Epitaxial Single‐Layer MoS2 on GaN with Enhanced Valley Helicity

Engineering the substrate of 2D transition metal dichalcogenides can couple the quasiparticle interaction between the 2D material and substrate, providing an additional route to realize conceptual quantum phenomena and novel device functionalities, such as realization of a 12‐time increased valley spitting in single‐layer WSe₂ through the interfacial magnetic exchange field from a ferromagnetic EuS substrate, and band‐to‐band tunnel field‐effect transistors with a subthreshold swing below 60 mV dec⁻¹ at room temperature based on bilayer n‐MoS₂ and heavily doped p‐germanium, etc. Here, it is demonstrated that epitaxially grown single‐layer MoS₂ on a lattice‐matched GaN substrate, possessing a type‐I band alignment, exhibits strong substrate‐induced interactions. The phonons in GaN quickly dissipate the energy of photogenerated carriers through electron–phonon interaction, resulting in a short exciton lifetime in the MoS₂/GaN heterostructure. This interaction enables an enhanced valley helicity at room temperature (0.33 ± 0.05) observed in both steady‐state and time‐resolved circularly polarized photoluminescence measurements. The findings highlight the importance of substrate engineering for modulating the intrinsic valley carriers in ultrathin 2D materials and potentially open new paths for valleytronics and valley‐optoelectronic device applications.     

Tailoring MoS2 Valley‐Polarized Photoluminescence with Super Chiral Near‐Field

Transition metal dichalcogenides with intrinsic spin–valley degrees of freedom hold great potentials for applications in spintronic and valleytronic devices. MoS₂ monolayer possesses two inequivalent valleys in the Brillouin zone, with each valley coupling selectively with circularly polarized photons. The degree of valley polarization (DVP) is a parameter to characterize the purity of valley‐polarized photoluminescence (PL) of MoS₂ monolayer. Usually, the detected values of DVP in MoS₂ monolayer show achiral property under optical excitation of opposite helicities due to reciprocal phonon‐assisted intervalley scattering process. Here, it is reported that valley‐polarized PL of MoS₂ can be tailored through near‐field interaction with plasmonic chiral metasurface. The resonant field of the chiral metasurface couples with valley‐polarized excitons, and tailors the measured PL spectra in the far‐field, resulting in observation of chiral DVP of MoS₂‐metasurface under opposite helicities excitations. Valley‐contrast PL in the chiral heterostructure is also observed when illuminated by linearly polarized light. The manipulation of valley‐polarized PL in 2D materials using chiral metasurface represents a viable route toward valley‐polaritonic devices.     

Ruthenium Nanoparticles on Cobalt‐Doped 1T′ Phase MoS2 Nanosheets for Overall Water Splitting

2D MoS₂ nanostructures have recently attracted considerable attention because of their outstanding electrocatalytic properties. The synthesis of unique Co–Ru–MoS₂ hybrid nanosheets with excellent catalytic activity toward overall water splitting in alkaline solution is reported. 1T′ phase MoS₂ nanosheets are doped homogeneously with Co atoms and decorated with Ru nanoparticles. The catalytic performance of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is characterized by low overpotentials of 52 and 308 mV at 10 mA cm^?2 and Tafel slopes of 55 and 50 mV decade^?1 in 1.0 m KOH, respectively. Analysis of X‐ray photoelectron and absorption spectra of the catalysts show that the MoS₂ well retained its metallic 1T′ phase, which guarantees good electrical conductivity during the reaction. The Gibbs free energy calculation for the reaction pathway in alkaline electrolyte confirms that the Ru nanoparticles on the Co‐doped MoS₂ greatly enhance the HER activity. Water adsorption and dissociation take place favorably on the Ru, and the doped Co further catalyzes HER by making the reaction intermediates more favorable. The high OER performance is attributed to the catalytically active RuO₂ nanoparticles that are produced via oxidation of Ru nanoparticles.     

Intrinsic Ferromagnetism in Mn‐Substituted MoS2 Nanosheets Achieved by Supercritical Hydrothermal Reaction

Doping atomically thick nanosheets is a great challenge due to the self‐purification effect that drives the precipitation of dopants. Here, a breakthrough is made to dope Mn atoms substitutionally into MoS₂ nanosheets in a sulfur‐rich supercritical hydrothermal reaction environment, where the formation energy of Mn substituting for Mo sites in MoS₂ is significantly reduced to overcome the self‐purification effect. The substitutional Mn doping is convincingly evidenced by high‐angle annular dark‐field scanning transmission electron microscopy and X‐ray absorption fine spectroscopy characterizations. The Mn‐doped MoS₂ nanosheets show robust intrinsic ferromagnetic response with a saturation magnetic moment of 0.05 µ _B Mn^?1 at room temperature. The intrinsic ferromagnetism is further confirmed by the reversibility of the magnetic behavior during the cycle of incorporating/removing Li codopants, showing the critical role of Mn 3d electronic states in mediating the magnetic interactions in MoS₂ nanosheets.     

Giant Enhancements of Perpendicular Magnetic Anisotropy and Spin‐Orbit Torque by a MoS2 Layer

2D transition metal dichalcogenides have attracted much attention in the field of spintronics due to their rich spin‐dependent properties. The promise of highly compact and low‐energy‐consumption spin‐orbit torque (SOT) devices motivates the search for structures and materials that can satisfy the requirements of giant perpendicular magnetic anisotropy (PMA) and large SOT simultaneously in SOT‐based magnetic memory. Here, it is demonstrated that PMA and SOT in a heavy metal/transition metal ferromagnet structure, Pt/[Co/Ni]₂, can be greatly enhanced by introducing a molybdenum disulfide (MoS₂) underlayer. According to first‐principles calculation and X‐ray absorption spectroscopy (XAS), the enhancement of the PMA is ascribed to the modification of the orbital hybridization at the interface of Pt/Co due to MoS₂. The enhancement of SOT by the role played by MoS₂ is explained, which is strongly supported by the identical behavior of SOT and PMA as a function of Pt thickness. This work provides new possibilities to integrate 2D materials into promising spintronics devices.     

Conversion of Intercalated MoO3 to Multi-Heteroatoms-Doped MoS2 with High Hydrogen Evolution Activity

Lack of effective strategies to regulate the internal activity of MoS₂ limits its practical application for hydrogen evolution reactions (HERs). Doping of heteroatoms without forming aggregation or an edge enrichment is still challenging, and its effect on the HER needs to be further explored. Herein, a two-step method is developed to obtain multi-metal-doped H-MoS₂, which includes intercalation of the layered MoO₃ precursor with a following sulfurization. Benefiting from the capability of the intercalation method to uniformly and simultaneously introduce different elements into the van der Waals gap, this method is universal to obtain multi-heteroatoms co-doped MoS₂ without forming clusters, phase separation, and an edge enrichment. It is demonstrated that the doping of adjacent cobalt and palladium monomers on MoS₂ greatly enhances the HER catalytic activity. The overpotential at 10 mA cm⁻² and Tafel slope of Co and Pd co-doped MoS₂ is found to be 49.3 mV and 43.2 mV dec⁻¹, respectively, representing a superior acidic HER catalytic activity. This intercalation-assisted method also provides a new and general strategy to synthesize uniformly doped transition metal dichalcogenides for various applications.     

Probing magnetic-proximity-effect enlarged valley splitting in monolayer WSe<Subscript>2</Subscript> by photoluminescence

Possessing a valley degree of freedom and potential in information processing by manipulating valley features (such as valley splitting), group-VI monolayer transition metal dichalcogenides have attracted enormous interest. This valley splitting can be measured based on the difference between the peak energies of σ⁺ and σ^? polarized emissions for excitons or trions in direct band gap monolayer transition metal dichalcogenides under perpendicular magnetic fields. In this work, a well-prepared heterostructure is formed by transferring exfoliated WSe₂ onto a EuS substrate. Circular-polarization-resolved photoluminescence spectroscopy, one of the most facile and intuitive methods, is used to probe the difference of the gap energy in two valleys under an applied out-of-plane external magnetic field. Our results indicate that valley splitting can be enhanced when using a EuS substrate, as compared to a SiO₂/Si substrate. The enhanced valley splitting of the WSe₂/EuS heterostructure can be understood as a result of an interfacial magnetic exchange field originating from the magnetic proximity effect. The value of this magnetic exchange field, based on our estimation, is approximately 9 T. Our findings will stimulate further studies on the magnetic exchange field at the interface of similar heterostructures.

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Intrinsic excitonic emission and valley Zeeman splitting in epitaxial MS<Subscript>2</Subscript> (M = Mo and W) monolayers on hexagonal boron nitride

Two-dimensional (2D) semiconductors, represented by 2D transition metal dichalcogenides (TMDs), exhibit rich valley physics due to strong spin-orbit/spin-valley coupling. The most common way to probe such 2D systems is to utilize optical methods, which can monitor light emissions from various excitonic states and further help in understanding the physics behind such phenomena. Therefore, 2D TMDs with good optical quality are in great demand. Here, we report a method to directly grow epitaxial WS₂ and MoS₂ monolayers on hexagonal boron nitride (hBN) flakes with a high yield and high optical quality; these monolayers show better intrinsic light emission features than exfoliated monolayers from natural crystals. For the first time, the valley Zeeman splitting of WS₂ and MoS₂ monolayers on hBN has been visualized and systematically investigated. This study paves a new way to produce high optical quality WS₂ and MoS₂ monolayers and to exploit their intrinsic properties in a multitude of applications.

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Engineering 2D Metal–Organic Framework/MoS2 Interface for Enhanced Alkaline Hydrogen Evolution

2D metal–organic frameworks (MOFs) have been widely investigated for electrocatalysis because of their unique characteristics such as large specific surface area, tunable structures, and enhanced conductivity. However, most of the works are focused on oxygen evolution reaction. There are very limited numbers of reports on MOFs for hydrogen evolution reaction (HER), and generally these reported MOFs suffer from unsatisfactory HER activities. In this contribution, novel 2D Co‐BDC/MoS₂ (BDC stands for 1,4‐benzenedicarboxylate, C₈H₄O₄) hybrid nanosheets are synthesized via a facile sonication‐assisted solution strategy. The introduction of Co‐BDC induces a partial phase transfer from semiconducting 2H‐MoS₂ to metallic 1T‐MoS₂. Compared with 2H‐MoS₂, 1T‐MoS₂ can activate the inert basal plane to provide more catalytic active sites, which contributes significantly to improving HER activity. The well‐designed Co‐BDC/MoS₂ interface is vital for alkaline HER, as Co‐BDC makes it possible to speed up the sluggish water dissociation (rate‐limiting step for alkaline HER), and modified MoS₂ is favorable for the subsequent hydrogen generation step. As expected, the resultant 2D Co‐BDC/MoS₂ hybrid nanosheets demonstrate remarkable catalytic activity and good stability toward alkaline HER, outperforming those of bare Co‐BDC, MoS₂, and almost all the previously reported MOF‐based electrocatalysts.     

Stabilizing MoS2 Nanosheets through SnO2 Nanocrystal Decoration for High‐Performance Gas Sensing in Air

The unique properties of MoS₂ nanosheets make them a promising candidate for high‐performance room temperature sensing. However, the properties of pristine MoS₂ nanosheets are strongly influenced by the significant adsorption of oxygen in an air environment, which leads to instability of the MoS₂ sensing device, and all sensing results on MoS₂ reported to date were exclusively obtained in an inert atmosphere. This significantly limits the practical sensor application of MoS₂ in an air environment. Herein, a novel nanohybrid of SnO₂ nanocrystal (NC)‐decorated crumpled MoS₂ nanosheet (MoS₂/SnO₂) and its exciting air‐stable property for room temperature sensing of NO₂ are reported. Interestingly, the SnO₂ NCs serve as strong p‐type dopants for MoS₂, leading to p‐type channels in the MoS₂ nanosheets. The SnO₂ NCs also significantly enhance the stability of MoS₂ nanosheets in dry air. As a result, unlike other MoS₂ sensors operated in an inert gas (e.g. N₂), the nanohybrids exhibit high sensitivity, excellent selectivity, and repeatability to NO₂ under a practical dry air environment. This work suggests that NC decoration significantly tunes the properties of MoS₂ nanosheets for various applications.     

Spectroscopic study of Zn1−xCoxO thin films showing intrinsic ferromagnetism

Dilute magnetic semiconductors are widely studied due to their potential applications in spin-resolved electronics. We report the direct evidences of intrinsic ferromagnetism in the primarily ferromagnetic ZnO:Co thin films using near-edge X-ray absorption fine structure (NEXAFS) and soft X-ray magnetic circular dichroism (XMCD). The single phase Zn_1−xCo_xO thin films with nominal compositions (0.00 ≤ x ≤ 0.15) were synthesized by a spray pyrolysis technique, which exhibit room temperature ferromagnetism as revealed by alternating gradient force magnetometer (AGFM) measurements. The spectroscopic measurements indicate that most of Co dopants have substituted for Zn sites in ZnO matrix and they are present in divalent Co²⁺ (d⁷) state with tetrahedral symmetry according to the atomic multiplet calculations. The O 1s NEXAFS spectra suggest strong hybridization between O 2p and Co 3d electrons within ZnO matrix. The Co 2p XMCD measurements rule out the magnetism due to the presence of Co clusters, and show that Co–O–Co bonding provides localized magnetic moments leading to ferromagnetism.     

3D 1T‐MoS2/CoS2 Heterostructure via Interface Engineering for Ultrafast Hydrogen Evolution Reaction

Metallic phase (1T) MoS₂ has been regarded as an appealing material for hydrogen evolution reaction. In this work, a novel interface‐induced strategy is reported to achieve stable and high‐percentage 1T MoS₂ through highly active 1T‐MoS₂/CoS₂ hetero‐nanostructure. Herein, a large number of heterointerfaces can be obtained by interlinked 1T‐MoS₂ and CoS₂ nanosheets in situ grown from the molybdate cobalt oxide nanorod under moderate conditions. Owing to the strong interaction between MoS₂ and CoS₂, high‐percentage of metallic‐phase (1T) MoS₂ of 76.6% can be achieved, leading to high electroconductivity and abundant active sites compared to 2H MoS₂. Furthermore, the interlinked MoS₂ and CoS₂ nanosheets can effectively disperse the nanosheets so as to enlarge the exposed active surface area. The near zero free energy of hydrogen adsorption at the heterointerface can also be achieved, indicating the fast kinetics and excellent catalytic activity induced by heterojunction. Therefore, when applied in hydrogen evolution reaction (HER), 1T‐MoS₂/CoS₂ heterostructure delivers low overpotential of 71 and 26 mV at the current density of 10 mA cm^?2 with low Tafel slops of 60 and 43 mV dec^?1, respectively in alkaline and acidic conditions.     

Structural and Magnetic Properties of Transition Metal-Adsorbed MoS<Subscript>2</Subscript> Monolayer

The electronic and magnetic properties of transition metal (TM) atoms (TM = Co, Cu, Mn Fe, and Ni) adsorbed on a MoS₂ monolayer are investigated by density functional theory (DFT). Magnetism appears in the case of Co, Mn, and Fe. Among the three magnetic cases, the Co-adsorbed system has the most stable structure. Therefore, we further study the interaction in the two-Co-adsorbed system. Our results show that the interaction between the two Co atoms is always ferromagnetic (FM) and the p–d hybridization mechanism results in such ferromagnetic states. However, the FM interaction is obviously suppressed by increasing the Co–Co distance, which could be well explained by the Zener–Ruderman–Kittel–Kasuya–Yosida (RKKY) theory. Moreover, similar magnetic behavior is observed in the two-Mn-adsorbed system and a longrange FM state is shown. Such interesting phenomena suggest promising applications of TM-adsorbed MoS₂ monolayer in the future.     

Carrier Transport and Photoresponse in GeSe/MoS2 Heterojunction p–n Diodes

Simple stacking of thin van der Waals 2D materials with different physical properties enables one to create heterojunctions (HJs) with novel functionalities and new potential applications. Here, a 2D material p–n HJ of GeSe/MoS₂ is fabricated and its vertical and horizontal carrier transport and photoresponse properties are studied. Substantial rectification with a very high contrast (>10⁴) through the potential barrier in the vertical‐direction tunneling of HJs is observed. The negative differential transconductance with high peak‐to‐valley ratio (>10⁵) due to the series resistance change of GeSe, MoS₂, and HJs at different gate voltages is observed. Moreover, strong and broad‐band photoresponse via the photoconductive effect are also demonstrated. The explored multifunctional properties of the GeSe/MoS₂ HJs are expected to be important for understanding the carrier transport and photoresponse of 2D‐material HJs for achieving their use in various new applications in the electronics and optoelectronics fields.     

High-concentration dispersions of exfoliated MoS<Subscript>2</Subscript> sheets stabilized by freeze-dried silk fibroin powder

Liquid-phase exfoliation (LPE) is an attractive method for the scaling-up of exfoliated MoS₂ sheets compared to chemical vapor deposition and mechanical cleavage. However, the MoS₂ nanosheet yield from LPE is too small for practical applications. We report a facile method for the scaling-up of exfoliated MoS₂ nanosheets using freeze-dried silk fibroin powders. Compared to MoS₂ dispersion in the absence of silk fibroin powder, sonicated MoS₂ dispersions with silk fibroin powder (MoS₂/Silk dispersion) show noticeably higher exfoliated MoS₂ nanosheet yields, with suspended MoS₂ concentrations in MoS₂/Silk dispersions sonicated for 2 and 5 h of 1.03 and 1.39 mg·mL^–1, respectively. The MoS₂ concentration in the MoS₂/Silk dispersion after centrifugation above 10,000 rpm is more than four times that without the silk fibroin. The size of the dispersed silk fibroin is controlled by the change of centrifugation rate, showing the removal of silk fibroin above tens of micrometers in size after centrifugation at 2,000 rpm. Size-controlled silk fibroin biomolecules combined with MoS₂ nanosheets are expected to increase the practical use of such materials in fields related to tissue engineering, biosensors and electrochemical electrodes. Atomic force microscopy and Raman spectroscopy provide the height of the MoS₂ nanosheets spin-cast from MoS₂ /Silk dispersions, showing thicknesses of 3–6 nm. X-ray photoelectron spectroscopy and X-ray diffraction indicate that the outermost surface layer of the hydrophobic MoS₂ crystals interact with oxygen-containing functional groups that exist in the hydrophobic part of silk fibroins. The amphiphilic properties of silk fibroin combined with the MoS₂ nanosheets stabilize dispersions by enhancing solvent-material interactions. The large quantities of exfoliated MoS₂ nanosheets suspended in the as-synthesized dispersions can be utilized for the fabrication of vapor and electrochemical devices requiring high MoS₂ nanosheets contents.

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Ultrathin Transition Metal Dichalcogenide/3d Metal Hydroxide Hybridized Nanosheets to Enhance Hydrogen Evolution Activity

The vast majority of the reported hydrogen evolution reaction (HER) electrocatalysts perform poorly under alkaline conditions due to the sluggish water dissociation kinetics. Herein, a hybridization catalyst construction concept is presented to dramatically enhance the alkaline HER activities of catalysts based on 2D transition metal dichalcogenides (TMDs) (MoS₂ and WS₂). A series of ultrathin 2D‐hybrids are synthesized via facile controllable growth of 3d metal (Ni, Co, Fe, Mn) hydroxides on the monolayer 2D‐TMD nanosheets. The resultant Ni(OH)₂ and Co(OH)₂ hybridized ultrathin MoS₂ and WS₂ nanosheet catalysts exhibit significantly enhanced alkaline HER activity and stability compared to their bare counterparts. The 2D‐MoS₂/Co(OH)₂ hybrid achieves an extremely low overpotential of ≈128 mV at 10 mA cm^?2 in 1 m KOH. The combined theoretical and experimental studies confirm that the formation of the heterostructured boundaries by suitable hybridization of the TMD and 3d metal hydroxides is responsible for the improved alkaline HER activities because of the enhanced water dissociation step and lowers the corresponding kinetic energy barrier by the hybridized 3d metal hydroxides.     

Au NPs@MoS2 Sub‐Micrometer Sphere‐ZnO Nanorod Hybrid Structures for Efficient Photocatalytic Hydrogen Evolution with Excellent Stability

MoS₂ shows promising applications in photocatalytic water splitting, owing to its uniquely optical and electric properties. However, the insufficient light absorption and lack of performance stability are two crucial issues for efficient application of MoS₂ nanomaterials. Here, Au nanoparticles (NPs)@MoS₂ sub‐micrometer sphere‐ZnO nanorod (Au NPs@MoS₂‐ZnO) hybrid photocatalysts have been successfully synthesized by a facile process combining the hydrothermal method and seed‐growth method. Such photocatalysts exhibit high efficiency and excellent stability for hydrogen production via multiple optical‐electrical effects. The introduction of Au NPs to MoS₂ sub‐micrometer spheres forming a core–shell structure demonstrates strong plasmonic absorption enhancement and facilitates exciton separation. The incorporation of ZnO nanorods to the Au NPs@MoS₂ hybrids further extends the light absorption to a broader wavelength region and enhances the exciton dissociation. In addition, mutual contacts between Au NPs (or ZnO nanorods) and the MoS₂ spheres effectively protect the MoS₂ nanosheets from peeling off from the spheres. More importantly, efficiently multiple exciton separations help to restrain the MoS₂ nanomaterials from photocorrosion. As a result, the Au@MoS₂‐ZnO hybrid structures exhibit an excellent hydrogen gas evolution (3737.4 μmol g^?1) with improved stability (91.9% of activity remaining) after a long‐time test (32 h), which is one of the highest photocatalytic activities to date among the MoS₂ based photocatalysts.     

Synergetic Exfoliation and Lateral Size Engineering of MoS2 for Enhanced Photocatalytic Hydrogen Generation

Generally, exfoliation is an efficient strategy to create more edge site so as to expose more active sites on molybdenum disulphide (MoS₂). However, the lateral sizes of the resultant MoS₂ monolayers are relatively large (≈50–500 nm), which retain great potential to release more active sites. To further enhance the catalytic performance of MoS₂, a facile cascade centrifugation‐assisted liquid phase exfoliation method is introduced here to fabricate monolayer enriched MoS₂ nanosheets with nanoscale lateral sizes. The as‐prepared MoS₂ revealed a high monolayer yield of 36% and small average lateral sizes ranging from 42 to 9 nm under gradient centrifugations, all exhibiting superior catalytic performances toward photocatalytic H₂ generation. Particularly, the optimized monolayer MoS₂ with an average lateral size of 9 nm achieves an apparent quantum efficiency as high as 77.2% on cadmium sulphide at 420 nm. This work demonstrates that the catalytic performances of MoS₂ could be dramatically enhanced by synergistic exfoliation and lateral size engineering as a result of increased density of active sites and shortened charge diffusion distance, paving a new way for design and fabrication of transition‐metal dichalcogenides‐based materials in the application of hydrogen generation.     

Oxygen condensation stacking faults in crystallized Co found during magnetic annealing of Co-rich amorphous alloy

Amorphous Co_75.26–x Fe_4.74(BSi)_20+x (0 < x) magnetic alloys were examined with Transmission Electron Microscopy (TEM) after magnetic field annealing. TEM analysis revealed that the crystallized Co layer under the surface oxide could be highly faulted with planar defects depending on the composition. Based on the electron diffraction, we have proposed a new form of stacking fault in which two oxygen atoms are substituted for FCC Co in the {111} planes to create randomly distributed clusters of oxygen atoms on the Co {111} planes. Such clusters would explain the absent {200} peak in the highly faulted composition of crystallized FCC Co and the streaks observed in the electron diffraction pattern of the material. The oxygen fault was also closely related to the magnitude of the induced magnetic anisotropy of the material, suggesting the Co–O bonds acting as the localized antiferromagnetic region.     

0D–2D Quantum Dot: Metal Dichalcogenide Nanocomposite Photocatalyst Achieves Efficient Hydrogen Generation

Hydrogen generation via photocatalysis‐driven water splitting provides a convenient approach to turn solar energy into chemical fuel. The development of photocatalysis system that can effectively harvest visible light for hydrogen generation is an essential task in order to utilize this technology. Herein, a kind of cadmium free Zn–Ag–In–S (ZAIS) colloidal quantum dots (CQDs) that shows remarkably photocatalytic efficiency in the visible region is developed. More importantly, a nanocomposite based on the combination of 0D ZAIS CQDs and 2D MoS₂ nanosheet is developed. This can leverage the strong light harvesting capability of CQDs and catalytic performance of MoS₂ simultaneously. As a result, an excellent external quantum efficiency of 40.8% at 400 nm is achieved for CQD‐based hydrogen generation catalyst. This work presents a new platform for the development of high‐efficiency photocatalyst based on 0D–2D nanocomposite.