion; photocatalytic hydrogen evolution; photodegradation; rGO.

Figure 1

Field emission scanning electron microscopy (FESEM) image of P25-rGO at different magnifications (a,b) and 5% MoS₂@P25-rGO (c). Yellow arrows in (c) highlight dispersed MoS₂ nanosheets.

Figure 2

High-resolution transmission electron microscopy (HRTEM) images of the synthesized materials: (a) P25-rGO composite; (b) P25-rGO at higher magnification, with the inset highlighting lattice fringes corresponding to an interplanar spacing of approximately 0.35 nm, assigned to the (101) plane of anatase TiO₂; and (c) a monolayer MoS₂ sheet, with the inset displaying the selected area electron diffraction (SAED) pattern characteristic of hexagonal 2H-MoS₂. This image is provided to illustrate the structural quality of the exfoliated MoS₂ precursor and is not intended as direct evidence of its distribution within the final composite.

Figure 3

XRD patterns of commercial TiO₂-P25 (a), rGO (b), exfoliated MoS₂ (c), and 5% MoS₂@P25-rGO (d).

Figure 4

Tauc plots of (αhν)² versus energy (eV) and determination of the bandgap energy of TiO₂ (P25) (a), TiO₂-rGO (b), and 5% MoS₂@TiO₂-rGO (c).

Figure 5

PL spectra of P25-rGO, 1% MoS₂@P25-rGO, 3% MoS₂@P25-rGO, 5% MoS₂@P25-rGO, and 10% MoS₂@P25-rGO.

Figure 6

Raman spectra of the TiO₂-P25 (a), rGO (b), exfoliated MoS₂ (c), and 5% MoS₂@TiO₂-rGO (d). The black, red and green asterisks represent peaks assigned to TiO₂-P25, rGO and MoS₂, respectively.

Figure 7

XPS core level spectra for 5% MoS₂@TiO₂-rGO: Ti2p (a), O1s (b), C1s (c), and Mo3d-S2s (d).

Figure 8

Malathion degradation profiles for pristine P25, P25-rGO, and MoS₂@P25-rGO composites under simulated solar irradiation. Each data point represents the average of three independent experiments. Error bars were omitted for clarity, as the standard deviation was below 4% and the overlap of multiple curves impeded visual interpretation.

Figure 9

Proposed photocatalytic degradation pathways of malathion (m/z 330) under UV-visible irradiation using the 5% MoS₂@P25-rGO catalyst, based on GC-MS analysis. Five main degradation routes (Pathways A–E) were identified, involving hydrolysis, desulfuration, C–O and P–S bond cleavage, oxidative demethylation, and ring-opening reactions.

Figure 10

Schematic diagram of the proposed mechanism for the photodegradation of malathion, using the 5% MoS₂@P25-rGO catalyst under irradiation.

Figure 11

Hydrogen production profiles of the synthesized catalysts under irradiation at different wavelengths. All experiments were performed in triplicate. Although data variability was consistently below 5%, error bars were omitted to preserve visual clarity due to the overlap of multiple data series.

Figure 12

Transient photocurrent response in the light-on−light-off processes of the synthesized catalysts under irradiation at 500 nm.

Figure 13

Schematic diagram of the proposed mechanism for hydrogen production under irradiation.