Plasmonic Au–MoS2 Nanohybrids Using Pulsed Laser-Induced Photolysis Synthesis for Enhanced Visible-Light Photocatalytic Dye Degradation

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.


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.−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 TiO 2 , exhibit limited absorption and utilization capabilities in this region due to visible light's relatively low energy. 10Therefore, 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.TiO 2 has been a research priority due to its exceptional photocatalytic performance. 11However, with a bandgap of approximately 3.2 eV, TiO 2 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-TiO 2 photocatalytic materials, such as ZnO, Fe 2 O 3 , CdS, and ZnS. 12−19 As a layered semiconductor with a suitable bandgap in the visiblelight region, MoS 2 exhibits tremendous potential in visiblelight photocatalysis and organic pollutant degradation.However, MoS 2 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.
MoS 2 is a typical two-dimensional (2D) layered semiconductor belonging to the transition metal dichalcogenide family. 20Its unique properties have garnered significant attention in optoelectronics, energy storage, biosensing, nanoelectronics, and photocatalysis.The tunable bandgap structure of MoS 2 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 MoS 2 , 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 MoS 2 include controlling its morphology and structure, 23 doping, 24 forming heterojunctions, 25 modifying with carbon nanomaterials, 26 and compositing with noble metal nanoparticles. 27,28The photocatalytic process of MoS 2 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 MoS 2 surface, generating reactive oxygen species (ROS) such as O 2 − and OH radicals.Recent studies have highlighted the potential of metalloaded semiconductor photocatalysts, 29−37 particularly Au nanoparticles (NPs) composited with MoS 2 , for enhanced visible light photocatalysis.−42 The synergistic effects of MoS 2 's optical and electronic properties, Au NPs' localized surface plasmon resonance (LSPR), and the formation of Schottky junctions at the Au/MoS 2 interface contribute to the enhanced performance of these NCPs.Au−MoS 2 is a heterogeneous NCP material formed by the integration of Au NPs with MoS 2 nanosheets, achieving a synergistic combination of the advantages of both materials.Optically, MoS 2 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−MoS 2 NCPs with exceptional visible light absorption and photoelectric conversion capabilities.
Compared to traditional photocatalytic materials (such as TiO 2 and CdS), Au−MoS 2 exhibits superior photocatalytic activity, primarily arising from the intrinsic excellent optical and electronic properties of MoS 2 , the LSPR surface effect of Au NPs, and the formation of Schottky junctions at the Au/ MoS 2 interface, favoring charge separation and transfer.Additionally, MoS2's relatively high chemical stability and the protective effect of Au NPs modification render the Au− MoS 2 composite more stable during photocatalytic reactions.−37 The remarkable photocatalytic activity of Au−MoS 2 NCPs can be attributed to the large surface area of MoS 2 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−MoS 2 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 MoS 2 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−MoS 2 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−MoS 2 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 MoS 2 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 MoS 2 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 (HAuCl 4 ) precursor effectively increases the loading of Au nanoparticles, thereby enhancing the photocatalytic degradation performance of the Au−MoS 2 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−MoS 2 NCPs photocatalysts on their photocatalytic performance.This research provides theoretical guidance and experimental evidence for the development of efficient visible light-driven photocatalysts.

EXPERIMENTAL SECTION
Layered MoS 2 was prepared using a sonication-assisted exfoliation method from bulk MoS 2 crystals in an aqueous surfactant solution, following a procedure adapted from previous studies. 43Initially, 250 mg of MoS 2 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 MoS 2 nanosheet dispersion.Subsequent to ultrasonication, the dispersion was centrifuged at 3000 rpm for 30 min to separate the green supernatant containing the exfoliated MoS 2 nanosheets from the bulk MoS 2 .The supernatant was further centrifuged at 12,000 rpm for 30 min to isolate the layered MoS 2 .To eliminate sodium cholate adsorbed on the nanosheet surfaces, the isolated MoS 2 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 MoS 2 .To ensure high reproducibility for optical absorption detection, the stock solution of layered MoS 2 was utilized after sonication treatment for 2 min.The experimental procedure for preparing Au−MoS 2 NCPs is as follows.First, a nanosecond pulsed laser was employed to induce the photolysis reaction of auric acid (HAuCl 4 ) in an aqueous solution in the presence of a reducing agent (H 2 O 2 ), with layered MoS 2 prepared by a sonicationassisted exfoliation method.The specific reaction mechanism for the formation of Au NPs has been described in other studies. 44To fabricate Au−MoS 2 NCPs, 1 mL of MoS 2 at a concentration of 0.1 mg/mL was added to the auric acid/H 2 O 2 precursors containing various concentrations of HAuCl 4 •3H 2 O (1 mL fixed volume), as well as a fixed concentration (2 mM) and volume (1 mL) of H 2 O 2 .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/cm 2 .The laser beam was directed at the middle of the precursor solution to ensure uniform light exposure.This method primarily involves the addition of H 2 O 2 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−MoS 2 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 (C 0 ).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−MoS 2 solution as working electrode, conducting carbon as the counter electrode, and Ag as the pseudo reference electrode.

RESULTS AND DISCUSSION
The absorption spectrum is one of the most crucial properties of layered MoS 2 .As a 2D material, the absorption spectrum of MoS 2 is closely related to its layered structure and electronic configuration.The visible light absorption of layered MoS 2 primarily originates from its band structure, which comprises both direct and indirect bandgaps.For monolayer MoS 2 , the direct bandgap is approximately 1.8−1.9eV, while the indirect bandgap is around 1.2−1.3eV. 45These bandgaps give rise to distinct absorption characteristics in the visible range, which are of significant importance for applications such as photocatalysis.When illuminated, MoS 2 absorbs light, leading to the excitation of electrons from the VB to the CB, forming electron−hole pairs.Within the band structure of MoS 2 , there exist different states, denoted as A1 and B1, representing distinct bandgap transitions associated with specific energy level changes during the electron excitation process. 46The 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 MoS 2 and Au−MoS 2 NCPs. Figure 2a displays the absorption spectra of the fabricated MoS 2 using the sonication-assisted exfoliation method and the PLIP-fabricated Au−MoS 2 NCPs over the wavelength range of 200−800 nm.The black curve represents the layered MoS 2 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 MoS 2 . 45The presence of these peaks confirms the existence of MoS 2 nanosheets with different layer numbers in the prepared sample.Beyond 700 nm in the near-infrared region, the absorption of MoS 2 rapidly decreases.In contrast, the absorption spectrum of the Au−MoS 2 (red curve) undergoes a significant change.The original A1 and B1 characteristic peaks of MoS 2 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. 47The PLIP method successfully incorporates Au nanoparticles into the MoS 2 matrix, endowing the hybrid with new optical properties.The difference of optical property between layered MoS 2 and Au−MoS 2 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 MoS 2 , resulting in a redistribution of the electronic energy levels. 48Additionally, the local electric field effect alters the distribution of electronic states within MoS 2 .The combined influence of these microscopic effects reshapes the band structure of MoS 2 , 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−MoS 2 nanoheterostructure.
Figure 2b presents the XRD patterns of the fabricated MoS 2 using the sonication-assisted exfoliation method and the PLIPfabricated Au−MoS 2 NCPs.The black XRD pattern of fabricated MoS 2 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− MoS 2 hexagonal phase, confirming the successful preparation of 2H−MoS 2 . 49Notably, the intense (002) peak arises from the layered structure of MoS 2 , where the (002) planes are exposed on the surface.For the Au−MoS 2 NCP (red XRD pattern in Figure 2b), in addition to the characteristic peaks of MoS 2 , 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 MoS 2 matrix via the PLIP method.Compared to as fabricated MoS 2 , the Au−MoS 2 sample exhibits a significant decrease in the intensity of the (002) peak, possibly due to the intercalation of Au NPs between the MoS 2 layers during the PLIP process, disrupting the long-range stacking order along the c-axis. 51dditionally, the presence of Au NPs on the surface of MoS 2 sheets could lead to X-ray absorption or scattering effects, affecting the relative intensities of MoS 2 peaks. 43The laser irradiation during PLIP might have also caused localized structural changes or defects in MoS 2 , further influencing its diffraction pattern. 38Interestingly, a slight protrusion is observed at around 44.2°on the shoulder of the 44.4°Au peak in the Au−MoS 2 sample, which coincides with the (006) plane of layered MoS 2 .This suggests that despite the potential disruption of the layered stacking by Au incorporation, a small fraction of MoS 2 nanosheets may have retained some degree of ordered stacking.The weak (006) peak could arise due to the coupling interaction between Au NPs and MoS 2 , potentially inducing partial rearrangement and preferential orientation of some MoS 2 layers. 52igure 3 presents the surface morphologies of sonication− exfoliated MoS 2 and Au−MoS 2 NCPs investigated by SEM Figure 3a reveals that the sonication−exfoliated MoS 2 sample comprises a large quantity of stacked nanosheet structures, exhibiting the unique 2D layered morphology characteristic of MoS 2 .This morphology endows MoS 2 with a high specific surface area, beneficial for charge separation and transport of photogenerated carriers.However, severe agglomeration and restacking of the MoS 2 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−MoS 2 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 MoS 2 nanosheet matrix.Second, compared to the pristine MoS 2 sample, the MoS 2 nanosheets in the Au−MoS 2 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 MoS 2 , facilitating its exfoliation.Such morphological changes are highly beneficial for enhancing the photocatalytic performance of the Au−MoS 2 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 MoS 2 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 MoS 2 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 TEM and high-resolution TEM (HRTEM) analyses provide direct insights into the nanostructural features of sonication− exfoliated MoS 2 and the Au−MoS 2 NCPs.The lowmagnification TEM image in Figure 4a reveals that the sonication−exfoliated MoS 2 sample comprises stacked nanosheet structures, consistent with the SEM observations.The high-resolution HRTEM image in Figure 4b    Figure 5a shows the photocatalytic degradation behavior of MB solution under visible light irradiation, using the Au−MoS 2 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−MoS 2 for effectively degrading MB molecules.Interestingly, a blue shift in the MB absorption peak is observed during the photocatalytic degradation process with Au−MoS 2 .This phenomenon suggests that MB may undergo an initial step where its conjugated aromatic ring structure is disrupted, potentially generating linear intermediates such as polyketonelike 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 CO 2 and H 2 O. 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−MoS 2 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.
Moreover, as shown in Figure 5b, a gradual decrease in absorbance is observed for the Au−MoS 2 /MB solution system even in the absence of light irradiation.This phenomenon is likely attributed to the adsorption of MB molecules onto the Au−MoS 2 nanosheets, which often precedes and induces the subsequent photocatalytic reaction.Indeed, as a cationic dye molecule, MB is readily attracted to the negatively charged MoS 2 surface through electrostatic interactions, facilitating chemical adsorption. 56The observed blue shift in the MB absorption peak further corroborates that upon electrostatic or chemical adsorption onto the Au−MoS 2 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. 54This 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, MoS 2 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−MoS 2 NCPs.As the concentration of the Au precursor solution (HAuCl 4 ) increases, the catalytic activity of the resulting Au−MoS 2 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−MoS 2 NCPs.These results indicate that a higher Au nanoparticle loading is more favorable for realizing the maximum photocatalytic potential of the Au−MoS 2 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−MoS 2 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−MoS 2 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.
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 MoS 2 under visible light irradiation is negligible, and no significant adsorption is observed in the absence of light, unlike the case of Au−MoS 2 .The lack of photocatalytic activity toward MB over sonication−exfoliated MoS 2 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 MoS 2 surface.Consequently, the Aumodified MoS 2 exhibits improved visible-light photocatalytic activity toward MB degradation, and the adsorption of MB is more pronounced compared to pristine MoS 2 .It is noteworthy that in the absence of light irradiation, the concentration of MB in the Au−MoS 2 /MB mixed solution slowly decreases, which can be ascribed to the adsorption of MB molecules on the Au−MoS 2 surface, a common precursor step for photocatalytic reactions.Compared to sonication−exfoliated MoS 2 , the Au-modified MoS 2 NCP demonstrates superior visiblelight 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−MoS 2 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 MoS 2 nanosheets.The injected electrons in the MoS 2 CB participate in photoreduction reactions with adsorbed oxygen molecules, forming superoxide radical anions (O 2 •− ).On the other hand, the holes in the VB of MoS 2 can oxidize water or hydroxide ions to produce highly reactive hydroxyl radicals (OH − ).These ROS, including O 2 •− 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 MoS 2 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−MoS 2 interface may also contribute significantly to the enhanced photocatalytic activity of the Au−MoS 2 NCPs. 54Considering the small band gap of MoS 2 , the LSPR effect of Au NPs may have a limited contribution to the overall photoactivity.Instead, the Schottky barrier formed between Au and MoS 2 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 MoS 2 nanosheets, creating an internal electric field at the interface.This electric field promotes the directional migration of photogenerated electrons from MoS 2 to Au, while the holes remain in the MoS 2 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−MoS 2 interface, further facilitating the photocatalytic reactions.

CONCLUSIONS
The results demonstrate the successful synthesis of Au−MoS 2 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 MoS 2 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/MoS 2 nanoscale interface facilitates favorable charge transfer between the two components.Furthermore, the electrostatic adsorption of cationic MB molecules onto the negatively charged Au− MoS 2 surface enhances interfacial reactions, contributing to the overall photocatalytic efficiency.These complementary effects synergistically boost the visible-light photocatalytic performance of the Au−MoS 2 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−MoS 2 heterojunction structure, coupled with tuning the Au content and charge state of MoS 2 , 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 MoS 2 nanosheets via the PLIP method.The rational design and optimization of Au−MoS 2 heterojunctions, coupled with tuning the Au content and charge state of MoS 2 , offer a promising strategy for developing highly efficient visible-lightdriven nanocomposite photocatalytic systems.These Au− MoS 2 NCPs hold immense potential for applications in organic pollutant remediation, water splitting for hydrogen production, CO 2 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.

■ AUTHOR INFORMATION
Figure 1 shows in detail the synthesis process and photocatalytic application of the Au− MoS 2 NCP photocatalyst.The as purchased MoS 2 powders are first subjected to ultrasonication and centrifugation to produce exfoliated MoS 2 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 MoS 2 , thereby yielding the Au−MoS 2 NCPs.A balland-stick model illustrates the structure of the Au−MoS 2 hybrid, showing uniform distribution of Au NPs on the layered MoS 2 matrix.The growth of Au NPs on the layered MoS 2 is facilitated by a 532 nm pulsed laser in a photolysis process involving H 2 O 2 and HAuCl 4 .

Figure 1 .
Figure 1.Schematic illustration of the synthesis of plasmonic Au−MoS 2 NCP photocatalysts and their photocatalytic applications.(Upper left) Exfoliation of layered MoS 2 into nanosheets by ultrasonication and centrifugation.(Center) Au nanoparticles uniformly decorated on the surface of MoS 2 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 H 2 O 2 and HAuCl 4 .The resultant plasmonic Au−MoS 2 hybrids exhibit exceptional light absorption and charge separation capabilities, enabling their application in photocatalytic degradation of organic dyes, MB.
further illustrates the atomic arrangement within the layered MoS 2 structure, where each alternating bright and dark rectangular region represents the sandwich-like "S−Mo−S" configuration of a single MoS 2 layer.Meticulous analysis yields an interlayer spacing of approximately 0.62 nm, closely matching the standard value of 0.615 nm for the 2H−MoS 2 hexagonal phase, 49 reaffirming the typical 2H phase structure.For the Au−MoS 2 NCPs, the low-magnification TEM image in Figure 4c clearly displays few Au NPs uniformly distributed on the surface of the MoS 2 nanosheets.The HRTEM image in Figure 4d reveals that Au NPs are anchored to the edges and interlayer regions of the MoS 2 nanosheets, exhibiting intimate

Figure 4 .
Figure 4. (a) TEM image of sonication−exfoliated MoS 2 showing stacked nanosheet structure.(b) HRTEM image highlighting the atomic configuration and 0.62 nm interlayer spacing of MoS 2 .(c) TEM image of Au−MoS 2 NCPs with Au NPs distributed on MoS 2 nanosheets.(d) HRTEM image of Au−MoS 2 exhibiting Au nanoparticles anchored at MoS 2 edges and interlayers.(e) TEM−EDS elemental mapping confirming the nanoscale distribution and coupling of Au and MoS 2 .

Figure 5 .
Figure 5. (a) Photocatalytic degradation of MB by Au−MoS 2 under visible light, showing gradual absorbance decrease and a blue shift in the MB absorption peak.(b) Absorbance decay of MB/Au−MoS 2 solution in the dark, attributed to MB adsorption onto the Au−MoS 2 nanosheets.

Figure 6 .
Figure 6.(a) Effect of Au loading on MB photodegradation over Au−MoS 2 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−MoS 2 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.

Figure 7 .
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−MoS 2 /dark (red) exhibits adsorption of MB onto the Au−MoS 2 NCPs.MB/Au−MoS 2 /light (blue) demonstrates efficient photocatalytic degradation of MB over the Au−MoS 2 NCPs under white light irradiation.MB/MoS 2 /light (green) shows no absorption or photocatalytic degradation of MB with sonication−exfoliated MoS 2 .(b) Proposed mechanism for photocatalytic degradation of MB over the Au−MoS 2 NCPs under white light irradiation.Au NPs facilitate light absorption and charge separation via LSPR, while MoS 2 nanosheets act as the semiconductor.Photogenerated electrons reduce O 2 to superoxide radicals (O 2 •− ), and holes oxidize H 2 O/OH − to hydroxyl radicals ( • OH), enabling MB degradation.