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ACS Omega
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2470-1343
American Chemical Society

10.1021/acsomega.4c04687
Article
Plasmonic Au–MoS2 Nanohybrids Using Pulsed Laser-Induced Photolysis Synthesis for Enhanced Visible-Light Photocatalytic Dye Degradation
Lin Yung-Mei †
Wu Chieh-Ming †
Lin Yi-Hsuan †
Chang Jui-Hao ‡
Liang Ling-Yi ‡
Hsiao Vincent K. S. *†
https://orcid.org/0000-0001-6712-0064
Chu Chih-Chien *‡§
† Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou 545301, Taiwan
‡ Department of Medical Applied Chemistry, Chung Shan Medical University, Taichung 40201, Taiwan
§ Department of Medical Education, Chung Shan Medical University Hospital, Taichung 40201, Taiwan
* Email: kshsiao@ncnu.edu.tw.
* Email: jrchu@csmu.edu.tw.
04 09 2024
17 09 2024
9 37 3877838787
17 05 2024
02 09 2024
29 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by/4.0/ Permits the broadest form of re-use including for commercial purposes, provided that author attribution and integrity are maintained (https://creativecommons.org/licenses/by/4.0/).

The Au–MoS2 nanocomposites (NCPs) exhibit excellent visible-light photocatalytic activity and potential applications in the photocatalytic degradation of organic dyes. In this study, an Au–MoS2 heterojunction structure with Au nanoparticles (NPs) deposited on MoS2 nanosheets was synthesized via the pulsed laser-induced photolysis method. The influence of Au content on the photocatalytic performance was systematically investigated, and the working mechanism under visible light excitation was elucidated. The optimal Au–MoS2 NCPs exhibited efficient degradation of methylene blue (MB) dye, mainly attributed to the plasmon resonance effect of Au NPs which facilitated the visible light harvesting and hot electron injection. The Au/MoS2 interface promoted the separation and transfer of photogenerated charge carriers. The electrostatic adsorption between positively charged MB molecules and the negatively charged MoS2 surface favored the affinity toward active sites. Furthermore, the photogenerated electrons and holes participated in generating reactive oxygen species such as superoxide and hydroxyl radicals, which initiated the oxidative degradation of MB. The PLIP-introduced Au NPs not only endowed the material with excellent visible light responsivity but also possibly modulated the electronic structure and photocatalytic active sites of MoS2 through an intrinsic effect, providing new insights for further enhancing the photocatalytic performance of Au–MoS2 NCPs.

National Science and Technology Council 10.13039/501100020950 MOST 110-2221-E-260-008-MY3 National Science and Technology Council 10.13039/501100020950 NSTC 111-2113-M-040-001 document-id-old-9ao4c04687
document-id-new-14ao4c04687
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pmc1 Introduction

Photocatalysis plays a crucial role in environmental remediation and energy conversion. In the environmental domain, photocatalysis effectively degrades organic pollutants, heavy metal ions, and hazardous substances into harmless products,1−3 improving water and air quality and mitigating pollution’s adverse impacts. In the energy sector, photocatalysis efficiently converts solar energy into chemical energy, such as hydrogen production via water splitting,4−6 promoting clean energy generation and utilization. Additionally, photocatalysis can enhance resource utilization by converting wastewater pollutants into renewable resources. Developing visible-light-responsive photocatalysts is both necessary and challenging.7−9 The visible light region comprises a significant portion of the solar spectrum, and photocatalysts capable of absorbing and utilizing visible light can fully exploit solar resources, improving photocatalytic efficiency and reducing energy consumption and environmental impact. However, traditional photocatalytic materials, such as TiO2, exhibit limited absorption and utilization capabilities in this region due to visible light’s relatively low energy.10 Therefore, developing emerging materials to enhance visible light absorption and conversion efficiency is crucial, alongside addressing technical challenges like reaction condition optimization, catalyst stability, and activity.

TiO2 has been a research priority due to its exceptional photocatalytic performance.11 However, with a bandgap of approximately 3.2 eV, TiO2 can only respond to ultraviolet light below 400 nm, resulting in low visible light utilization efficiency and weak photocatalytic reduction capability for hydrogen production. To address these limitations, researchers have developed non-TiO2 photocatalytic materials, such as ZnO, Fe2O3, CdS, and ZnS.12−15 Among them, MoS2 has garnered significant attention due to its large surface area, open nanostructure, and adjustable interlayer spacing.16−19 As a layered semiconductor with a suitable bandgap in the visible-light region, MoS2 exhibits tremendous potential in visible-light photocatalysis and organic pollutant degradation. However, MoS2 still suffers from limited visible-light utilization and high recombination rates of photogenerated electron–hole pairs, restricting its large-scale applications, such as photocatalytic water splitting and organic pollutant degradation. Therefore, developing new photocatalytic materials with high visible light responsiveness and efficient charge carrier separation is crucial for improving overall catalytic efficiency and reducing energy consumption.

MoS2 is a typical two-dimensional (2D) layered semiconductor belonging to the transition metal dichalcogenide family.20 Its unique properties have garnered significant attention in optoelectronics, energy storage, biosensing, nanoelectronics, and photocatalysis. The tunable bandgap structure of MoS2 can be continuously adjusted from 1.9 eV (direct bandgap) to 1.2 eV (indirect bandgap) depending on the number of layers,21 enabling efficient absorption and utilization of visible and infrared light. The conduction band (CB) and valence band (VB) positions of MoS2, approximately −0.2 and +1.8 eV (vs NHE),22 suggest sufficient reduction and oxidation potentials to form superoxide and hydroxyl radicals during photocatalysis. Strategies to enhance photocatalytic activity of MoS2 include controlling its morphology and structure,23 doping,24 forming heterojunctions,25 modifying with carbon nanomaterials,26 and compositing with noble metal nanoparticles.27,28 The photocatalytic process of MoS2 involves the generation and migration of electron–hole pairs upon irradiation. The photogenerated electrons and holes participate in reduction and oxidation reactions, respectively, with adsorbed species on the MoS2 surface, generating reactive oxygen species (ROS) such as O2– and OH radicals.

Recent studies have highlighted the potential of metal-loaded semiconductor photocatalysts,29−37 particularly Au nanoparticles (NPs) composited with MoS2, for enhanced visible light photocatalysis. These Au–MoS2 nanocomposites (NCPs) have found applications in organic dye degradation, electrochemical sensing, hydrogen evolution, carbon dioxide reduction, and water splitting.38−42 The synergistic effects of MoS2’s optical and electronic properties, Au NPs’ localized surface plasmon resonance (LSPR), and the formation of Schottky junctions at the Au/MoS2 interface contribute to the enhanced performance of these NCPs. Au–MoS2 is a heterogeneous NCP material formed by the integration of Au NPs with MoS2 nanosheets, achieving a synergistic combination of the advantages of both materials. Optically, MoS2 possesses a narrow bandgap, enabling efficient visible light absorption, while Au NPs enhance light absorption and localized electric fields due to the LSPR effect, providing the Au–MoS2 NCPs with exceptional visible light absorption and photoelectric conversion capabilities.

Compared to traditional photocatalytic materials (such as TiO2 and CdS), Au–MoS2 exhibits superior photocatalytic activity, primarily arising from the intrinsic excellent optical and electronic properties of MoS2, the LSPR surface effect of Au NPs, and the formation of Schottky junctions at the Au/MoS2 interface, favoring charge separation and transfer. Additionally, MoS2’s relatively high chemical stability and the protective effect of Au NPs modification render the Au–MoS2 composite more stable during photocatalytic reactions. Au–MoS2 NCPs have been employed for the efficient and eco-friendly degradation of various organic dye molecules.35−37 The remarkable photocatalytic activity of Au–MoS2 NCPs can be attributed to the large surface area of MoS2 nanosheets, facilitating efficient loading of Au NPs, and the unique metal–semiconductor interface effect, significantly enhancing the separation and transfer efficiency of photoinduced charges.

In this study, we successfully prepared visible light-driven Au–MoS2 NCP photocatalysts using the pulsed-laser-induced photolysis [pulsed laser-induced photolysis (PLIP)] technique and evaluated their visible-light photocatalytic degradation performance toward methylene blue (MB) dye. Compared to conventional synthesis techniques, the PLIP approach offers several distinct advantages. First, it enables the rapid, one-step formation of Au NPs directly on the MoS2 surface, ensuring intimate contact and strong coupling between the two components. Second, the PLIP process allows precise control over the size, distribution, and loading of Au NPs by tuning the laser parameters and precursor concentrations. This level of control is crucial for optimizing the photocatalytic performance of the resulting Au–MoS2 hybrids. Furthermore, the PLIP method is a green and efficient synthesis route, as it does not require harsh chemicals, high temperatures, or prolonged reaction times. These advantages make the PLIP technique a promising strategy for fabricating high-quality Au–MoS2 photocatalysts with tailored properties. Through material characterization techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM), we confirmed the tight integration of Au NPs on the surface of the layered MoS2 substrate, forming an ideal heterogeneous nanostructure. The experimental results demonstrate that after modification by the PLIP process, the Au NPs grown on the surface of MoS2 significantly enhance its light absorption and photocatalytic activity in the visible light region. Electrochemical cyclic voltammetry (CV) studies revealed that increasing the concentration of the auric acid (HAuCl4) precursor effectively increases the loading of Au nanoparticles, thereby enhancing the photocatalytic degradation performance of the Au–MoS2 composite material. Regarding the key factors influencing photocatalytic activity, we believe that the adsorption of organic dye molecules on the photocatalyst surface plays a crucial role. The adsorption of organic molecules not only promotes their enrichment and activation on the photocatalyst surface but also affects the transfer and transport of photoinduced charges, thereby profoundly influencing the photocatalytic efficiency. Therefore, we focused on the adsorption behavior on the photocatalyst surface and explore its intrinsic relationship with photocatalytic activity. Through this work, we aim to reveal in depth the effects of Au modification and adsorption behavior in Au–MoS2 NCPs photocatalysts on their photocatalytic performance. This research provides theoretical guidance and experimental evidence for the development of efficient visible light-driven photocatalysts.

2 Experimental Section

Layered MoS2 was prepared using a sonication-assisted exfoliation method from bulk MoS2 crystals in an aqueous surfactant solution, following a procedure adapted from previous studies.43 Initially, 250 mg of MoS2 powder and 75 mg of sodium cholate (Aldrich) were added to a 50 mL aqueous solution. The mixture was subjected to ultrasonic crushing in an ice–water bath. Ultrasonication was conducted for 3 h at 50% power, followed by an additional 2 h at 70% power to achieve a black–green MoS2 nanosheet dispersion. Subsequent to ultrasonication, the dispersion was centrifuged at 3000 rpm for 30 min to separate the green supernatant containing the exfoliated MoS2 nanosheets from the bulk MoS2. The supernatant was further centrifuged at 12,000 rpm for 30 min to isolate the layered MoS2. To eliminate sodium cholate adsorbed on the nanosheet surfaces, the isolated MoS2 was dispersed in ultrapure water with sonication assistance. The dispersion was then centrifuged at 12,000 rpm for 30 min, and the sediment was collected to complete the washing process. This washing procedure was repeated twice more to ensure thorough removal of sodium cholate. Finally, the sediments were dispersed in a specific quantity of ultrapure water to prepare a uniform dispersion of layered MoS2. To ensure high reproducibility for optical absorption detection, the stock solution of layered MoS2 was utilized after sonication treatment for 2 min. The experimental procedure for preparing Au–MoS2 NCPs is as follows. First, a nanosecond pulsed laser was employed to induce the photolysis reaction of auric acid (HAuCl4) in an aqueous solution in the presence of a reducing agent (H2O2), with layered MoS2 prepared by a sonication-assisted exfoliation method. The specific reaction mechanism for the formation of Au NPs has been described in other studies.44 To fabricate Au–MoS2 NCPs, 1 mL of MoS2 at a concentration of 0.1 mg/mL was added to the auric acid/H2O2 precursors containing various concentrations of HAuCl4·3H2O (1 mL fixed volume), as well as a fixed concentration (2 mM) and volume (1 mL) of H2O2. The solution was then irradiated for 10 min with a pulsed Q-switch Nd:YAG laser (LS-2137U; LOTIS TII, Minsk, Belarus) with a wavelength of 532 nm, a pulse duration of 6–7 ns, a pulse repetition rate of 5 Hz, and a fluence of approximately 50 mJ/cm2. The laser beam was directed at the middle of the precursor solution to ensure uniform light exposure. This method primarily involves the addition of H2O2 precursor into auric acid and triggering of a photolysis reaction through the energy of intense pulsed laser light. In the photocatalytic degradation experiment of MB, a fixed volume of Au–MoS2 composite material (0.5 mL) and a fixed concentration (10–5 M) and volume (1.5 mL) of MB were placed in a standard cuvette. The absorption spectra of the cuvette under white light illumination (Thorlabs, OSL1) and time durations were recorded. The photocatalytic degradation efficiency was calculated by dividing the remaining MB concentration (C) at each time point by the initial MB concentration (C0). The sample was positioned 20 cm from the fiber guided light source. The electrochemical measurements, CV, were carried out on a wireless potentiostat (Zensor, ECWP100) that was connected to a screen printed electrode where the added 5 μL Au–MoS2 solution as working electrode, conducting carbon as the counter electrode, and Ag as the pseudo reference electrode. Figure 1 shows in detail the synthesis process and photocatalytic application of the Au–MoS2 NCP photocatalyst. The as purchased MoS2 powders are first subjected to ultrasonication and centrifugation to produce exfoliated MoS2 nanosheets. These nanosheets are then exposed to a PLIP process using a gold precursor solution, leading to the in situ growth of Au nanoparticles (NPs) on the surface of MoS2, thereby yielding the Au–MoS2 NCPs. A ball-and-stick model illustrates the structure of the Au–MoS2 hybrid, showing uniform distribution of Au NPs on the layered MoS2 matrix. The growth of Au NPs on the layered MoS2 is facilitated by a 532 nm pulsed laser in a photolysis process involving H2O2 and HAuCl4.

Figure 1 Schematic illustration of the synthesis of plasmonic Au–MoS2 NCP photocatalysts and their photocatalytic applications. (Upper left) Exfoliation of layered MoS2 into nanosheets by ultrasonication and centrifugation. (Center) Au nanoparticles uniformly decorated on the surface of MoS2 nanosheets via the PLIP process using a gold precursor solution. (Right) Depiction of the PLIP process involving a 532 nm pulsed laser for the in situ growth of Au NPs in the presence of H2O2 and HAuCl4. The resultant plasmonic Au–MoS2 hybrids exhibit exceptional light absorption and charge separation capabilities, enabling their application in photocatalytic degradation of organic dyes, MB.

3 Results and Discussion

The absorption spectrum is one of the most crucial properties of layered MoS2. As a 2D material, the absorption spectrum of MoS2 is closely related to its layered structure and electronic configuration. The visible light absorption of layered MoS2 primarily originates from its band structure, which comprises both direct and indirect bandgaps. For monolayer MoS2, the direct bandgap is approximately 1.8–1.9 eV, while the indirect bandgap is around 1.2–1.3 eV.45 These bandgaps give rise to distinct absorption characteristics in the visible range, which are of significant importance for applications such as photocatalysis. When illuminated, MoS2 absorbs light, leading to the excitation of electrons from the VB to the CB, forming electron–hole pairs. Within the band structure of MoS2, there exist different states, denoted as A1 and B1, representing distinct bandgap transitions associated with specific energy level changes during the electron excitation process.46 The absorption of light at particular wavelengths corresponds to electron transitions to these different bandgap states, resulting in characteristic absorption peaks in the spectrum. Absorption spectroscopy can provide strong evidence for the significant differences in optical and electronic properties between MoS2 and Au–MoS2 NCPs. Figure 2a displays the absorption spectra of the fabricated MoS2 using the sonication-assisted exfoliation method and the PLIP-fabricated Au–MoS2 NCPs over the wavelength range of 200–800 nm. The black curve represents the layered MoS2 sample, exhibiting distinct A1 and B1 absorption peaks at 610 and 670 nm, respectively. According to quantum confinement theory, these two characteristic peaks correspond to the direct and indirect bandgap transitions in monolayer MoS2.45 The presence of these peaks confirms the existence of MoS2 nanosheets with different layer numbers in the prepared sample. Beyond 700 nm in the near-infrared region, the absorption of MoS2 rapidly decreases. In contrast, the absorption spectrum of the Au–MoS2 (red curve) undergoes a significant change. The original A1 and B1 characteristic peaks of MoS2 disappear completely, being replaced by a broad and intense absorption band with a maximum at 550 nm. The emergence of this absorption band is closely associated with the LSPR absorption of the Au NPs in the visible region.47 The PLIP method successfully incorporates Au nanoparticles into the MoS2 matrix, endowing the hybrid with new optical properties. The difference of optical property between layered MoS2 and Au–MoS2 NCP may be attributed to the delocalized nature of the Mo–S bonds in the rock salt structure leads to a reorganization of the electron cloud in MoS2, resulting in a redistribution of the electronic energy levels.48 Additionally, the local electric field effect alters the distribution of electronic states within MoS2. The combined influence of these microscopic effects reshapes the band structure of MoS2, eliminating its original A1 and B1 electronic transition levels. Instead, a new absorption band arises due to the LSPR effect. This remarkable change in the absorption behavior directly reflects the reshaping of electronic processes within the Au–MoS2 nanoheterostructure.

Figure 2 (a) UV–vis spectra of sonication–exfoliated MoS2 (black) showing A1 and B1 excitonic peaks, and Au–MoS2 NCPs (red) exhibiting a broad plasmonic band centered at 550 nm. (b) XRD patterns of sonication–exfoliated MoS2 (black) with peaks corresponding to 2H phase, and Au–MoS2 (red) with additional Au diffraction peaks at 38.2 and 44.4°. The slight protrusion at 44.2° in Au–MoS2 is assigned to residual ordered MoS2 (006) planes.

Figure 2b presents the XRD patterns of the fabricated MoS2 using the sonication-assisted exfoliation method and the PLIP-fabricated Au–MoS2 NCPs. The black XRD pattern of fabricated MoS2 exhibits characteristic peaks at around 2θ values of 14.4, 39.6, 44.2, and 49.8°, corresponding to the (002), (103), (006), and (105) planes, respectively. The positions and relative intensities of these diffraction peaks align well with the standard card (JCPDS no. 37-1492) of the 2H–MoS2 hexagonal phase, confirming the successful preparation of 2H–MoS2.49 Notably, the intense (002) peak arises from the layered structure of MoS2, where the (002) planes are exposed on the surface. For the Au–MoS2 NCP (red XRD pattern in Figure 2b), in addition to the characteristic peaks of MoS2, two distinct diffraction peaks emerge at 38.2 and 44.4°, assigned to the (111) and (200) planes of the face-centered cubic phase of Au (JCPDS no. 04-0784),50 verifying the successful incorporation of Au nanoparticles into the MoS2 matrix via the PLIP method. Compared to as fabricated MoS2, the Au–MoS2 sample exhibits a significant decrease in the intensity of the (002) peak, possibly due to the intercalation of Au NPs between the MoS2 layers during the PLIP process, disrupting the long-range stacking order along the c-axis.51 Additionally, the presence of Au NPs on the surface of MoS2 sheets could lead to X-ray absorption or scattering effects, affecting the relative intensities of MoS2 peaks.43 The laser irradiation during PLIP might have also caused localized structural changes or defects in MoS2, further influencing its diffraction pattern.38 Interestingly, a slight protrusion is observed at around 44.2° on the shoulder of the 44.4° Au peak in the Au–MoS2 sample, which coincides with the (006) plane of layered MoS2. This suggests that despite the potential disruption of the layered stacking by Au incorporation, a small fraction of MoS2 nanosheets may have retained some degree of ordered stacking. The weak (006) peak could arise due to the coupling interaction between Au NPs and MoS2, potentially inducing partial rearrangement and preferential orientation of some MoS2 layers.52

Figure 3 presents the surface morphologies of sonication–exfoliated MoS2 and Au–MoS2 NCPs investigated by SEM Figure 3a reveals that the sonication–exfoliated MoS2 sample comprises a large quantity of stacked nanosheet structures, exhibiting the unique 2D layered morphology characteristic of MoS2. This morphology endows MoS2 with a high specific surface area, beneficial for charge separation and transport of photogenerated carriers. However, severe agglomeration and restacking of the MoS2 nanosheets are observed, likely induced by interlayer van der Waals interactions, which may reduce the accessible effective surface area to some extent. In contrast, the Au–MoS2 nanocomposite material in Figure 3b displays distinctly different morphological features. First, numerous small metallic nanoparticles with diameters ranging from 20 to 50 nm are observed decorating the surface of the MoS2 nanosheet matrix. Second, compared to the pristine MoS2 sample, the MoS2 nanosheets in the Au–MoS2 NCPs exhibit a certain degree of exfoliation and delamination, resulting in a more dispersed and separated nanosheet morphology with increased exposure of edge and basal plane surfaces. This deagglomeration effect is likely attributed to the incorporation of Au NPs, which disrupts the interlayer van der Waals interactions in MoS2, facilitating its exfoliation. Such morphological changes are highly beneficial for enhancing the photocatalytic performance of the Au–MoS2 NCPs. On one hand, the presence of Au nanoparticles imparts the material with excellent visible light absorption and electron trapping capabilities.47 On the other hand, the exfoliated and dispersed MoS2 nanosheets provide more efficient pathways for charge transport and separation of photogenerated carriers.45 These complementary effects synergistically promote the overall photocatalytic reaction. Therefore, the successful incorporation and surface decoration of Au NPs onto MoS2 nanosheets via the PLIP method not only endows the composite with unique optical properties but also optimizes its morphological structure, favoring enhanced photocatalytic activity. These findings will provide strong support for subsequent investigations of visible light photocatalytic performance and mechanistic analyses.53

Figure 3 SEM images of (a) sonication–exfoliated MoS2 showing stacked and agglomerated nanosheet morphology, and (b) Au–MoS2 NCPs exhibiting well-dispersed MoS2 nanosheets decorated with Au nanoparticles of 20–50 nm diameter.

TEM and high-resolution TEM (HRTEM) analyses provide direct insights into the nanostructural features of sonication–exfoliated MoS2 and the Au–MoS2 NCPs. The low-magnification TEM image in Figure 4a reveals that the sonication–exfoliated MoS2 sample comprises stacked nanosheet structures, consistent with the SEM observations. The high-resolution HRTEM image in Figure 4b further illustrates the atomic arrangement within the layered MoS2 structure, where each alternating bright and dark rectangular region represents the sandwich-like “S–Mo–S” configuration of a single MoS2 layer. Meticulous analysis yields an interlayer spacing of approximately 0.62 nm, closely matching the standard value of 0.615 nm for the 2H–MoS2 hexagonal phase,49 reaffirming the typical 2H phase structure. For the Au–MoS2 NCPs, the low-magnification TEM image in Figure 4c clearly displays few Au NPs uniformly distributed on the surface of the MoS2 nanosheets. The HRTEM image in Figure 4d reveals that Au NPs are anchored to the edges and interlayer regions of the MoS2 nanosheets, exhibiting intimate contact favorable for rapid charge transfer. Furthermore, the TEM–EDS elemental mapping in Figure 4e corroborates the precise distribution and coupling of Au and Mo elements at the nanoscale, providing direct evidence of the successful formation of the Au–MoS2 NCPs. Notably, the incorporation of Au NPs induces a certain degree of exfoliation and tilting of the MoS2 nanosheets, as observed in Figure 4d, exposing more edge and basal plane surfaces. This morphological change significantly increases the effective surface area, facilitating charge separation and transport, thereby enhancing the photocatalytic efficiency. In conjunction with the XRD results, the microscopic analyses provide a comprehensive understanding of the structural features of the composite photocatalyst. As previously discussed, the XRD patterns confirm the existence of the Au–MoS2 NCP structure through characteristic peak positions and intensities. Concurrently, the weakening of the MoS2(002) peak and the emergence of a faint protrusion near the Au(200) peak suggest that the incorporation of Au nanoparticles via the PLIP method induces structural distortions and reorganization within the layered MoS2 matrix. Such structural changes not only increase the surface area but may also modulate the band structure and active sites of MoS2 through mechanisms like quantum confinement effects, collectively promoting the overall photocatalytic performance.

Figure 4 (a) TEM image of sonication–exfoliated MoS2 showing stacked nanosheet structure. (b) HRTEM image highlighting the atomic configuration and 0.62 nm interlayer spacing of MoS2. (c) TEM image of Au–MoS2 NCPs with Au NPs distributed on MoS2 nanosheets. (d) HRTEM image of Au–MoS2 exhibiting Au nanoparticles anchored at MoS2 edges and interlayers. (e) TEM–EDS elemental mapping confirming the nanoscale distribution and coupling of Au and MoS2.

Figure 5a shows the photocatalytic degradation behavior of MB solution under visible light irradiation, using the Au–MoS2 NCPs as the photocatalyst. The absorbance of the MB solution gradually decreases with prolonged irradiation time, indicating the significant visible light photocatalytic activity of Au–MoS2 for effectively degrading MB molecules. Interestingly, a blue shift in the MB absorption peak is observed during the photocatalytic degradation process with Au–MoS2. This phenomenon suggests that MB may undergo an initial step where its conjugated aromatic ring structure is disrupted, potentially generating linear intermediates such as polyketone-like species.54−56 These intermediates, with reduced conjugation, would exhibit a blue-shifted absorption maximum. Subsequently, these intermediates are further oxidized and ring-opened, eventually leading to complete mineralization into small molecules like CO2 and H2O. Therefore, the blue shift in the MB absorption peak likely reflects the presence of degradation intermediates. Furthermore, when MB molecules undergo electrostatic or chemical adsorption interactions with the Au–MoS2 nanosheet surface, the specific surface forces may influence their molecular orbitals and electron cloud densities, resulting in a shift of the absorption maximum.54 This molecular–surface interaction is analogous to the effect of MB interacting with other ions or molecules in solution. Generally, MB exists predominantly in an aggregated state in solution, whereas it tends to adsorb as individual molecules on the catalyst surface. The absorption maxima of the aggregated and monomeric forms of MB can exhibit a blue or red shift due to differences in intra- and intermolecular conjugation effects. The observed blue-shifted peak may reflect the transition of MB from an aggregated to a monomeric state upon adsorption.

Figure 5 (a) Photocatalytic degradation of MB by Au–MoS2 under visible light, showing gradual absorbance decrease and a blue shift in the MB absorption peak. (b) Absorbance decay of MB/Au–MoS2 solution in the dark, attributed to MB adsorption onto the Au–MoS2 nanosheets.

Moreover, as shown in Figure 5b, a gradual decrease in absorbance is observed for the Au–MoS2/MB solution system even in the absence of light irradiation. This phenomenon is likely attributed to the adsorption of MB molecules onto the Au–MoS2 nanosheets, which often precedes and induces the subsequent photocatalytic reaction. Indeed, as a cationic dye molecule, MB is readily attracted to the negatively charged MoS2 surface through electrostatic interactions, facilitating chemical adsorption.56 The observed blue shift in the MB absorption peak further corroborates that upon electrostatic or chemical adsorption onto the Au–MoS2 nanosheet surface, MB molecules may experience changes in their molecular orbitals and electron cloud densities due to the specific surface forces exerted by the catalyst, leading to a shift or blue shift in their absorption maximum.54 This molecular–surface interaction is fundamentally similar to the effect observed when MB interacts with other ions or molecules in solution. As a negatively charged semiconductor material, MoS2 can readily attract the positively charged MB cations through electrostatic forces, potentially inducing a rearrangement of the electronic states and energy level structure of MB during the adsorption process, ultimately resulting in changes in its absorption spectrum.

Figure 6a further elucidates the influence of Au NPs loading on the photocatalytic degradation efficiency of MB over the Au–MoS2 NCPs. As the concentration of the Au precursor solution (HAuCl4) increases, the catalytic activity of the resulting Au–MoS2 samples gradually improves. The sample prepared using a 3 mM gold precursor (Au-3) exhibits optimal catalytic performance, achieving nearly 70% degradation of MB within 20 min of visible light irradiation. The Au-5 sample, synthesized with a 5 mM gold precursor, displays the highest photocatalytic activity, degrading approximately 80% of MB within the same time frame. In contrast, a lower gold precursor concentration (1 mM, Au-1 sample) results in a significantly reduced catalytic activity of the Au–MoS2 NCPs. These results indicate that a higher Au nanoparticle loading is more favorable for realizing the maximum photocatalytic potential of the Au–MoS2 hybrid material. An increased quantity of Au NPs not only enhances visible light absorption through LSPR effects but also provides additional pathways for photogenerated charge separation and transfer, thereby significantly boosting the overall photocatalytic activity. The above observations are further corroborated by electrochemical CV analyses, as shown in Figure 6b. With increasing gold precursor concentration, the Au–MoS2 samples exhibit more pronounced redox peak pairs in the CV curves, accompanied by a gradual increase in peak current intensities. This implies that a greater number of Au NPs are involved in the charge transfer processes, facilitating the photocatalytic reactions. Conversely, at lower Au loadings, the characteristic CV peaks become less distinct, indicating a reduced efficiency in charge carrier transfer. Collectively, these findings demonstrate that the PLIP synthesis method allows for precise control over the Au nanoparticle loading within the Au–MoS2 NSPs. An appropriate increase in Au content is beneficial for achieving optimal charge separation and transfer efficiencies, thereby maximizing the visible light photocatalytic activity. However, it is crucial to note that excessively high Au loadings beyond a certain threshold may lead to detrimental effects, such as nanoparticle aggregation, which could hinder the photocatalytic reactions. Therefore, maintaining an optimal Au content range is essential.

Figure 6 (a) Effect of Au loading on MB photodegradation over Au–MoS2 under visible light. Au-3 (3 mM Au) shows optimal ∼70% degradation in 20 min. Au-5 (5 mM Au) exhibits highest ∼80% degradation. Lower Au loading (Au-1, 1 mM) leads to much lower activity. (b) CV curves of Au–MoS2 with varying Au loadings. Higher Au content enhances redox peak intensities, indicating improved charge transfer for photocatalysis. Distinct redox peaks at high Au loadings, less pronounced peaks at low loadings.

As shown in Figure 7a, the absorption spectrum of MB alone under light irradiation exhibits no photodegradation. Similarly, the photocatalytic degradation efficiency of MB over sonication–exfoliated MoS2 under visible light irradiation is negligible, and no significant adsorption is observed in the absence of light, unlike the case of Au–MoS2. The lack of photocatalytic activity toward MB over sonication–exfoliated MoS2 under visible light illumination is likely due to the absence of effective visible light absorption and catalytic active sites. This adsorption and activation effect can be attributed to the localized electric field enhancement induced by the Au NPs, enhancing the adsorption and activation of organic molecules on the MoS2 surface. Consequently, the Au-modified MoS2 exhibits improved visible-light photocatalytic activity toward MB degradation, and the adsorption of MB is more pronounced compared to pristine MoS2. It is noteworthy that in the absence of light irradiation, the concentration of MB in the Au–MoS2/MB mixed solution slowly decreases, which can be ascribed to the adsorption of MB molecules on the Au–MoS2 surface, a common precursor step for photocatalytic reactions. Compared to sonication–exfoliated MoS2, the Au-modified MoS2 NCP demonstrates superior visible-light photocatalytic activity for MB degradation. This highlights that the introduction of Au nanoparticles not only endows the material with excellent visible light responsivity but also potentially promotes the visible-light photocatalytic reaction through synergistic mechanisms, thereby enhancing the overall catalytic efficiency. The schematic, as shown in Figure 7b, illustrates the photocatalytic degradation mechanism of MB dye over Au–MoS2 NCPs under visible light irradiation. Upon white light illumination, the LSPR effect in the Au NPs is excited, leading to the generation of hot electrons that can be injected into the CB of the MoS2 nanosheets. The injected electrons in the MoS2 CB participate in photoreduction reactions with adsorbed oxygen molecules, forming superoxide radical anions (O2•–). On the other hand, the holes in the VB of MoS2 can oxidize water or hydroxide ions to produce highly reactive hydroxyl radicals (OH–). These ROS, including O2•– and OH–, are responsible for the degradation of the adsorbed MB dye molecules through oxidation reactions. Notably, the adsorption of positively charged MB molecules on the negatively charged MoS2 surface is facilitated by electrostatic attractions, ensuring close proximity between the dye and the catalytically active sites. This adsorption process plays a crucial role in enhancing the photocatalytic degradation efficiency by enabling efficient charge transfer and ROS-mediated oxidation reactions at the interface. In addition to the plasmonic effects of Au NPs, the formation of a Schottky barrier at the Au–MoS2 interface may also contribute significantly to the enhanced photocatalytic activity of the Au–MoS2 NCPs.54 Considering the small band gap of MoS2, the LSPR effect of Au NPs may have a limited contribution to the overall photoactivity. Instead, the Schottky barrier formed between Au and MoS2 could play a key role in the efficient separation and transfer of photogenerated charge carriers. The Schottky barrier arises from the band alignment between the Au NPs and the MoS2 nanosheets, creating an internal electric field at the interface. This electric field promotes the directional migration of photogenerated electrons from MoS2 to Au, while the holes remain in the MoS2 VB. The spatial separation of electrons and holes effectively suppresses their recombination, leading to an enhanced photocatalytic activity. Moreover, the Schottky barrier can also lower the activation energy for charge transfer across the Au–MoS2 interface, further facilitating the photocatalytic reactions.

Figure 7 (a) Photocatalytic degradation profiles of MB under different conditions. MB/light (black) shows no degradation of MB without a catalyst under light irradiation. MB/Au–MoS2/dark (red) exhibits adsorption of MB onto the Au–MoS2 NCPs. MB/Au–MoS2/light (blue) demonstrates efficient photocatalytic degradation of MB over the Au–MoS2 NCPs under white light irradiation. MB/MoS2/light (green) shows no absorption or photocatalytic degradation of MB with sonication–exfoliated MoS2. (b) Proposed mechanism for photocatalytic degradation of MB over the Au–MoS2 NCPs under white light irradiation. Au NPs facilitate light absorption and charge separation via LSPR, while MoS2 nanosheets act as the semiconductor. Photogenerated electrons reduce O2 to superoxide radicals (O2•–), and holes oxidize H2O/OH– to hydroxyl radicals (•OH), enabling MB degradation.

4 Conclusions

The results demonstrate the successful synthesis of Au–MoS2 heterogeneous NCP photocatalysts via the PLIP method and elucidate their efficient visible-light-driven mechanism for the degradation of MB dye. The incorporation of an optimal amount of Au NPs significantly enhances the photocatalytic activity of MoS2 through several synergistic pathways. First, the plasmonic Au NPs improve the visible light harvesting capability of the composite. Second, hot electron injection from the Au NPs promotes charge separation within the heterojunction. Third, the Au/MoS2 nanoscale interface facilitates favorable charge transfer between the two components. Furthermore, the electrostatic adsorption of cationic MB molecules onto the negatively charged Au–MoS2 surface enhances interfacial reactions, contributing to the overall photocatalytic efficiency. These complementary effects synergistically boost the visible-light photocatalytic performance of the Au–MoS2 NPs toward efficient MB degradation. The photogenerated electrons and holes participate in the generation of superoxide and hydroxyl radicals, respectively, subsequently initiating the oxidative degradation of MB. Therefore, rational design of the Au–MoS2 heterojunction structure, coupled with tuning the Au content and charge state of MoS2, offers a promising strategy for developing highly efficient visible-light-driven nanocomposite photocatalytic systems with potential applications in organic pollutant remediation and energy catalysis. The results provide valuable insights for developing efficient visible-light-driven metal–semiconductor composite photocatalysts, suggesting that tuning the noble metal component loading can optimize charge separation and transfer pathways, thereby maximizing photocatalytic activity. The complementary characterization techniques provide mutually corroborating evidence, unambiguously demonstrating the efficient integration of Au NPs with MoS2 nanosheets via the PLIP method. The rational design and optimization of Au–MoS2 heterojunctions, coupled with tuning the Au content and charge state of MoS2, offer a promising strategy for developing highly efficient visible-light-driven nanocomposite photocatalytic systems. These Au–MoS2 NCPs hold immense potential for applications in organic pollutant remediation, water splitting for hydrogen production, CO2 reduction, and other energy conversion processes driven by visible light irradiation. The insights gained from this study pave the way for the design and development of advanced nanocomposite photocatalysts for addressing environmental and energy challenges.

The authors declare no competing financial interest.

Acknowledgments

This research was funded by National Science and Technology Council, grant nos MOST 110-2221-E-260-008-MY3 and NSTC 111-2113-M-040-001.
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